INFRARED ABSORPTION ANALYSIS METHOD AND APPARATUS FOR DETERMINING GAS CONCENTRATION

• US 3,781,910 A
• Filed: 09/22/1972
• Issued: 12/25/1973
• Est. Priority Date: 09/28/1971
• Status: Expired due to Term

• Current Assignee: Pierre Herrmann
• Original Assignee: Pierre Herrmann
• Inventors: Herrmann, Pierre
• Primary Examiner(s): Lindquist, William F.
• Application Number:
  US05/291,379
• Time in Patent Office: 459 Days
• Field of Search: 250/43.5R
• US Class Current: 250/341.5
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis

Abstract

There is disclosed a dual beam gas analyzer and the mode of operation thereof for measuring small concentrations of a gas of interest in the presence of another gas having absorption bands overlapping those of the gas of interest. This is encountered in the determination of the concentration of CO2, CO, CH4, C2H2, NH3 or SO2 in the presence of CH4, CO2, CO2, CO2 or CH4 respectively. The dual beam gas analyzer comprises a source of infrared rays providing an infrared beam which is directed into one end of an analysis chamber through an optical commutator. The optical commutator directs alternately and periodically the infrared beam to a comparison filter transparent to the infrared beam and to a gaseous filter absorbing completely the energies of the absorption bands of the gas to be measured. The output signals of a non-selective radiation detector disposed at the other end of the analysis chamber are applied to electronic circuits which provide a signal representative of the difference between the energies of the two successive beams passing through the analysis chamber. A movable shutter permits one to modify the energy of the beam corresponding to the comparison filter and is adjusted to eliminate the influence of the other gas on the determination of the concentration of the gas of interest. According to one embodiment an auxiliary tube is placed along the path of the beam passing through the gaseous filter to increase the precision of the gas analyzer in cases where the concentrations to be measured are in the range of a few tens ppm.

Claims

1. Gas analyzer of the infrared absorption type for measuring small concentrations of gas of interest in a gase mixture containing a parasitic gas which has absorption bands overlapping those of the gas of interest, comprising an analysis chamber;

means for alternately and periodically directing into one end of the analysis chamber a first infrared beam which contains no energy at the wavelengths corresponding to the absorption bands of the gas of interest and a second infrared beam;

an auxiliary tube filled with an inert gas vis-a-vis the infrared beam and placed along the analysis chamber so as to divert a portion of the first infrared beam, a movable shutter associated with said auxiliary tube for varying the amount of energy passing therethrough, a non-selective radiation detector disposed at the other end of the analysis chamber for providing signals proportional to the energies of the beams passing through the analysis chamber and the auxiliary tube;

circuit means associated with the non-selective radiation detector for providing a signal representative of the difference between the energies of the first and second beams; and

means for adjusting the intensity of the second beam so as to obtain the same signal when the analysis chamber is filled with either an inert gas or the parasitic gas, whereby the influence of the parasitic gas is eliminated when the analysis chamber is filled with the gas mixture to be analyzed.

2. A method of determining the concentraton of a gas M in a mixture consisting of the gas M and a parasitic gas P, wherein the two gases have overlapping regions of infrared absorption spectra, the method comprising the steps of filling a test chamber with a gas N which is optically inert in the absorption spectra of interest, which spectra include the overlapping regions;

irradiating the test chamber with infrared energy alternately passed through a cell which is filled with the gas N that is optically inert in the absorption spectra of interest and a cell filled with the gas M alone;

detecting and displaying the magnitude of the difference between levels of radiation passing through the test chamber, the magnitude thus displayed constituting a reference level;

filling the test chamber with the gas P;

irradiating the test chamber alternately through the cells;

adjusting the level of radiation passing through the optically inert gas cell in a direction to equalize the readings obtained in the steps of irradiating the test chamber when filled with the parasitic and optically inert gases, respectively;

repeating the steps of filling the test chamber alternately with the optically inert and parasitic gases, N and P, and of adjusting the displayed magnitude and level of radiation until the two readings are equal; and

filling the test chamber with the gaS mixture of M and P, repeating the step of irradiating the chamber through the cells and displaying the resulting difference in magnitudes as a measure of the concentration of gas M.

3. A method according to claim 2 which further includes, before the step of filling the test chamber with the gas mixture of M and P, the step of calibrating the display by filling the test chamber with the gas N;

adjusting the display to a zero reading;

filling the test chamber with a mixture of M and p wherein the gas M constitutes a known concentration equal to the maximum anticipated concentration; and

adjusting the display to a predetermined maximum scale reading.