(a) Field of the Invention
This invention relates to detectors or sensors for determining the presence of a combustible gas in air and more particularly relates to detectors which detect a change in temperature when a combustible gas is oxidized at the surface of the detector.
(b) History of the Prior Art
In the prior art, gas detectors are known which comprise a metal wire which is connected as a resistance into a wheatstone bridge circuit and is electrically heated to a temperature of about 900.degree. C. At that temperature a combustible gas will oxidize at the surface of the wire, further increasing the temperature of the wire, which in turn alters the electrical resistance of the wire which is detected by the wheatstone bridge circuit. Such detectors are unsatisfactory for many reasons. The wire must be manufactured from a high melting metal such as palladium or platinum which has catalytic properties. Even when such exotic metals are used, palladium or platinum metal vaporizes from the surface of the wire at the high operating temperatures, thus causing early failure of the metal wire detector. Additionally, even slight vaporization results in a change in the resistance of the wire, thus necessitating frequent zero adjustments to the bridge circuit.
Such detectors additionally are insufficiently sensitive for many applications since they are usually unable to detect the presence of a combustible gas at concentrations lower than about 300 ppm. Furthermore, the large size and high operating temperatures of such detectors utilize an undesirably high amount of electrical energy. These detectors also had undesirably long response times and were frequently position sensitive.
A few of the disadvantages of the wire type combustible gas detector were overcome when it was recognized that the wire could be wound in a coil and coated with a ceramic material containing a compound such as palladium chloride or chloroplatinic acid (H.sub.2 PtCl.sub.6.6H.sub.2 O) which would convert to a catalytic metal upon application of sufficient heat. Alternatively, a catalytic metal could be condensed upon the ceramic coating by heating a catalytic metal in close proximity to the coating.
Such detectors are disclosed in U.S. Pat. Nos. 3,092,799 and 3,200,011 to A. R. Baker; 3,564,474 to Firth et al and 2,816,863 to Page.
While such detectors were an improvement over the wire type detectors by preventing vaporization of the wire by means of the ceramic or refractory coating material, such detectors continued to have the disadvantage of vaporization of the catalytic metal at the surface of the detector, thus substantially reducing sensitivity. Additionally such detector sometimes continued to have varying responses depending upon the orientation of the detector. In order to maintain at least some sensitivity, such detectors were dependent upon diffusion of additional catalytic metals through the ceramic material to the surface. Such detectors therefore continued to have characteristics which varied over their useful life and continued to have poor sensitivity. The prior art detectors were relatively large and in order to maintain their catalytic surfaces at usuable temperature, much energy was dissipated through conduction, convection, and radiation. Additionally, the operating temperatures, although slightly lower than the operating temperatures frequently utilized with a wire detector, continued to be undesirably high, thus substantially reducing the useful life of the detector and utilizing undesirably high quantities of electrical energy. Such energy requirements were not only wasteful but also required large batteries in portable instruments.
Of particular importance is the fact that prior art sensors lack zero stability in the absence of combustible gas and failed to stay in calibration when subjected to prolonged exposure to the low levels of combustible gas frequently present in industrial environments. These effects are due to the fact that although some prior art sensors used palladium oxide, the oxide was only a thin veneer on palladium metal.
When viewed under a microscope, prior art sensors have a blue-to-violet iridescent black appearance characteristic of thin oxide films on palladium metal. Some patches on the sensor frequently show gray metal. By contrast, the sensor of the present invention varies in appearance from prior art sensors since iridescence and gray metal are not visible.
Furthermore, such sensors could not be made sufficiently small since the most suitable metals could not be wound in sufficiently small coils. It is known that the lamp industry uses proprietary machines to wind very small coils of tungsten wire on mandrels; however, these machines are unable to wind such coils from metals such as platinum or palladium since the coils do not retain their shape.