accellerometer faq 092003

8/3/2019 Accellerometer FAQ 092003

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Pressure Load Torque Acceleration Displacement Instrumentation

Frequently Asked Questions

Honeywel l Sensotec(800) 867-3892

2080 Arl ingate LaneColumbus, Ohio 43228USA

Tel: 614-850-6000Fax: 614-850-1111

www.honeywell.com/sensotecwww.sensotec.com


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Honeywel l Sensotec | www.sensotec.com | (800) 867-3892Copyr ight 2003 Honeywel l In ternat iona l Inc .

Table of Contents

TABLE OF CONTENTS .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

ACCELEROMETERS .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

HO W D O ES A P I EZO-ELEC TR I C AC C ELER O METER WO R K? . . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . 6WH AT AR E TH E D I FFER EN T TYPES O F AC C ELER O METER? . . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . . .. . . . . .. . . . 6WH AT I S A S I N G LE EN D ED C O MPR ESSI O N AC C ELER O METER? . . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . 7WH AT I S AN I SO LATED C O MPR ESSI O N AC C ELER O METER? . . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . . .. . . . . .. . . . 7WH AT I S A SH EAR TYPE AC C ELER O METER? . . . . . . .. . . . . .. . . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . . .. . . . . .. . . . . . . 8WH AT I S A P I EZO-R ESI ST I VE AC C ELER O METER? . . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . . .. 8WH AT I S A STR AI N G AG E BASED AC C ELER O METER? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9WH AT I S TH E U SEABLE FR EQ U EN C Y R AN G E? . . . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . 9WH AT I S AN IEPE AC C ELER O METER?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10WH AT I S AN ICP AC C ELER O METER? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11WH AT I S A C H AR G E O U TPU T AC C ELER O METER?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11WH AT I S TH E N ATU R AL FR EQ U EN C Y O F AN AC C ELER O METER?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11WH AT I S TH E MO U N TED N ATU R AL FR EQ U EN C Y?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

WH AT I S BASE STR AI N SEN SI T I V I TY? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13WH AT I S C R O SS SEN SI T I V I TY O R TR AN SVER SE SEN SI T I V I TY? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13WH AT I S D YN AMI C R AN G E?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14WH AT I S AMPL I TU D E L I N EAR I TY?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14WH AT AR E TH E D I FFER EN C ES BETWEEN Q U AR TZ C R YSTAL BASED AN D C ER AMI C C R YSTAL BASEDAC C ELER O METER S?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15HO W D O I MO U N T AN AC C ELER O METER? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15WH AT I S A Q U I C KF I T MO U N T?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17WH EN SH O U LD I U SE A VELO C I TY O U TPU T AC C ELER O METER? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17WH AT S I G N AL C O N D I T I O N I N G D O I N EED FO R MY AC C ELER O METER? . . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . . .. .18WH AT AR E G R O U N D I SO LATED AC C ELER O METER S? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19WH AT I S AN I SO LATED STU D?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22HO W D O I I N STALL A C H AR G E AMPLI F I ER? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22WH AT I S TH E TR I BO-ELEC TR I C EFFEC T?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23HO W D O I C H O O SE TH E SEN SI T I V I TY O F AN AC C ELER O METER? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24

WH AT I S TH E O U TPU T O F AN IEPE AC C ELER O METER? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24WH AT I S AN FFT? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24WH AT I S C O N D I T I O N MO N I TO R I N G? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25WH AT FR EQ U EN C Y R ESPO N SE D O I WAN T FR O M MY AC C ELER O METER?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25WH AT TYPE O F AC C ELER O METER BEST SU I TS MY APPL I C AT I O N? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28

CALIBRATION.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29

WH Y SH O U LD I C ALI BR ATE?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30WH AT I S NIST? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30WH AT I S A NIST TR AC EABLE C AL I BR AT I O N? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30CAN A LO AD C ELL BE MAD E TR AC EABLE TO NIST?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31CAN AN O R G AN I ZAT I O N BE NIST TR AC EABLE?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31WH AT I S A2LA O R NVLAP AC C R ED I TAT I O N?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32WH AT I S U N C ER TAI N TY? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32

WH EN I S U N C ER TAI N TY I MPO R TAN T?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32HO W I S U N C ER TAI N TY MEASU R ED? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33

CALIBRATION CLASS LOAD CELLS .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

WH AT D O ES A LO AD C ELL C AL I BR AT I O N C O N SI ST O F? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35WH AT LO AD C ELL C AL I BR AT I O N STAN D AR D SH O U LD I AD O PT? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35WH AT AR E I MPO R TAN T PAR AMETER S FO R A C AL I BR AT I O N C LASS LO AD C ELL?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35WH AT AR E TH E C H AR AC TER I ST I C S O F A LO W U N C ER TAI N TY C AL I BR AT I O N C LASS LO AD C ELL? . . .. . .. . .. 36WH Y D O ES A CAL I BR AT I O N CLASS LO AD C ELL H AVE A BASE PLATE AN D A C AL I BR AT I O N AD APTER?. . . .36WH AT I S TH E U N C ER TAI N TY O F A SEN SO TEC LO AD C ELL? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37WH AT U N C ER TAI N TY SH O U LD MY C AL I BR AT I O N R EFER EN C E LO AD C ELL H AVE?. . . . . . . . . . . . . . . . . . . . . . . . . . . . .37


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DO ES A LO AD C ELL H AVE TO BE C AL I BR ATED WI TH I TS D I SPLAY? . . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . .38WH AT D O ES TH E ASTM E74 STAN D AR D SPEC I F I C ALLY SAY ABO U T C AL I BR AT I O N C LASS LO AD C ELLS?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38

GENERAL.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .40

HO W D O I KN O W WH AT AC C U R AC Y C LASS TO U SE FO R MY SEN SO R? . . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . . .. .41WH AT I S SEN SI T I V I TY? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41WH AT I S NON -L I N EAR I TY? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43WH AT I S HYSTER ESI S? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46WH AT I S STAT I C ER R O R BAN D? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46WH AT I S CAL I BR AT I O N FAC TO R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47WH EN SH O U LD I F I T A CO N N EC TO R O R IN TEG R AL CABLE? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48WH AT I S TH E D I FFER EN C E BETWEEN SU BMER SI BLE AN D WATER PR O O F? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50WH AT AR E NEMA AN D IP DEFI N I T I O N S FO R EN VI R O N MEN TAL PR O TEC TI O N?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51

LOAD CELLS .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53

WH AT I S OVER LO AD PR O TEC TI O N O N A LO AD CELL?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54WH AT I S A CO MPR ESSI O N ON LY LOAD CELL?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55WH AT I S A TEN SI O N ONLY LOAD CEL L?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .56WH AT I S A TEN SI O N AN D CO MPR ESSI O N ONL Y LOAD CEL L?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .58WH AT I S A RO D END BEAR I N G?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .58WH AT I S A LO AD BU TTO N?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .59WH AT I S LOAD CELL SYMMETR Y? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60

