Analytical Instrumentation

Infrared Gas Detector Working Principle

IR gas Detector

  • Determines the gas based on an emitted infrared light source that is absorbed through a specific air sample. Different air molecules absorb various light frequencies. 
  • A sensor can detect a high concentration of a certain molecule by reporting that air with a particular gas present will absorb more of a particular frequency.

Infrared Gas Analyzer 


Infrared energy was employed by spectrum IR analyzers to analyze gases while only using a small percentage of the entire electromagnetic spectrum. For instance, the gas CH4 exhibits a significant absorption peak at 3.4 microns and an absorption wave length of 3.4 microns. Gases such as CO, CO2, SO2, Nitrous Oxide, CH4, and many more hydrocarbons are analyzed using IR analyzer.The following CO2: 4.26 m & CO: 4.7 m.


Types of IR Gas Analyzer

There are two types of IR Gas Analyzer.IR analyzers that are non-dispersive and dispersive

Dispersive infrared light 

The energy is reflected from surface to surface until it reaches the absorbing cell of this type thanks to polished reflecting surfaces. These types don’t use optical systems. 

Non-dispersive Infrared light 

These varieties use discrete optical band pass filters, much as the sunglass filters intended to block harmful UV rays for eye protection. Since the infrared energy is permitted to flow through the sampling chamber without deforming, there is optical dispersion in this.

Principle of IR gas Detector

  • When certain gases are present in the atmosphere, they have characteristic absorption bands in the infrared spectrum.
  • An IR gas detector utilizes an infrared source that emits radiation at specific wavelengths.
  • The detector measures the amount of infrared radiation that passes through a sample chamber containing the gas being detected.
  • If the target gas is present, it absorbs specific wavelengths of the emitted infrared radiation.
  • By analyzing the difference between the emitted and detected radiation, the gas concentration can be determined.


IR Source

  • Any source that can produce sufficient radiation at the desired wavelength for the purpose of detecting the particular target gas may be employed as an IR source. These are electrically modulated thermal infrared emitters that are micro-machined. 1–5 microns of energy for detection 
  • The infrared source emits radiation at specific wavelengths that correspond to the absorption bands of the target gas. It is usually a heated filament or an LED.

Examples of common IR sources: Nichrome heated filament wire, Nerst glower

Sample Chamber

  • The sample chamber is a space through which the gas to be detected flows. It allows the gas to come into contact with the emitted infrared radiation.

Modulating Light 

  • The light source must be modulated in order for the detector and associated circuitry to operate correctly. By directing the light via a chopper blade, which mimics a fan blade, this is accomplished. The frequency pulses as a result of the blade cutting off the light. 

Optical System

  • To detect and quantify the concentration of a gas, optical system is used to extract the wavelength of interest from an IR source. DIR types make use of an optical component like a prism or grating. Discrete optical band pass filters are used by NDIR types


  • The detector measures the intensity of the infrared radiation that passes through the sample chamber. It can be a photodiode, photovoltaic cell, or other suitable detectors.

Signal Processing Unit

  • The signal processing unit receives the detector’s output and performs necessary calculations to determine the gas concentration. It may include amplifiers, filters, and data processing algorithms.
IR gas Detector

Operation of IR detector

  • The infrared source emits radiation at specific wavelengths, including those absorbed by the target gas.
  • The emitted radiation passes through the sample chamber, where it interacts with the gas.
  • If the target gas is present, it absorbs specific wavelengths of the emitted radiation, reducing the intensity of the transmitted radiation.
  • The detector measures the intensity of the transmitted radiation after it has passed through the sample chamber.
  • The signal processing unit compares the intensity of the transmitted radiation with the original emitted radiation.
  • By analyzing the difference in intensity, the signal processing unit calculates the gas concentration and provides an output, such as a digital display or an alarm signal.

