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Infrared Gas Sensors - International Sensor …

55 Chapter 5 Infrared Gas SensorsChapter 5 Infrared Gas SensorsInfrared (IR) gas detection is a well-developedmeasurement technology. Infrared gas analyzershave a reputation for being complicated, cumber-some, and expensive. However, recent technical ad-vancements, including the availability of powerful am-plifiers and associated electronic components, haveopened a new frontier for Infrared gas analysis. Theseadvancements have resulted from an increase in de-mand in the commercial sector, and these demandswill likely continue to nourish the advancement of to be detected are often corrosive and reac-tive.

55 Chapter 5 Infrared Gas Sensors Chapter 5 Infrared Gas Sensors I nfrared (IR) gas detection is a well-developed measurement technology. Infrared gas analyzers have a reputation for being complicated, cumber-

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Transcription of Infrared Gas Sensors - International Sensor …

1 55 Chapter 5 Infrared Gas SensorsChapter 5 Infrared Gas SensorsInfrared (IR) gas detection is a well-developedmeasurement technology. Infrared gas analyzershave a reputation for being complicated, cumber-some, and expensive. However, recent technical ad-vancements, including the availability of powerful am-plifiers and associated electronic components, haveopened a new frontier for Infrared gas analysis. Theseadvancements have resulted from an increase in de-mand in the commercial sector, and these demandswill likely continue to nourish the advancement of to be detected are often corrosive and reac-tive.

2 With most Sensor types, the Sensor itself is directlyexposed to the gas, often causing the Sensor to driftor die main advantage of IR instruments is that thedetector does not directly interact with the gas (orgases) to be detected. The major functional compo-nents of the analyzer are protected with optical other words, gas molecules interact only with a lightbeam. Only the sample cell and related componentsare directly exposed to the gas sample stream. Thesecomponents can be treated, making them resistant tocorrosion, and can be designed such that they are eas-ily removable for maintenance or , many IR instruments are available for a widevariety of applications.

3 Many of them offer simple,Fig. 1 An example of an IRgas monitor with the gas cellassembly Cell56 Hazardous Gas Monitorsrugged, and reliable designs. In general, for toxic andcombustible gas monitoring applications, IR instrumentsare among the most user friendly and require the leastamount of maintenance. There is virtually an unlimitednumber of applications for which IR technology can beused. Gases whose molecules consist of two or more dis-similar atoms absorb Infrared radiation in a unique man-ner and are detectable using Infrared techniques. Infra-red Sensors are highly selective and offer a wide rangeof sensitivities, from parts per million levels to 100 per-cent concentrations.

4 This chapter provides general in-formation, with a special emphasis on instruments usedfor area air quality and safety of OperationThe Infrared detection principle incorporates only asmall portion of the ver y wide electromagnetic portion used is that which we can feel as heat. Thisis the region close to the visible region of the spectrumto which our eyes are sensitive. Electromagnetic radia-tion travels at close to 3 x 108 m/sec and has a wave-likeprofile. Let s review the basic physics of electromagneticradiation by defining the terminology involved with : Similar to a wave in the ocean, the electro-magnetic radiation waves oscillate, one wave followedby another.

5 There are both electromagnetic and me-chanical waves, with mechanical waves having a muchlonger wavelength. Figure 2 illustrates a mechanical sec2 sec12345678910111213141516171819201 cm2 cm Frequency = 10 Hz. Wavelength = cm, Wave Number = 10 cm-1 Fig. 2 A simple mechanical wave showing 10 waves per centimeter toillustrate the concept of the 5 Infrared Gas SensorsFrequency: Number of waves per second passingthrough a point. An electromagnetic wave travels atthe speed of light which is 300 million meters per sec-ond, or 3 x 108 m/sec. Therefore, the frequency is thespeed of light divided by the wavelength, and is ex-pressed as the number of waves per second, or hertz(Hz).

6 Wavelength: The distance between two peaks ofthe wave, or the spacing between two waves. It is com-monly expressed in microns. It is a very popular termused in representing gas molecular absorption bandsas well as optical component number:The number of waves in one centi-meter. It is the reciprocal of wavelength. Since 1 mi-cron = 10-6 m = 10-4 cm, the reciprocal of one micron is1/10-4 (10,000 wave numbers per cm), and 2 microns =5000 wave numbers per cm. The formula is:Wave number = 1/wavelengthMicron:A common unit used to express wave-length in the Infrared region. It is one millionth of ameter(10-6m) or a micrometer, and is abbreviated as.

7 Transmittance:The ratio of transmitted radiationenergy to the incident energy. The energy not trans-mitted is absorbed and reflected. It is used to specifyoptical : Opposite of transmittance. Used todescribe the amount of energy absorbed by gas mol-ecules. Both percent absorption and percent transmittanceare used as the y-axis versus wave number or wavelengthas the x-axis in the Infrared number and wavelength are common termsused by scientists to describe the Infrared region forgas analysis because they provide a convenient methodto express radiation frequency and the mechanisms ofinteraction between Infrared radiation and gas mol-ecules.

8 Mathematically, they are the reciprocals of eachTransmittance = 100 = 65%6565 Transmitted Energy25 AbsorbedEnergy100 Incident Energy10 Reflected Energy58 Hazardous Gas Monitorsother. For example, methane gas has the absorption wave-length of microns, or a wave number of 2941 cm-1.(Figure 4, on page 59, shows a spectroscopic descriptionof methane gas which illustrates that methane has a strongabsorption peak at , or a wave number of 2941 cm-1.)Electromagnetic waves propagate through space ormatter by oscillating electric and magnetic fields. In avacuum, they travel at the speed of light. The completerange of frequencies of these waves is called the electro-magnetic frequencies range from gamma rays of 1020 Hzto radio waves of 106 Hz.

9 They are classified from higherto lower frequencies as gamma rays, x-rays, ultravioletlight, visible light, Infrared light, microwaves, and radiowaves. Figure 3 shows the electromagnetic light, at about 4 x 1014 Hz (or to mi-crons), is actually only a very narrow portion of the spec-trum. Infrared is just below visible light, and this explainswhy we feel, but do not see, temperature. The infraredregion is most useful for gas analysis because absorptionby gas molecules is unique and selective in this Gas Absorption Fingerprints. The com-plexity of the gas molecules determines the number ofabsorption peaks.

10 The more atoms that form a molecule,the more absorption bands that will occur. The regionin which this absorption occurs, the amount of absorp-tion, and the specific character of the absorption curveis unique to each gas. Gas molecules can be fingerprintedusing their absorption characteristics and archived forFig. 3 Location of Infrared in the Electromagnetic Spectrum4 x 1014 Frequency (cycles/sec.)Radio8 x 10141061020101810151012109 TelevisionMicrowaveInfraredUltra-violetX -rays Gamma rays59 Chapter 5 Infrared Gas Sensorsgas analysis and identification purposes. A library of thesecurves can then be stored in the memory inside an in-strument.


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