WH AT I S ZER O BALAN C E FO R A LO AD C ELL?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .61WH AT I S ZER O BALAN C E TEMPER ATU R E EFFEC T? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62WH AT I S OU TPU T SPAN TEMPER ATU R E EFFEC T?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63WH EN SH O U LD I H AVE ZER O AN D SPAN AD JU STMEN TS O N MY LO AD C ELL?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65WH AT AR E TH E AD VAN TAG ES AN D D I SAD VAN TAG ES O F H AVI N G A SEN SO R I N TER N AL AMPLI F I ER O N ALO AD C ELL? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .66WH Y I S TH E O U TPU T O F MY PR ESSU R E D ETEC TO R Q U O TED I N MV/V?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69HO W D O I KN O W WH AT AC C U R AC Y C LASS TO U SE FO R MY SEN SO R? . . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . . .. .70HO W DOES TEMPER ATU R E AFFEC T A LO AD C ELL? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71HO W D O YO U C O MPEN SATE FO R TEMPER ATU R E I N A LO AD C ELL?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .71WH AT I S TH E TEMPER ATU R E C O MPEN SATI O N R AN G E?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .71WH AT I S TH E TEMPER ATU R E O PER ATI N G R AN G E? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71HO W D O I PI C K TH E R I G H T FU LL SC ALE O U TPU T FO R A LO AD C ELL? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .72WH AT I S TH E EFFEC T O F DYN AMI C LO AD S O N A LO AD C ELL?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .72HO W D O ES A LO AD C ELL WO R K? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73

WH AT LO AD R AN G E SH O U LD I C H O SE FO R A LO AD C ELL? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73WH AT TH I N G S D O I N EED TO C O N SI D ER WH EN MO U N TI N G A LO AD CELL? . . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . . .. .74

PRESSURE SENSORS.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .76

HO W DO ES A BO N D ED FO I L STR AI N GAGE -BASED PR ESSU R E SEN SO R WOR K? . . . . . . .. . . . . .. . . . . .. . . . . . ..77HO W DO ES A S I L I C O N-BASED PR ESSU R E SEN SO R WO R K? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .78WH AT AR E AD VAN TAG ES BETWEEN BO N D ED FO I L STR AI N GAG E -BASED AN D S I L I C O N -BASEDPR ESSU R E SEN SO R S? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .79WH AT I S A GAG E PR ESSU R E SEN SO R?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .79WH AT I S A TR UE GAGE PR ESSU R E SEN SO R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .80WH AT I S AN ABSO LU TE PR ESSU R E SEN SO R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .81WH EN SH O U LD I U SE AN ABSO LU TE PR ESSU R E SEN SO R R ATH ER TH AN A G AG E PR ESSU R E SEN SO R? . . 82WH AT I S A D I FFER EN TI AL PR ESSU R E SEN SO R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83WH AT I S A VAC U U M PR ESSU R E SEN SO R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .83WH AT I S A BAR O METR I C PR ESSU R E SEN SO R?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .84WH I C H PR ESSU R E REFER EN C E SH O U LD I U SE FO R MY APPL I C AT I O N? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .85WH AT I S OVER LO AD PR O TEC TI O N O N A PR ESSU R E SEN SO R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .86WH AT C O N SI D ER ATI O N S SH O U LD I MAKE WH EN MO U N TI N G A PR ESSU R E SEN SO R? . . . . . . .. . . . . .. . . . . . .. . . .87HO W C AN I PR O TEC T AG AI N ST WATER H AMMER? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88WH AT I S ZER O BALAN C E FO R A PR ESSU R E TR AN SD U C ER?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .89WH AT I S ZER O BALAN C E TEMPER ATU R E EFFEC T? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90WH AT I S OU TPU T SPAN TEMPER ATU R E EFFEC T?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .92WH EN SH O U LD I H AVE ZER O AN D SPAN AD JU STMEN TS O N MY PR ESSU R E D ETEC TO R? . . . . . .. . . . . .. . . . . . .94WH AT AR E TH E AD VAN TAG ES AN D D I SAD VAN TAG ES O F H AVI N G A SEN SO R I N TER N AL AMPLI F I ER O N APR ESSU R E D ETEC TO R?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .95


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WH Y I S TH E O U TPU T O F MY PR ESSU R E D ETEC TO R Q U O TED I N MV/V?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .97

TORQUE SENSORS .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

WH AT I S TO R Q U E? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101WH AT I S R EAC TI O N TO R Q U E?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101WH AT I S R O TAR Y TO R Q U E? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102HO W D O YO U MEASU R E R O TAR Y TO R Q U E? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102HO W D O R O TAR Y TO R Q U E SEN SO R S WO R K? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103WH EN SH O U LD I C H O O SE AN I N-L I N E SEN SO R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105WH EN SH O U LD I C H O O SE A C LAMP O N C O LLAR? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105WH EN SH O U LD I STR AI N G AG E MY SH AFT AS MY TO R Q U E TR AN SD U C ER? . . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . .. . 106WH AT AR E TH E MAJO R D I FFER EN C ES BETWEEN TH E VAR I O U S FO R MS O F TO R Q U E MEASU R EMEN T? . . 1 0 7


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ACCELEROMETERS


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HOW DOES A PIEZO -ELECTRIC ACCELEROMETER WORK?

Piezo-electric crystals are man-made or naturally occurring crystals thatproduce a charge output when they are compressed, f lexed or subjected toshear forces. The word piezo is a corruption of the Greek word for squeeze. In

a piezo-electric accelerometer a mass is attached to a piezo-electric crystal,which is in turn mounted to the case of t he accelerometer. When the body ofthe accelerometer is subjected to vibration the mass mounted on the crystalwants to stay st i l l i n space due to inert ia and so compresses and stretches thepiezo electric crystal. Thi s force causes a charge to be generated and due toNewton law F=ma this force is i n turn proport ional to acceleration. The chargeoutput is either is converted to a low impedance voltage output by the use ofintegral electronics (example: in an IEPE accelerometer) or made available as acharge output (Pico-coulombs /g) in a charge output piezo-electricaccelerometer.

WHAT ARE THE DIFFERENT TYPES OF ACCELEROMETER?

There are many different type of accelerometers and each has uniquecharacterist ics, advantages and disadvantages. The different types include:

Different technologiesPiezo-electric accelerometersPiezo-resist ive accelerometersStrain gage based accelerometers

Different output accelerometersCharge outputIEPE output (2-wire voltage)Voltage output (3 wire)4-20mA outputVelocity output accelerometers

Different designs of accelerometerShear type design


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Single ended compression designIsolated compressionInverted compressionFlexural design

WHAT IS A SINGLE ENDED COMPRESSION ACCELEROMETER?

A single ended compression accelerometer is where the crystal is mounted tothe base of the accelerometer and the mass is mounted to the crystal by asetscrew, bolt or fastener.

A single ended compression accelerometer

WHAT IS AN ISOLATED COMPRESSION ACCELEROMETER?

Single ended compression accelerometers can be susceptible to base strainand so to alleviate this problem the crystal is isolated from the base bymounting on an isolation washer or by reducing the mounting area by which thecrystal is mounted to the base.


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Isolated compression accelerometers

WHAT IS A SHEAR TYPE ACCELEROMETER?

A shear type accelerometer is where the seismic mass is attached to the crystal

so that it exerts a shear l oad on the crystal rather than a compressive load.Shear type accelerometers are designed for applications that are l ikely toencounter signif icant base distort ion from thermal transients or where they aremounted onto f lexible structures.

Shear type piezo-electric accelerometer

WHAT IS A PIEZO -RESISTIVE ACCELEROMETER?

A piezo-resist ive accelerometer is an accelerometer that uses a piezo-resist ivesubstrate in place of the piezo electric crystal and the force exerted by theseismic mass changes the resistance of the etched bridge network and awhetstone bridge network detects this. Piezo-resist ive accelerometers have theadvantage over piezo-electric accelerometers in that they can measureaccelerations down to zero Hertz.


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WHAT IS A STRAIN GAGE BASED ACCELEROMETER?

