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Chapter 1 Introduction

Chapter 1

Introduction

T he detection of hazardous gases has always beena complex subject and makes choosing an appro-priate gas monitoring instrument a difficult task.

To address this problem, this book aims to providethe following essential tools:

• A simple guide to the various sensor technolo-gies available

• Information to help you intelligently select theproper instruments for specific applications

• Information engineers can use to design a com-plete monitoring system

• Technical data and practical procedures thattechnicians can use to check and maintain a gasmonitoring system

The main emphasis of the book is on gas detec-tion technology that is used in the field of area airquality and safety. This field primarily involves theprotection of personnel and property against toxicand combustible gases. The discussion includes thetypes of sensors used, the various instruments avail-able, and the applications that incorporate these in-struments.

Analy tical Instruments and Monitoring Systems

To date, no gas sensors exist that are 100% selec-tive to a single gas. Achieving such selectivity requiresthe use of instruments that employ analytical tech-niques to identify gases.

Examples of such instruments include Fourier trans-form infrared (FTIR) instruments that use the infraredspectral characteristics of gases, gas chromatographs thatuse analytical columns, and mass spectrometers that iden-tify molecules through characteristic variable deflec-tions from a magnetic field.

These instruments provide fairly accurate and se-lective gas readings. Some typical applications forthese kinds of instruments include airport bombdetection, drug abuse screening, and analyzing airpollutants. However, these analytical instruments re-quire skilled and knowledgeable operators, and aregenerally very expensive and designed for laboratorytabletops or specific on-line applications for in-plantinstallations.

In addition, many suffer from limitations such ashigh maintenance, slow response time, and large size,making them impractical monitors for area air qual-ity and safety. Thus, they are typically used only as alast resort for applications in which a suitable sensoris not available.

For work area air quality and safety applications,monitoring systems must meet a number of practicalcriteria. These monitoring systems must be:

• rugged and corrosion-resistant

• weather- and dust-proof

• capable of being installed in hazardous areas

• durable and long-term

• operationally stable

• easy to maintain

• operated by a minimally skilled person

An Analytical Instrument. Theexample shown above automatesgas chromatography with the helpof its built-in robotic technology.(Courtesy of CE Instruments)

A Gas Monitor. IST’s MP-204is a wall-mounted unit housedin weatherproof enclosure withfour sensor channels.

2

Hazardous Gas Monitors

3


1 Any device that converts input en-ergy of one form to output energyof another.

• suitable for multisensor systems that, for ex-ample, can be used for an entire chemical plant

• low cost

This book deals with gas monitors for work areaair quality and safety applications. For practical pur-poses, we will not delve into the realm of the muchmore complex analytical instruments which, for themost part, do not meet these criteria.

Gas Sensors

A gas sensor is a transducer1 that detects gas mol-ecules and which produces an electrical signal with amagnitude proportional to the concentration of the gas.

Unlike other types of measurement, types that arerelatively straightforward and deal with voltage, tem-perature, and humidity, the measurement of gases ismuch more complicated. Because there are literallyhundreds of different gases, and there is a wide ar-ray of diverse applications in which these gases arepresent, each application must implement a uniqueset of requirements. For example, some applicationsmay require the detection of one specific gas, whileeliminating readings from other background gases.Conversely, other applications may require a quan-titative value of the concentration of every gas presentin the area.

Types of Gas Sensors. There are many differenttechnologies currently available for the detection ofgases, each with certain advantages and disadvantages.The following sensing methods are the focus of ourdiscussion and are the five types most suitable andwidely used as gas monitors for area air quality andsafety applications:

• electrochemical • infrared

• catalytic bead • photoionization

• solid state

Gas Molecules

GAS TRANSDUCER ElectricalSignal

Some Examples of Gas Sensors1. Catalytic Bead. 2. Infrared.3. Solid State. 4. Electrochemical.5. Photoionization.

1

2

34

5

4


All of these sensors are commonly used for detection oftoxic and combustible gases in the work area for hu-man and property protection, or for process control.

One common characteristic of these sensors,despite what is often claimed or implied, is that theyare not specialized to detect any one specific gas.Each sensor is sensitive to a group or a family ofgases. In other words, the sensor is non-specific andis subject to interference by other gases much likea smoke detector in a house cannot distinguish be-tween the smoke caused by a furniture fire and thesmoke caused by food burning in the stove or oven.

In limited cases, a chemical filter can be installedto filter out interference chemicals while permittingthe target gas to pass through to the sensor. Alterna-tively, an analytical column can be installed to iden-tify chemicals qualitatively and quantitatively.

