Patent GB-1451381-A (GB)
Title: Gas analysis apparatus
IPC: G01N21/35
Filed: 1973-12-17
Abstract: 1451381 Photo-electric gas analyzers AERONUTRONIC FORD CORP 17 Dec 1973 [2 Jan 1973] 58250/73 Heading G1A In a gas analyzer which measures the amount of radiation passing through a gaseous sample to determine the amount of a particular gas in the sample, two gaseous filters are provided in the radiation paths, each filter containing a quantity of the particular gas but the pressure in the filters being different.  Apparatus: In Fig. 1 radiation from source 1 is chopped at frequency f c  by wheel 3 and then passes through one of the two gaseous filters 6, 7 carried by wheel 5. This latter wheel rotates at a lower speed f m  to alternately place filters 6 and 7 in the radiation path. The filters extend over nearly 180 degrees as in Fig. 2. The filtered radiation passes through the sample region 9 and a spectral filter 10 to a detector 11. The detector output is amplified at 12 by an amplifier whose passband centres on f c  and is demodulated at 13 against a frequency of f c  derived from a wheel speed sensor 14. The demodulated output has a D.C. component V c  and an A.C. component; the latter is demodulated at 15 against a frequency of f m  from a wheel speed sensor 16 to give a D.C. output V m . The ratio Vm/Vc is taken at 17 as a measure of the concentration of the gas to be determined. The D.C. signal V c  can form an AGC signal for the amplifier 12, in  which case the signal V m  is automatically referred to the signal V c  and the ratio stage is unnecessary. In an alternative circuit, Fig. 4, the detector feeds amplifier 40, whose output is demodulated at 41 against a frequency of f c  to give V c  and is also fed to a low pass filter 42 before demodulation to give V m . The ratio stage 44 can again be dispensed with if AGC is used. Theory: the low-pressure filter 6 has sharp absorption lines, while the high-pressure filter 7 has wider absorption lines due to pressure broadening. The transmission of the two filters against wavenumber is shown in Fig. 3A by lines 31 and 30 respectively; there are two absorption lines, at numbers 0 and 4, in the wavenumber region shown. The two filters are adjusted so that, taken over the whole spectral range passed by filter 10 of Fig. 1, they pass the same amount of radiation; this is done by making the path lengths of the two filters different, or preferably by using a non- absorbing gas such as N2 or O 2  to provide the higher-pressure in filter 7. As a result there is substantially no difference in the transmission of the two filters except at absorption lines, as shown by the solid line 32 representing the difference in Fig. 3A. Fig. 3B shows the transmission of the gaseous sample. Consider the radiation received by the photo-detector at wavenumber O when filter 6 is in the path; because the filter absorbs all this wavenumber, the detector receives no such radiation. However, as seen in Fig. 3A, when filter 7 is in the path it transmits about 0À6 of such radiation, and the amount received by the detector will depend on the amount of the wavenumber O absorbed in the sample. It can be seen that the amount of radiation received at the detector is modulated at f m  by this process; by synchronous demodulation at f m  this modulation is recovered as a measure V m  of the sample absorption. It can also be seen that at most wavenumbers the filters have equal absorptions, so that by synchronous demodulation at f c  the mean radiation transmitted by the sample over the spectrum passed by filter 10 is measured, to give V c . This is compared with V m  to remove the effects of particulate matter or other broad-band absorbing constituents of the sample. Because the gas in the filters is chosen to be the same as whichever constituent of the sample is to be measured, the filter absorption lines coincide with those of the constituent; the spectrum passed by filter 10 is wide enough to include several such lines, and the chance of a narrow band absorbing constituent of the sample (e.g. another gas) interfering with many of these lines is small. The gaseous filters thus provide good discrimination against other gases. Modifications: the gaseous filters may follow the sample if desired, and this can eliminate the effects of emission by the sample. The filters may be stationary, with mirrors alternating the radiation through them. Two sources may direct radiation through respective gaseous filters and then through the sample. The filter 10 and chopper 3 may be placed elsewhere on the radiation path. The chopper and filter wheels may have a common motor. Specific gases, wavelengths and detectors are discussed.