Non-Dispersive Infrared Gas Analyzer

  • The Non-Dispersive Infrared (NDIR) detection method is based on the infrared radiation’s absorption at particular wavelengths as it passes through sample volume. 
  • Modern analyzers can also employ NDIR to identify a specific gas by observing the characteristic infrared wavelength absorption of that gas. A heated filament emits infrared radiation. 
  • The radiation spectra constrained to the absorption band of the gas being studied by optical energy filtering. After the infrared radiation has travelled through the gas to be examined, a detector monitors the energy. When there is no absorption, this energy is used as a benchmark. A “detector” chamber heats up and swells as a result of absorbing some of the infrared light. As a result, the pressure inside the sealed vessel rises, which can be measured using a mass flow sensor or a pressure transducer.
Non Dispersive IR gas analyzer

Why it is called as Non-dispersive?

It is referred to as non-dispersive because the optical filter placed in front of the detector prevents all light except that which the chosen gas molecules may absorb from passing through the sample chamber without being pre-filtered. The detector measures the attenuated signal, which is proportional to the measured gas concentration and is dependent on the degree of gas absorption.

Double Beam NDIR Gas Analyzer

  • The sample beam and the reference beam are two distinct optical channels or beams that make up the analyzer. The gas sample being studied is traversed by the sample beam, but not by the reference beam. The target gas in the sample beam absorbs the infrared radiation, which reduces the amount of radiation that reaches the detector. 
Double beam NDIR gas analyzer

Working of Double beam NDIR gas analyzer

  • A wide range of infrared radiation, including the particular wavelength absorbed by the target gas, is emitted by the infrared source. Only the desired wavelength is allowed to enter the sample and reference beams thanks to the optical filters. 
  • The reference beam avoids the sample while the sample beam travels through the sample cell. Target gas in the sample beam absorbs infrared light at a certain wavelength, lowering the intensity of the radiation that reaches the detector. 
  • Both the sample beam and the reference beam’s radiation intensity are measured by the detector. 
  • The signal processing unit determines the intensity difference between the sample and reference beams, which is inversely proportional to the target gas concentration. The reading for the gas concentration is shown or recorded.
Detailed Diagram of Double Beam NDIR Gas Analyzer

Features of Double beam NDIR gas analyzer

  • Great sensitivity and low concentration measurements with great accuracy Mass flow sensor with a dual detector system
  • Minimum cross sensitivity
  • Flexibility in component measurement used most often for lower range measurements

What is meant by term FTIR & PAS Gas detector?

Fourier Transform Infrared (FTIR) Gas Detector

A Fourier Transform spectrometer is used in this sort of detector to identify gases. In order to calculate the quantity of gases present in the sample, FTIR detectors analyze the absorption of IR light over a wide range of wavelengths. For the detection of a variety of gases, such as hydrocarbons, CO, CO2, NOx, and SO2, FTIR gas detectors are utilized. 

Photo-acoustic Spectroscopy (PAS)

A laser is used in a gas detector to stimulate a gas sample, which then absorbs IR light. A pressure wave is emitted as a result of this absorption and is picked up by a microphone. The ratio of the gas content in the sample to the pressure wave’s intensity shows this relationship. The detection of a variety of gases, such as hydrocarbons, CO, CO2, and SF6, is done using PAS detectors. 

Compare IR gas detector types based on measurement.

Two different types of gas detectors are used to measure the presence of hazardous gases in varied environments: point type gas detectors and open path IR gas detectors. 

ParameterOpen path IR gas detectorsPoint type gas detectors
Detection PrincipleThe presence of a gas is detected by the receiver due to a shift in the way that gas molecules absorb particular IR wavelengths.Point type gas detectors rely on a reactive or non-reactive sensor that comes into direct contact with the gas or its byproducts. 
Area of CoverageThe range of an open path IR gas detector between the transmitter and receiver can reach hundreds of meters. They are perfect for keeping an eye on big outdoor areas including storage complexes, tank farms, and pipelines.Point type gas detectors are made for localized gas detection and are frequently positioned close to possible hazard sources, like a gas storage tank or process machinery. Within a few meters, they can detect the presence of gas.

Response Time
Since it takes some time for the gas to build up in the IR radiation’s path, open path IR gas detectors typically have a response time of 10 to 30 seconds.Due to their immediate reaction to the gas or its byproducts, point type gas detectors respond significantly more quickly, usually within a few seconds. 
Maintenance RequirementsOpen path IR gas detectors need routine maintenanceLess maintenance is required for point-type gas detectors, but regular sensor calibration and replacement are still necessary

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