For gas monitoring applications, a proper sensoris usually selected to match the specific applicationrequirements and circumstances, with the user inter-preting the readings based on an awareness of thesensor’s limitations.

Terms, Definitions, and AbbreviationsUnits of Measure for Gas Concentration

ppm: parts per million by volume (see Table 1, oppo-site page)

ppb: parts per billion by volumemg/m3: milligrams per cubic metermg/cc: milligrams per cubic centimeterg/m3: grams per cubic meterg/cc: grams per cubic centimeter

For Combustible Gases

Flash Point (Fl.P): The temperature at which a com-bustible liquid gives off enough vapor to form

Fuel

Oxygen Ignition Source

The Combustion Triangle

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Table 1. Equations for Deriving Units of Gas Concentration

Gas concentration is commonly expressed as percent (%), ppm, or ppb. Mathematically, these areunitless expressions since they do not carry a unit for volume or weight but simply express the ratio ofgases in relation to background air.

For instance, one ppm of CH4 simply means one part of methane amongst 999,999 parts of back-ground air. It is expressed as

Vg =Vg

Va + Vg VT

Vg = volume of gas; Va = volume of air; VT = total volume of air and gas

Multiply the fraction derived from the formula above:a) by 102 % to obtain the percentageb) by 106 ppm to obtain the ppmc) by 109 ppb to obtain the ppb

For example, if you mix 1 cc of gas with 99 cc of air, the calculation is as follows:1 cc

= 0.011 cc + 99 cc

Thus, 0.01 x 102 % = 1%0.01 x 106 ppm = 10,000 ppm0.01 x 109 ppb = 10,000,000 ppb = 107 ppb

In cases like this, one would normally not cumbersomely express the units as ten million parts perbillion. Instead, the simpler expression, 1%, is preferred.

This volumetric expression of concentration is straightforward. Additionally, the volume ratio is equalto the pressure ratio according to Dalton’s law of partial pressures. It is expressed as

Pg =Pg =

Vg

Pa + Pg PT VT

Pg is the partial pressure of the gas within the total pressure PT and Pa is the partial pressure of air.As an example, 1 psi of gas within 99 psi of air with total pressure of 100 psi has a concentration of 1%as in the volume expression.

Another unit, commonly used in the medical and metallurgical industries, is mg/m3 or milligrams percubic meter, in situations wherein the chemical is either in liquid or solid state at room temperature. Toconvert mg/m3 to percent or ppm, the ideal gas law must be used. Chemical conversion factors areincluded in the Gas Data section in Appendix II, page 199, and the conversion formula is discussed onpage 173 in Chapter 11, Gas Sensor Calibration.

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TOO RICH FOR COMBUSTION

TOO LEAN FOR COMBUSTION

0%

100%

COMBUSTIBLE MIXTURE

GA

S CO

NCE

NTR

ATIO

N(M

IXTU

RE IN

AIR

)

UEL

LEL

Fig. 1. A Window of Combustibility

an ignitable and combustible mixture when airis present near the liquid’s surface.

That is, it is the temperature at which a com-bustible liquid chemical has sufficient partialpressure2 in the air to be ignited. The partialpressure curves of chemicals are available fromchemical libraries or manufacturers of thechemicals.

Lower Explosive Limit (LEL) or Lower FlammableLimit (LFL) : The minimum concentration of gas or

vapor mixed with air (percent by volume, at roomtemperature) that will cause the propagation offlames when it comes in contact with a source ofignition. In common terminology, mixtures be-low the LEL or LFL are too lean to ignite.

Upper Explosive Limit (UEL) or Upper FlammableLimit (UFL) : The maximum concentration of gas

or vapor mixed with air (percent by volume, atroom temperature) that will cause the propaga-tion of flames when it comes in contact withan ignition source. In common terminology,mixtures above the UEL or UFL are too rich tosupport combustion. The combustible range is,therefore, between the LEL and the UEL (seeFigure 1 below).

2 The law of partial pressures, firstformulated by James Dalton in1802, states that the pressure ofa mixture of gases, P, which do notreact chemically, is the sum of theindependent pressures (partialpressures) that each gas exerts:

P = P1 + P2 + . . . + Pn

P = PO2 + PN2 + PH2O+

P = 3.087 psi + 10.29 psi + 1.323 psiP = 14.7 psi

N2 = 70%= 10.29 psi

O2 = 21%= 3.087 psi

H2O+= 9%= 1.323 psi

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Specific Gravity (Sp.Gr.): The ratio of the weightper unit volume or mass of a substance at 68°F(20°C) to the mass of an equal volume of dis-tilled water at 39.2°F (4°C).3

Vapor Density: The weight per volume of gas orvapor compared to dry air; both componentshaving the same temperature and pressure. Forexample, air has a vapor density of 1; carbondioxide, 1.52; hydrogen, 0.07; methane, 0.55;and propane, 1.52.

