92-1015 v2

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A Practical Guide to Gas Detection Combustible and toxic gas detection principles DET - TRONICS Detector Electronics Corporation 6901 West 110th Street • Minneapolis, Minnesota 55438 USA Tel: 952.941.5665 or 800.765.3473 • Fax: 952.829.8750 92-1015 $19.95

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Page 1: 92-1015 v2

A Practical Guide to Gas Detection

Combustible and toxic gas detection principles

DET-TRONICS Detector Electronics Corporation6901 West 110th Street • Minneapolis, Minnesota 55438 USATel: 952.941.5665 or 800.765.3473 • Fax: 952.829.8750 92-1015

$19.95

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Due to the large number of existing hazardous vapors, all with specific physicaland chemical properties, there are many different technologies available fordetection including catalytic, infrared (both point and open path systems), metaloxide semiconductor (MOS) and electrochemical detection and measurementtechniques.

The intent of this guide is to furnish the user with information concerning theprinciples of combustible and toxic gas detection, and to provide assistance in theselection, placement, and use of gas detection equipment.

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Table of Contents i

Table of Contents

Chapter 5

Gas Detection Helpful Hintsand Applications

Table 4Table 5

Chapter 6

Approval Organizations and Guidelines

Table 6Table 7Table 8Table 9Table 10

Table 11Table 12Table 13

Appendix

Chapter 1

Basic Concepts for Combustible Substances

• Definitions & Concepts

• Combustible Materials

Figure 1Table 1

Chapter 2

Detection of Combustible Substances

• Combustible Gas Detection

• Types of IR Detection

Figure 2Figure 3Figure 4Figure 5

Chapter 3

Introductory Concepts for Toxic Substances

• Definitions & Concepts

• Toxic Gases

Table 2Table 3

Chapter 4

Detection of Toxic Substances

• Toxic Gas Detection

Figure 6Figure 7Figure 8

1

23

9

29

15 19

35

37 Glossary

39 References

41 Worldwide Locations

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ii

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Definitions & Concepts Combustible Material

A combustible material is a solid,liquid, or gas that may undergothe chemical reaction ofcombustion. Combustion occurswhen an organic chemical (orother substances such ashydrogen or sulfur) is oxidized toproduce energy, water, andcarbon dioxide. For example, thecombustion of the commoncombustible gas, methane:CH4 + 2O2 � CO2 + 2H2O + energy

If the concentrations of thereactants (i.e., methane andoxygen) fall within the explosivelimits of methane, and an ignitionsource is present, the releasedenergy will produce an explosion.

Explosive Range

The explosive range is theconcentration range of acombustible gas or vapor (% byvolume in air) in which anexplosion can occur uponignition. In other words, theregion between the LFL and theUFL (see following definitions) of agas or vapor. The explosive rangevaries with the particular gas orvapor. See Figure 1, a genericexample of Explosive Range, forclarification.

Lower Flammable Limit (LFL),Lower Explosive Limit (LEL)

The lower flammable limit orlower explosive limit is the lowerconcentration limit of an explosiverange for a combustible mixture.At or above the LFL a combustiblegas or vapor will support a self-propagating flame when mixedwith air and ignited. A mixturebelow this concentration level isconsidered too "lean" to burn.

Upper Flammable Limit (UFL),Upper Explosive Limit (UEL)

The upper flammable limit orupper explosive limit is the upperconcentration limit of the explosiverange for a combustible mixture.Above the UFL, the mixture isconsidered “too rich” to cause anexplosion.

Flash Point (FP)

The flash point is the temperatureat which a liquid or volatile solidgives off vapor sufficient to forman ignitable mixture with the airnear the surface of the liquid orwithin the test vessel. In general,any gas or vapor with a flashpoint below ambient temperatureshould be monitored.

Ignition Point (IP)

The ignition point is the minimumtemperature required to initiate orcause self-sustained combustion inany substance in the absence of aspark or flame.

Relative Vapor Density

The vapor density is the weight ofvapor per unit volume, comparedto the weight of the same volumeof air at a given temperature andpressure. The substance is lighterthan air if the vapor density is lessthan 1, and heavier than air if thevapor density is greater than 1.

Basic Principles of Combustible Materials 1

Chapter 1 Basic Principles of Combustible Materials

100%

0%LEL UEL

OXYGEN

COMBUSTIBLETOO LEANTO BURN

TOO RICHTO BURN

FIGURE 1Explosive Limits of Gases and Vapors.

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Combustible Materials

The combustibility of a particularsubstance depends on its physicalproperties, the ignition point, andthe flash point.

PHYSICAL AND CHEMICALPROPERTIES

Table 1 provides physical andchemical properties of manycombustible materials including:its name, formula, molecularweight, boiling point, relativevapor density, flash point,flammability limits in air, andignition temperature. If confusedas to what these properties mean,refer back to the “Definitions &Concepts” portion of thischapter for clarification.

Note:This is NOT a complete listing.Refer to NFPA 325 “Guide to FireHazard Properties of FlammableLiquids, Gases and Solids” for amore complete listing.12

Basic Principles of Combustible MaterialsChapter 1

2

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Basic Principles of Combustible Materials 3

Chapter 1 Basic Principles of Combustible Materials

TABLE 1 — Properties of Combustible Materials______________________________________________________________________________________________________________________________________

Compound Formula MW B.P. (°C) Rel. Vap. F.P. (°C) Flammability Limits in Air I.T. (°C)Dens. LEL (%Vol) UEL (%Vol)

______________________________________________________________________________________________________________________________________acetaldehyde CH3CHO 44.1 20 1.52 -38 4 57 140acetic acid CH3COOH 60.0 118 2.07 40 5.4 16 485acetic anhydride (CH3CO)2O 102.1 140 3.52 54 2.7 10 (334)acetone (CH3)2CO 58.1 56 2.0 -19 2.15 13 535acetonitrile CH3CN 41.1 82 1.42 5 – 4.4 523acetyl chloride CH3COCl 78.5 51 2.7 4 5.0 – 300acetylene CH=CH 26.0 -84 0.9 – 1.5 100 305acrylonitrile CH2=CHCN 53.1 77 1.83 -5 3 17 480allyl chloride CH2=CHCH2Cl 76.5 45 2.64 <-20 3.2 11.2 485allylene CH3C=CH 40.1 -23 1.38 – 1.7 – –ammonia NH3 17.0 -33 0.59 – 15 28 630amyl acetate CH3COOC5H11 130.2 147 4.48 25 1.0 7.1 375aniline C6H5NH2 93.1 184 3.22 75 1.2 8.3 617benzaldehyde C6H5CHO 106.1 179 3.66 65 1.4 – 190benzene C6H6 78.1 80 2.7 -11 1.2 8 560benzyl chloride C6H5CH2Cl 126.6 179 4.36 60 1.2 – 585bromobutane CH3(CH2)2CH2Br 136.9 102 4.72 <21 2.5 – 265bromoethane C2H5Br 84.9 38 3.76 <-20 6.7 11.3 510butadiene H2C=CHHC=CH2 54.09 -4 1.87 – 2.1 12.5 430butane C4H10 58.1 -1 2.05 -60 1.5 8.5 365butanol CH3(CH2)2CH2OH 74.1 118 2.55 29 1.7 9.0 340 butene CH2=CHCH2CH3 56 -6 1.94 – 1.6 10 440butanone C2H5COCH3 72.1 80 2.48 -1 1.8 11.5 505butyl acetate CH3COOOCH2(CH2)2CH3116.2 127 4.01 22 1.4 8.0 370 butylamine (CH3)3CNH2 73.1 63 2.52 -9 – – 312 butyraldehyde CH3CH2CH2CHO 72.1 75 2.48 <-5 1.4 12.5 230

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Basic Principles of Combustible MaterialsChapter 1

4

TABLE 1 — Properties of Combustible Materials (continued)______________________________________________________________________________________________________________________________________

Compound Formula MW B.P. (°C) Rel. Vap. F.P. (°C) Flammability Limits in Air I.T. (°C)Dens. LEL (%Vol) UEL (%Vol)