A strain gauged based accelerometer is based on detecting the deflection of a

seismic mass by using a si l icon or foi l strain gauged element. A whetstonebridge network detects the deflection. The deflection is dir ectly proport ional tothe acceleration applied to the sensor. Like the piezo-r esist ive accelerometer ithas a frequency response down to zero Hz.

WHAT IS THE USEABLE FREQUENCY RANGE?

For an accelerometer to be useful the output needs to be directly proport ionalto the acceleration that it is measuring. This f ixed ratio of output to input is onlytrue for a range of frequencies as described by the frequency response curve.


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Typical Piezo-electric frequency response curve.

The usable frequency response is the f lat area of the frequency response curveand extends to approximately 1/3 to of the natural frequency. The definit ion

of f lat also needs to be qualif ied and is done so by quoting the roll off of thecurve in either percentage terms (typically 5% or 10%) or in dB terms (typically+/- 3db)

WHAT IS AN IEPE ACCELEROMETER?

IEPE stands for Integrated Electronics Piezo Electric and defines a class ofaccelerometer that has built in electronics. Specif ically it defines a class ofaccelerometer that has low impedance output electronics that works on a twowire constant current supply with an voltage output on a DC voltage bias. IEPEtwo wire accelerometers are easy to install, have a wide frequency response,can run over long cable lengths and are relat ively cheap to purchase. The IEPEtechnology has generally replaced most 3 wire accelerometers and is broadlyused for most applications except for specialist applications such as zero Hz


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accelerometers, high temperature applications or 4-20mA accelerometers usedin the process industries.

WHAT IS AN ICP ACCELEROMETER?

ICP is the trademarked PCB name for IEPE accelerometers. I t stands forIntegrated circuit-piezo electric

WHAT IS A CHARGE OUTPUT ACCELEROMETER ?

All piezo-electric accelerometers work by measuring the charge generated by acrystal that is being compressed or shear loaded by a mass inf luenced byacceleration. In most applications this high impedance charge output isconverted to a low impedance voltage output by the use of integral electronics.However in some applications integral electronics are not appropriate such ashigh temperature or high radiation applications. Charge output accelerometersare self-generating and would typically have amplifying electronics mountedseveral feet away from the local heat or l ocal radiation source.

WHAT IS THE NATURAL FREQUENCY OF AN ACCELEROMETER?

The natural frequency of an accelerometer is the frequency where the ratio ofoutput is at i t highest. The natural fr equency of an accelerometer is defined by

the equation

From a frequency roughly 1/3 to of the natural frequency the ratio of outputto input becomes non-l inear and therefore makes measurements from thisregion diff icult to interpret. Therefore the higher the natural frequency of anaccelerometer the higher frequencies where the output to input is l inear and the

higher the frequencies that can be measured.


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I t can be seen from the formula for natural frequency that to i ncrease thenatural frequency the mass needs to be as small as possible and the st if fnessneeds to be as high as possible. A small mass usually means a lower sensit ivityand this is true of most high fr equency accelerometers.

WHAT IS THE MOUNTED NATURAL FREQUENCY?

An accelerometer has a different natural frequency when it is in free space tothat when it is mounted. The only frequency that is of interest to the user is ofcourse the mounted natural frequency and is often the one quoted in thespecif ications. The mounted natural frequency is of course dependent on thestif fness of the mounting structure to which i t is attached and is thereforequoted as the natural frequency of the accelerometer as installed according tomanufacturers instructions. Gluing, magnetically mounting or loose bolt ing

down to a surface wil l si gnif icantly reduce the mounted natural frequency.


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WHAT IS BASE STRAIN SENSITIVITY?

Base strain sensit ivity is the erroneous signal that is generated by anaccelerometer when the base is subjected to bending, torque or distort ioneither by mechanical movement or thermal str essing. The relat ive movement ofthe base of the accelerometer squeezes the crystal in an accelerometer and the

seismic mass mounted on the crystal. Base strain is where the base distorts themass while acceleration causes the seismic mass to distort the crystal. Thesetwo forces on the crystal are indist inguishable and so reduction of the basestrain is vital for good signals only to be generated. The more indirectly that acrystal is mounted to the base under strain the l ess sensit ive the accelerometeris to base strain. Single ended compression sensors are the most prone to basestrain sensit ivity and shear type accelerometers the least. Isolated compressionaccelerometers are a good compromise between have good base strainimmunity and the disadvantages that shear type accelerometers bring in termsof sensit ivity and robustness.

Base Strain Sensit ivity

WHAT IS CROSS SENSITIVITY OR TRANSVERSE SENSITIVITY?

An accelerometer produces a charge output when the crystal is compressed.That same crystal also produces a charge, albeit a much smaller one, when ashear load is exerted on the crystal. The accelerometer therefore produces acharge when it is vibrated in the axis 90 degrees to the main axis ofmeasurement, which is indist inguishable from acceleration in the main axis.Conversely shear type accelerometers produce an erroneous signal when theyexperience cross axis acceleration only this t ime it loads the crystal incompressive mode.


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Cross axis sensit ivity

The sensit ivity of the accelerometer to a transverse v ibration is known as thetransverse sensit ivity and is typically less than 5% of the sensit ivity to an onaxis acceleration.

WHAT IS DYNAMIC RANGE?

The dynamic range of an accelerometer is the range between the smallestacceleration detectable by the accelerometer to the largest. A piezo-electricaccelerometer produces a charge proport ional the force applied to the crystal,which due to the seismic mass on the crystal is proport ional to accelerationapplied. The piezo electric effect can be detected for very small forces oraccelerations all the way through to very l arge accelerations. In most cases thesmallest acceleration is dictated by the amplifying electronics noise f loor and

for high g levels to the voltage r ail used by the power supply. The design of theaccelerometer wil l also play a part in what shock g levels an accelerometer canwithstand before the crystal is ir reparably damaged or the structure holding thecrystal is distorted. Compression accelerometers are the most shock resistantdesign of accelerometer.

Accelerometers with integral electronics have a maximum output voltagedetermined by the circuit design and the input voltage. The maximum output foran IEPE accelerometer is typically 4-8 volts. An accelerometer with a sensit ivityof 100mV/g with electronics that has a maximum output of 5V wil l obviouslyhave a dynamic range of +/- 50g while an accelerometer of sensit ivity of10mV/g wil l have a dynamic range of +/- 500g

WHAT IS AMPLITUDE LINEARITY?

The amplitude l inearity of an accelerometer is the degree of accuracy that anaccelerometer reports the output in voltage terms as it moves from beingexcited at the smallest detectable acceleration levels to the highest. This


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accuracy is qualif ied by the l inearity. Typically the amplitude l inearity is 1%.The dynamic range describes the minimum to maximum accelerations that canbe detected. The output of an IEPE accelerometer can typically go from 100micro g to 500g. This dynamic range is dependent on the electronics used wi ththe accelerometer either internal or external, as is the output l inearity over thedynamic range.

WHAT ARE THE DIFFERENCES BETWEEN QUARTZ CRYSTAL BASED AND CERAMIC CRYSTALBASED ACCELEROMETERS?

Ceramic Crystals Quartz Crystals

Man made piezo electric crystals Natural piezo electric crystals

Higher output sensit ivity Lower output sensit ivity

Less expensive More expensive

Higher pyro-electric effect atelevated temperatures

Lower pyro-electric effect atelevated temperatures

Higher crystal decay rates atelevated temperatures

No crystal decay rates with t ime ortemperature

Lower temperature of operation Higher temperature operation

HOW DO I MOUNT AN ACCELEROMETER?