For Toxic Gases

The National Institute for Occupational Safety andHealth (NIOSH) is a branch of the U.S. Departmentof Health and Human Services, Public Health Service,Centers for Disease Control and Prevention. Actingunder the authority of the Occupational Safety andHealth Act of 1970 and the Federal Mine Safety andHealth Act of 1977, it publishes recommended exposurelimits (RELs) for hazardous substances or conditionsin the work place.

To formulate these recommendations, NIOSH col-lects and evaluates data from the fields of industrialhygiene, toxicology, occupational medicine, and ana-lytical chemistry. These recommendations are thenpublished and transmitted to the Occupational Safetyand Health Administration (OSHA) and the MineSafety and Health Administration (MSHA) for use inpromulgating legal standards.

OSHA published the permissible exposure limits(PELs) that are known as the General Industry AirContaminants Standard.

The American Conference of Governmental In-dustrial Hygienists (ACGIH) is a professional society,not an official government agency. Membership is lim-ited to professionals in governmental agencies or edu-cational institutions engaged in occupational safetyand health programs in the United States and around

3 Water at 4°C has the LOWESTvolume per gram. Ice expandswhen its temperature goes below0°C and will crack a rigid encase-ment of it even if it was made ofcement.

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the world. ACGIH publishes the exposure standard,threshold limit values (TLV).

These three different standards—NIOSH’s RELs,OSHA’s PELs and ACGIH’s TLVs—are similar to eachother, yet in some instances there are variations amongthem. All are based on time-weighted-average (TWA),short-term exposure limit (STEL), ceiling (C) and immedi-ately dangerous to life or health (IDLH) concepts. Thefollowing definition of these terms are for informa-tional purposes only. It is beyond the scope of this bookto include detailed discussions of each.

Time-Weighted Average (TWA) is the average con-centration of contaminants over a specified time

period. Mathematically,TWA is the integrated areaunder the concentrationcurve over time divided bytime period. To illustratethe concept of TWA (Figure2), let us assume that dur-ing a three-hour period, COconcentration is constant at50 ppm during the firsthour; then the CO concen-tration increases steadily to100 ppm at the end of thethird hour. The TWA at anygiven moment is repre-sented by the red line. Many microprocessor-based instruments and datalogging programs are ca-pable of performing theTWA calculation, and thetime interval used by theinstruments to calculate theTWA is much shorter thanone hour.

56.25 ppm

GA

S CO

NCE

NTR

ATIO

N(%

MIX

TURE

IN A

IR)

1 2 3HOURS

0

TWA

0

50

100

66.7 ppm

100 ppm

Fig. 2 Simplified Three-Hour TWA. PELs are based on an eight-hour TWA. This is a simplified version to illustrate the TWA concept.

During the first hour, the TWA is:(50 ppm x 1 hr.) / 1 hr. = 50 ppm

Up to the end of the second hour, the TWA is: (50 ppm x 1 hr.) + (50 ppm x 1 hr.) +

(25 ppm x 1 hr.)

2 = 56.25 ppm 2

Up to the end of the third hour, the TWA is:

(50 ppm x 3 hrs.) + (50 ppm x 2 hrs.)

2 = 66.7 ppm3

The numbers within the parentheses represent areas under the curve.

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Recommended Exposure Limit (REL) is TWA con-centration permissible for up to ten-hour work-days during a forty-hour work week.

Permissible Exposure Limit (PEL) and Thresh-old Limit Value (TLV) are TWA permissibleconcentrations, to which workers may be ex-posed continuously, day after day, without ad-verse effects, for a normal eight-hour workdayand a forty-hour work week.

Short-Term Exposure Limit (STEL) is defined as afifteen-minute TWA exposure which should notbe exceeded at any time during a work day evenif the eight-hour TWA is withinlimits. Exposures at the STELshould not be longer than fif-teen minutes and should not berepeated more than four timesper day. There should be at least60 minutes between successiveexposures at the STEL. The con-cept of STEL is illustrated in Fig-ure 3.

Ceiling Limit : The concentrationwhich should not be exceededat any time.

Immediately Dangerous to Life or Health (IDLH)concentration is the maximum concentrationabove which only a highly reliable breathingapparatus providing maximum protection forworkers is permitted. The IDLH value is basedon the ability of a worker to escape without lossof life, irreversible health effects, or otherhealth effects such as disorientation or incoor-dination that could prevent escape.