______________________________________________________________________________________________________________________________________carbon disulfide CS2 76.1 46 2.64 <-20 1.0 60 100 carbon monoxide CO 28.0 -191 0.97 – 12.5 74.2 605 chlorobenzene C6H5Cl 112.6 132 3.88 28 1.3 7.1 637 chlorobutane CH3(CH2)2CH2Cl 92.5 78 3.2 <0 1.8 10.1 460chloroethane C2H5Cl 64.5 12 2.22 – 3.6 15.4 510chloroethanol CH2ClCH2OH 80.5 129 2.78 55 5 16 425chloroethylene CH2=CHCl 62.3 -14 2.15 – 3.8 29.3 470chloromethane CH3Cl 50.5 -24 1.78 – 10.7 13.4 6252-chloropropane (CH3)2CHCl 78.5 47 2.7 <-20 2.6 11.1 520cresol CH3C6H4OH 108.1 191 3.73 81 1.1 – 555 crotonaldehyde CH3CH=CHCHO 70.1 102 2.41 13 2.1 15.5 (230)cumene C6H5CH(CH3)2 120.2 152 4.13 36 .88 6.5 420cyclobutane CH2(CH2)2CH2 56.1 13 1.93 – 1.8 – –cyclohexane CH2(CH2)4CH2 54.2 81 2.9 -18 1.2 7.8 259 cyclohexanol CH2(CH2)5CHOH 100.2 161 3.45 68 1.2 – 300 cyclohexanone CH2(CH2)5CO 98.1 156 3.38 43 1.4 9.4 419 cyclohexene C6H10 82.1 83 2.83 <-20 1.2 – 310 cyclohexylamine C6H11NH2 99.2 134 3.42 32 – – 290cyclopropane CH2CH2CH2 42.1 -33 1.45 – 2.4 10.4 495 decahydronaphthalene C10H18 138.2 196 4.76 54 0.7 4.9 260decane CH3(CH2)8CH3 142.3 173 4.9 96 0.8 5.4 205diacetone alcohol CH3COCH2C(CH3)2OH 116.2 166 4.0 58 1.8 6.9 640 diaminoethane NH2CH2CH2NH2 49.0 116 2.07 34 – – 385dibutyl ether (C4H9)2O 130.2 141 4.48 25 1.5 7.6 185dichlorobenzene C6H4Cl2 147.0 179 5.07 66 2.2 9.2 (640)1,1-dichloroethane CH3CHCl2 99.0 57 3.42 -10 5.6 16 440

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Basic Principles of Combustible Materials 5

Chapter 1 Basic Principles of Combustible Materials

TABLE 1 — Properties of Combustible Materials (continued)______________________________________________________________________________________________________________________________________

Compound Formula MW B.P. (°C) Rel. Vap. F.P. (°C) Flammability Limits in Air I.T. (°C)Dens. LEL (%Vol) UEL (%Vol)

______________________________________________________________________________________________________________________________________dichloroethylene ClCH=CHCl 97.0 32 3.55 -10 9.7 12.8 440dichloropropane CH3CHClCH2Cl 113.0 96 3.9 15 3.4 14.5 555diethylamine (C2H5)2NH 45.1 56 2.53 <-20 1.7 10.1 310 diethylaminoethanol (C2H5)2NCH2CH2OH 117 161 4.04 60 – – –diethyl ether (C2H5)2O 74.1 34.5 2.55 <-20 1.7 36 170 diethyl oxalate (COOC2H5)2 146.1 180 5.04 76 – – –diethyl sulfate (C2H5)2SO4 154.2 208 5.31 104 – – –di-isobutylene C8H16 112 105 3.87 2 – – 305 dimethylamine (CH3)2NH 45.1 7 1.55 – 2.8 14.4 400 dimethylaniline C6H3(CH3)2NH2 121.2 194 4.17 63 1.2 7 370 dimethyl ether (CH3)2O 46.1 -25 1.59 – 3.7 27.0 –dioxane OCH2CH2OCH2CH2 88.1 101 3.03 11 1.9 22.5 379 dioxolane OCH2CH2OCH2 74.0 74 2.55 2 – – –ethane CH3CH3 30.1 -89 1.04 – 3.0 15.5 515ethanethiol C2H5SH 62.1 35 2.11 -20 2.8 18 295ethanol C2H5OH 46.1 78 1.59 12 3.3 19 425 ethanolamine HOCH2CH2NH2 61.0 172 2.1 85 – – –ethoxyethanol HOCH2CH2OC2H5 90.0 135 3.1 95 1.8 15.7 235ethyl acetate CH3COOCH2CH3 88.1 77 3.04 -4 2.1 11.5 460 ethyl acrylate CH2=CHCOOC2H5 100.0 100 3.45 9 1.8 – –ethylbenzene C2H5C6H5 106.2 136 3.66 15 1.0 6.7 431 ethyl cyclobutane C2H5CHCH2CH2CH2 84.2 – 2.0 <-16 1.2 7.7 210ethyl cyclohexane C2H5CH(CH2)4CH2 112.2 131 3.87 14 0.9 6.6 262ethyl cyclopentane C2C5CH(CH2)3CH2 98.2 103 3.4 1 1.1 6.7 260ethyl formate HCOOCH2CH3 74.1 54 2.55 <-20 2.7 16.5 440ethyl methyl ether CH3OC2H5 60.0 8 2.09 – 2.0 10.1 190

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Basic Principles of Combustible MaterialsChapter 1

6

TABLE 1 — Properties of Combustible Materials (continued)______________________________________________________________________________________________________________________________________

Compound Formula MW B.P. (°C) Rel. Vap. F.P. (°C) Flammability Limits in Air I.T. (°C)Dens. LEL (%Vol) UEL (%Vol)

______________________________________________________________________________________________________________________________________ethylene CH2=CH2 28.1 -104 0.97 – 2.7 34 425 ethylene oxide CH2CH2O 44.0 11 1.52 – 3.7 100 440 ethyl mercaptan C2H5SH 62.1 35 2.11 <-20 2.8 18 295 ethyl methyl ketone CH3COCH2CH3 72.0 80 2.48 -1 1.8 11.5 505formaldehyde HCHO 30.0 -19 1.03 – 7 73 424hexane CH3(CH2)4CH3 86.2 69 2.79 -21 1.2 7.4 233 hexanol CH3(CH2)4CH2OH 102.2 157 3.5 63 1.2 – –heptane C7H16 100.2 98 3.46 -4 1.1 6.7 215 hydrogen H2 2.0 -253 0.07 – 4.0 75.6 560 hydrogen cyanide HCN 27.0 26 .90 -18 5.6 40 (538)hydrogen sulfide H2S 34.1 -60 1.19 – 4.3 45.5 270 isopropylnitrate (CH3)2CHONO2 105 105 – 20 2 100 175kerosene 150 – 38 0.7 5 210 metaldehyde (CH3CHO)n n=4 to 6 112 6.07 36 – – –methane (firedamp) CH4 16.0 -161 0.55 – 5 15 595 methanol CH3OH 32.0 65 1.11 11 6.7 36 455 methoxyethanol CH3OCH2CH2OH 76.1 124 2.63 39 2.5 14 285methyl acetate CH3COOCH3 74.1 57 2.56 -10 3.1 16 475 methyl acetoacetate CH3COCH2CO2CH3 116 170 4.0 67 – – 280methyl acetylene CH3C=CH 40.1 -23 1.4 – 1.7 – –methyl acrylate CH2=CHCOOCH3 86.1 80 3.0 -3 2.8 25 –methylamine CH3NH2 31.1 -6 1.07 – 5 20.7 430 methylcyclohexane CH3CH(CH2)4CH2 98.2 101 3.38 -4 1.15 6.7 260 methylcyclohexanol CH3C6H10OH 114 168 3.93 68 – – 295 methyl formate HCOOCH3 60.1 32 2.07 <-20 5 23 450 2-methylpentane C3H7CH(CH3)C2C5 86.2 2.97 <-20 1.2 – –

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Basic Principles of Combustible Materials 7

Chapter 1 Basic Principles of Combustible Materials

TABLE 1 — Properties of Combustible Materials (continued)______________________________________________________________________________________________________________________________________

Compound Formula MW B.P. (°C) Rel. Vap. F.P. (°C) Flammability Limits in Air I.T. (°C)Dens. LEL (%Vol) UEL (%Vol)

______________________________________________________________________________________________________________________________________naphtha (Petroleum) 35 2.5 -6 0.9 6 290naphthalene C10H8 128.2 218 4.42 77 0.9 5.9 528nitrobenzene C6H5NO2 123.1 211 4.25 88 1.8 – 480 nitroethane CH3CH2NO2 75 115 2.58 27 – – 410 nitromethane CH3NO2 61.0 101 2.11 36 – – 415nitropropane CH3CH2CH2NO2 89.0 131 3.06 49 – – 420 nonane C9H20 128.3 151 4.43 30 0.8 5.6 205 nonanol CH3(CH2)7CH2OH 144.3 178 4.97 75 0.8 6.1 –octane CH3(CH2)6CH3 114.2 126 3.93 13 1.0 3.2 210octaldehyde C4H9CH(C2H5)CHO 128.0 163 4.42 52 – – –octanol CH3(CH2)6CH2OH 130.2 195 4.5 81 – – –paraformaldehyde HO(CH2O)nH n=8 to 100 25 – 70 – – 300paraldehyde C6H12O3 132.0 124 4.56 17 1.3 – 235pentane C5H12 72.2 36 2.48 <-20 1.4 8.0 285 pentanol C5H11OH 88.0 138 3.04 34 1.2 10.5 300 pentylacetate CH3COOC5H11 130.0 147 4.48 25 1.0 7.1 375phenol C6H5OH 94.1 182 3.24 75 – – 605propane CH3CH2CH3 44.1 -42 1.56 – 2.0 9.5 470 propanol CH3CH2CH2OH 60.1 97 2.07 15 2.15 13.5 405 propylamine C3H7NH2 59.1 32 2.04 <-20 2.0 10.4 320propylene CH3CH=CH2 42.1 -48 1.5 – 2.0 11.7 455 pyridine N(CH)4CH 79.1 115 2.73 17 1.8 12.0 550 p-cymene CH3C6H4CH(CH3)2 134.2 177 4.62 47 0.7 5.6 435styrene C6H5CH=CH2 104.2 145 3.6 30 1.1 8.0 490 tetrahydrofuran CH2(CH2)2CH2O 72.1 64 2.49 -17 2.0 11.8 260 tetrahydrofurfuryl alcohol C4H7OCH2OH 102.1 178 3.52 70 1.5 9.7 280

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MW = Molecular Weight (g/mol)B.P. = Boiling PointRel. Vap. Dens. = relative vapor

densityF.P. = flash pointLFL = lower flammable limitUFL = upper flammable limitI.T. = ignition temperature

Note:Data from IEC (North America)and Hawley's CondensedChemical Dictionary.