The mounting of an accelerometer affects its frequency response. The mountednatural frequency is dependent directly on the st if fness of the mounting. The

higher the st if fness the more the mounted natural frequency approaches itsmaximum. The least st if f mounting of an accelerometer is magnetic mountingand the highest st if fness is using a hi gh tensile setscrew t ightened to thecorrect torque mounted on a hard f lat surface. Other mounting methods come inbetween these two extremes.


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Incorrectly mounted accelerometer

WHAT IS A QUICKFIT MOUNT?

A quickfit mount is used in install at ions where the accelerometer wil l beremoved between monitoring the acceleration or velocity vibration yet wil l berepeatedly place back in the same location. Such installat ions i nclude machinehealth monitoring using data collectors.

Quickfit mounting for an accelerometer

WHEN SHOULD I USE A VELOCITY OUTPUT ACCELEROMETER?

Velocity output accelerometers are usually used in condit ion monitoringapplications where velocity is a much better parameter for judging the health ofa machine. Doubling of velocity vibration equates to a doubling of thedeterioration of the health of the machine. Velocity can also be used i n lowerfrequency applications where the acceleration amplitude of vibration is toosmall to measure and the velocity vibration maybe of a higher and more


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meaningful value. Velocity vibration accelerometers are only really effective if the frequency of vibration is higher than 2Hz but more ideally 5 Hz.

WHAT SIGNAL CONDITIONING DO I NEED FOR MY ACCELEROMETER?

All internally amplif ied accelerometers need a power supply be it a constantcurrent IEPE supply, a 4-20mA loop, a 10V bridge excitat ion or a bipolar +/-15V supply for a three wire accelerometer. The output of the accelerometer isnow condit ioned to an AC voltage whose amplitude is proport ional to theamplitude of vibration with a frequency the same as the frequency of thevibration. An AC voltage signal needs further signal condit ioning to retrieve anyuseful data.

This signal condit ioning takes three main forms:a) Overall voltage levels in either RMS or peak to peakb) Spectral content analysis

c) Snap shot t ime domain analysis

Overall acceleration levels in RMS ter ms

Overall acceleration levels in Peak-Peak terms


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Breaking the acceleration signal into its f requency components

Viewing the acceleration signal on a storage scope or t ransient recorder

WHAT ARE GROUND ISOLATED ACCELEROMETERS?

Ground loops can be a signif icant problem to all t ype of sensors where thesignal is un-amplif ied or the signal levels are low. Ground loops occur whendifferent parts of the structure lab or building have different electrical grounds.These grounds may only dif fer by a few mil l ivolts or less. When areas withdifferent grounds are connected by sensor cables then unless measures aretaken to prevent it a ground loop are set up in the cable that can be signif icantwhen compared to low level voltage signals that come from the sensor.


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Ground loops are often very diff icult to detect so it is prudent to takeprecautions to prevent their effects.

There are a number of ways that ground loops can be prevented. The f irst is tohard wire different parts of the str ucture to ensure that each area has exactlythe same ground.

Preventing ground loops by ensuring all parts of structure have same ground

Ensuring different parts of a plant have the same ground may not be so easypart icularly when long distances are involved or structures carry noisegenerating machinery. In these cases it may be better not to eliminate groundloops but to prevent their effects inf luencing the sensor output. This can beachieved by mounting the accelerometer on an electrically isolated mountingstud. In this way the accelerometer sits on a locally constructed instrument


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ground and ensures that now ground loop exists between this and themeasuring instrument.

Isolated mounting bases eliminate problems with ground loops

The same effect as mounting the accelerometer on an electrically isolatedmounting base can be achieved by isolating the accelerometer internals fromthe outer case of the accelerometer. This is done by t he manufacturer.Mounting the accelerometer on an isolating base or internally i solat ing theaccelerometer does reduce the st if fness of the accelerometer and thereforereduces the mounted natural frequency. I t is for this r eason that not allaccelerometers come automatically with internal isolat ion.

Internally isolated accelerometers can prevent ground loops but have a reducedfrequency response as a result


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WHAT IS AN ISOLATED STUD ?

An accelerometer isolated stud is used in application where the possibil i ty for

ground loops exists which can corrupt the output of the sensor. Isolated studsdo reduce the frequency response of the accelerometer somewhat so cautionshould be taken if high frequency data needs to be measured.

HOW DO I INSTALL A CHARGE AMPLIFIER?

Charge output accelerometers are used in applications where:High temperatures environments are encounteredHigh radiation environments are encounteredVery high frequency accelerometers are used where no room exists for internalelectronics

Charge output accelerometers are self-generating and so no excitat ion isrequired but a local charge amplif ier is used to convert the charge output to avoltage. The charge output accelerometers do however have high outputimpedance. This high output impedance makes them susceptible to noise, cablemovement (tr ibo-electric effect) and low insulation resistance. To minimizethese effects it is important to have; a charge amplif ier-impedance convertermounted as close to the accelerometer as possible, prevent cable movement,use low noise co-axial cable and ensure all surfaces are kept clean and dry.


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Install ing a charge output accelerometer and clamping low noise co-axial cable

Charge amplif ier is located as close to accelerometer as possible but away fromthe hosti le environment

WHAT IS THE TRIBO -ELECTRIC EFFECT?

Tribo-electric effect is when a spurious signal is generated by a charge outputaccelerometer by the movement of the co-axial cable. To prevent the tribo-electric effect the low noise c able needs to be clamped down as close to theaccelerometer as possible. See How do I install a charge amplif ier?

Tribo-electric effect


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HOW DO I CHOOSE THE SENSITIVITY OF AN ACCELEROMETER?

Accelerometers with integral electronics have a maximum output voltagedetermined by the circuit design and the input voltage. The maximum output foran IEPE accelerometer is typically 4-8 volts. An accelerometer with a sensit ivityof 100mV/g with electronics that has a maximum output of 5V wil l obviously

have a dynamic range of +/- 50g while an accelerometer of sensit ivity of10mV/g wil l have a dynamic range of +/- 500g

If the maximum g levels l ikely to be experienced is known then dividing thisnumber by 5 volts wil l give the maximum sensit ivity that should be used to getthis dynamic range

Example; Vibration expected to be seen is 300g. Sensit ivity wil l be 5 divided by200, which equals 16.6 mV/g. The nearest sensit ivity would be a 10mV/gaccelerometer.

WHAT IS THE OUTPUT OF AN IEPE ACCELEROMETER?

An IEPE accelerometer is a two-wire sensor that r equires a constant currentsupply and outputs an AC voltage output on a DC voltage bias. The DC bias isoften removed by the use of a decoupling capacitor.

WHAT IS AN FFT?

An FFT is short for Fast Fourier Transform and is an algorithm that is used toobtain frequency content data from time domain signal. Spectral analysis,frequency analysis are terms also used to describe obtaining frequency contentdata from time domain signals.


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WHAT IS CONDITION MONITORING?

Condit ion monitoring is where the health of a rotating machine is monitoredusing vibration levels. As the health of a machine (example becomesunbalanced, fan blades corrode, bearing surfaces degrade) deteriorates so theamplitude of the vibration the machine generates increases. By monitoring thevibration levels over a long period of t ime this gradual deterioration of thehealth of the machine can be assessed unti l the vibration levels get to a pointwhere the machine needs to be taken out of service and overhauled. Analysisof the frequency content of the machine vibration signal wil l indicate not onlythat the health of the machine has deteriorated but also root causes can beattributed to the problem.