The preceding information is a simplified versioninterpreted from the NIOSH Pocket Guide to ChemicalHazards. Appendix II at the end of this book lists data

GA

S CO

NCE

NTR

ATIO

N(M

IXTU

RE IN

AIR

)STEL

PEL

5 10 15MINUTES

0

CEILING

TWA (red line)

Fig. 3. A graphical example of permitted TWA excursionsabove STEL, provided the 15-minute TWA does not exceedthe STEL.

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extracted from the same pocket guidethat includes REL, PEL and IDLHconcentrations.

Figure 4 illustrates the overall sum-mary of the toxic gas safety concept.

Combinations of Substances. Whentwo or more hazardous substancesare present in the area, the additiveeffects should be considered. Thatis, if the sum of the fractions of haz-ardous substance concentrations di-vided by the respective PEL exceeds

unity, then the threshold limit of the mixture mustbe considered excessive. This case is illustrated inthe following formula:

C1 + C2 + C3 + . . . + Cn < 1 PEL1 PEL2 PEL3 PELn

If it is greater than one, then the PEL is exceeded.C1,C2 and C3 represent the TWA of various hazardoussubstances.

For example, if the ambient air contains 35 ppmof carbon monoxide (PEL 50 ppm) and 350 ppm ofcarbon dioxide (PEL 5000 ppm), the calculation is asfollows:

35 + 350 = 0.77 50 5000

The threshold limit is not exceeded.This discussion is for conceptual purposes only. It does

not take into account the effect of the combining ofchemicals that can react with each other, resulting ina final mixture that can be more toxic to humans thanthe individual toxicities of each gas.

Performance Specifications

Accuracy: Webster’s dictionary defines accuracy as

GA

S CO

NCE

NTR

ATIO

N(M

IXTU

RE IN

AIR

)

STEL

PEL

HOURS

CEILING

IDLH

90 1 2 3 4 5 6 7 8

TWA

Fig. 4. Basic Rules for Compliance: TWA for eightworking hours does not exceed PEL/TLV. Observe thedefinition of STEL and never exceed the ceiling limit.

“the quality or state of being accurate or exact;precision; exactness.” Accurate is defined as “freefrom mistake or error; precise; adhering closelyto a standard.” Precise is defined as “strictly de-fined; minutely exact; low tolerance; etc.” Accord-ingly, a measurement can be precise but not nec-essarily accurate. The accuracy can only be deter-mined when compared to a standard.

Accuracy is the most important definition ofthe quality of performance for most of the ob-jects we deal with every day; for example, yourwatch, the weather thermometer, the bathroomscale, and the measuring tape, to name a few. Thedefinition of these measurement standards arewell-defined.

At the National Institute of Science and Tech-nology (NIST; formerly National Bureau of Stan-dards), standards for weight, length, temperature,etc. are kept. Internationally, there is a total agree-ment about the “absoluteness” of those standards.

In real life, the most accurate instrument maynot necessarily be the best. For example, the mea-suring tape which is used by the tailor is not veryaccurate, but it is practical for the task. Calipersused in a machine shop are more accurate thanthe tailor’s tape but would not be suitable for useby the tailor. Thus, each instrument serves a dif-ferent objective. The gas monitors serve more likea tailor’s tape than a machine shop’s caliper.

The Challenge of Accuracy. With gas moni-toring systems, there is no standard by which tocompare accuracy. There are hundreds of differ-ent chemicals, each having its own unique chemi-cal and physical properties.

As an example, what is 100 ppm of carbon mon-oxide (CO) in air? Mathematically, this equates to0.01% of CO and 99.99% air. After the mixture ismade, its accuracy is difficult to determine since

MeasuredWeight

StandardWeight

ACCURACY

The International Vocabulary of Basic andGeneral Terms in Metrology defines theterms for result of the measurement asfollows:

1. Accuracy: Closeness of the agreementbetween the result of a measurementand a true value of the measurand.*

2. Repeatability: Closeness of the agree-ment between the results of successivemeasurements of the same measurandcarried out under the same conditionsof measurement.

3. Reproducibility: Closeness of the agree-ment between the results of measure-ments of the same measurand carriedout under changed conditions of mea-surement.

4. Linear scale: Scale in which each spac-ing is related to the corresponding scaleinterval by a coefficient of proportional-ity that is constant throughout the scale.

* measurand–a particular quantity subjectto measurement; e.g., vapor pressureof a given sample of water at 20°C.

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there is no standard 100 ppm of CO to comparewith and there is no common agreement that de-fines a 100 ppm CO mixture.

Because calibration standards are difficult todefine in practice, accuracy is the most misunder-stood and abused term in gas monitoring. Thereare few agreements amongst manufacturers of in-struments, and there is no common understand-ing in general.

