impr ind temp cal 2013
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Improving Industrial TemperatureCalibration Accuracy
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©2012 Fluke Corporation NPI Team Intro 2Jason Titmas Temperature Calibration Tour 2009 © 2009 Fluke Corporation 2
Improve temperature calibration by considering thecalibration system components:
1. Thermometer Readout
2. Reference Probe3. Temperature Source (Heat Source)
Drywell
Liquid bath
Agenda
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©2012 Fluke Corporation NPI Team Intro 3Jason Titmas Temperature Calibration Tour 2009 © 2009 Fluke Corporation 3
1. Readout for the UUT
2. Reference (readout and probe) to compare to (canbe the drywell controller display)
3. Heat Source - Drywell or Liquid Bath
Calibration system components
Your Sensor andReadout
External ReferenceThermometer
Dry-Well’s Thermometer/Controller
Comparison
1502C
Probe
F
Comm
K
Sample Exit
Menu
Enter
Or
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©2012 Fluke Corporation NPI Team Intro 4Jason Titmas Temperature Calibration Tour 2009 © 2009 Fluke Corporation 4
Digital Multi Meters
• Adequate in some situations
• Not as accurate or efficient to use
Readout designed for
temperature measurement
• More accurate
• Designed for ease of use
Thermometer Readouts
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©2012 Fluke Corporation NPI Team Intro 5Jason Titmas Temperature Calibration Tour 2009 © 2009 Fluke Corporation 5
•Temperature is calculated from the readoutmeasuring the sensor (resistance or voltage)
•The more accurately the readout measures
the sensor, the more accurate the displayedtemperature will be
•The more sensitive the sensor is, the lesseffect measurement errors have on the
displayed temperature
Principles of measurement
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©2012 Fluke Corporation NPI Team Intro 6Jason Titmas Temperature Calibration Tour 2009 © 2009 Fluke Corporation 6
S
E E
R
T
Basic Formula for Error
• E T : equivalent temperature error
• E R : readout measurement error
• S : sensor sensitivity
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• A readout measures the resistance of a sensor with anerror of 0.01W
• The sensor has a sensitivity of 0.4 W/°C
• What is the equivalent temperature error of the
readout’s measurement?
Simple Example
C C
E T
W
W 025.0
4.0
01.0
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• 100 W PRT: 0.4 W/°C
• 25 W SPRT: 0.1 W/°C
• 10 kW thermistor: 0.04 W/W/°C x resistance
• Type K thermocouple: 0.04 mV/°C• Type S thermocouple: 0.01 mV/°C
Typical Sensitivities
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• Accuracy likely varies over the range
• Accuracy spec might have two or more parts, likepercent of reading and percent of scale
• Absolute accuracy of the measurement must becalculated
Readout Specifications
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•Current selection
•Filter settings
•2 or 3 wire vs. 4 wire
•Long cables, poor quality•Thermoelectric EMF
•Drift, calibration, environment
Other Readout Issues
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©2012 Fluke Corporation NPI Team Intro 11Jason Titmas Temperature Calibration Tour 2009 © 2009 Fluke Corporation 11
•The temperature calculation might not beaccurate
• Sensor drift
• Erroneous characterization coefficients
• Poor calibration
•The temperature of the sensor might bedifferent from the target
• Temperature gradient• Self-heating
Other Sources of Error
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©2012 Fluke Corporation NPI Team Intro 12Jason Titmas Temperature Calibration Tour 2009 © 2009 Fluke Corporation 12
•Specified for stability, not accuracy
•Stability is affected by time and use
•Stability must be verified
Reference Probe- Properties
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• A probe that passes calibration doesn’t necessarilymean it is “In Tolerance”
• Tolerance is calculated by comparing currentcalibration data with previous calibration data
Is My Probe In Tolerance?
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• Periodically measure the resistance at the triple point ofwater (RTPW)
• Maintain a control chart of the values
• If TPW not available, use ice-point, other fixed-point, etc.
Reference Probes – Prevent Out of Tolerance!
