Optics: measuring and testing – For light transmission or absorption
Patent
1991-05-08
1992-10-27
Rosenberger, Richard A.
Optics: measuring and testing
For light transmission or absorption
356437, G01N 2117
Patent
active
051594110
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method and apparatus for the detection of a gas using photoacoustic spectroscopy.
All gases have characteristic absorption spectra disclosing the ability of the material to absorb energy as a function of the wavelength of a direct influx of energy. This absorption spectrum is characteristic for each specific gas and may be considered a sort of fingerprint for the gas. Thus, a specific gas in a gas mixture may be detected by measuring the energy absorption of the gas mixture at selected wavelengths of energy where the gas has a high absorption. A method and an apparatus for such measuring is known from U.S. Pat. No. 4,740,086. Another very appropriate measurement method for measuring such absorption is photoacoustic spectroscopy as mentioned for instance by A. G. Bell in "Philosophical Magazine" 11, 510 (1981). According to this method the gas mixture is influenced by a pulsated energy source, e.g. a laser, c.f. E. L. Kerr and J. G. Atwood, "Applied Optics" 7, 915 (1968). The energy absorption of the gas mixture will cause a pressure increase and a pressure decrease, respectively, proportional to the absorption, the gas being heated during absorption of the energy and being cooled again when releasing the energy absorbed to the surroundings. The absorption may thus be recorded using a pressure transducer wherein the pressure is proportional to the absorption.
However, this measurement method gives rise to problems e.g. in case of interference between the absorption spectra of two gases. In that event it is very difficult to distinguish the absorption of each of the specific gases from each other.
This problem is known in particular from measurements carried out on atmospheric gas mixtures using a CO.sub.2 laser. Herein the large amount of CO.sub.2 in the gas mixture in combination with the particularly intensive absorption ability of CO.sub.2 at the wavelengths of radiation which may be generated with a CO.sub.2 laser to a particularly high degree obstructs the detection of less dominant gases, c.f. U.S. Pat. No. 4,457,162.
As disclosed in the article "Selectivity in Optoacoustic Trace Gas Monitoring with Waveguide CO.sub.2 lasers": 11th International Conference on Infrared and Milimeter Waves, Tirreria, Pisa, 24-24 October 1086, the selectivity as regards different gases may be increased by letting the laser scan a wavelength range around a centre wavelength instead of measuring the photoacoustic amplitude at some of the fixed wavelengths of the laser. The laser is pulsated in a conventional manner where the radiation emitted within each pulsation has a constant wavelength, but the wavelength is changed in the course of several pulsations. Combination of this scanning with a reduced pressure in the measurement chamber, thus concentrating the absorption to a narrowed wavelength range, has resulted in a significantly increased selectivity. As to measurement in e.g. atmospheric environments, however, this method is still far from satisfactory, the CO.sub.2 content of the gas mixture still drowning the photoacoustic signals from less dominant gases.
It is the object of the invention to provide a method for the detection of a first gas in a gas mixture through photoacoustic spectroscopy, wherein the gas mixture further comprises a second gas the absorption spectrum of which interferes with the absorption spectrum of the first gas, wherein the gas mixture is irradiated by pulsating laser light having a constant pulsation frequency and therefore a uniform wavelength during the measurement and wherein the wavelength of the laser light is varied gradually and wherein the measurement comprises at least one reading of the phase of the photoacoustic signal as a function of the uniform wavelength during measurement wherein, however, the above inconveniences are eliminated.
This is achieved whereby a photoacoustic measurement is carried out in the presence of a third gas in the gas mixture, said third gas being present in the gas mixture or being added to the mixture immediately before the mea
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Wood et al., "Effects of 10.6-.mu. Laser Induced Air Chemistry on the Atmospheric Refractive Index", Applied Optics, vol. 10, No. 8 (Aug. 1971), pp. 1877-1884.
Hammerich Mads
Henningsen Jes
Olafsson Ari
FLS Airloq A/S
Rosenberger Richard A.
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