Numerous gas detection instruments are known and most include a gas sensor which responds to concentrations of a particular gas of interest. One type of gas detection instrument which includes an oxygen sensor is described in U.S. Pat. No. 3,886,785 issued June 3, 1975 to H. Stradler for "Gas Sensor and Method of Manufacture". The oxygen sensor, and its method of manufacture as described in this patent, has a sintered ceramic body of transition metal oxide with a pair of spaced-apart electrodes. As the partial pressure of oxygen in the gas being sensed varies in response to variations in the inlet air/fuel mixture ratio, the resistance of the ceramic material varies.
Another type of gas detection instrument which includes a solid state gas sensor is described in U.S. Pat. No. 4,013,943 issued Mar. 22, 1977 to J. Chau et al for "Solid State Electronic Cell Gas Sensor Head". The sensor described in this patent is capable of measuring low concentration levels of oxygen as well as up to about 20% by volume. The solid state material from which the sensor is fabricated is produced by the addition of one or more metal oxide to a nonmetal oxide. The sensor includes a collector and a heater element made from a stable metal such as platinum.
Another semiconductor-type gas sensor is described in U.S. Pat. No. 3,865,550 issued Feb. 11, 1975 to B. Bott for "Semi-Conducting Gas Sensitive Devices". This device includes a semiconductor of a first metal with at least one additional metal incorporated therein. The conductivity of the active oxide changes in response to exposure to the gas of interest.
Still another type of gas detection instrument includes a catalytic gas sensor that uses a small coil of platinum wire, or beads, which is exposed to a combustible gas. The beads have a resistance which varies upon oxidation and the change in resistance is identified and measured with a Wheatstone bridge, or similar type circuitry, to determine the amount of combustible gas which is present in a sample.
As is well known, the beads must be heated for oxidation to occur. With methane, catalytic oxidation with platinum wire begins at about 500.degree. C. Oxidation will occur at a lower temperature if the platinum beads are finely divided. However, generally the hotter the catalyst, up to a point, the stronger the change in resistance to a given level of gas.
If the gas detection instrument is to be portable, most often one or more batteries provide the source voltage. The catalytic sensor is heated by the source voltage and the operating temperature of the platinum beads is somewhat critical in order to produce a linear output on the Wheatstone bridge circuit when exposed to a combustible gas. Linearity in the sense can be expressed as % LFL (percent of lower flammable limit) difference between a known gas sample (in % LFL) and that level depicted by the electronic network (also expressed in % LFL). Typically linearity of less or equal to about 3% is observed when beads are operated near the critical temperature for LFL ranges up to 50%.
A particular problem with many prior art gas detection instruments which use catalytic-type gas sensors incorporating an active and reference element in one arm of a Wheatstone bridge-type circuit is that the catalytic oxidation of combustible gases results in an increase in the temperature of the active element which increases its resistance. At the same time, the resistance of the reference element decreases because of a reduction caused by the drop in current that occurs when the resistance of the active element increases. Temperature rise of the active element can exceed 100.degree. C., or more, depending on various factors, e.g., the initial temperature in the reference air, type of combustible gas, type of catalyst, energy density of coil and thermal coupling between the burning vapor and the resistive coil wire. The active element temperature also increases even if the reference element is in the opposing arm of a Wheatstone bridge-type circuit.
An adverse effect of temperature increases in the active sensor element is the potential restructuring in size of the finely divided catalytic agglomerates. This size restructuring can affect the sensitivity of the instrument. In extreme over-temperature situations, the sensor can be irreversibly damaged. For example, palladium can be irreversibly converted to palladium oxide making it useless as a catalyst.