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SPRTSN1234CONTROLRTPWCHART
date
RTPW
25.532155
25.532186
25.532215
25.53215
25.53216
25.53217
25.53218
25.53219
25.53220
25.53221
25.53222
25.53223
25.53224
25.53225
MAR01_93
APR02_93
OCT20_93
MAR13_94
MAY04_94
OCT19_94
DEC15_94
JAN15_95
RTPW Drift Control Chart
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©2012 Fluke Corporation NPI Team Intro 16Jason Titmas Temperature Calibration Tour 2009 © 2009 Fluke Corporation 16
• Drift• Drift is a natural occurrence with resistors (including resistancethermometers) and can increase or decrease with time.
• Usually the drift is minimal and does not adversely affect probeperformance
• Drift is not reversible• Suggested Measurements
• Frequent Triple Point of Water Checks
• Others (Ice point, Gallium)
Reference Probes
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• Stress, strain, and shock• Caused by vibration and slight physical impulses, ordropping (shock)
• Introduces mechanical deformations in element withresulting increase in Rtpw
• Changes can be as large as 0.050°
C• Most effects are reversible by annealing
Causes of Drift
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Cold quenching
• Caused by rapid cooling of a hot (> 500 °C) probe
• Introduces crystal lattice vacancies with resulting increase in Rtpw
• Changes generally do not exceed 0.025°
C• Most effects are reversible by annealing
Causes of Drift
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©2012 Fluke Corporation NPI Team Intro 20Jason Titmas Temperature Calibration Tour 2009 © 2009 Fluke Corporation 20
• Oxidation and strain can be reduced by annealing inhigh-quality PRT probes
• Platinum begins annealing ~400°C
• Typically 480°C or 665 C is used
Probe Annealing
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• When a probe is calibrated, check theRTPW:
1. Before sending to calibration lab
2. When probe is returned to calibration lab
3. Periodically during use
• Know probe’s drift before the cal lab tellsyou the bad news
• If possible, update RTPW in readout to
remove drift• Save money, send the probe out for
calibration when it needs it
Summary: Close the Loop
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©2012 Fluke Corporation NPI Team Intro 22Jason Titmas Temperature Calibration Tour 2009 © 2009 Fluke Corporation 22
• Drywells (& Metrology Wells)
• Liquid Baths
• Others?
Heat Sources
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Drywells work in industrial calibration because:
• Portable
• Fast
• Wide temperature ranges• Relatively inexpensive
• Easy to use
• Performance is typically adequate
Drywell Strengths
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Uncertainty contributions for drywells
• Axial and radial uniformity
• Stability
• Display accuracy
• Block loading• Immersion depth
• Environmental conditions
• Procedural variance
• Probe fit and positioning issues
Drywell limitations
H d d t i d ll
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To answer this question you need to know:
•How the drywell will be used
•Important sources of error
•How drywell manufacturers’
specifications are written and applied
How do you determine dry-wellaccuracy?
How you use a dry well greatly
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Temperature range• Generally errors are greater the further away from ambient
temperature
Will dry-well temperature be measured using anexternal reference or the internal control sensorand display?
• Each method is valid, but an external reference will generallyprovide better uncertainties
How you use a dry-well greatlyaffects performance!
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• Metrology wells and drywells are used forcomparison calibrations
• Comparison calibrations require thermal equilibrium
and consistency
• Without thermal equilibrium no comparison can be made
• Equilibrium requires stability
• Consistency allows comparisons to have meaning over
time and between different tests
• Good consistency requires similar loading, low drift, goodhandling practices and verification
What errors are significant?
