Abstract: An apparatus for determining the composition of a gas by measuring the acoustic impedence and absorption of a gas at two distinct frequencies. The velocity variations due to the presence of a gas in a fixed length column are measured by comparing the phase of the voltage applied to a sending transducer to the phase of the voltage in a receiving transducer. Since the distance between the transducers is fixed, the phase variation is indicative of the velocity of the sound at any particular frequency. The output voltage obtained from a phase comparator is proportional to the phase difference and since the relationship between the concentration of the gas and the output voltage is known, the concentration of the gas is easily ascertained. The influence of temperature changes in the column are eliminated by summing the phase differences measured at the two distinct frequencies. In addition, by a judicious choice of frequencies, the amount of water vapor present in a particular gas may be compensated for.

Abstract: An apparatus for time delay acoustic spectrographic analysis of fluid composition uses frequency modulated ultrasonic vibrations which are transmitted through the fluid from a sending transducer to a receiving transducer. By scanning the frequency of the modulated vibrations at a sufficiently high rate, standing waves which give erroneous indications of fluid component concentrations, are eliminated. The frequency of the signal at the sending transducer is compared with that at the receiving transducer and a comparing means whose output is related to the frequency difference of the signal at the receiving transducer from the sending transducer measures the number of times this signal shifts 360.degree. during a scanning period. This measurement is directly related to a property of the fluid being measured. The method and apparatus have particular application in the investigation of coagulation in human blood.