GB1474080A - Gas detector - Google Patents

Abstract

1474080 Detecting gases electrically NOHMI BOSAI KOGYO CO Ltd 3 July 1974 29570/74 Heading G1N In order to discriminate differing gaseous components which may be present in a gas, a gas detector includes at least two metallic oxide semiconductor gas sensing elements each having a different sensitivity for at least one of the gaseous components to be detected. In one embodiment two sensing elements are connected in series with a resistor across a pair of terminals to which a voltage is supplied and the detector output is taken from across the resistor. By suitable choice of the catalyzer or temperature to which the elements are heated the elements are made to exhibit different characteristics for various gases Figs. 2, 3 where Fig. 2 shows the output voltage when the first element above is connected in the circuit and Fig. 3 shows that for the second element above for various gases hydrogen H, propane P, methane M, isobutane I. Thus knowing the temperature of operation and the output voltage the gases can be identified more precisely. More than two sensing elements may be used to differentiate the gases from each other. In a second arrangement the two sensing elements are connected in adjacent arms of a resistive bridge circuit, Fig. 6 (not shown) so that at a temperature of 130 C hydrogen present in the gas being tested affects both elements the same and there is no output from the bridge whereas propane or methane gas unbalance the bridge and the polarity of the output signal indentifies the gas. Alternatively the sensing elements may be connected in opposite arms of the bridge, Fig. 7 (not shown) in which case Hydrogen produces an output and not the other gases, the bridge being normally in an unbalanced state. In an A.C. energized arrangement Fig. 8 hydrogen gas produces a full wave A.C. voltage across terminals 4, 3, methane a positive half wave and isobutane a negative halfwave, the elements operating at different temperatures. More than one pair of output terminals may be used to detect and identify the gases e.g. an output may be provided across one of the sensing elements as well as across the resistor Fig. 10 (not shown). This arrangement enables individual detection of hydrogen, a compound of propane and isobutane, and methane to be activated. Alternatively the resistor may be omitted and the output taken just across one of the elements, Fig. 11 (not shown). One of the sensing elements may be replaced by a plurality of parallel connected gas sensing elements exhibiting different sensitivities Fig. 12 (not shown). The sensing elements may be formed integrally with each other Fig. 13 (not shown).