GB978236A - Means and methods for gas detection - Google Patents

Abstract

978,236. Analysing gases electrically. MINE SAFETY APPLIANCES CO. March 28, 1962, No. 11854/62. Heading G1N. [Also in Division H1] A gas mixture is tested for the presence of a polar component e.g. a halogen-containing compound, by exposing a semi-conductor diode to the gas and subjecting it to two different voltages, the change in current through the diode being indicative of the type of polar compound. As shown (Fig. 1), two semi-conductor diodes 1a, 2a are connected into a bridge circuit, energized with DC from a battery 8 through a DPDT switch 9 whereby the diodes are subjected to a large voltage in the reverse direction and a voltage reduced by rheostat 10 in the forward direction. The voltage across the diodes is measured by a voltmeter 2V. The gas mixture to be analysed is fed through a bath 14 containing a saturated salt solution at a constant temperature, whereby its water vapour content is standardised. The bridge is initially balanced for the water vapour content only, the additional polar component being absorbed by a filter 27, balance being produced by adjustment of potentiometer 5 in the forward biasing direction, and adjusted by a rheostat 1p in the reverse biasing direction. The filter 27 is now by-passed by valves 28, 29, but a similar filter 17b is introduced in the branch feeding the housing 2 containing the diode 2a, whereby the test gas affects diode 1a only. The unbalance currents indicated on meter 6 for the forward and reverse bridge potentials are noted, and from preliminary calibrations these reading- are converted to specific concentrations of particular polar components. Alternatively,the gas mixture may be fed alternately over diodes 1a and 2a, and the difference in unbalance currents utilised to determine the test gas content of the mixture. In a modification only one diode is utilized, the change in unbalance current with and without the test gas for the same applied potential being indicative of the quantity of test gas in the mixture. In another modification, the differences in unbalance current for forward and reverse potentials, with and without the test gas in the mixture, are compared. When the gas mixture contains a non-polar component which can be converted into a polar compound, this may be effected prior to the above analysis. As shown (Fig. 2), the gas mixture entering at 13 is divided to flow through reactors 30a, 30b, where the non-polar compound is converted to a polar compound by burning, hydrolosis, or reaction with hydrogen, the former by a heater 51a, and the latter by supplying hydrogen from an electrolytic cell 43. Humidity stabilisers and gas filters are used as before. Diode construction. A germanium or silicon wafer 88 (Fig. 5) on a conducting base 87 of metal or graphite is clamped to a metal base plate 90 by springs 91. Point contact is effected by a steel ball 89 held by a threaded terminal post 86 in an insulating bracket 93. The unit is contained in a sealed housing 80 with gas entry and exit tubes 83a, 83b. The inlet hole may be continued through the post 86 and the steel ball. A second diode may be introduced into the housing to compensate for temperature changes. In one form (Fig. 8), two semiconductor wafers 211 are mounted on a common conducting block 210 carried by a disc 206 which divides the housing 200 into two compartments, each of which may contain its own gas. In another embodiment (Fig. 9), the measuring and compensating diodes constitute a single semi-conductor layer 311 on a carbon block 310, separated by a slot 314 locating a wall 306 dividing the housing into two compartments. In other arrangements (Figs. 3, 4, 11, not shown), compensation is provided by a plurality of compensating diodes, preferably biased in the forward direction, while the detector diode is biased in the reverse direction, all the diodes being contained in a common housing and subjected simultaneously to the test gas mixture. A multiple diode arrangement may be formed by attaching a thin strip of germanium or silicon to a quartz or glass base and subjecting selected separated areas of the surface to irradiation. Alternatively gold or indium may be melted on to the selected areas. Specifications 909,411, 909,412 and 909,413 are referred to.