Figure 3 In the detector, there are reference and measuring chambers in each of which a thin platinum wire is stretched. The reference chamber is filled with reference gas and through the measuring chamber, sample gas is flowed. Each platinum wire composes a bridge circuit in combination with an external fixed resistor and it is heated by a flowing constant current. The resulting thermal change is taken out as a change in electric resistance according to which the concentration of measured gas is calculated. When all four resistances are the same Vour is zero, and the bridge is considered balanced. When zeroing, the reference gas is passed over all the filaments, the resistances will be the same (because filaments are matched) and the bridge is balanced. When the sample gas is passed over half of the bridge, then Vour value correlates to the content of the sample gas in the reference. The detector is a four element katharometer having two elements situated in the reference gas, and two elements in the sample gas as shown in Fig. 3.