Example: An 8 bladed pump running at 6000 rpm (100Hz) wil l produce avibration signal with 100 Hz fr equency if i t becomes unbalanced, 200 Hz if i tbecomes misaligned 800 Hz if the blades become corroded and 43-47 Hz if thebearings start to go into oil whirl.

WHAT FREQUENCY RESPONSE DO I WANT FROM MY ACCELEROMETER?

The frequency response of the accelerometer needed for testing depends onwhat frequencies of vibration are required to be measured. An accelerometershould have a high enough natural frequency as to capture all the frequenciesrequired to be measured.


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Natural frequency suff iciently high to capture all frequencies in signal

Problems start to arise however when the vibration content of the accelerationto be measured gets close to the natural frequency of the accelerometer.

Frequencies to be measured approach the natural frequency of theaccelerometer

In these instances distort ion of the acceleration by the high gains seen near thenatural frequency can give a false picture of t he reported accelerationamplitudes at high frequencies.


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Acceleration signal is misrepresented by non-unity gain of the hi gherfrequencies

To overcome this problem one of two things needs to happen:

a higher frequency accelerometer needs to be used

A higher natural frequency accelerometer solves the problem of measuring highfrequency accelerations

If the higher frequencies are not required to be measured then using a low passfi l ter should f i l ter them out.


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A low pass f i l ter removes high frequency components of the measured signal

WHAT TYPE OF ACCELEROMETER BEST SUITS MY APPLICATION?

Accelerometer Type Advantages Disadvantages

Single endedcompression

RobustHighest naturalfrequencyHigh shock resistance

Poor base straincharacterist ics

Isolated basecompression

RobustHigh natural frequency

Better base strainperformance

Shear

Best base strainperformanceBest temperaturetransients immunitySmallest size

Less robustLower shockresistance

Charge output

High temperatureoperationSuitable for radiationenvironmentsSmall size

Requires local chargeamplif ierSusceptible to tr ibo-electric effect

Piezo-resist ive Measures down to zeroHz

Limited high frequencyresponse

Strain Gage basedMeasures down to zeroHzHigh shock resistance

Limited high frequencyresponse


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CALIBRATION


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WHY SHOULD I CALIBRATE?

Any measurement is subject to degradation due to use, abuse, drif t or ageing.To understand this degradation calibration at regular intervals needs to becarried out to characterize the instrument after degradation, to r estore the

instrument to an as new condit ion as regards it s measurement performanceand to reference the measurement to National Standards.ISO9000 and many other standards specify the maximum period between re-calibration as once every two years and more frequently if the instrumentdegradation is signif icant during that period. (Typicall y 1% degradation) Manyusers adopt an annual calibration as the standard interval between calibrations.

Sensotec NoteMany customers adopt the annual calibration but very few do a comparisonbetween the current calibration and the previous calibration to ascertain thedegree of degradation and determine a suitable re-calibration t ime period.

WHAT IS NIST?

NIST is the National Institute of Standards and Technology, which i s the USfederal government agency responsible for the maintenance of NationalStandards.

WHAT IS A NIST TRACEABLE CALIBRATION?

A NIST traceable calibration means that the calibration can be tr aced by anunbroken chain of documented steps, comparisons and stated uncertaint iesright back to the national standards.


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CAN A LOAD CELL BE MADE TRACEABLE TO NIST?

No. Only the results produced by that load cell are traceable to NIST providedthat the condit ions under which the results are obtained are clearly understoodand under control. For example if a load cell and signal condit ioning arecert if ied by NIST the readings taken by the l oad cell are not NIST traceableunless the way the measurements are taken is clearly understood and thecondit ions under which they are taken is under control.

CAN AN ORGANIZATION BE NIST TRACEABLE?

No an organization cannot be NIST traceable. Only the results of theorganization can be traceable


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WHAT IS A2LA OR NVLAP ACCREDITATION?

In order to reduce the uncertainty of a l oad cells measurements or results thecalibration needs to be carried out by a competent person using appropriatecalibration equipment and adopting good calibration practicesA2LA and NVLAP are two cert ifying bodies that audit companies against

ISO17025, which is a standard that ensures competent people, carry out goodcalibration practices using good calibration equipment

WHAT IS UNCERTAINTY?

Uncertainty is a tolerance band around any measurement result that indicatesthe range of results that would be reported if the test were carried out oninfinitely accurate equipment using standards held at NIST or any otherinternationally recognized standards.Uncertainty is used to qualify measurements and their absolute accuracy and isexpressed as a tolerance foe example:

422 lbf +/- 0.28 lbs. (+/- 0.28 is the uncertainty)

That means that if NIST standards were on inf initely accurate equipment thatthe results that would be reported would l ie s omewhere in the range of 400.28lbf and 399.72 lbf.

Uncertainty estimations are developed by very detailed mathematical analysisand by observations

WHEN IS UNCERTAINTY IMPORTANT?

Uncertainty is important when carrying out crit i cal absolute measurements andis not important when making relat ive measurements. When measuring breakingforces for seat belts it might well be important to know if i t breaks at 740 lbs or760lbs. I f however measurements are being made on the force required topress f it gearbox bearings on an automotive production l ine the absolute valueof the load may not be as important as to ensure that the same load is appliedevery t ime.

When absolute measurements are stated they should always be qualif ied byadding the uncertainty example 28 lbf +/- 0.16 lbf or somewhere a qualif icationstatement should be stated such as all readings have an uncertainty of 0.07%.Uncertainty is very important when calibrating load cells, as calibration is thetime that a cell is c hecked on how it measures absolute loads and checkedagainst national standards or loads that are traceable to national standards.Obviously a load cell that has an uncertainty of +/- 0.15lbf is better than a l oadcell that has an uncertainty of +/- 1.2 lbf. This might be because it is a better


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load cell or alternatively the load cell wi th the better uncertainty might just havebeen calibrated to a higher standard.

HOW IS UNCERTAINTY MEASURED?

Uncertainty is not measured but is estimated. Uncertainty estimations aredeveloped by very detailed mathematical analysis and by observations.Uncertainty and its determination is a science all of i ts own and is often verydiff icult, complicated and t ime consuming. Many calibration labs employ peoplefull t ime just to determine uncertainty of measurement results obtained withintheir facil i ty.

NIST recognizes two main methods of obtaining uncertainty.

Method ADetermined by analysis, measurements and observations of results. All the

contributing elements that give rise to any error i n measurement are measuredand the standard deviation obtained. The measurement results of al l thecontributing components are then added together using the square root of thesum of the squares and an overall standard deviation determined. The overalluncertainty is then stated as either 2 t imes the standard deviation or 3 t imesthe standard deviation.

Method BDetermination of the uncertainty by a theoretical and mathematical analysisrather than by observation.

Sensotec NoteAll uncertainty measurements carried out at Sensotec are carried out usingMethod A


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CALIBRATION CLASS LOAD CELLS


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WHAT DOES A LOAD CELL CALIBRATION CONSIST OF?

There is no specif ic standard for l oad cell calibration except ASTM E74, whichis targeted toward the force testing machines. A number of large companiesand organizations such as Boeing and the USAF also have standards for load

cell calibration. Standards such as ISO 17025 and Z54 lay out guidelines forcalibration but do not specif ically address load cells. As a result eachmanufacturer has their own standard of load cell calibration. However a typicalcalibration wil l indicate sensit iv ity while more comprehensive calibration mightindicate l inearity, error, best f i t straight l ine hysteresis. A more elaboratecalibration might calibrate the load cell in tension and compression and mightuse more data points. All calibrations should be tr aceable to NIST standards.

