Explosive gas detector

Claim 1

An explosive gas detector comprising: a container having a gas entrance; a source of DC power having a ground reference terminal, a full voltage terminal, and a fractional voltage terminal; impedance means positioned in said container and having first and second terminals, and comprising: a conductor which varies in impedance directly with temperature, a catalytic material coated on said conductor, which material increases in temperature in the presence of an explosive gas, and said first terminal being connected to said reference terminal of said power source; a transistor having its collector and emitter connected in series between said full voltage terminals of said power source and said second terminal of said impedance means; first amplification means comprising an operational amplifier having its non-inverting input connected to said second terminal of said impedance means and its inverting input connected to said fractional voltage terminal of said power source; a container temperature responsive circuit comprising a thermistor positioned within said container and a resistor connected in series with said thermistor between said reference terminal and one of said power terminals of said power source; a summing circuit comprising: a summing junction, a first input to said summing junction comprising a resistor connected between the output of said first operational amplifier and said summing junction, a second summing junction input comprising a container temperature responsive input including a variable resistor, one terminal of said variable resistor being connected to said summing junction, and the other terminal being connected through said resistor of said temperature responsive circuit to one of said power terminals of said power source, and a third input to said summing junction comprising an operating level circuit, in turn including a second variable resistor connected between said junction and one of said power terminals of said power source; a second operational amplifier, its inverting input being connected to said summing junction, and its non-inverting input being connected to said fractional voltage terminal of said power source; drive means connected to the output of said second operational amplifier for applying a drive signal to the base of said transistor for controlling the impedance of said transistor as a direct function of current flow through said impedance means, whereby the voltage applied to said impedance means is decreased as a function of explosive gas sensed by said catalytic material; and indication means responsive to the voltage across said impedance means for indicating the quantity of gas present.