Abstract: Atmospheric concentrations of reducing gases are measured and indicated by a gas sensor whose conductivity varies in accordance with variations in the concentration of ambient reducing gases, a resistance in series with the gas sensor for establishing a reference voltage, a circuit interconnecting the resistance with a linearizing circuit including the non-inverting inputs of a plurality of voltage comparators and with the inverting input of a fault detecting comparator, an adjustable resistance of a magnitude substantially equal to the response of the sensor at its maximum value, a thermistor in parallel with a fixed resistance and in series with the adjustable resistance and with a voltage divider including a plurality of fixed resistors respectively connected with the inverting inputs of the plurality of voltage comparators together with a light emitting diode in series with the output of each voltage comparator, each light emitting diode indicating a concentration of an ambient reducing gas in terms of pa