Basic Principles of Combustible MaterialsChapter 1

8

TABLE 1 — Properties of Combustible Materials (continued)______________________________________________________________________________________________________________________________________

Compound Formula MW B.P. (°C) Rel. Vap. F.P. (°C) Flammability Limits in Air I.T. (°C)Dens. LEL (%Vol) UEL (%Vol)

______________________________________________________________________________________________________________________________________toluene C6H5CH3 92.1 111 3.18 6 1.2 7 535 toluidine CH3C6H4NH2 107.2 200 3.7 85 – – 480triethylamine (C2H5)3N 101.2 89 3.5 <0 1.2 8 –trimethylamine (CH3)3N 59.1 3 2.04 – 2.0 11.6 190trimethylbenzene (CH3)3C6H3 120.0 165 4.15 – – – 470trioxane OCH2OCH2OCH2 90.1 115 3.11 45 3.6 29 410turpentine mixture – 149 – 35 0.8 – 254 xylene C6H4(CH3)2 106.2 144 3.66 30 1.0 6.7 464

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Combustible GasDetection

Two types of combustible gasdetection are addressed in thischapter:

• Catalytic• Infrared Absorption

A general description of thesensing theory, the advantages,and the limitations are presented.

CATALYTIC SENSOR

Theory of Operation

The most traditional andfrequently used combustible gasdetection method employscatalytic oxidation.

The catalytic gas sensor (orPellistor) usually consists of a pairof platinum wire wound resistorsencased by a bead of ceramic,such as alumina. The activecatalytic bead is coated with acatalyst, such as platinum orpaladium; the reference catalyticbead remains untreated. Thismatched pair is then enclosedbehind a flameproof sinter. Inoperation, the beads areresistively heated to temperaturesranging from 300°C to 700°C.

When a combustible gas comes incontact with the catalytic surface,it is oxidized. Heat is released,causing the resistance of the wireto change. The concentration ofgas present may be found byplacing the sensor pair into aWheatstone bridge circuit to sensethe resistance change. Dependingon the concentration of gaspresent, the Wheatstonearrangement produces adifferential voltage between theactive and reference beads.

The reference bead, or passivebead, maintains the sameelectrical resistance in clean air asthe active bead, but does notcatalyze the combustible gas. Thisattempts to compensate forvariations in the ambienttemperature, humidity, altitude,etc.

An example of a typical catalyticgas sensor is given in Figure 2.

CATALYTIC SENSOR

Advantages

There are several benefits inchoosing catalytic sensors fordetecting the presence ofcombustible gases. • Capable of detecting the

greatest possible range ofcombustible vapors, includinghydrocarbon and acetylene

• Good repeatability andaccuracy

• Fast response time• Low initial cost• Good service life (typically 3

to 6 years)

Detection of Combustible Substances 9

Chapter 2 Detection of Combustible Substances

FIGURE 2Diagram of a Catalytic Sensor

ACTIVE ELEMENT

REFERENCE ELEMENT

THERMAL BARRIER

FLAME ARRESTER

RE

D

WH

T

BL

K

B1124

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CATALYTIC SENSOR

Limitations

The output of the catalytic sensor,in millivolts, has a linearrelationship with the concentrationof the combustible gas present.Although, it is possible to measureup to 100% LFL with catalyticsensors, use of catalytics is notrecommended over 100% LFL. Athigh combustible gasconcentrations, there may beinsufficient oxygen to catalyze allof the combustible gas. In thisinstance, the output may decreaseand indicate a concentration ofless than 100% LEL. Otherrestrictions of catalytic sensingtechnology include:• Requires oxygen levels greater

than 10% to ensure oxidation• Requires routine calibration

(typically every three months)• Require either a constant

current or voltage powersource (i.e., transmitter/card)

• Can only determine thegeneral presence ofcombustible gases, not thespecific chemicals or the typesof gases.

• Susceptible to poisoning froma variety of substances, suchas silicones, halogens (halon,chlorine, fluorine, bromine,freon), phosphate esters,tetraethyl lead, acid and pvcvapors, and other corrosivematerials.

• Consumptive technology –exposure to highconcentrations of combustiblegas or for extended periods oftime can consume the sensingelement, thus requiringrecalibration or replacement ofthe sensor.

• Non fail-safe, meaning thesensor is unable to detect if itis poisoned.

• As with any diffusion basedpoint detector, a number ofdetectors may be required tomonitor a hazardous area.

Detection of Combustible SubstancesChapter 2

10

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INFRARED ABSORPTION

Theory of Operation

The infrared (IR) method of gasdetection relies on the IRabsorption characteristics of gasesto determine their presence andconcentration. IR gas detectorsconsist of an IR light source(transmitter) and light detector(receiver) to measure the intensityboth at the absorption wavelengthand a non-absorbed wavelength.If gas is present in the opticalpath, it will affect the intensity oflight transmitted between the lightsource and the detector. Thischange in intensity provides thedata for determining that aspecific gas, or type of gas ispresent.

This method works only for gasesthat can absorb infraredradiation. Most hydrocarbonbased gases absorb IR radiationat around 3.4 micrometers, whichis transparent to both water andcarbon dioxide vapors. SeeFigure 3 for an example of an IRspectrum.

IR gas detection is rooted in theprinciples expressed in the Beer-Lambert Law. T = I / Io = exp (-aLc)

Defined as: the ratio of theintensity of light leaving (I) thesampling area to the intensity oflight entering (Io) the samplingarea (otherwise known as the T,the transmittance of IR) isproportional to the exponential ofthe absorption coefficient of theparticular gas (a), theconcentration of the gas (c), andthe length of sampling area (L).

Note:IR gas detectors measuretransmission. This signal is not,however, linear with gasconcentration. Thus, signalprocessing algorithms arerequired to provide a linear outputwith gas concentration.

Detection of Combustible Substances 11

Chapter 2 Detection of Combustible Substances

TR

AN

SM

ITTA

NC

E (

%)

WAVELENGTH2 3 4 5 6 7 8

FIGURE 3Spectrum Example

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Types of IR Detection

Combustible IR gas detection cantake one of two forms: either thepoint detector or the openpath detector. The primarydifference between point detectorsand open path is the size of the IRpath and its relationship to thegas/vapor sample source. Theself-contained point detector has asmaller IR path than the open pathdetector, and is used to monitorfixed areas of space. The openpath detector usually consists of aseparate transmitter and receiver,which monitor much larger areasof space. Consequently, in someinstances a single open pathdetector serves the function ofmultiple point detectors.

POINT DETECTION

Theory of operation

Point detectors follow the sameprinciples of operation asdescribed previously for infrareddetection. The point detectoroperates on a short fixed path (30to 150 mm) between the IR lightsource and the light detector,assuming uniform concentrationalong this path. IR light is passedthrough a sample of thegas/vapor, and received at thesensor(s). The active sensor is setin the absorption band of the gasbeing monitored, while thereference sensor is not. The ratioof the wavelengths are comparedto determine if the gas/vapor ispresent.

POINT DETECTION

Advantages

A rapidly growing trend incombustible gas detection is theuse of point IR detectors ratherthan catalytic detectors. Point IRdetectors offer:• Immune to poisoning from

contaminants.• Low maintenance.• Factory calibrated.• Unaffected by prolonged

exposure to gas or high gasconcentrations.

• Unaffected by changes inoxygen level.

• Fail-safe, meaning the sensoris able to detect and indicateconditions in which it is blindto gas.

POINT DETECTION

Limitations

Point IR detectors do havelimitations, such as: • Only detect hydrocarbon-

based gases/vapors. It is notpossible to use IR in thedetection of hydrogen (H2),carbon disulfide (CS2) oracetylene, for example.

• As with all diffusion-basedpoint detectors, a number ofindividual detectors may berequired to monitor ahazardous area.

