GB369137A - Improvements in or relating to the detection or measurement of combustible ingredients in gaseous mixtures - Google Patents

Abstract

In a device for measuring the proportion of combustible ingredients in gaseous mixtures by comparison of the electrical resistances of two heated wires, of which one serves as a standard, the heated wires form two arms Rx, Rn of a Kelvin bridge, as shown in Fig. 1. The remaining arms R1 .. R4 are resistances of the order of 100-400 ohms, and the arm Ro has a very low resistance. The test and comparison wires Rx, Rn are of low resistance and made of catalytic material such as a member of the platinum group, e.g. palladium, and are as far as possible identical to compensate for temperature changes. To compensate for slight differences in their resistances, potentiometers with movable arms G1, G2 are provided, the galvanometer G being connected between these arms, and a switch 11 is arranged between the arm G2 and the mid-point of arm Ro to enable the network to be balanced. A suitable source of E.M.F. is connected to terminals T1, T2, and the wires Rx, Rn are heated to above 300 DEG C. A suitable arrangement of apparatus is shown in Fig. 2, wherein the <PICT:0369137/III/1> <PICT:0369137/III/2> measuring cell 22 is mounted on a bracket 21 in a casing 20, the comparison cell (not shown) being mounted directly behind it. Each cell 24 is provided with a suitable liner 24a. The measuring cell 22 is provided with inlet and outlet pipes 25, 26 for the gas to be tested, and the comparison cell has several small openings to allow air to circulate. The catalytic wires are welded to conductors 30, 31 passing through insulating discs 32 which, together with strips 34 extending between them, form a carrier whereby the wires are readily withdrawn from the cells. This unit is held in position by caps 35 and washers 36, contact being made by members 38, 39 formed with openings for the conductors 30, 31. In the lower part of the casing are mounted the resistances R1 .. R4 and the potentiometers 45, 46, the arms of which are adjusted by screws 61, 62. The arm Ro is mounted above them, and the switch 11 is arranged to contact with a stud 60 connected to the arm G2 (not shown) of the potentiometer 46. To prevent incorrect adjustment, the switch 11 is arranged to cover the adjusting screws 61, 62 respectively in its open and closed positions. The galvanometer and source of E.M.F. are connected to terminals 51.