Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1997-06-02
1999-05-18
Allen, Stephone B.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
356 51, 250343, G01N21/61
Patent
active
059052708
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a method of and apparatus for detecting the presence of a light absorbing gas within an atmosphere containing the gas.
The invention has application to the detection of a combustible, toxic or other light absorbing gas, and the invention is hereinafter described in the context of detecting the presence of infra-red absorbing combustible gas in air. However, it will be understood that the invention does have broader application, to detection of the presence of any light absorbing gas in an atmosphere containing or consisting of any other gas. It will also be understood that the term "light absorbing gas" means a gas which characteristically absorbs radiation falling within the infra-red, visible and ultra-violet regions of the electromagnetic spectrum, and that the terms "radiation source" and "light source" mean a source which, when energised, emits radiation falling within the infra-red, visible and ultra-violet regions of the electromagnetic spectrum.
BACKGROUND OF THE INVENTION
The presence of hydrocarbon gas in air is conventionally monitored by beaming infra-red radiation through a chamber containing the gas-air mixture and detecting for absorption, or the degree of absorption, of radiation within the chamber. An output signal is derived which provides a measure of the amount of infra-red radiation that impinges on a detector, with an increase in the level of the output signal signifying a reduction in the amount of infra-red absorbing gas within the chamber. A bandpass filter is normally located between the radiation source and the detector to permit selective detection of the presence of a specific gas, that is a gas which absorbs radiation having a frequency in the filter pass band.
Determination of the volumetric proportion of hydrocarbon gas in air is determined by comparing a measured level of the output signal with a reference level, whilst accommodating variables introduced by ambient temperature change, component heat contributions, drifting of source and detector sensitivity, and accumulation of pollutants on optical elements within the system. One of the steps taken toward accommodating variables has involved driving the infra-red source by a square wave voltage, typically at a rate in the range of 0.5 to 50 Hz, and thermal drifts have been accommodated by measuring the peak-to-peak value of the output signal.
SUMMARY OF THE INVENTION
It has now been determined by the applicant that performance stability may be improved and the effect of variables may more readily be accommodated by driving the radiation source from a power supply which delivers substantially constant power at the or each level at which the radiation source is energised. Thus, the applicant has become aware that the output of the radiation source varies with age induced variations in the structural/chemical characteristics and the applicant has determined that the aberrant behaviour may be improved by driving the source with constant power.
Thus, the invention may be defined broadly as providing an apparatus for detecting the presence of a light absorbing gas within an atmosphere containing the gas. The apparatus comprises a light source, a detector arranged to detect the presence of incident radiation from the source and to provide an output signal representative of the level of incident radiation, electronic circuitry arranged to process the output signal to provide an indication of incident radiation, and a power supply for cyclically energising the light source. The apparatus is characterised in that the power supply is arranged to deliver substantially constant power at the or each level of energisation.
The invention may be defined also in terms of a method of detecting for the presence of a light absorbing gas within an atmosphere containing the gas. The method comprises the steps of beaming radiation from a light source through a chamber containing the atmosphere and toward a detector which is arranged to provide an output signal which is representative of a level of
REFERENCES:
patent: 5070244 (1991-12-01), Simpson
patent: 5369278 (1994-11-01), Lehto
McCaughey Ian
Mikkelsen Peter Frederick
Allen Stephone B.
Gas Tech Australia PTY Limited
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