Detection of Combustible SubstancesChapter 2

12

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OPEN PATH DETECTION

Theory of operation

The open path IR detectors consistof a IR light source (transmitter)and an IR light detector (receiver),usually positioned at opposite endsof the open surveillance path(typically 10 – 100 m). As the IRbeam reaches the receiver, itpasses through a focusing lens andthen passes through two narrowband IR filters – an active and areference wavelength. The detectorcomputes the ratio between thetwo signals (Figure 4).

The instrument's output is afunction of gas concentrationtimes the length of the surveillancepath, and is expressed in units ofLFL - m, ppm - m or % LFL - m.(The unit of LFL - meters is definedas: one LFL - m equals 100% LFLover a path of one meter.)Consequently, a small dense cloudof gas and a large dispersedcloud of gas could produce thesame output. See Figure 5 forclarification.

Detection of Combustible Substances 13

Chapter 2 Detection of Combustible Substances

UNIT OF MEASUREMENT = CONCENTRATION (PPM, % LFL) TIMES DISTANCE (METERS)

P

RUPTURED PUMP SEAL

TRANSMITTER RECEIVER

TRANSMITTER RECEIVER

20,000 PPM (100% LFL PROPYLENE) IN A 1 METER (3.28 FT) CLOUDOR 20,000 PPM TIMES 1 METER = 20,000 PPM-MOR 100% LFL TIMES 1 METER = 100% LFL-M

SLOW LEAKING VALVE

2,000 PPM (10% LFL PROPYLENE) IN A 10 METER (33 FT) CLOUDOR 2,000 PPM TIMES 10 METERS = 20,000 PPM-MOR 10% LFL TIMES 10 METERS = 100% LFL-M

V

FIGURE 5Illustration of a 2000 ppm-m gas open path output

for various concentrations and cloud sizes

SIGNALPROCESSINGELECTRONICS

RATIOMETER

STATUSALARMRELAY

ALARMRELAY

OPTIONALANALOGOUTPUT

ANALOGOUTPUT

MOTOR

INFRAREDPHOTO

DETECTOR

THERMOELECTRICCOOLER

CONTROL

TRANSMITTERLAMP

RECEIVER OPTICS

DENOM.

NUM.

RATIO

OPTICAL FILTERS

FILTER WHEEL

TRANSMITTEROPTICS

POWERSUPPLY

ADJUST

VAPOR CLOUDBEING DETECTED

TR

AN

SM

ITT

ER

RECEIVER

HYDROCARBONLEAK SOURCE

THRESHOLDDETECTOR

OPTIONAL4 TO 20 MA

CONVERTER

THRESHOLDDETECTOR

FIGURE 4Open Path Detection Example

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OPEN PATH DETECTION

Advantages

Open path IR is most effective forgeneral area, perimeter andstraight line monitoring because itcan monitor large areas with aminimum number of instruments.Some examples include,monitoring a bank of heatexchangers, a row of pumps,tunnels, and pipelines. Thebenefits experienced with openpath detection include:• Greatest possible coverage –

some commercially availableunits are capable of detectingvapors over distances of 100meters.

• Immune to poisoning fromcontaminants.

• Low maintenance.• Unaffected by prolonged

exposure to gas or high gasconcentrations.

• Unaffected by changes inoxygen level.

• Fail-safe, meaning the sensoris able to detect and indicateconditions in which it is blindto gas.

OPEN PATH DETECTION

Limitations

Since a larger space is underevaluation it is more susceptible tothe following:• Obstructions in the beam path

such as dust, rain, snow,mobile equipment, personnel,etc.

• Weight of the gas/vapor incomparison to the surroundingatmosphere.

• Only detect hydrocarbon-based gases/vapors. It is notpossible to use IR in thedetection of hydrogen (H2),carbon disulfide (CS2) oracetylene based substances.

• It is more difficult to identifythe specific location of agas/vapor leak, or cloudconcentration.

• Susceptible to condensationunless external optical surfacesare heated.

Open Path systems arerecommended for use inconjunction with point IR detectorsor catalytic sensor systems foroptimal combustible gas detectionwithin a hazardous area.

Detection of Combustible SubstancesChapter 2

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Toxic gases form another group ofpotentially hazardous chemicals.These differ from combustiblematerials in terms of themechanisms and effects ofexposure. Many toxic gases arealso combustible, in which casethe potential for personnelexposure and hazards will be thedetermining factors in selectingthe appropriate detection method.

A toxic substance may be definedas a chemical compound that cancause a wide range of damage tohumans, ranging from minorirritations to the most extremesituation leading to death. Toxicchemicals may be ingested,inhaled, or absorbed through theskin. The effects of toxicsubstances vary by:• Physical properties of the

chemical.• Length of exposure in units of

time.• Concentration in air of the

toxic chemical often measuredin parts per million (ppm) ormilligrams per cubic meter (mgm-3).

• Individual susceptibility.

Gas detection methods are usedto continuously monitor andmeasure the concentration of toxicgases, and insure that it fallswithin safe limits. Limits for shortand long term exposure to toxicgases have been established byoccupational safety and healthagencies such as OccupationalSafety and Health Organization(OSHA) in the United States andControl of Substances Hazardousto Health (COSHH) in the UnitedKingdom. For specific informationon individual toxic chemicals referto the appropriate Material SafetyData Sheets (MSDS), whichinclude:• Chemical's identity• Manufacturer and contact

information• Hazardous Ingredients and

Identity information (i.e.,exposure limits)

• Fire and Explosion HazardData (i.e., flash point,firefighting procedures,flammability limits)

• Reactivity Data (i.e., healthhazards, emergency first aid,routes of entry)

• Precautions for Safe Handlingand Use

• Control Measures (i.e., gloves,protective equipment)

The MSDS in Europe differ fromthose in the US; additionaltransport and regulatoryinformation is provided in Europe.There are many sources forobtaining MSDS on toxicchemicals such as: • Company's collection of MSDS

on chemicals used at yourfacility

• Internet• Chemical manufacturer• Local Environmental or

Occupational Health Offices• A chemistry library• Chemical handbooks.

Note:The format may vary dependingon the source, however, theinformation provided will beconsistent.

Introductory Concepts for Toxic Substances 15

Chapter 3 Introductory Concepts for Toxic Substances

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Definitions & Concepts Threshold Limit Value (TLV)

The threshold allowable time-weighted average, expressed inparts per million (ppm) or partsper billion (ppb) or milligrams percubic meter (mg m-3), forpersonnel exposure over a normal8 hour workday and 40 hourworkweek.4 For optimal protectionfrom toxic gases, typically a fullscale range of 3 to 10 times theTLV of gas being monitored isselected. Exposure below the TLVgenerally does not have negativehealth effects.

Short Term Exposure Limit(STEL)

The STEL is the maximumconcentration for a continuousexposure for up to 15 minutes,provided that the TLV is notexceeded.4

Long Term Exposure Limit(LTEL)

The LTEL is the maximumconcentration for exposure underan 8 hour time weighted averageperiod.10 TLV is the morecommonly used term for thisconcentration,.

Immediate Danger to Lifeand Health (IDLH)

The maximum concentration fromwhich one could escape withinminutes without any escape-impairing symptoms or irreversibleharm effects.4

Lethal Concentration 50(LC50)

The concentration of a material inair that is expected to kill 50% ofa group of test animals inhalingit.4

Introductory Concepts for Toxic SubstancesChapter 3

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Toxic Gases

The occupational exposurestandards for several commontoxic substances are provided inTable 2.

Introductory Concepts for Toxic Substances 17

Chapter 3 Introductory Concepts for Toxic Substances

TABLE 2 — Occupational Exposure Standards___________________________________________________________________________________________________

Toxic Gas Formula Density* LTEL STELppm mg m-3 ppm mg m-3

___________________________________________________________________________________________________Ammonia NH3 .596 25 17 35 24Carbon Dioxide CO2 1.53 5000 9000 15000 27000Carbon Monoxide CO .968 50 55 300 330Chlorine Cl2 2.49 1 3 3 9Hydrogen Sulfide H2S 1.19 10 14 15 21Nitrogen Dioxide NO2 1.04 3 5 5 9Ozone O3 1.66 0.1 0.2 0.3 0.6Sulfur dioxide SO2 2.26 2 5 5 13

*With reference to air = 1.00

Note:For a complete listing of occupational exposure standards for toxic substances refer to OSHA in the United Statesand COSHH in the United Kingdom.

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To demonstrate the hazardouseffects of toxic gases Table 3presents the effects of exposure toone of the most common toxicgases encountered in industry,hydrogen sulfide (H2S).

Introductory Concepts for Toxic SubstancesChapter 3

18

TABLE 3 — Effects of H2S________________________________________________________________

PPM Level Time Effect________________________________________________________________170 – 300 1 hour Maximum permissible exposure level50 – 100 1 hour mild eye / respiratory irritation200 – 300 1 hour marked eye / respiratory irritation500 – 700 .5 to 1 hour unconsciousness, death+1000 minutes rapid unconsciousness,

cessation of respiration, death

Note:For further information consult the IDLH Sheets from NIOSH.11

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Toxic Gas Detection

Two types of toxic gas detectionare addressed in this portion ofthe manual:• Electrochemical• Metal Oxide Semiconductor

(MOS)

A general description of thetheory behind the technology, theadvantages, and the limitationsare presented.

