Circuit arrangement for an apparatus for measuring and indicating the concentration of combustible gases and vapors contained in air

Claim 1

In a circuit arrangement for an apparatus for the measurement and indication of the concentration of combustible gases and vapors contained in air by the thermal effect principle, including a bridge circuit for measurement of thermal effects produced during a combustion process of combustible gases and vapors and for producing a concentration-dependent measuring signal fed to an indicating apparatus, the bridge circuit having gas sensors in respective branches thereof which are in the form of a measuring sensor and a comparison sensor, and a comparator connected for transmitting an additional signal to the indicating apparatus at a specified measuring-bridge-voltage, the improvement comprising: a. the comparator being connected in parallel with the measuring bridge and, via a diode means, in parallel to the indicating apparatus; b. a resistance connected in parallel with the comparator and connected in series with the bridge such that concentration-dependent bridge current is measured as a voltage drop across the resistance; c. the indicating apparatus being a digital indicating device, and d. a voltage divider connected to the comparator and adapted to provide a reference voltage which is fed to the comparator for comparison of said voltage drop with said voltage reference, said comparator adapted such that when the voltage drop attains or is less than the reference voltage an additional measuring signal is superimposed on the bridge-measuring signal such that the measuring range of the indicating device is exceeded whereby indication is switched off.