Infrared Gas Sensor

I
nfrared (IR) gas detection is a well-developed
measurement technology. Infrared gas analyzers
have a reputation for being complicated, cumbersome, and expensive. However, recent technical advancements, including the availability of powerful amplifiers and associated electronic components, have
opened a new frontier for infrared gas analysis. These
advancements have resulted from an increase in demand in the commercial sector, and these demands
will likely continue to nourish the advancement of this
technology.
Gases to be detected are often corrosive and reactive. With most sensor types, the sensor itself is directly
exposed to the gas, often causing the sensor to drift
or die prematurely.
The main advantage of IR instruments is that the
detector does not directly interact with the gas (or
gases) to be detected. The major functional components of the analyzer are protected with optical parts.
In other words, gas molecules  interact only with a light
beam. Only the sample cell and related components
are directly exposed to the gas sample stream. These
components can be treated, making them resistant to
corrosion, and can be designed such that they are easily removable for maintenance or replacement.
Today, many IR instruments are available for a wide
variety of applications. Many of them offer simple

Hazardous Gas Monitors

rugged, and reliable designs.  In general, for toxic and
combustible gas monitoring applications, IR instruments
are among the most user friendly and require the least
amount of maintenance. There is virtually an unlimited
number of applications for which IR technology can be
used. Gases whose molecules consist of two or more dissimilar atoms absorb infrared radiation in a unique manner and are detectable using infrared techniques. Infrared sensors are highly selective and offer a wide range
of sensitivities, from parts per million levels to 100 percent concentrations. This chapter provides general information, with a special emphasis on instruments used
for area air quality and safety applications.
Principle of Operation
The infrared detection principle incorporates only a
small portion of the very wide electromagnetic spectrum.
The portion used is that which we can feel as heat. This
is the region close to the visible region of the spectrum
to which our eyes are sensitive. Electromagnetic radiation travels at close to 3 x 10
8
 m/sec and has a wave-like
profile. Let’s review the basic physics of electromagnetic
radiation by defining the terminology involved with it.
Wave: Similar to a wave in the ocean, the electromagnetic radiation waves oscillate, one wave followed
by another. There are both electromagnetic and mechanical waves, with mechanical waves having a much
longer wavelength. Figure 2 illustrates a mechanical wave.
1 sec 2 sec
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1 cm 2 cm
Frequency = 10 Hz.  Wavelength = 0.1 cm, Wave Number = 10 cm
-1
Fig. 2    A simple mechanical wave showing 10 waves per centimeter to
illustrate the concept of the wave