WHAT LOAD CELL CALIBRATION STANDARD SHOULD I ADOPT?

Calibration costs t ime and money so it is important to adopt a standard that iscomprehensive enough to cover the needs of the application but not socomprehensive that t ime and money is spent needlessly. The calibration shouldbe comprehensive enough to ensure that the uncertainty is four t imes betterthan the system that is to be calibrated.If measuring the uncertainty is too cumbersome, too complex or insuff icientt ime can be invested in the project then at l east an appreciation of theuncertainty should be attempted.For example:If the test r ig on which the load cell is being used can only consistentlyreproduce loads to an accuracy of 5lbs then a l oad cell calibration that ensuresbetter than 1.25 lbs is l ikely to be suff icient.

WHAT ARE IMPORTANT PARAMETERS FOR A CALIBRATION CLASS LOAD CELL?

Any load cell can be a calibration class load cell provideda) It s uncertainty is knownb) It is used to cali brate load cells whose uncertainty only needs to be 4 x more(the generally accepted ratio) than this calibration standardc) It is only used over part of i ts measurement range (say 20% to 100%) whichis dictated by the uncertainty of the cell

For exampleA 1000 lb load cell with an uncertainty of 0.5 lbs (which means that if i tmeasures a load and it indicates 760 lbs it could actually be anywhere between759.5 lbs to 760.5 lbs.) Is used as a reference standard to calibrate other loadcells provided that these cells under test are not required to report loads with agreater uncertainty of +/- 2 lbs. In addit ion this reference cell would not be ableto carry out calibrations with loads less than 200 lbs. This lower l imit is


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determined by mult iplying the uncertainty by 400 for class A l oad cells and2000 for class AA load cells.

WHAT ARE THE CHARACTERISTICS OF A LOW UNCERTAINTY CALIBRATION CLASS LOAD CELL?

A low uncertainty calibration class load cell has the following attributes:a) The cell has a high repeatabil i ty (or a low repeatabil i ty error)b) The l inearity curve is well known. (It does not have to be highly l inear. I t justhas to be well known either as a polynomial curve or a look up table)c) The cell should have a low hysteresisd) The cell should have very low creepe) The cell should have low drif tf) The cell should be calibrated with a low uncertainty i.e. with good calibrationpractices on good calibration equipment.g) The read out should be highly accurate and have a good uncertaintyh) The f ixturing used should be carefully designed that ensures accurate

calibration with high repeatabil i ty

WHY DOES A CALIBRATION CLASS LOAD CELL HAVE A BASE PLATE AND A CALIBRATIONADAPTER?

In order to get high repeatabil i ty (low repeatabil i ty error) and low creep from acalibration class load cell Then the following condit ions need to be met:

a) The cell sensing element must be mounted on a f lat surfaceb) The load cell sensing element must be rigidly f ixed to the structure.

Rigidly f ixed usually means bolted downc) The load cell element should be torqued down evenly to avoid distort iond) Any threads used in the loading path should be pre-tensioned to avoid

thread creep during the load cyclee) All compressive forces should be applied absolutely perpendicular to the

sensing element and any side loading should be avoided.f) All tensile forces should be applied to the load cell absolutely

perpendicular to the sensing element.

The calibration adapter and base plate help achieve all of these goals as shownin the diagram.


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WHAT IS THE UNCERTAINTY OF A SENSOTEC LOAD CELL?

The uncertainty of a Sensotec load cell is determined by a number of factors

a) The type of load cellb) The calibration procedure used in its calibrationc) The range of the load celld) The calibration test stand that was used to do the calibratione) If the low uncertainty option was specif ied at t ime of manufacturef) I f the load cell includes pull plate and calibration adaptor.

An 10,000 lb imperial class calibration load cell with pull plate and calibrationadapter and SC2000 calibration class signal condit ioning may have anuncertainty of 0.75 lbf or 0.0075%

A 100 lbf model 41 would have an uncertainty of 0.05 lbf or 0.05%

WHAT UNCERTAINTY SHOULD MY CALIBRATION REFERENCE LOAD CELL HAVE?

The simple answer is that a calibrati on reference load cell should have anuncertainty that is 4 t imes better that the load cell i t is going to be used incalibrating. The more complete answer is however that it i s not strict ly the loadcell that has the uncertainty but the results obtained by that load cell. In orderto get results from a calibrati on reference load cell the load cell needs adisplay, i t needs signal condit ioning, it needs a calibration stand or means ofloading and it needs a calibration procedure. I f you l ook at the contributing


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uncertaint ies in the results obtained from a cali bration load cell the largestcontributors to error in descending order are l ikely to be:

Fixturing

Calibration rig or means of loading

Reference load cell display and signal condit ioning

Reference calibration load cell calibration

Reference Load cell creep

Reference Load cell hysteresis

Reference Load cell l inearity

If a low uncertainty calibration result is desired then it makes sense to reduceindividual contributors to the overall uncertainty budget. I f the f ixturing,calibration rig, display and signal condit ioning are poor it would seem pointlessto spend t ime and money employing a low (good) uncertainty load cell. I f thefixturing, calibration rig, di splay and signal condit ioning are good then it wouldbe worthwhile spending t ime and money gett ing a low uncertainty calibrationcarried out on a low creep, low hysteresis, high l inearity load cell. .

DOES A LOAD CELL HAVE TO BE CALIBRATED WITH ITS DISPLAY?

Strict ly speaking the answer is no. H owever when using the load cell theuncertainty of the signal condit ioning cabling and the display unit need to bedetermined and added to that of the load cell. In addit ion the signalcondit ioning/display unit needs to be within its annual calibration. I t is for thesereasons that the signal condit ioning/display unit is often included in thecalibration of the load cells as it a) determines the uncertainty of the load celland signal condit ioning unit combined and b) ensures that the signalcondit ioning unit gets its annual or biannual cali bration.

WHAT DOES THE ASTM E74 STANDARD SPECIFICALLY SAY ABOUT CALIBRATION CLASS LOADCELLS?

a) It outl ines a procedure on how load cells should be calibrated and theterminology used

b) Load cells should be tested by deadweight testing machines andhydraulic test machines and specif ies the uncertainty of these weightsand how local gravity needs to be determined.

c) I t defines how the l inearity curve should be defined as a polynomial curvewith a 2nd order f i t but that up to 5 t h order can be used.d) It states that a calibration should be carried out at 10%, 20%, 30%, 40%

50%, 60%, 70%, 80%, 90%, 100% in ascending loading only. I t alsostates that if increments in deadweights cannot be obtained to carry outthese percentages that alternatives can be used but need to be specif iedin the calibration cert if icate

e) A calibrated load cell cannot be used below 10% unless weights wereapplied below 10% and calibration results obtained.


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f) A load cell can never be used below 2000 t imes the uncertainty of thecell for a class AA load cell and 400x the uncertainty

g) A load cell should never be used below 2% of its ful l scale output.h) I t specif ies good calibration practices to adopt l ike temperature control

and application of the weights etc.i) I t specif ies that after the cell is taken through one 19 point calibration

cycle that the cell should be r otated by 120 degrees and a 19 point

calibration carried out again and then rotated again through 120 degreesfor a third 19 point calibration carried out.

j) The uncertainty of the cell is reported as 2.4 x the standard deviation ofthe results obtained during the calibration process.

k) The uncertainty for a class A standard load cell should never exceed0.25% and 0.05% for a class AA load cell

l) The temperature error over the stated temperature range should notexceed 0.01% for Class AA load cells and 0.05% for Class A load cells

m) Load cells need to be re-calibrated 1 year after manufacture and thenevery 2 years provided the calibration has not changed by more than0.1%. If the calibration has changed then the cells need to be re-calibrated more frequently unti l a new t ime interval is established.

n) It specif ies a format for the report or calibration cert if icate

ASTM E74 does not specify the required accuracy of a load cell nor does itspecify l inearity or hysteresis


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GENERAL


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HOW DO I KNOW WHAT ACCURACY CLASS TO USE FOR MY SENSOR?

