Infrared Gas Analyzers
Theory
Non-Dispersive Infrared (NDIR) techniques for the measurement of various gases rely on the energy absorption characteristics of a particular gas in the infrared region. In a simple NDIR instrument, Infrared energy passes through two identical tubes and falls on a detector. The first tube is the reference cell and is filled with a non-absorbing gas such as nitrogen. The second tube is the measurement cell and contains the gas sample to be analyzed. Energy in the region of interest is absorbed by the gas in the measurement cell, attenuating the energy passing through the cell and falling on the detector. This attenuated energy is compared to the unattenuated signal from the reference cell. The difference is proportional to the amount of absorbing gas in the measurement cell.
Application Techniques
A simple method to make the comparison easily with analog electronics is to use a "chopper wheel" assembly which prevents the energy from both cells from falling on the detector simultaneously. As the wheel spins, energy from the reference and measurement cells fall on the detector alternately, producing an AC signal with a magnitude proportional to the diference in energy.
An important enhancement to this basic technique is Gas Filter Correlation (GFC). A rotating, gas-filled filter wheel contains two chambers. One chamber is filled with the species of interest while the other contains an optically inert gas (i.e. Nitrogen). This filter wheel is interposed between the IR source and the single sample chamber. As the wheel rotates, the light passes through one side, then the other as well as the sample chamber. IR energy in the region of interest is attenuated by one side of the wheel, but not the other. As a result, the difference in energy is inversely proportional to the amount of the species in the sample chamber. This has the advantage of providing more sensitivity at lower measured concentrations. Interferent species present in the gas sample attenuate both the sample and reference signals equally and will not be measured.
A further enhancement of the GFC technique employs multiple filter wheels which align in combinations to permit the detection of multiple species within the same instrument.
Detector Types
Over the years, various detectors have been developed for use with infrared gas analyzers. While most modern instruments employ a solid state, photo-conductive detector, Luft-type and microflow detectors are commonly encountered.
Generally, all detectors translate the difference in infrared energy into a sine wave that has a frequency derermined by the chopping rate and a magnitude proportional to the energy difference.
The solid state detector is constructed of a material that changes its electrical conductance in response to IR light energy falling on it. Changes in current flowing through the detection circuit are proportional to the difference in transmitted IR energy. The solid state detector has the advantage of being insensitive to external mechanical vibration. In addition, careful selection of detector material allow for simplified optical filter design.
The Luft type detector is the oldest type found in common use. It is found in dual beam analyzers and uses a pair of chambers that are filled with the adsorbing gas interest. One chamber receives light through the inert gas-filled reference cell, the other receives light through the sample gas-filled cell. The chambers are separated by a capacitive diaphragm which distends in response to the difference in adsorption-induced gas expansion between the two detector chambers. This detector design has the disadvantages of being sensitive to mechanical vibration and expensive to produce.
The Microflow detector is similar to the Luft detector, but employs a small orifice between the two detection chambers. Instead of differential pressure, the resulting flow is measured. A thermocouple arrangement in the orifice is used for this purpose. This detector is relatively immune to mechanical vibration, but is not as simple as the solid state type.
Interferences
Many species of gas have overlapping infrared absorption spectra which can cause cross-interference in the basic technique. Example are water vapor, CO, CO2, NO, NO2, NH3 and SO2. Selectivity and interference rejection in NDIR analyzers is achieved either through the addition of optical band-pass filters, or gas-filled cells. More advanced examples of this technique, employ multiple filters in combination on rotating wheels, which align to allow the detection of several different gases within the same instrument.