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Errors depend upon mode of use
External Reference
1. Axial Uniformity
2. Radial Uniformity
3. Stem conduction4. Loading Effect (Very Little)
5. Stability
6. Reference temperaturemeasurement
– Reference Probe – Reference Readout
– Hysteresis
Internal con trol sensor andcal ibrated d isplay
1. Axial Uniformity
2. Radial Uniformity
3. Stem conduction
4. Loading effect
5. Stability
6. Reference temperature measurement
– Sensor and display drift – Hysteresis
– Sensor Calibration
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Estimate of Significance
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Axial Uniformity
Insert
Measureme
nt Zone
ProbeSensors
• The temperature difference between thetop and bottom of the well
• Measurement zone is where axialuniformity is smallest
• EA-10/13 requires 40mm (1.5 inches)
• Hart recommends 60mm (2.25 inches)
• Axial uniformity in the measurement zoneneeds to be known to determineuncertainty
• Hart has a special probe designed to measurethis error
• Axial uniformity error is minimized by• Dual-zone control
• Ensuring probe sensor fits in measurement zone
• Aligning the centers of the sensing elements inthe reference and UUT
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©2012 Fluke Corporation NPI Team Intro 32Jason Titmas Temperature Calibration Tour 2009 © 2009 Fluke Corporation 32
Stem Conduction
d
15-20d
• Heat conducted up the sheathof the probe• Causes non-equilibrium between
sensor and source
• This is a function of the sizeand type of material
• Large diameter probes conduct moreheat
• Alumina conducts more than Inconel
• This error is minimized bydeeper immersion
• Fluke suggests 15 times the diameter of
the probe + sensor length
• An 8mm probe should have about150mm
• Metrology wells have the extra depthneeded to minimize this error
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• Added Heat Loss due to increased numbers orsize of thermometers, creates a shift in thetemperature gradient in the insert (block) of thedry-well.
• The Temperature Controller cannot completelycompensate for this shift.
• The result can be a temperature error that is
particularly apparent in the Direct Mode.
Block Thermal Loading
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Loading Effect
• The number of probes willimpact the amount of heatdrawn from or into the well
• Loading effect is minimizedby well design
• Deeper immersion• Dual-zone control
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Temperature Stability
• Stability is temperature variation
with time
• Time frame needs to be
specified
• EA10/13 recommends 30min
• Should be stated with high
confidence
Use of TYPICAL to be avoided !
Temperature Stability
249.75
249.8
249.85
249.9
249.95
250
250.05
250.1
250.15
250.2
250.25
1 0: 00 :0 0 1 0: 03 :2 0 1 0: 06 :4 0 1 0: 10 :0 0 1 0: 13 :2 0 1 0: 16 :4 0 1 0: 20 :0 0 1 0: 23 :2 0 1 0: 26 :4 0 1 0: 30 :0 0
±0.181°C – 30 min
±0.046°C – 15 min
±0.017°C – 30 min
±0.011°C – 15 min
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Temperature Stability
• Stability is required to reachthermal equilibrium• Probes need time to reach
equilibrium with surroundings
• Multiple measurements rarely
instantaneous
• Temperature stability error
minimised by design• Accurate control with good
resolution• Off the shelf controllers do not
provide exceptional stability
Temperature Stability
249.75
249.8
249.85
249.9
249.95
250
250.05
250.1
250.15
250.2
250.25
1 0 :0 0: 00 1 0: 03 :2 0 1 0: 06 :4 0 1 0: 10 :0 0 1 0 :1 3: 20 1 0 :1 6: 40 1 0: 20 :0 0 1 0 :2 3: 20 1 0 :2 6: 40 1 0 :3 0: 00
±0.181°C – 30 min
±0.046°C – 15 min
±0.017°C – 30 min
±0.011°C – 15 min
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Control Sensor Hysteresis
Control Sensor Hysteresis
0
100
200
300
400
500
600
700
0 100 200 300 400 500 600 700
Set Point Temperature, C
ActualTempera
ture,C
Heating
Cooling
Average Value
Temperature Range
Midpoint
Error Error
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est PracticeProbe fit and position
• Fully Immerse Probes.
• Reference and UUT at Same Depth.
• Similar Diameters.
• Similar sheath heat conduction characteristics.
• Snug Fit Into Sleeves.
• Allow Ample Time For Stability.
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Liquid Baths
•Liquid baths are moreuniform heat sources
for comparison
measurements
• Always use aReference Thermometer
•Follow same guidelines
as drywells for bestmeasurement
uncertainty
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Other heat sources
• Air chamber• Poor uniformity & stability
• Not Recommended
• “In-situ calibration”
heat source• Provides the same test
environment as the test
probe sees.