Realistically, it is best to establish a calibrationmethod that can yield consistent and precise calibra-tion data. The accuracy of this calibration methodcan be compared to an accepted standard whenchallenged.

As long as the calibrations are done with highprecision, the accuracy of your calibration can beestablished when an accepted standard is available.

Repeatability: Repeatability is the ability of sensors torepeat the measurements of gas concentrations whenthe sensors are subjected to precisely calibrated gassamples.

Zero Air : “Zero air” is available on the market in theform of a mixture of oxygen and nitrogen in ahigh pressure tank. In normal applications, how-ever, gas monitoring sensors are put to work in“non-ideal” environments and, consequently,there are many other components in the ambientair besides O2 and N2 , such as water vapor, car-bon dioxide, carbon monoxide, and other tracegases.

Therefore, it is not practical to zero a sensorto a simple mixture of oxygen and nitrogen. Somesensors can be zeroed with dry air or N2 but somecannot. For instance, most infrared (IR) detectorscan be zeroed with dry air or N2 as long as thewavelength being detected has a minimum watervapor effect. However, with solid-state sensors orphotoionization detectors (PIDs), very different

N2

O2

N2

O2

H2O

CO2 & others

A. Artificial Air

or

B. Environmental Air

Zero Air vs. Ambient Air

13


readings may result from dry air compared withwet air.

In many applications, sensors may be satisfac-torily zeroed by exposing the sensor to a bag ofair collected from a location where the air is “nor-mal.” In fact, this is the easiest way to verifywhether a sensor is giving a real alarmor a false alarm.

Linearity : Quantitatively, linearity refersto the output signal in relation to thegas concentration: If 1 volt equals 10ppm and a full scale 5 volts represent50 ppm, then the output will be lin-ear. With most sensors, the initialoutput of the sensor is linear or closeto it, but as the gas concentration in-creases, the output signal is graduallyreduced. Figure 5 shows a typical sen-sor response curve.

Specificity or Selectivity: This is the ability of an in-strument to detect a target gas without being af-fected by the presence of other interfering gases.

Most sensors are sensitive to a family of gases,and there are no sensors specific to only one gas.Among the more specific sensors is an electro-chemical sensor for the detection of oxygen.

Different techniques are employed in orderto achieve some degree of selectivity to suit prac-tical applications. For example, a charcoal filteris used to filter out most hydrocarbons while let-ting only CO, H2, and CH4 pass through.

In solid-state sensors, the surface temperatureof the sensor can be set differently in order tomake it more sensitive to one gas and less sensi-tive to other gases.

The most common practice is to use an ana-lytical column, in which the gas sample stream isintroduced into the column and the chemical

0Gas Concentration

Nonlinear Saturation(Poor Resolution)

NearLinear

Sensor Outputin volts

5

10

Fig. 5. Typical Sensor Output Curve.As the gas concentration increases,the output signal becomes smaller inrelation to the increase in gasconcentration, resulting in poorresolution. Most sensors providebetter accuracy at lower concen-trations than at very high concentra-tions. Thus, instruments on themarket today commonly have outputsignals that are digitally linearized.

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components are separated and come out at theend of the column at different times, where theyare detected by a sensor.

This method works well for laboratory use buthas proven to be impractical for gas monitoringbecause it requires a high degree of user knowl-edge and a high degree of maintenance. Addi-tionally, since the sample must be drawn throughthe column, the time necessary to take readingscan be relatively excessive. Sample times of 15 to30 minutes are not unusual.

For ambient air monitoring, it is much morepractical to use a sensor directly installed at thelocation being monitored and compensated forthe different gases which may be present.

Interference Ratio: As mentioned earlier, sensors arenot selective to a single gas and will read othergases as well. Thus, a common practice for manu-facturers of gas monitoring equipment is to pro-vide data indicating the ratios that different gaseswill read on the sensor. For instance, on a 100ppm carbon monoxide sensor, hydrogen mayread at a 3-to-1 ratio. This means that 3 ppm ofH2 will read the same as 1 ppm of CO.

In many cases, even though it is stated that acertain gas will not interfere, if the concentra-tion of this gas is high enough, it may in fact ac-tually interfere. For example, while a CO sensorwith a charcoal filter has little interference fromcertain solvents at 100 ppm, when the concen-tration is increased to 1,000 ppm, they may in-terfere drastically.

