Gas monitors

Claim 1

In a gas monitoring device having a gas sensor of the semi-conductor type, a heating device located adjacent to said gas sensor such that increasing or decreasing the temperature of the heating device increases or decreases the temperature of the gas sensor, and a power source for energizing the sensor and the heating device, the improvements comprising: (a) first comparing means to compare an output from said sensor to a first gas concentration threshold value and provide a first output signal when said sensor output exceeds said first threshold value; (b) second comparing means to compare the sensor output to a second gas concentration threshold value and provide a second output signal when said sensor output exceeds said second threshold value; and, (c) stepped current source means connected to said power source, said heating device, and said first and second comparing means so as to maintain said heating device and, consequently, said sensor at a quiescent temperature under normal operating conditions and wherein said stepped current source reduces the power supplied to the heating device to thereby lower its temperature and the temperature of said sensor upon receipt of a signal from said first comparing means and increases the power supplied to the heating device to thereby raise its temperature and the temperature of the sensor upon receipt of a signal from said second comparing means.