Many gases have infra red absorption bands which can act as a finger print and identify them.
Infra Red filters are a key enabling device used for NDIR gas detection, a typical system is shown below.
The filter is tuned for the gas being detected.
The graph below shows the absorption bands of some common gases
Many more can be found on the NIST web book.
Specifying a Filter
- Chose a unique absorption feature to base your filter around, for example most of these features fall within the 3 to 7µm window.
The bands associated with water should be avoided, these can be seen below.
- The profile of the filter should 'hug' the profile of the absorption feature, see example below for a Carbon Dioxide filter (a suitable reference filter is also shown)
- Many systems require a reference filter, this should be chosen well away from any gas, 4µm is a commonly used wavelength
- All gas filters should block for the full range of the detector being used with it, Vortex filters bock from UV to 30 µm+
- The temperature stability is important, the filter will fail to function properly if the transmission band moves with temperature, a typical specification of <0.2 nm per degree rise in temperature will be sufficient in most situations. Vortex filters are magnetron sputtered and move <0.1 per Deg C. Also if the filter operates at a temperature outside +/- 100 Deg from room temperature the standard specifications may not be suitable
- The other features of specifying the 'close up' properties of the filter are shown below:
- Centre Wavelength (CWL), it should be in the middle of the gases absorption feature
- Peak transmission required, typically 70% +
- Half power width of the filter, the wavelengths where the peak transmission/2 occurs
- Blocking range 1, i.e. From the UV to the filter feature and the level of transmission acceptable, for example R <0.2% ave for UV to transmission feature
- Blocking range 2, i.e. From the far IR back to the filter feature and the level of transmission acceptable, for example R <0.2% ave for transmission feature to 30µm
- The specifications for Vortex filters are given on the datasheets below
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