ELECTROCHEMICAL

Theory of operation

Electrochemical sensors, or cells,are most commonly used in thedetection of toxic gases. They areavailable with different numbersof electrodes determined by thetarget gas, device stability andcost.

The two-electrode electrochemicalcell consists of an active (orsensing electrode) and a counterelectrode connected externally viaa load resistor, as shown in Figure6. These electrodes are enclosedin a case with a permeablemembrane for diffusion of the gas,which is submerged in anelectrolyte.

As diffusion occurs, an oxidationor reduction reaction occurs at theactive electrode. An exchange ofelectrons between the activeelement and the gas produces achange in the potential of theactive electrode, while the counterelectrode balances the reaction.The active electrode potential mustfall within a specific range for areaction to occur. As the targetgas concentration increases,current flows causing the counterelectrode to become polarized. Ifthe gas level continues to rise, thepotential of the active electrodewill eventually exceed thepermitting range.

When this occurs the outputbecomes non-linear, thus limitingthe maximum measurableconcentration point.

In an attempt to compensate forthis limitation a third electrode,called the reference electrode isemployed. The active electrode isheld at a fixed potential relative tothe reference electrode. Thecounter electrode while free topolarize, no longer has any affecton the active electrode. Thus, thecell is able to detect vapors over amuch greater concentration range,The three-electrode electrochemicalcell configuration is shown inFigure 7.

Detection of Toxic Substances 19

Chapter 4 Detection of Toxic Substances

O-RING SEAL

SENSING ELECTRODE

SEPARATOR

REFERENCE ELECTRODE

COUNTER ELECTRODE

ELECTROLYTE RESERVOIR

CAPILLARY DIFFUSION BARRIER

FIGURE 7Three-electrode electrochemical cell

LOADRESISTOR

AIR SUPPLY

ELECTROLYTE

ANODE

CATHODE

V

4e-

FIGURE 6Two-electrode electrochemical cell

(oxygen cell shown)

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ELECTROCHEMICAL

Advantages

Advantages of electrochemicalsensors include:• Highly sensitive.• Low power (i.e., intrinsically

safe operation).• Good specificity to target

gas/vapor.• Direct linear output of current

to gas concentration.• Real zero.

ELECTROCHEMICAL

Limitations

There are limitations to usingelectrochemical cells such as:• Consumptive technology,

electrolyte evaporates (thisvaries with the sensor's designand manufacturer) in aridconditions.

• Restrictions in some coldtemperature environments.

• Requires routine calibration.• As with all point detectors a

number of devices may berequired to monitor ahazardous area.

• Non fail-safe, meaning thesensor is unable to detect if itis poisoned.

Detection of Toxic SubstancesChapter 4

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MOS DETECTORS

Theory of operation

Metal oxide semiconductor (MOS)detectors are most frequently usedfor detecting hydrogen sulfidegas. An MOS device usuallyconsists of a gas sensitive resistivefilm, a platinum heater elementand an insulation medium. Thegas sensitive film, traditionally, ismade from a basic material of tinoxide or tungsten oxide combinedwith other oxides, catalyst orinhibitors to increase the selectivityof the device. Figure 8 presents adiagram of a typical MOSdetector.

MOS sensors measure changes inelectrical conductivity due to thepresence of gas/vapor. In thepresence of gas/vapor the MOSsensor produces a change inelectrical conductivity.

In clean air, oxygen is adsorbedat the surface of the MOS sensorwhich strips electrons from theactive material and forms apotential barrier, causing theresistance to increase. If areducing gas is present, such asH2S, the sensor surface willadsorb the vapor and becomereduced, lowering the sensorresistance. This change inresistance may be logarithmicallycorrelated to the concentration ofthe gas/vapor present. Theoperating temperature of the MOSsensor usually exceeds 100°C tospeed up the rate of reaction,reduce ambient temperatureeffects, and provide greaterdegree of selectivity.

MOS DETECTORS

Advantages

Advantages of MOS sensorsinclude:• Quick response• Long life• Operate in low humidity• Detects refrigerant gases, e.g.

Freons, CFCs and HFCs.• Wide operating temperature

range• Able to detect a broad range

of vapors, includingcombustible gases

Detection of Toxic Substances 21

Chapter 4 Detection of Toxic Substances

HEATER CIRCUITRYTO MAINTAIN CONSTANT TEMPERATURE

GAS SENSITIVE RESISTORCONNECTED TO MEASUREMENT CIRCUITRY

FIGURE 8 Diagram of a MOS detector

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MOS DETECTORS

Limitations

Despite the advantages mentionedabove, this method is notcommonly employed due to:• Must be frequently checked as

the calibration tends to driftmore than other technologies,they "go to sleep."

• Non-specific to targetgas/vapor.

• High sensitivity to atmosphericdisturbances such as rain andhumidity changes.

• Affected by changes in oxygenlevel.

• As with all diffusion-basedpoint detectors, a number ofindividual detectors may berequired to monitor ahazardous area.

• Indirect measurementtechnique.

• Non fail-safe, meaning thesensor is unable to detect if itis poisoned.

Detection of Toxic SubstancesChapter 4

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This chapter of the guide containssuggestions as to what type ofdetection method is suitable fordifferent applications. Typical gasdetection applications, along withthe relevant gases to bemonitored, basic guidelines fordetector placement, and otherconsiderations for properinstallation and usage will beprovided.

What type of gas detectionmethod should be used?

As illustrated in Chapter 2 andChapter 4 of this manual, thereare 4 main types of gas detection: • Catalytic and Infrared (IR) for

combustible gases.• MOS and Electrochemical for

toxic gases.

Suggestions for selectingdetection technology for anapplication:

1. The type of gas – combustibleor toxic – and its associateddanger potential.In toxic gas applications,a full scale range should bechosen that is greater than theTLV. (This allows you to setalarm points at concentrationranges above or below theTLV in situations where gaslevels are hazardous.)

2. The potential risks that couldresult from a leak, such as• level of toxicity and the

effects of exposure topersonnel for toxicsubstances

• explosion potential andmagnitude for combustiblegases (Remember, that anexplosion will only occur ifthe concentration fallswithin the explosiverange, oxygen is present,and there is an ignitionsource)

• value of the equipment orprocess within the areaunder investigation

3. Review plant safety policy,insurance requirements/incentives, and regulatory orlegal requirements/incentivesfor additional informationregarding hazard protectionand the need for gasdetection equipment.

4. The amount of funds availableto purchase gas detectionsystems, and life cycle costanalysis for maintenance,particularly calibration.

After these issues are addressed,the appropriate detection methodmay be determined. Some tips forwhen to use which method areprovided in Table 4. Table 5,which lists many commonindustries and the types ofhazardous gases in theirprocesses may be of use.

Note:These tables are intended asguides only, and do not includeevery possible industry or everypossible hazard within eachindustry. Each situation should becarefully examined to determineall possible risks and to determinethe best possible gas detectionequipment.

Gas Detection Helpful Hints and Applications 23

Chapter 5 Gas Detection Helpful Hints and Applications

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Gas Detection Helpful Hints and ApplicationsChapter 5

24

TABLE 4 — Guidelines for Detection Method Selection_____________________________________________________________________________________________________

Combustible Gas Detection Toxic Gas DetectionIR for: Catalytic for: Electrochemical for: MOS for:_____________________________________________________________________________________________________

• Hydrocarbonapplications

• Anaerobic areas• Corrosives/caustics• Wastewater• Areas with constant

backgroundcombustible gases

• Situations where costof ownership is aconcern

• Hydrocarbonapplications with widevariety of vaporspresent

• Cost of ownership isnot a concern

• Non-hydrocarbonmonitoring (hydrogen)

• Low initial costs• Applications where

oxygen is alwayspresent

• Application wheretarget gas should notnormally be present

• Low humidity is not anissue

• Require highrepeatability andconsistency

• Applications whereoxygen is normallypresent

• Application wheretarget gas should notnormally be present

• High temperatureand/or low humidityapplications (e.g.,deserts)

• Applications whereoxygen is alwayspresent

• Areas where ambientconditions are fairlystable

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Gas Detection Helpful Hints and Applications 25

Chapter 5 Gas Detection Helpful Hints and Applications

TABLE 5 — Typical Gas Detection Installations and Gases___________________________________________________________________________________________________

Industry Gas___________________________________________________________________________________________________

Aircraft Hangars CombustiblesBattery & UPS Rooms H2Biowaste/Mulch Pits Methane, H2SChemical, Solvent, & Paint Storage CombustiblesCoal Storage Areas and Domes Combustibles, CO, O2Furnace Rooms, Kilns, Incinerators Combustibles, CO, O2, SO2, NO2Gaseous Fuel Storage Spheres CombustiblesGround Vaults Combustibles, O2, other toxicsIntegrated Circuit Manufacturing Combustibles and toxicsLandfill & Land Reclamation Sites Methane, H2SLiquid Fuel Storage, Tank Farms CombustiblesNuclear Power Plants Combustibles, H2Oil and Natural Gas Drilling Sites Combustibles, H2SOffshore Platforms Combustibles, H2S, other toxicsPaint & Powder Spray Booths CombustiblesPaper & Pulp Mills Combustibles, H2S, Cl2, other toxicsPetroleum & Petrochemical Refineries Combustibles, H2S, O2, COPipeline Compressors & Drive Packages Combustibles and toxicsPrinting Operations CombustiblesPropane-powered Vehicles CombustiblesPublic Institutions Combustibles and toxicsRestaurants & Kitchens CombustiblesRocket Staging & Launch Pads Combustibles and toxicsShips & Boats CombustiblesSteam & Gas Turbines Combustibles, H2, CO, O2Tunnels & Parking Garages Combustibles, CO, O2Wastewater Treatment Plants Methane, H2S, Cl2, O2Welding Shops Combustibles and toxics

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Where should the sensor beplaced?