Because there are so many gases, it is not pos-sible for manufacturers to present data on crosssensitivity ratios for all gases. Therefore, if inter-ference data is not provided for a gas that is ofinterest to you, you should inquire with themanufacturer if the sensor is selective to a spe-

9 ppmH2

CO Sensor 33 ppmCO Signal

30 ppmCO

9 ppm H2 shows as 3 ppm CO

15


cific gas being targeted; in which case, the manu-facturer could provide the data needed.

Response/Recovery Time: This is typically definedas the time it takes for a sensor to read a certainpercentage of full-scale reading after being ex-posed to a full-scale concentration of a given gas.For example, T80 = 30 seconds means that thesensor takes 30 seconds to reach 80% of the full-scale reading after being exposed to a full-scalegas concentration.

Temperature and Humidity: Specifications for theseparameters are easy to understand, but beware ofthe humidity specification. Relative humidity isan indication of the amount of water vapor in airas a percentage of the total amount possible at agiven temperature.

Quantitatively, the amount of actual water vaporin air is a function of temperature. For instance, at80% relative humidity and a temperature of 25°C,water vapor is present at a level of 3%. However, thesame 80% relative humidity at a temperature of 48°Cproduces a water vapor level of 10%. In the pres-ence of chemicals, combined with the changing oftemperature between day and night, the possiblewater condensation and resultant corrosive mixturescan compromise the life expectancy of a sensor.

0% Time

Scal

e

50%

100%

T100T80

Fig.6. A Typical Sensor Response

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Hysteresis: The difference in response of the sensor whencalibrating from a zero level to mid-scale comparedto the response when calibrating from full scale tomid-scale, is known as hysteresis. This quantity is nor-mally expressed as a percentage of full scale. For ex-ample, a 100 ppm instrument, when calibrated from0 to 50 ppm and exposed to a 50 ppm calibration gas,will indicate 50 ppm. However, when the sensor is cali-brated to 100 ppm gas but is exposed to 50 ppm thesensor may indicate 55 ppm. This variation of 5 ppmis 5% full-scale hysteresis. Most infrared and photo-ionization instruments do not exhibit hysteresis, butmany other sensors, including electrochemical, solidstate, and catalytic sensors do exhibit hyteresis.

In alarm setting, the difference between the onpoint and off point alarm is also referred to as hyster-esis. For instance, if the alarm comes on at 100 ppm,the alarm will not turn off until the gas is below 90ppm. This hysteresis is needed; otherwise, an alarmcan be chattering at the set point of 100 ppm.

Zero and Span Drift: While there is no specific definitionfor these two terms, common understanding holds thatthis drift is the percentage change of the zero or spancalibration over a specified period of time, typically30 days or more.

Hazardous Locations

Gas monitoring instruments are often installed in in-dustrial process and production areas. These areas areoften classified as hazardous locations. Industrial fa-cilities in which potentially explosive gas atmospheresexist or may exist must utilize proper explosion proofprotection methods when using these types of instru-ments. It is beyond the scope of this book to providefull details; however, the following information maybe helpful. The reader is advised to consult with themanufacturer for specific needs.

North America and other parts of the world that

17


have been influenced by North American practiceshave traditionally used the National ElectricCode(NEC®) articles 500-503. They employ a classand division system: Classes identify the type of haz-ard present as gases or vapors, combustible dusts,and flammable fibers. Divisions define the condi-tion under which the hazardous materialmay be present. The devices designed andmanufactured for these hazardous loca-tions should be tested and approved foruse by a nationally recognized laboratorysuch as Under writer’s Laboratories(UL®), Factory Mutual (FM), or the Ca-nadian Standards Association (CSA®).The NEC 500 hazard classifications areas follows:

Class I: Flammable gases or vapors.Class II: Combustible dustsClass III : Easily ignitable fibers and flyingsGroups are based on flame propagation characteris-

tics, ignition temperature, and pressure generated duringexplosion of various gases and vapors. There are four dif-ferent groups in Class I. These groups are as follows:

Group A: Acetylene

Group B: Acrolein, butadiene, ethylene oxide, form-aldehyde, hydrogen, propylene oxide, and propylnitrate

Groups C and D: All other combustible gases belongto Groups C and D

Division 1: Where ignitable concentrations of gases,vapors, dusts, and fibers can exist all the time or someof the time under normal operating conditions.

Division 2: Where ignitable concentrations of gases,vapors, dusts, and fibers do not exist under normaloperating conditions. Hazardous conditions only ex-ist in the event of abnormal conditions, such as acci-

North American Certification Agencies

Underwriters Laboratories Factory Mutual

Canadian Standards Association

18


dental rupture or breakdown of a container, stor-age tank, etc.

Gas monitoring instruments are typically designedand certified for use in Class I, Division 1, Group B, Cand D hazardous locations for use in North Americanmarkets.

