An infrared gas analyzer is used to measure the quantity of various gas. The amount of gas is determined by amount of a particular frequency of light absorbed by the gas when the light is passed through the gas. Different molecules in the air absorb different frequencies of light, measuring the absorbed frequency clearly gives the relation to the amount of particular gas in the air.
There are dispersion and non-dispersion types of infrared analyzers. Dispersion infrared analyzers are used in laboratories as spectrophotometers; non-dispersion infrared analyzers (NDIR) are used for continuous measurement in industrial applications.
Principle of Operation:Â
The absorption spectrum of infrared radiation absorbed by a gas, is unique to the type of gas, In every absorption-type, optical analyzer, the fundamental equation relating photon absorption to substance concentration is the Beer-Lambert Law (sometimes called the Lambert-Beer Law):
Where,
A = Absorbance
a = Extinction coefficient for photon-absorbing substance(s)
b = Path length of light traveling through the sample
c = Concentration of a photon-absorbing substance in the sample
I0 = Intensity of source (incident) light
I = Intensity of received light after passing through the sample
Construction and Working:
There is two tubes in the analyzer one is filled with reference gas and other with the sample gas or process gas which absorbs light. The reference gas is usually gases like nitrogen which will not absorb light.
The incident light (infrared) is split into two parallel beams by the upper mirror. One beam is used for the measurement and the other is used as the reference.
The reference light beam passes through the reference cell, which is filled with air or N2 , and is then reflected off the bottom mirror onto a semiconductor detector. The measurement light beam passes through the measurement cell, and is reflected onto a semiconductor detector in the same fashion as the reference light.
The component to be measured in the gas sample flowing through the measurement cell absorbs some of the measurement light, thus reducing the strength of the light relative to the strength of the reference light. The two light beams are alternately cut off from the detector by a semi- circular rotary sector. This allows the detector to convert the difference in the strengths of the measurement and reference light beams into an alternating electrical signal that represents the concentration of the gas being measured.
Advantages:
- Gas molecule doesn’t interact directly with the gas.
- Non-destructive analysis.
- Infrared gas analysers are standard detectors for the measurement of gas in any given environment.
- Monitoring emission levels over longer periods of time.
Disadvantages:
- A simple measurement becomes a complex measurement
- Cost of measuring the gas normailisation parameter is greater than the cost of primary dust measurement
Also, read:
Gas analyzer spectroscopy – Working Principle and Types
Working & Automated calibration of Gas analyzer
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