A non-dispersive gas analyzing apparatus is described having a first chamber for containing a first gas, the density of which it is desired to determine. A source of radiant energy is provided for passing radiant energy through the first chamber. Modulation means are provided for modulating the radiant energy passing through the first chamber by modulating the volume of the chamber at the acoustic resonance frequency of the first gas and the chamber. Signal generating means including a second chamber for containing a gas which is heated by radiant energy emerging from the first chamber and a microphonic means responsive to the resulting pressurization in the second chamber is provided for generating a signal having a frequency and amplitude corresponding to the modulation of the radiant energy in the first chamber. In one embodiment this signal is used to modulate in phase opposition radiant energy passing through an intermediate gas-filled third chamber so as to cancel in the second chamber the effects of the modulation in the first chamber. In a second embodiment this signal is used to modulate in phase opposition the intensity of the radiant energy emerging from the energy source so as to cancel in the second chamber the effects of the modulation in the first chamber. Output means are further provided responsive to the amplitude of the signal generated by the signal generating means and the amplitude of the modulation in the first chamber for providing an output proportional to the density of the gas in the first chamber.
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