Zones. European countries, as well as a majority ofother nations of the world, have been influenced bythe International Electrotechnical Commission’s (IEC)three-tiered zone approach. The IEC separates thepotentially explosive atmosphere into Zones 0, 1, and2 based on the probability of occurrence and length oftime a potential explosive mixture may be present.Apparatuses designed for use in these areas are usuallytested and approved for use by the European Commit-tee for Electrotechnical Standardization (CENELEC)test authorities using Euronorm (EN) standards. Thedivision of these three zones are:

Zone 0: An area in which an explosive gas atmosphereis continuously present for long periods.

Zone 1: An area in which an explosive gas atmosphereis likely to occur in normal operation.

Zone 2: An area in which an explosive gas atmosphereis not likely to occur in normal operation, and ifit does, it will exist for a short period only.

Zones 20, 21, and 22 are a subset of Codes 0, 1, and 2that refer to ignitable dust clouds.

Definition Comparisons. In accordance with theirexplosive properties, the combustible gases and vaporsare divided into temperature classes and explosion pro-tection subgroups. There are no direct comparisonsbetween the current NEC and IEC standards. NationalFire Protection Agency (NFPA) in America adoptedarticle NEC 505 which is comparable to IEC standards.A brief comparison of IEC (world), CENELEC (Eu-rope) and NEC (USA) are as follows:

1. Condition: Hazardous conditions exist continuously

Example: Ex EEx d IIb T3

Approved mark forapparatus certifiedby an EC testauthority

Symbol for apparatusbuilt in accordancewith a Europeanstandard

Flameproof enclosure(type of protection)

Explosion group

Temperature class

CENELEC Marking

19


or for long periods of time.NEC 505: Class 1, Zone 0NEC 500: Class 1, Division 1IEC: Zone 0.CENELEC: Zone 0

2. Condition: Hazardous condition is likely to occur innormal operation.

NEC 505: Class 1, Zone 1NEC 500: Class 1, Division 1IEC: Zone 1CENELEC: Zone 1

3. Condition: Hazardous condition is not likely to oc-cur in normal operation and if it does, only infre-quently and for a short period.

NEC 505: Class 1, Zone 2NEC 500: Class 1, Division 2.IEC: Zone 2CENELEC: Zone 2

Types of ProtectionThere are several acceptable types of protection

for electrical equipment in hazardous locations. Themore common types are the following:

A. Flameproof Enclosure (d)4 : The enclosure,such as the one shown in Figure 6, will with-stand an internal explosion, without causingignition of an external explosive atmosphere.The enclosure joints and structure covers aredesigned and manufactured for such purposes.This type of protection is most commonly usedfor gas monitoring applications and can meetthe requirements of:

NEC 500 - Class 1, Division 1 & 2.NEC 505-Class 1, Zone 1 & 2, AExd.IEC-Exd.CENELEC-EExd.

Fig. 6. An Explosion-ProofEnclosure

4 The lower case letter in a CENELEC-approved marking designates thetype of protection offered by anenclosure.

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B. Intrinsic Safety (i): The electrical energy inan intrinsically safe circuit which enters into theenclosure is not sufficient to generate a sparkand ignite a combustible mixture in the haz-ardous area, in any worst case scenario. To meetthis requirement, safety barriers or other devices

limiting the electrical energyare placed on the wires tolimit the electrical energy al-lowed to flow through the cir-cuit before the wire entersinto the hazardous location.Safety barriers are a combi-nation of zener diodes, powerresistors, and fuses which aredesigned to limit the amountof electrical energy allowed to

flow through the wires. Various approved andcertified safety barriers are available as standardelectrical components. These are limited to lowpower device applications only. This method ofprotection can meet the requirements of:

NEC 500 - Class 1, Division 1 & 2.NEC-505 - Class 1, Zones 0, 1 & 2.

AExi(a).IEC - Exi(a).CENELEC - EExi(a).

Types of Flamepaths in Flameproof Enclosures

Joint Threaded Spigot

Materials used: Aluminum, Iron (Courtesy of Cullen Associates)

ZenerDiodes

Resistor Fuse

IntrinsicallySafe Ground

Single Channel

(+) 24 v

RM

Power supply mustnot be grounded

ProcessController

SensorModule

SAFE AREAHAZARDOUSAREA

(Courtesy of Cullen Associates)

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C. Purged and Pressurized (p): This is the pro-cess of supplying sealed electrical enclo-sures with a protective gas to prevent theentrance of flam-mable gases whilemaintaining a positiveenclosure pressure.This type of protec-tion can meet the requirements of:

NEC 500 - Type X, Y & Z.NEC 505 - Type X, Y & Z.IEC - Exp.CENELEC - EExp.

