Measuring and testing – Gas analysis – By vibration
Patent
1992-09-01
1994-08-23
Johnston, Jill A.
Measuring and testing
Gas analysis
By vibration
356432, G01N 2100
Patent
active
053396740
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method and an apparatus for transmission of an acoustic signal in a photoacoustic or optoacoustic cell which constitutes part of an apparatus for measuring physical or chemical material properties in a medium and/or for analysis of substances/gases or mixtures of these. In the method known as "Photoacoustics" or "Optoacoustics," an acoustic signal produced optically in the medium is measured and the amplitude, frequency or phase of the signal provides information as to the substance composition, density, homogeneity, structure or other properties of the analyzed medium, for instance a smoke gas.
BACKGROUND OF THE INVENTION
From GB-A-2148487 an apparatus for optical analysis of a smoke gas is known. Said apparatus requires, however, an optically clear view, if necessary assisted by mirrors or lenses, of a detection zone through a smoke gas. The zone may be identical to the width of a duct for transporting the smoke gas in that the two nearly identical parts of the apparatus are mounted perpendicularly to the duct and the sensors contained in each part are protected against the harmful effect from the smoke gas by a protective gas fed into each part. The optical signal may vary corresponding to the gas density in the detection zone and is transferred into an electric signal to be read in a meter. The apparatus is only suited for measuring the density of a smoke gas and gives no information as to other essential properties of a gas, such as composition, homogeneity or structure. For that purpose a photoacoustic measurement of the smoke gas has to be made by means of an apparatus allowing the analyzed amount of gas to be introduced into a separate analyzing zone for further examination.
U.S. Pat. No. 3,659,452 discloses a method and an apparatus for photoacoustic spectroscopy of a gas by means of an acoustic resonance cell.
U.S. Pat. No. 4,253,770 discloses an apparatus for optoacoustic analysis by means of the light from a laser resonator and a differential cell assembly.
CH-B-344234 discloses a solely acoustic gas analyzer in which the microphone may be mounted in a side tube to the tube containing the gas sample and be protected against the harmful effects from the analyzed gas by a thin layer of inert gas. A mixing between the analyzed gas and the protecting inert gas has, however, to be considered, and the protection of the microphone is only to be ensured when the relation between the two gases is kept at 100:1. In addition, the mounting of the microphone in a side tube inevitably implies an unwanted weakening of the acoustic signal.
SUMMARY OF THE INVENTION
To optimize the sensitivity of a microphone in a photoacoustic cell for the above analyzing purpose, a mounting of the microphone as close as possible to the source of the acoustic signal is preferred. If this signal is generated at high temperatures in chemical aggressive surroundings or in other surroundings damaging to the microphone, which may often be the case, such mounting may make very great and costly demands on the microphone construction to secure a satisfactory function unless other measures are taken to protect the microphone.
It is therefore the object of the invention to provide a method for isolating a microphone unit in a photoacoustic cell from said damaging surroundings and without the disadvantages associated with the hitherto known mountings of such a unit. It is a further object of the invention to provide an apparatus for carrying out the method.
The foregoing object is attained, according to one aspect of the present invention, by a method for transferring an acoustic signal to a microphone in a photoacoustic cell that is part of an apparatus for photoacoustic analysis of at least one substance and in which apparatus the microphone is protected against the harmful effects from the analyzed substance. The method comprises the steps of placing a microphone for receiving the acoustic signal from an excitation zone, which is located in an environment which is damaging to the microphone, outside of
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Hammerich Mads
Henningsen Jes O.
Olafsson Ari
Collins Laura E.
FLS Airlog A/S
Johnston Jill A.
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