Patent GB-1256010-A (GB)
Title: Infra-red gas analyser unit
IPC: G01N21/31
Filed: 1969-08-29
Abstract: 1,256,010. Infrared gas analyzer. MINE SAFETY APPLIANCES CO. Aug. 29, 1969 [Oct. 9, 1968 (2)], No.43183/69. Heading G1A. In an infrared gas analyzer unit having sample and reference cells 7 and 14 respectively which are traversed by modulated beams of radiation 3 and 4, these beams are received by gas chambers 12 and 16 having a common gas flow chamber 22 in which is situated a thermistor 23 responsive to cooling by gas flow. Gas containing the component of interest is introduced into cell 7, this component being one that absorbs infrared energy, whereas cell 14 contains a non-absorbing gas. Shutter 54 is initially adjusted so that the two beams incident on the cells are of equal intensity. Thus, the pulsed beam falling on chamber 12 contains less infrared energy than that falling on chamber 16. Accordingly, if the two chambers contain gas that absorbs infrared, the gas in  chamber 16 will be heated more and will expand more, resulting in a pulsed flow of gas through flow chamber 22. This gas flow cools thermistor 23 whose resistance increases, resulting in a change of current in the external circuit which is read on meter 38. The pulsed nature of the pressure fluctuations results in an A.C. output from the thermistor. In a further embodiment Fig. 2 (not shown) the chambers 12 and 16 are dispensed with, the two cells being themselves interconnected through the chamber 22. A gas filled filter cell (56) may be placed between each chamber and the chopper to remove unwanted spectral components Fig. 3 (not shown). Alternatively Fig. 4 (not shown) one filter cell may be filled with air and the other with the particular gaseous component, say CO 2 , whose concentration in the sample gas is to be measured. Both cells 7 and 14 are filled with the sample gas at the same pressure. The incident beam that has passed through the CO 2  - filled filter will be attenuated in the infrared region that is absorbed by CO 2 . If there is CO 2  in the sample, the consequent absorption differential between the two cells will result in the pulsed flow of gas previously described. In this latter embodiment, the sample gas may be flowed through the two cells 71, 72 - see Fig. 5. The flow is kept constant, so that any differential absorption between the two cells will disturb the steady-state condition of the thermistor and cause a pulsed current output to the meter. To compensate for fluctuations in flow-rate, a further thermistor flow chamber assembly is mounted where the flowing gas leaves the second chamber, the two thermistors being made adjacent arms of a bridge arrangement. In the embodiment of Fig. 6 infrared energy from a heated helix of "nichrome" (Registered Trade Mark) wire 81 is chopped, then dispersed by a rotatable prism 84 and focused on to a sample cell 91, the cell 92 also containing the sample gas. Any infrared absorbing component of the sample will cause an A.C. output from the thermistor, as previously described. The prism drive mechanism is also coupled to a chart drive, and the thermistor output is plotted on this chart to give a record of infrared absorption as a function of wavelength.