While various regulatoryauthorities have put forth gasdetection system design andperformance requirements (seeChapter 6), there are no “thirdparty”* rules that define optimumdetector placement, or quantityrequirements for an application.

There are, however, factors thatshould be considered whendetermining the best sensorlocations within a hazardousarea. While some factors result inhard-fast rules that apply to allgas detection applications, otherfactors will vary depending uponthe specific situation. Generalguidelines are provided below tohelp identify the best locations ofdetectors for optimal performance.

* Instrumentation Society ofAmerica (ISA) offers thefollowing standards:• RP92.0.02, Installation,

Operation and Maintenanceof Toxic Gas Instruments,Hydrogen Sulfide

• RP12.13, Part II, Installation,Operation and Maintenanceof Combustible GasDetection Instruments

Indoor vs. Outdoor HazardousAreas:

No other single property has alarger impact on vapor dispersioncharacteristics and gas detectioncapability than the environmentalsetting of the application.

Indoor settings usually mean thatthe overall hazardous area is wellcontained. In addition, air flowcharacteristics are usually fairlyconsistent, and can be identifiedand often controlled. Ceilings andwalls usually are the likely areasfor gas accumulation and areadelineation. Point(s) of humancontact are usually easilyidentifiable. These factors help theapplication engineer to select theoptimum detector locations.

Outdoor settings mean the airflow characteristics of thehazardous area are lesscontrollable. Typically, there areno clear areas of likely gasaccumulation, and often there aremultiple points for potential humancontact. Prevailing winds, ifpresent, may be variable at best.These areas present theapplication engineer with achallenge which requirescomprehensive applicationanalysis, and ultimately sound

engineering judgement, to identifyand select optimum detectorlocations.

General Sensor PlacementGuidelines:

1. Density of gas(es) to bemonitored• If the density of the

gas/vapor is less than air(1.29 g/cc at normalconditions) the gas/vaporwill rise, in still air, so thedetector should be placednear the ceiling. (Refer toTable 1 and Table 2 forthe relative densities ofmany combustible andtoxic gases.)

• If the density of thegas/vapor is greater thanair (1.29 g/cc at normalconditions) the gas/vaporwill settle, in still air, thedetector should typicallybe placed 18 to 24 in(45.7 to 61 cm) above thefloor or ground, or insome instances inchesabove where the gaswould settle. (Refer toTable 1 and Table 2 forthe relative densities ofmany combustible andtoxic gases.)

• If the density of thegas/vapor isapproximately equal tothat of air (1.29 g/cc atnormal conditions), thedetector should be placedat or near breathing level,or near leakage point.

• The density of a gas iseffected by temperature.Heating will decrease thedensity, making the gaslighter. Cooling willincrease the density of thegas, making it heavier.Heating or cooling a gasby 30°C (54°F) willchange the density of thegas by approximately11%.

2. Air movementThe velocity and direction ofair will affect the distributionof vapors or gases or both. Toresolve this issue, thedetectors should be placedwhere the currents with themaximum concentration of thegas being monitored reside.Smoke generator tests areoften helpful to identify deadair spaces.• Pressurized vapors will be

distributed based uponlocation of leak.

Gas Detection Helpful Hints and ApplicationsChapter 5

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3. Gas/vapor source• The detector will only

correctly sense gases orvapors for which it iscalibrated. Verify that thepotential gas or vaporsource is in agreementwith the detectorcalibration.

• The location and nature ofpotential sources ofgases/vapors need to beevaluated. These mayinclude pressure, amountof source, sourcetemperature, and distance.The detector should beplaced where leaks ofgas/vapor may occur. Forhelp in finding potentialgas/vapor sources reviewprocess andinstrumentation diagrams(PID), facility maps, andhazardous-areaclassification drawings.

• Common areas forgas/vapor releasesinclude pump andcompressor seals,instrumentation sources,valve seals, gaskets,sample points and lessfrequently process piping.

4 Ignition sourceFor combustible gas detectionapplications, sources ofignition should be identified.The detector should be placedbetween the potential sourceof the gas/vapor and theignition source.

5. Ambient temperatureDetermine the maximumambient high temperature,including all hot surfaces(e.g., motors, pumps, orsteam lines). • As a general precaution,

the maximum ambienttemperature plus a safetyfactor of 50°C to 60°Cshould be less than theflash point of the gas beingmonitored. (Refer to Table1 for the flash points ofmany combustible gases.)

6. Gas DispersionA hazardous gas or vapormay travel from its source orcreate a cloud that expandsas it moves. Other factors suchas density, temperature, andair movement as mentionedbefore effect this behavior. If itis possible to predict wherethe gas will move, place asensor at that location.

7. VolatilityDetectors should be placednear the source for hazardousliquids with low volatility.Heavy vapors such asgasoline require some airmovement to disperse vapors.

8. VibrationExcessive vibration candamage the detector,producing unreliable results.To avoid this, attach thedetector to a firm base.

9. AccessibilityEnsure that the detector isplaced in a convenientlocation for futuremaintenance and calibrationrequirements.

10. Structural arrangementsPhysical or thermal barriers(e.g., walls, troughs, orpartitions) may disrupt typicalgas dispersion patterns –allowing the vapor/gas toaccumulate. If this is apotential problem, determinethe dispersion patterns of thegas/vapor and placedetectors accordingly.

Note:If an explosion occurs obstaclessuch as vessels and pipes maybecome dislodged in a violentmanner, thus creating furtherdanger.

11. Location of PersonnelParticularly in situationsdealing with toxic gases, it isextremely important toconsider where people at yourfacility are likely to gather.Review Process InstrumentationDiagrams (PIDS), facility maps,and hazardous-areaclassification drawings to findthese locations. A sensorshould always be placedbetween the leak source andthe people.

12. Mechanical damage andcontamination• Detectors should be placed

in suitable locations toavoid damage from normaloperations (e.g., cranes,traffic, exhausts,washdowns) at your facility.

• Precautions (i.e., additionalsensors) should be taken toprotect any expensiveequipment or valuables ata facility from damagecaused by gas leaks, suchas corrosion.

Gas Detection Helpful Hints and Applications 27

Chapter 5 Gas Detection Helpful Hints and Applications

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Are there any otherprecautions to be takenwhen installing GasDetectors?

1. When multiple gases/vaporsare present, sensors should becalibrated to detect the gasthat provides the least amountof relative sensor output atconcentrations up to 100%LFL.

2. Generally, a diffusion-basedgas detector can beconsidered to provideadequate coverage for adistance of 18 ft (5.5 m),according to the ChristianMichelsen Institute, Norway.

Note:This is for unobstructed spaceonly. It is not suitable for indoorapplications.

3. For outside applications,careful consideration ofprevailing wind conditions,which can disperse the gas invarious directions and greatlyreduce the measuredconcentration at the detector,is required.

4. For indoor applications, beaware that a confined areawill increase the danger orhazard potential, or both, ofthe gas due to higherexplosion pressures causedby contaminant of theexplosion.

5. A dust cover or rain shield isrecommended for dirty, dusty,wet, and outside applications.

6. For catalytic applications,verify that the sensor isinstalled facing downwards.

7. Sensor wiring should berouted away from other highcurrent ac or dc wiring.

Gas Detection Helpful Hints and ApplicationsChapter 5

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Government law, insurancecompanies, and/or companypolicy require product approvalsfrom authorized organizationsthroughout the world to maintainprotection of personnel andproperty. This chapter describesthe different organizations aroundthe world and the guidelines thatare used to ensure product qualityand performance, thus eliminatingpotential hazards.

APPROVAL AGENCIES

Approval agencies perform testingin accordance with standardsdeveloped by committees whichare made up of representativesfrom industry, government, andapproval agency authorities.These organizations differgeographically. Det-Tronics workswith many approval bodies fromacross the globe; Table 6 liststhese organizations and theirrespective countries.