I t depends on the application. How much does error in your project matter? If youre f i l l ing a tub with water then it may not matter at al l. I f youre f i l l ing of atub with chemicals, the consequence of error could be severe.

If the consequences of error are signif icant, you should identify all sources oferror and their contribution to total error.

You can then snap pressure sensors with differ ent levels of accuracy into yourevaluation and see the impact of sensor error on each.

If you have other error factors that ar e much larger than the transducer thenupgrading the transducer accuracy may not matter.

Remember to translate the accuracy into hard numbers to get a betterperspective. Is a 1000 PSI pressure transducer with .1% accuracy good

enough? The real question is whether an accuracy of +/- 1 PSI is accurateenough. Is detecting between 999 and 1001 PSI at a true pressure of 1000 PSIacceptable? It depends on the application.

You can always opt for the highest accuracy, but it can be more beneficial toanalyze accuracy in the context of the applications needs, transducer costsand transducer lead t imes.

WHAT IS SENSITIVITY?

The ratio of change between a transducers output and input is known as itssensit ivity. For example, a transducer that produces 1 mV for every 100 psihas a sensit ivity value of .01 mV/psi.

Under ideal condit ions, a transducers sensit ivity value does not changebetween zero and full scale. A transducer that produces 1 mV for every 100 psiwould then, under ideal condit ions, also produce 2 mV for an applied pr essureof 200 psi, 3 mV for an applied pressure of 300 psi, and so.

A transducers ideal sensit ivity can therefore be mapped as a straight l ine, andthe transducers sensit ivity value, expressed as the r atio of output to input,then equates to the slope of that l ine


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Ideal Sensit ivity is Represented as a Straight Line

Notice also that under ideal condit ions, there is zero output when there is zeroinput.

However, the actual sensit ivity of a tr ansducer f luctuates slightly between zerobalance and full scale. Some reasons for this might be due to manufacturing

and materials imperfections, electrical interference, and even the age of thetransducer. In addit ion, a transducer usually produces some amount of outputeven at zero balance. Thus, true sensit ivity actually equates to a non-l inearfunction with a zero offset.

True Sensit ivity is Represented as a Curve

Because true sensit ivity is non-l inear, the tr ue sensit ivity value of a transducer(the ratio of output to input) wil l not always be the same at any point between

zero balance and full scale. In order for a sensit ivity value to be constant, thesensit ivity must be expressed l inearly. Most manufacturers use a best f i tstraight l ine to represent sensit ivity.


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Example of a Best Fit Straight Line

The sensit ivity value can then be expressed as the slope of the best f i t straightl ine, which becomes the value quoted on the transducers calibration cert i f icate.

Sensit ivity as the Slope of the Best Fit Straight Line

Sensotec NoteIn some cases, Sensotec uses the slope of a best f i t straight l ine as atransducers quoted sensit ivity on its calibration cert if icate. In other cases,Sensotec uses the slope of a terminal point straight l i ne. (See What is Non-Linearity?)

WHAT IS NON-L INEARITY?

In its broadest sense, non-l inearity refers si mply to a departure from somethingthat is l inear.


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In the world of transducers, non-l inearity is the maximum deviation in outputbetween a transducers sensit ivity curve and a l i near representation of i ts truesensit ivity curve drawn between nominal zero and full scale. Non-l inearity ismeasured on increasing input only, and is expressed as a percent of ful l scaleoutput. An example of non-l inearity for a transducer is 0.15% F.S.

Determining non-l inearity for a transducer raises a question of how to create

the l inear representation of a transducers true sensit ivity. Often a best f i tstraight l ine, which is based on the least squares method, is employed.

Best Fit Straight Line Compared to Ideal Sensit ivity

When a best f i t straight l ine is used, transducer non-l inearity is simply the

greatest deviation between the transducers sensit ivity curve and the best f i tstraight l ine obtained mathematically using the least squares f it method

Non-Linearity Based on Best Fit Straight Line


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In other cases, a terminal point straight l ine is used to determine transducernon-l inearity. The terminal point straight l ine is drawn between nominal zeroand output at ful l scale.

Terminal Point Straight Line

Terminal point straight l ine is often a more practical best straight l ine, as it iseasy to understand and implement. The user simply takes the output at zeroand the output at ful l scale and assumes a straight l ine relat ionship. Using aterminal point straight l ine results in a greater (worse) value for non-l inearitythan using a best f i t straight l i ne obtained mathematically.

Non-Linearity Based on Terminal Point Straight Line

Sensotec NoteSensotec uses the terms l inearity and non-l inearity interchangeably. Sensotecuses the terminal point straight l ine method and least squares f it best straightl ine to determine its transducers non-l inearit y. The datasheets indicate themethod used when quoting specif ications.


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WHAT IS HYSTERESIS?

Hysteresis refers to the behavior of a transducer to produce different outputvalues for a common input point depending on whether applied input isincreasing or decreasing.

Hysteresis is due to the behavioral patterns of metal crystals, which expandand contract dif ferently. As applied pressure on a transducer increases, thenon-l inear representation of the transducers output traces its true sensit ivitycurve. But as applied pressure on a transducer decreases, the non-l inearrepresentation of transducer output results in a different sensit i vity curve.

Hysteresis is then the greatest dif ference between output readings for acommon input point, one reading obtained while increasing from zero input, andthe other while decreasing from full scale output. The deviation is expressed asa percent of ful l scale. An example of hysteresis for a transducer is 0.10%F.S.

Hysteresis as the Deviation between Increasing and Decreasing Values

WHAT IS STATIC ERROR BAN D?

Static error band is a performance specif ication that takes into account theeffects of transducer non-l inearity and hysteresis.

The static error band is an err or envelope that is determined by drawing twolines parallel to the best f i t straight l ine (going through normalized zero point)with a width that is determined by the hysteresis curve. An example of stat icerror band for a transducer is 0.04% F.S.

Notice that a best f i t straight l ine, rather than a terminal point straight l ine, isused to calculate transducer error band. The best f i t straight l ine must takeinto account both curves. Once the best f i t straight l ine has been determined,

two l ines, which are both parallel to the best f i t straight l ine, are then drawnthrough the points of maximum deviation. The entire region between theseouter l ines is known as the stati c error band.


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Static Error Band and the Best Fit Straight Line

WHAT IS CALIBRATION FACTOR?

Calibration is the process of standardizing an instrument by determining itsdeviation from a desired standard. I t is through the calibration process that oneobtains the proper correction factors for the transducers deviation. Calibrationis essential ly the comparison of transducer outputs when compared to areference standard.Every transducer is shipped with a sensit ivity on its calibration cert if icate sothat the electronic equipment associated with the transducer can be set upcorrectly.

Sensotec NoteSensotec expresses the sensit ivity of the transducer by stating a calibrationfactor rather than a sensit ivity.

