Self-calibrating two path infrared gas analyzer having one enclosed path

Claim 1

IN AN INFRARED GAS ANALYZER COMPRISING IN COMBINATION AND IN OPTICAL ALIGNMENT, A SOURCE OF INFRARED RADIATION, TWO PATHS, AT LEAST ONE OF THEM BEING ENCLOSED, SAID PATHS BEING OF SUBSTANTIALLY EQUAL EFFECTIVE CROSSSECTION AND CAPABLE OF CONTAINING DIFFERENT CONSTITUENTS, MEANS FOR FORMING BEAMS FROM SAID INFRARED SOURCE AND FOR DIRECTING THEM THROUGH THE PATHS, AN INFRARED DETECTOR AND TWO FILTERING MEANS, ONE OF WHICH PASSES A BAND OF INFRARED RADIATION CONTAINING A STRONG ABSORBING BAND OF A PREDETERMINED CHEMICAL CAPABLE OF EXISTING IN ONE PATH, MEANS FOR IMAGING THE TWO BEAMS ALTERNATELY THROUGH EACH FILTER ONTO THE DETECTOR AT A PREDETERMINED ALTERNATING FREQUENCY BY RELATIVE ROTATION OF PATHS AND FILTERS, AND MEANS CONNECTED TO THE OUTPUT OF SAID DETECTOR AND RESPONSIVE AT THE FREQUENCIES OF SAID ALTERNATION OF THE BEAMS AND IN PHASE THEREWITH, THE IMPROVEMENT WHICH COMPRISES, (A) A RELATIVELY OPAQUE VANE MOVABLE TO BRIDGE THE AXIS OF SYMMETRY OF THE PATHS AND ON ONE SIDE ONLY OF THE CENTER OF ROTATION OF THE FILTERS AND PATHS, WHEREBY AN A.C. SIGNAL IN QUADRATURE TO THE OUTPUT SIGNAL IS PRODUCED WHEN THE FILTERS BECOME UNBALANCED TO PRODUCE A SIGNAL IN THE ABSENCE OF THE PREDETERMINED CHEMICAL, AND (B) MEANS FOR ADJUSTING THE POSITION OF THE VANE UNTIL NO SIGNAL IS PRODUCED IN PHASE WITH THE OUTPUT SIGNAL.