Approval Organizations and Guidelines 29

Chapter 6 Approval Organizations and Guidelines

TABLE 6 — Approval Agencies________________________________________________________________

World Area Approval Agency________________________________________________________________Australia SIMTARS, SSLBelgium ISSePBrazil CEPELCanada CSAChina Government of CHINADenmark DEMKO Germany PTB, TUV, BVS, DMTNorway DNV Russia VNIIEF, VNIIFTRI, VNIIPOUnited Kingdom BASEEFA, SIRAUnited States ITS, FMRC, UL ,NEMA

Note:Countries without approval agency may follow the IEC standards.

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HAZARDOUS LOCATIONPRODUCT APPROVALNational Electrical Code

In North America, equipment isclassified for use in hazardouslocations by the NationalElectrical Code (NEC) as part of aClass, Division, Group, andTemperature Code. The "Class"defines the general nature of thehazardous material in thesurrounding atmosphere (Table 7).The "Division" defines theprobability of hazardous materialbeing present in an ignitableconcentration in the surroundingatmosphere (Table 8). The"Group" defines the hazardousmaterial in the surroundingatmosphere (Table 9). The"Temperature Code" indicates themaximum surface temperature ofthe apparatus or componentsbased on –40°C to +40°Cambient temperature limits (–50°Cto +40°C in Canada) (Table 10).The temperature code is assumed"T5" unless specified otherwise.

Approval Organizations and GuidelinesChapter 6

30

TABLE 7 — Classes defined by the National Electrical Code___________________________________________________________________________________________________

Class I Locations where flammable gases or vapors may be present in the air in quantities sufficient toproduce explosive or ignitable mixtures.

Class II Locations where combustible or electrically conductive dusts may be present in the air in quantitiessufficient to produce ignitable mixtures.

Class III Locations where easily ignitable fibers or flyings not normally suspended in air may be present inquantities sufficient to produce ignitable mixtures.

TABLE 8 — Divisions defined by the National Electrical Code___________________________________________________________________________________________________

Division 1 Locations in which the probability of the atmosphere being hazardous is high due to flammablematerial being present continuously, intermittently, or periodically.

Division 2 Locations which are presumed to be hazardous only in an abnormal situation.

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Approval Organizations and Guidelines 31

Chapter 6 Approval Organizations and Guidelines

Acetylene, orgases or vapors ofequivalent hazards

Hydrogen, orgases or vapors ofequivalent hazards

Ethylene, or gasesor vapors ofequivalent hazards

Propane, or gasesor vapors ofequivalent hazards

Conductive metaldust

Carbon dust Flour, starch, orgrain dust

TABLE 9 — Groups as defined by the National Electrical Code______________________________________________________________________________________________________________________________________

Flammable Gases and Vapors (Class I) Flammable Dusts and Debris (Class II)Group A Group B Group C Group D Group E Group F Group G______________________________________________________________________________________________________________________________________

TABLE 10 — Temperature Codes______________________________________________________________________________________________________________________________________

North American IEC Max Surface Temp Max Surface TempTemperature Code Temperature Code °C °F______________________________________________________________________________________________________________________________________

T1 T1 450 842T2 T2 300 572

T2A 280 536T2B 260 500T2C 230 446T2D 215 419T3 T3 200 392

T3A 180 356T3B 165 329T3C 160 320T4 T4 135 275

T4A 120 248T5 T5 100 212T6 T6 85 185

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INTERNATIONALELECTROTECHNICALCOMMITTEE (IEC)

The IEC consists of close to 40countries, attempting to establishinternational standards forelectrical products. The IEC uses"Gas Groups," "Zones," and"Temperature Codes" to classifyequipment for use in hazardouslocations. The "Gas Groups"define the hazardous material inthe surrounding atmosphere (Table11). The "Zone" defines theprobability of hazardous materialbeing present in an ignitableconcentration in the surroundingatmosphere (Table 12). The"Temperature Code" indicates themaximum surface temperature ofthe apparatus or componentsbased on –20°C to +40°Cambient temperature limits (Table10).

Approval Organizations and GuidelinesChapter 6

32

TABLE 11 — Gas Groups defined by the IEC___________________________________________________________________________________________________

Gas Group Representative Atmosphere___________________________________________________________________________________________________

IIC Acetylene, hydrogen, or gases or vapors of equivalent hazards.IIB Ethylene, or gases or vapors of equivalent hazards.IIA Propane, or gases or vapors of equivalent hazards.I Methane, or gases or vapors of equivalent hazards.

TABLE 12 — IEC Zones___________________________________________________________________________________________________

Zone 0 An explosive concentration of gas or vapor is present continuously or is present for long periodsof time.

Zone 1 An explosive concentration of gas or vapor is likely to be present for short periods of time undernormal operating conditions.

Zone 2 An explosive concentration of gas or vapor is likely to be present for very short periods of timedue to an abnormal condition.

Zone 20 An explosive atmosphere of dust is present continuously or is present for long periods of time.

Zone 21 An explosive atmosphere of dust is likely to be present for short periods of time under normaloperating conditions.

Zone 22 An explosive atmosphere of dust is likely to be present for very short periods of time due to anabnormal condition.

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Comparison of ClassificationCodes

Table 13 presents a comparisonof the National Electrical Codeand International ElectrotechnicalCommittee hazardous locationclassification nomenclatures.

Approval Organizations and Guidelines 33

Chapter 6 Approval Organizations and Guidelines

TABLE 13 — NEC and IEC Comparison________________________________________________________________

NEC IEC________________________________________________________________

Class I, Division 1 Zone 0 and Zone 1Class I, Division 2 Zone 2Class II, Division 1 Zone 20 and Zone 21Class II, Division 2 Zone 22Class III, Division 1 NoneClass III, Division 2 None

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34

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Appendix 35

Appendix

Fill in the following information to guide you in the selection and placement for different applications at your site.

1. Provide a general description of the application in which the detector(s) will be placed (for example,environmental conditions, equipment conditions, equipment present and locations).________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

2. List all combustible and toxic gases and/or vapors which may be present at the site under surveillance.

3. Required measuring range(s): ____________________________________________________________________

4. Are there any other substances present (for example, dust)? If so, please list:____________________________________________________________________________________________________________________________________________________________________________________________

5. A) Is detection in:❑ Normal oxygen environment (21 Vol %)❑ Oxygen deficient environment❑ Oxygen enriched environment

B) Estimate the concentration of oxygen present in the atmosphere:__________________________________

6. Ambient temperature ranges for the application:Minimum: _________________ ❏ °C ❏ °FMaximum: _________________ ❏ °C ❏ °FNormal: _________________ ❏ °C ❏ °F

Gas or Vapor Name Toxic or Combustible Concentration Notes

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Appendix

36

7. Ambient humidity range for the application:Minimum RH: _______________%Maximum RH: _______________%

8. Pressure range for the application:Minimum: _________________ ❏ kPa ❏ psiMinimum: _________________ ❏ kPa ❏ psiMaximum: _________________ ❏ kPa ❏ psi

9. Velocity range of the application:Minimum: _________________ ❏ m/sec ❏ ft/secMaximum: _________________ ❏ m/sec ❏ ft/sec

10. List any potential contaminants (poisons) that may affect the performance of the detectors.____________________________________________________________________________________________________________________________________________________________________________________________

11. Define the classification for the application:Class: _________________Zone/Division: _________________Group: _________________

12. Are any additional accessories necessary?____________________________________________________________________________________________________________________________________________________________________________________________

13. Other considerations or precautions.____________________________________________________________________________________________________________________________________________________________________________________________

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Associated ApparatusApparatus in which thecircuits are not necessarilyintrinsically safe, but arerelied upon to maintainintrinsic safety in hazardouslocations by providingintrinsically safe outputs.

Boiling PointThe temperature of a liquid atwhich its vapor pressure isequal to the atmosphericpressure of the environment.

CalibrationThe capability to adjust theinstrument to "zero" and toset the desired "span."

Dust Ignition proofA protection concept thatprevents dust from entering anenclosure and will not allowarcs, sparks or heatgenerated inside theenclosure to cause ignition ofthe exterior accumulations oratmospheric suspensions of aspecified dust on or near theenclosure.

Explosion-proof/Flame-proofA protection concept thatrequires electrical equipmentto be capable of containingan internal explosion of aspecific flammable vapor-airmixture, thereby not allowingthe propagation of ignitiontemperature gases to theexternal environment whichmay be potentially explosive.In addition, the equipmentmust operate at a safetemperature with respect tothe surrounding atmosphere.

Flammable MaterialsFlammable materials are aspecial group of combustiblematerials that rapidly burnand are easily ignited. Theflash point of a liquid clarifiesthis further. The Flash Point(FP) of a flammable liquid isbelow 100°F (37.7°C), whilethe FP of a combustible liquidis at or above 100°F(37.7°C). The distinctionbetween the behavior of thevapors of the combustible andflammable liquids must beconsidered for hazardprotection, since it is thevapors that cause explosions.