The calibration factor for a tr ansducer is the transducers output value at ful lscale when the output has been normalized (i.e. zeroed). The l ine drawnthrough normalized zero and a transducers calibration factor equates to thebest f i t straight l ine of the transducer output. Thus, the transducers calibrationfactor in effect establishes the transducers sensit ivity.


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Calibration Factor Quoted on the Calibration Cert if icate

WHEN SHOULD I F IT A CONNECTOR OR INTEGRAL CABLE?

Using a connector makes it easy to disconnect (or reconnect) a sensorscabling which also makes it easier to remove or replace the sensor too. Thus,using connectors is an excellent choice for t emporary sensor applications.However, in addit ion to sometimes being more expensive than integral cableoptions, connectors introduce a point of vulnerabil i ty for potential water,moisture, and mechanical damage in the connection itself.

Connector version of sensor

Because of size constraints, i t is often impractical or impossible to f i tconnectors to small sensors; thus, integral cable technology is often the onlyoption for small sensors. As can be imagined, integral cables are often usedfor more permanent installat ions, where connecting and disconnecting asensors cabling is not planned. Because the cable is integrated with thesensor, the points of vulnerabil i ty with respect to water, moisture, and

mechanical damage that can occur with connector technology, are eliminated;on the other hand, integral cables require strain rel ief protection to prevent thecable from gett ing sheared off or ripped out. I f an integral cable is everdamaged, the entire sensor must be repaired or replaced.

Miniature sensor with integral cable


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While it is possible to f i t a connector to a submersible sensor (using asubmersible connector), using an integrated cable is much more cost effective.Because a submersible sensor (Fig 24c) is typicall y part of a permanentinstallat ion, an integrated cable becomes a much more practical choice. Insome cases, though, an application might not require true submersibil i ty, but

only a degree of water protection; thus one must be able to identify the trueneed.

Submersible Sensor

The following table summarizes the advantages and concerns for connectorsand integral cables:

Connector - Cable Comparison

ConnectorIntegral Cable Submersible

Easy to disconnectcabl ing f rom thesensor

Easy to replace thesensor

Ideal for temporaryinstal lat ions

More expensive

Point of vulnerabi l i ty

Subject to water,moisture, mechanicaldamage

Ideal for permanentinstal lat ions

Often the only opt ionon smal l sensors

Cable must beprotected to avoiddamage

Strain rel ief of ten

required

Cable damage meansreplacing or repair ingsensor

Submersible connectorsare very expensive

Integral cable is a morecost ef fect ive solut ion

More permanentinstal lat ion (bydef in i t ion)

Ensure of def in i t ionbetween waterproof and

submersible

Connector - Cable Comparison Table


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WHAT IS THE D IFFERENCE BETWEEN SUBMERSIBLE AND WATERPROOF?

There are many categories of environmental protection against water. Forexample, a product might be designed for water protection in various forms,such as protection from dripping; spraying; splashing; jet spr ay; immersion;submersion; and so on. A product with an environmental protection rating

against dripping water might not have suff icient protection against splashingwater or jet spray. Similarly, a product protected against spraying or splashingmight not have suff icient protection against submersion.

Three Environmental Protections for Enclosures

Waterproof is a general term with respect to environmental protection. Awaterproof sensor might actually be rated only against a specif ic type of wateringress, such as splashing, dripping, spraying, and so on. A sensor that is

rated against submersion, on the other hand, is probably also protected againstall other forms of water ingress.

Submersible sensors, which are rated by depth of submersion, enjoy thehighest environmental protection against water.


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WHAT ARE NEMA AN D IP DEFINITIONS FOR ENVIRONMENTAL PROTECTION?

The National Electrical Manufacturers Association (NEMA) has been developingstandards for the electrical manufacturing industry for more than 70 years.NEMAs environmental protection standards, which are used in America, are

expressed numerically as follows:

NEMA Number Definit ions: NEMA-#

# Meaning

1 General Purpose (Indoor)

2 Water Drip Proof (Indoor)

3R Dust Tight, Rain Tight, and Ice Resistant (Outdoor)

4 Water Tight and Dust Tight (Indoor/Outdoor)

4X

Water Tight, Dust Tight, and Corrosion Resistant

(Indoor/Outdoor)

9Indoor Hazardous Locations (Not Applicable to EMSEquipment)

12 Industrial Use - Dust Tight and Drip Tight (Indoor)

13 Oil Tight and Dust Tight (Indoor)

NEMA Number Definit ions Table

Thus, an enclosure that is rated as NEMA-4 i s both water t ight and dust t ight,whether indoors and outdoors.

Europe uses a different system (IP) to express environmental protection forenclosures. Protection categories are expressed by two numbers. Eachnumber defines the protection level. The f irst number refers to a part iclesprotection; the second number refers to water protection.


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IP Number Definit ions: IP##

1st # Meaning 2nd # Meaning

0No SpecialProtection

0 No Special Protection

1 Protected AgainstSolid Objects > 50mm in Diameter

1 Protected AgainstDripping Water

2Protected AgainstSolid Objects


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LOAD CELLS


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WHAT IS OVERLOAD PROTECTION ON A LOAD CELL?

Overload protection in a general sense refers to how a system or device isprotected from damage that can result from an input that exceeds a designedlimit. For example, overload protection in an electrical application might

involve using a fuse or circuit breaker to protect the system from a currentoverload.

Overload protection on a load cell refers specif ically to the means used toprevent the cell s diaphragm from deflecting beyond its designed elastic l imit.Without overload protection, a load cell s diaphragm experiences irreversibledamage under too much applied input.

To achieve overload protection in a load cell, a mechanical stop is inserted toprevent the diaphragm from deflecting beyond its elastic l imit The mechanicalstop bottoms out when excessive load is applied.

Overload Protection on a load cell ( in this case mounted between base plateand load cell)

Load cells that do not have mechanical overload protection enjoy an overloadcapacity by default typically 50%. This means that a 100 lbf load cell withoutmechanical overload protection can sustain a 150 lbf load without incurr ingdamage.

Notice that when a load cell s maximum designed input l imit is exceeded, theload cell s output does not increase further ( the output becomes asymtopic)

Mechanical overload protection lends itself more easily t o compression loadcells than to tension load cells. Because of the internal mechanical design of


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tension load cells, i t is dif f icult to insert a mechanical stop that providessuff icient overload protection.

Sensotec Note:Sensotec sensors operate in an elastic range of .002 - .003 inch.

Because of the typical design of most load cells, Sensotec can f it overload

stops on most compression load cells, but on only a few tension load cells.There are also some load cells that, because of their unique shape, are unableto have overload stops in either compression or tension.

At Sensotec, every load cell has a sl ightly dif ferent deflection. Becausetolerances on a load cell s internal dimensions are not t ight enough to permit ageneric overload protection stop, any protection stop that is i nserted must becustom made, custom fit ted, and then custom tested. This process increasesthe cost signif icantly.

WHAT IS A COMPRESSION ONLY LOAD CELL?

A compression only load cell is a load cell that has been designed specif icallyto measure only compression.

Most load cells work in both compression and tension to some degree.However, some load cells, by virtue of their physical construction, are bettersuited to either compression or tension.

Sometimes it is preferable to use a load cell that measures both compressionand tension without enabling its tension measuring capabil i ty. Rather than

design a load cell that measures only compression, a load cell capable ofmeasuring compression and tension can be shipped with calibration simplycarried out only for compression. This type of load cell might be considered acompression-only load cell, although it is technically a compression and tensionload cell being used only for compression.

Compression-only load cells are usually f i t ted wi th a load button to minimizeside loading.


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The physical design

accellerometer faq 092003

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