It is well known that many gas molecules can absorb radiation in the infrared portion of the electromagnetic spectrum. When infrared radiation is incident on the gas, the energy states of atoms vibrating in the molecules change in discrete steps when the wavelength of the infrared matches the molecules natural frequencies or resonances. Put simply, the vibrations of atoms held in their chemical bonds increase when excited by light of the right wavelength.
NOTE: Read our article about NDIR gas sensor usage.
Not every gas concentration can be measured with IR gas sensor. IR interacts with the gas only if it ‘sees’ a dipole on the molecule (an analogy is radio waves picked up by an aerial). Molecular dipoles exist if atoms in the molecules are arranged non-symmetically or if the modes of vibration are non-symmetrical. Symmetrical molecules, such as the binaries H2, N2and O2, are not excited by IR because their structures and modes of vibration do not create any net dipoles. Carbon dioxide is a good example of a symmetrical molecule that has symmetrical and non-symmetrical modes of vibration so it absorbs IR. Other gases and vapors, especially hydrocarbons are active in the IR largely through C-H stretching modes. Generally the greater the number of C-H bonds, the stronger the absorption lines, many of which merge into bands.
NON-DISPERSIVE INFRARED (NDIR). For the majority of IR gas sensing applications, the identities of the target gases are already known, so there is little need for gas spectrometry. The user must still measure the concentration of the target gas, but may be able to accept a certain degree of cross-sensitivity between different gases, if their absorption lines overlap. A Non-Dispersive Infrared sensing technique is more suited to these basic requirements.
For NDIR it is usual to regard the sensor as an individual component in the instrument. The key feature is that fixed narrow-band filters are used with individual IR detectors to identify a few gas absorption lines over a limited wavelength range. Relatively inexpensive sensor components can be resourced and smaller more rugged sensor packages can be fitted into instruments. The simplicity of design makes it easy for the sensor package to meet certifiable safety standards. Furthermore, gas concentrations can be derived in real time from simple algorithms used in the instrument’s microprocessor.
How to choose sensing wavelengths? It is important to avoid “water window” where influence of water absorption spectrum cannot be neglected. The water absorption spectrum shows strong absorptions below 3 microns, between 5 and 8 microns, and beyond 16 microns. Any attempt to sense gas spectral lines in these regions would be subject to strong interference if humidity were present with the target gas. Therefore it is safer to operate in either the 3 to 5 micron or 8 to 16 micron windows where many useful gas lines exist. The 3 to 5 micron window is chosen because:
- There are useful absorption lines at 4.2 microns for CO2 sensing and at 3.0 to 3.5 microns for hydrocarbon sensing.
- There are no gas absorption lines at 4.0 microns, enabling a reference signal to be taken at this wavelength
- IR lamps with glass envelopes radiate to 5 microns.
Gas sensing in the region beyond 8 microns is less popular as it requires an expensive IR source with a specialized IR transmissive and sealed window.
In cases where the ambient pressure and temperature are changing, the ideal gas law must be used to calculate the gas concentration. This is reason that most of NDIR sensors have embedded thermistor. By selecting suitable components and ensuring consistent build standards, the sensor responses to temperature are predictable and close to linear over the range -40 to +75 °C. This makes it relatively easy to compensate for in software once the sensor characteristics are fully understood.
Advantages of NDIR compared with catalytic gas sensing:
- It is a physical sensing technique which is unaffected by aggressive chemical environments.
-There are no poisoning effects as suffered by catalytic sensors.
- Gases may be sensed in anaerobic conditions, i.e. oxygen is not required for operation.
- Hydrogen is not detected and therefore causes no cross-sensitivity.
- Carbon dioxide is uniquely detectable with no interference from other gases.
- There are no problems with sensor deterioration or ‘burn-out’ if exposed to high gas concentrations or if monitoring gases for very long periods.
- Stable long-term operation requires less recalibration.
- Stable after long period of storage. - Life expectancy exceeds 5 years, much longer than catalytic sensors.
- Cost of ownership is low compared with catalytic sensors.
Principle of operation. As you can see on picture, NDIR gas senso has two outputs, reference and active. Reference output is not influenced by target gas variation in compare to active output. These to outputs are voltages and active output voltage will decrease when concentration of target gas increases. Output voltages are used Beer-Lambert Law for final gas concentration calculation. More details about NDIR sensors and Beer-Lambert Law you can read HERE.