Gas Detection

NDIR Gas Detection

Thermopiles (in combination with an infrared light source) can be used to measure the gas concentration of certain gases. The basic measurement principle is called nondispersive infrared (NDIR) gas detection, which is based on the absorption of infrared radiation
at certain gas specific wavelengths in accordance with the Beer-Lambert law. The most common gases that are measured with this NDIR principle are carbon dioxide (CO2), methane (CH4) and hydrocarbons (HC). Also gases like carbon monoxide (CO) and nitrogen oxide (NO) are possible to detect.


The most common applications for NDIR gas detection and gas analysis include, but not limited to:

 

 

NDIR Gas Measurement Principle

To measure the gas concentration with precision, you need two thermopile sensors and an infrared light source. One sensor serves
as reference to monitor the variation of IR source output due to source aging, power variations, etc.. The other sensor is equipped
with a gas-specific narrow band optical filter. The IR light source emits a broad spectrum of infrared light at all wavelengths which travels from the source to the sensor. On the way some wavelengths are absorbed by the gas molecules of interest. At the detector
the difference in light intensity between no absorption and gas specific absorption is measured to determine the actual gas concentration.


The picture below illustrates the principle using the example of a NDIR CO2 sensor:

NDIR gas detection measurement principle

On the right side we have an IR light source that emits radiation at all wavelengths. The reflector is used to direct as much light
as possible in the direction of the thermopile detector which is on the left side. In between there is a tube with IR reflecting walls,
which is used as measurement path. For CO2 detection there are two wavelengths of interest which are 3.91 µm (here light red)
and 4.26 µm (here dark red). IR radiation with 3.91 µm is not absorbed by either CO2 or other common gases in the atmosphere
and is transmitted without loss to the detector. Therefore, it is well suited as reference with no absorption. IR radiation with 4.26 µm wavelength is only absorbed by CO2 molecules and by no other gases in the atmosphere. This means, that the degree of radiation reduction on that channel depends only on the CO2 molecules in the air or in other words on the CO2 concentration.


The detector consists of two independent channels with narrow band filters and a thermopile behind in one TO-39 package.
The reference channel is equipped with a filter that has 3.91 µm center wavelength (CWL) and the CO2 sensing channel has a filter with 4.26 µm CWL. The thermopiles behind will therefore only detect radiation at their respective wavelength. As this principle uses
very narrow band filters, there will not be much radiation transmitted to the thermopiles. In turn the signal generated by the thermopiles will be rather low. To get a higher signal there are two options. One is to take a thermopile with bigger absorbing/active area
as the signal is proportional to the size of the absorber. The other option is to generate more light with the IR radiation source.
To generate more IR radiation, more electrical power is needed and this will also have a negative impact on the IR source lifetime. Therefore in most cases a bigger thermopile chip is the preferred choice if more signal is needed.


It is also possible to detect different gases at the same time. In this case you just need more than two independent channels.
One reference channel and one channel for each desired gas that is to be detected is needed. This is possible as each channel
is equipped with a different filter for a certain wavelength. Thus all channels are independent of each other.

NDIR Standard Filters

Heimann Sensor stock provides a bunch of standard filter products for gases suitable for detection by NDIR principle. If you want
to detect different gases, this is also possible with non-standard filters. A possible supplier of optical narrow band filters is Spectrogon . This manufacturer has a large variety of different products in his portfolio and custom designs are also possible. We can order
the desired filters from Spectrogon and use it to build a custom sensor.


Standard filters offered by Heimann without extra charge are listed in the table below:

Gas CWL in nm HPBW in nm Filter
CH4 3300 160 F3.3/160
HC 3375 190 F3.375/190
CO2 4260 180 F4.26/180
CO2 4300 110 F4.3/110
CO2 4430 60 F4.43/60
CO 4640 180 F4.64/180
CO 4640 90 F4.64/90
N2O 4530 85 F4.53/85
NO 5300 180 F5.3/180
- (ref.) 3910 90 F3.91/90

How to choose the right IR Light Source

Regarding IR light sources, there are basically two different types available from Heimann Sensor. One is a reliable low cost source useful at wavelenghts shorter than 4.5 microns, and a higher performance model. The low cost  HSL Series has two major limitations compared to the high performance source. First is speed, as it as much higher time constants for the heating and cooling
of the filament. Second is emission of IR radiation. The low cost IR source has a blub made of quartz glass. This glass blub absorbs more or less IR radiation depending its thickness and the respective wavelength. This is displayed in the below image for a bulb thickness of 0.25 mm. While the transmission at 4.26 µm which is used for CO2 detection is still okay at 40%, the transmission
for wavelengths above 4.5 µm is less than 10% which is in most cases not suitable for gas detection anymore.

low cost infrared radiation source Transmission vs. wavelength

The high performance IR sources do not require any window or filter. Thus their generated IR radiation is not limited by the window
to a certain wavelength range. To measure small concentrations of CO or NO at wavelengths of 4.64 µm and 5.3 µm, the high performance sources of the EMIRS Series  or similar are required.


You might also find this application note on NDIR gas detection from Analog Devices helpful.

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