Hazardous ChemicalAny chemical which is aphysical or health hazard.4

Ingress ProtectionProtection against ingress ofsolid foreign objects and/orwater.

Intrinsic SafetyA protection concept thatrequires electrical equipmentto be incapable of releasingsufficient electrical or thermalenergy to cause ignition of aspecific hazardous substanceunder "normal" or "fault"operating conditions.

Non-incendive/Non-sparkingA type of protection whichrequires electrical equipmentto be non-sparking andincapable of releasingsufficient electrical or thermalenergy to cause ignition of aspecific hazardous substanceunder "normal" operatingconditions.

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Glossary

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Ordinary LocationsA location where potentiallyexplosive substances are notpresent. Electrical equipmentcertified for use in theselocations must not present ashock or fire hazard.

Performance ApprovalAn approval stating the theinstrument is certified tooperate correctly under set ofstringent conditions.

PressurizedA type of protection by whichthe entry of a surroundingatmosphere into the enclosureof electrical apparatus isprevented by maintaining aprotective gas at a higherpressure inside the enclosurethan that of the surroundingatmosphere.

PurgingThe passing of a quantity ofprotective gas through anenclosure so that anyexplosive atmosphere presentin the enclosure is reduced toa concentration significantlybelow the lower explosivelimit.

ShippingEquipment designed to meetspecific criteria for use onocean-going vessels. Theequipment must comply withhumidity, vibration,temperature, and enclosurerequirements established byeach approval authority.

SpecialA concept which has beenadopted to permit thecertification of those types ofelectrical equipment which,by their nature, do not complywith the requirementsspecified for equipment withestablished types ofprotection; but which can beshown to be suitable for usein prescribed zones orhazardous areas.

Sprays (or Mists)Medium sized releases of fuelthat are mixed with air uponrelease. Found typically atpipe gaskets, pump seals andvalve stem failures under highpressure.

Vapor PressureThe pressure of a vapor atany given temperature that isin equilibrium with its liquidor solid form.

Glossary

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For additional information on anyof the topics mentioned in thismanual or any other issues relatedto the detection of combustibleand toxic gases, the following listof resources is suggested:

1. Mark E. Dundas, NewTechnologies in InfraredHydrocarbon Detection, ISSNVolume 31, Number 4, 1992

2. Robert H. Sonner, William V.Dailey, and David E. Wright,An Open Path InfraredHydrocarbon Detector, ISASpring Symposium, Chicago,IL, May 1-3 1984

3. Installation, Operation, andMaintenance of CombustibleGas Detection Instruments,ISA – RP12.13, Part II –1987

4. Genium PublishingCorporation, Worker SafetyResources,http://genium.com/glossary_search/, August, 1998

5. British Standard Code ofPractice for Selection,Installation and Maintenanceof Electrical Apparatus forUse in Potentially ExplosiveAtmospheres (Other thanMining Applications orExplosives Processing andManufacture, BS5345: Part1: 1989, British StandardsInstitution, London, England

6. Canadian Electrical CodePart 1; Safety Standard forElectrical Installations, CSAStandard C22. 1-1990,Sixteenth Edition, CanadianStandards Association,Rexdale, Ontario, Canada

7. Classification of Locations forElectrical Installations inPetroleum Refineries, APIRecommended Practice500A; Fourth Edition,Committee on RefineryEquipment, RefiningDepartment, AmericanPetroleum Institute, USA,1982

8. Earley, Mark W., and Murray,Richard H., and Caloggero,John M., The NationalElectrical Code – 1990Handbook, Fifth Edition,National Fire ProtectionAssociation, Quincy,Massachusetts, USA., 1989

9. Manual for Classification ofGases, Vapors, and Dusts forElectrical Equipment inHazardous (Classified)Locations, NFPA 497M,1991 Edition, National FireProtection Association,Quincy, Massachusetts, USA

10. Richard J. Lewis, Sr.,Hawley's CondensedChemical Dictionary,Thirteenth Edition, VanNostrand Reinhold, 1997

11. IDLH Documentation, NIOSH(National Institute forOccupational Safety andHealth),http://www.ded.gov/niosh/7783064.html, last updatedAugust 16, 1998

12. NFPA 325, Guide toProperties of FlammableLiquids, Gases, and VolatileSolids, 1194 Edition

13. Denis P. Nolan, P. E.“Handbook of Fire andExplosion ProtectionEngineering Principles for Oil,Gas, Chemical and RelatedFacilities,” NoyesPublications, 1996

References 39

References

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Glossary

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CorporateHeadquarters

6901 West 110th StreetMinneapolis, Minnesota USA55438

Tel: (952) 941-5665 or (800) 765-FIRE

Fax: (612) 829-8750Web site: www.detronics.comE-mail: [email protected]

FranceDet-Tronics France46, avenue des Frères Lumière78190 TrappesFrance

Tel: (33) (1) 30 13 15 28Fax: (33) (1) 30 13 15 29

GermanyDet-Tronics GermanyKidde Deugra GmbHHalskestrasse 30Ratingen 40880Germany

Tel: (49) 2102 405 152Fax: (49) 2102 405 151Telex: 8589029

ItalyDetector Electronics ItalyFenwal Italia S.p.A.Viale De Gasperi, 44 20010 Bareggio (Mi)Italy

Tel: (39) 02 90 36 16 20Fax: (39) 02 90 36 16 27

IndiaDetector Electronics127, Raheja Arcade,Koramangala,Bangalore – 560095India

Tel: (91) (80) 5531219 or (91) (80) 5531369

Fax: (91) (80) 5535670

JapanKidde International601-3, Asana Asaba-choIwata-gun, Shizuoka-ken437-11, Japan

Tel: (81) (538) 238562Fax: (81) (538) 238563

Middle EastDet-Tronics Middle Eastc/o Kidde InternationalP O Box 30791BUAMEEM II BuildingUmm Hureir RoadDubai, U.A.E.

Tel: (971) (4) 372498Fax: (971) (4) 375088

NetherlandsDet-Tronics BeneluxCosterweg 5NL-6702 AA WageningenThe Netherlands

Tel: (31) 317 497625Fax: (31) 317 427308

PolandDetector Electronics Polandul Ludowa 91, 42-200Czestochowa, Poland

Tel: (44) (0)1753687281Fax: (44) (0)1753684540

SingaporeKidde Int’l Protection Systems Pte. Ltd.207 Kallang BahruSingapore 339343

Tel: (65) 392 2282Fax: (65) 392 2272

Worldwide Offices 41

Worldwide Offices

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United KingdomDetector Electronics UK Ltd.Mathisen Way, Poyle RoadColnbrook, Slough, Berkshire SL3 OHB UK

Tel: (44) (1) (753) 683059Fax: (44) (1) (753) 684540E-mail:[email protected]: 848124 GRAVIN G

USADetector Electronics Corporation466 Conchester HighwayAston, Pennsylvania 19014 USATel: (610) 497-5593Fax: (610) 485-2078

USADetector Electronics Corporation13214 Poplar Glen LaneHouston, TX 77082 USATel: (281) 556-9861Fax: (281) 556-9860

USADetector Electronics Corporation7710-T Cherry Park Drive, #217Houston, TX 77095 USATel: (281) 970-2646Fax: (281) 970-2667

VenezuelaDet-Tronics South AmericaGustavo Mogollon & AsociadosCalle 72 con Avenida 3HCentro Comercial “Las Tinajitas”Local No. 18, 2do. NivelMaracaibo, VenezuelaTel: (58) (61) 926885Fax: (58) (61) 926525

Worldwide Offices

42

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Det-Tronics is a memberof the Fire ProtectionGroup of KiddeInternational, which is adivision of Williams Plc.

Since it was founded in1973, Det-Tronics hasmaintained a policy ofworldwide service,quality, and innovation.The company has aninternational network ofdistributors, salesrepresentatives andbranch offices to servethe global needs ofindustry. Det-Tronicshas a full force ofexperienced field serviceand applicationengineers who areavailable to give adviceand technical assistance.They analyze specificapplications and thendesign the best solutionfor the job.

Throughout the years, Det-Tronics hasestablished itself as aleader in innovation, anda provider of highquality solutions. Thisreputation was achievedby aggressively strivingto meet customer's nearand long term needs.Det-Tronics quality andservice are unsurpassed,and are based on thephilosophy: "We willnever leave a customerwith our problem."

Det-Tronics' Mission isto be the world's leadingmanufacturer ofdetection safety systemsfor high hazard, highrisk applications. Ourname must besynonymous withSAFETY, QUALITY,SERVICE and VALUEwhere the customers'goal is the protection oflife and property.

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DET-TRONICSDetector Electronics Corporation

6901 West 110th Street • Minneapolis, MN 55438 USATel: 952.941.5665 or 800.468.3244 • Fax: 612.829.8745 • Web site: www.detronics.com

© Detector Electronics Corporation