Gas analyzer

#The invention relates to a gas analyser comprising a Wheatstone bridge, consisting of resistance wires, which are provided in two mutually separated channels of a measuring block, wherein the resistance wires of opposite legs of the bridge are contained in the same channel, while a reference gas may be conducted through a first one of said channels and a gas to be analysed may be conducted through a second one of said channels.

#The operation of such a gas analyser is based on the thermal conductivity properties of gases. The bridge is adjusted in such a manner that the temperature of the resistance wires lies above the room temperature, the bridge being balanced when the same gas mixture is conducted through both channels. When, for example, air is used as reference gas and the gas to be analysed is a mixture of air and one other gas, the output voltage of the bridge varies in dependence on the concentration of said other gas because this gas conducts heat more or less than air.

#A problem of the conventional gas analysers of the type described is that the output voltage of the bridge is non-linearly dependent on the concentration of the gas to be analysed. This is caused by the fact that the thermal conductivity of gas increases with increasing temperature, whereby for gases conducting heat better than air, as for example helium, each next unit increment of concentration contributes less to the output voltage than the preceding unit increment, whereas for gases conducting heat less than air, as for example carbon dioxide, each next unit increment of concentration contributes more to the output voltage than the preceding unit increment. Therefore, the conventional gas analyser can only be used for measuring low concentrations of gas, for example up to a maximum concentration of 5%.

#The invention will hereinafter be explained in further detail by reference to the drawings, in which an embodiment of the gas analyser according to the invention is shown.

#FIG. 1 is a cross-section of a measuring block which can be used in the gas analyser according to the invention.

#FIG. 2 shows a schematic diagram of an embodiment of the gas analyser according to the invention.

#Referring to FIG. 1 there is shown a cross-section of a measuring block 1 having a channel 2 through which a gas to be analysed can be conducted, and a channel 3 through which reference gas can be conducted. In each of said channels 2 and 3 two resistance wires 4, 5 and 6, 7, respectively, are provided; the ends a-h of said resistance wires 4-7 are accessible by means of connection pins (not shown) mounted on the outside of the measuring block 1. The measuring block 1 can, for example, be made of aluminium or brass, while the resistance wires 4-7 usually consists of platinum.

#As shown in FIG. 2, the resistance wires 4-7 are connected as a Wheatstone bridge 8, wherein the resistance wires contained in the same channel are connected in opposite legs of the bridge 8. The current through the resistance wires 4-7 is such that the temperature of the wires 4-7 lies above the room temperature. The bridge 8 is adjusted in such a manner that the bridge 8 is balanced when the same gas is conducted through the channels 2 and 3, so that the voltage V.sub.o between the output terminals 9, 10 is zero.

#When, for example, air is conducted through the channel 3 as a reference gas, while a mixture of air and helium is conducted through the channel 2, the resistance wires 4, 5 will cool down stronger than the resistance wires 6, 7 because helium conducts heat better than air. Thereby, the output voltage V.sub.o changes in dependence on the percentage of helium.