Type X pressurizing: Reduces the classificationwithin the protected enclosure from Class 1, Di-vision 1 or Class 1, Zone 1 to unclassified.

Type Y pressurizing: Reduces the classificationwithin the protected enclosure from Division 1to Division 2 or Zone 1 to Zone 2.

Type Z pressurizing: Reduces the classificationwithin the protected enclosure from Class 1,Division 2 or Class 1, Zone 2 to unclassified.

D. Increased Safety (e): This is a type of explosionprotection applied to electrical apparatus thatdoes not produce arcs or sparks in normal ser-vice, in which additional measures are appliedso as to give increased security against the pos-sibility of excessive temperature and of the oc-currence of arcs and sparks. This method ofprotection can meet the requirements of:

NEC 500 - No Standard.NEC 505 - Class 1, Zone 1 & 2, AExe.IEC - Exe.CENELEC - EExe.

ProtectiveGas Supply

OptimalAlarm orPowerCut-OffSwitchPressure

Regulator ProtectiveEnclosure

PressureIndicatorOptional Pressure

Relief Device

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E. Other Protection Methods: Oil immersion(o),powder filling(q) and moulding(m).

Enclosure Classifications For Nonhazardous Areas

In North America, the National Electrical Manu-facturer’s Association (NEMA), as a way of standard-izing enclosure performance, classified the enclosuresin different ratings which are intended to provide in-formation for users to make proper product choices.This rating system identifies the ability of the enclo-sure to resist various possible conditions. The classifi-cations are as follows:

NEMA Type 1: For general-purpose indoor use, pro-vides protection against incidental contact withthe enclosed equipment.

NEMA Type 2: In addition to NEMA Type 1, pro-vides protection against a limited amount offalling water and dirt.

NEMA Type 3: For outdoor use. Provides protec-tion against windblown dust, rain and sleet, aswell as formation of ice on the enclosure. Pro-vides rust resistance.

NEMA Type 3R: Same as NEMA Type 3, but doesnot provide dust protection.

NEMA Type 4: Same as NEMA Type 3, except, it isfor indoors or outdoors, provides protectionagainst direct water hose down.

NEMA Type 4X: Same as NEMA Type 4, but it alsoprovides corrosion resistance for indoor use.

NEMA Type 6: Same as NEMA Type 4X, but pro-vides protection against water during tempo-rary submersion at a limited depth.

NEMA Type 7: For indoor use in hazardous loca-tions, Class 1, Groups A, B, C, and D.

NEMA Type 9: For indoor use in dust applications,Class II, Groups E, F, and G.

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NEMA Type 12: For indoor use, provides protec-tion against dust, falling dirt, and dripping non-corrosive liquids.

NEMA Type 13: For indoor use, provides protec-tion against dust, spraying water, oil, and non-corrosive coolant.

Internationally, the protections for electrical appa-ratuses are designated with ingress protection (IP) followedby a two-digit number which defines the degree of pro-tection. The first digit (0-6) defines the protectionagainst contact and entry of foreign objects while thesecond digit (0-8) defines the protection against water.

The IP classification is set by the InternationalElectrotechnical Commission. The definitions for pro-tections are different from that of NEMA. Therefore,the IEC enclosure classification designations cannot

Protection First Digit Second Digit

0 No protection No protection

1 Large objects of more Vertically falling water than 50 mm diameter

2 Medium-sized objects of Falling water at up tomore than 12 mm diameter 15 degrees from vertical

3 Small objects of more Falling water up to than 2.5 mm 60 degrees from vertical

4 Granular objects of more Water splashes fromthan 1 mm diameter any direction

5 Dust protected, Water from a nozzlenot completely tight from any direction

6 Dust tight Powerful water jet

7 Short-term immersion

8 Continuous immersion

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be exactly equated with NEMA enclosures.As examples, IP 66 is approximately equivalent to

NEMA 4 or 4X , IP 67 is equivalent to NEMA 6, andIP 55 is equivalent to NEMA 12.

Summary

There is no clear definition delineating a gas moni-tor from an analytical analyzer. The distinction be-tween the two is based largely upon their usage in theactual application. A gas monitor is most frequentlyused to monitor gases in toxic and combustible rangesfor area air quality and safety applications. For thistype of safety application, the concept of the units usedin the measurements and the definition of terminolo-gies is somewhat unique. It is important to understandthe terms used. However, the examples of chemicaltoxicity presented in this book do not consider thecomplexity of actual toxicology and interreactions be-tween contaminant chemicals. In critical applications,a specialist on the subject needs to be consulted.

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