Abstract: Disclosed is an electronic circuit for determining the phase difference between an input signal and a reference signal where both signals are of the same frequency. Furthermore, the circuit provides an amplitude output indicative of the input signal even when that signal is obscured by noise. A chopper is supplied with the obscured signal input and a reference signal input and provides an output to an integrator which provides a control voltage for a voltage controlled phase shifter which supplies an out of phase input to the chopper. The input to the voltage controlled phase shifter is of the same frequency as the signal obscured by noise. A further output of the voltage controlled phase shifter is 90.degree. out of phase with respect to the reference supplied to the chopper and thus is in phase with the signal obscured by noise and provides the desired marker output. This marker output is also supplied as a reference to a second chopper which is also supplied with the obscured signal as an input.

Abstract: A nonintrusive instrument for measuring the volume flowrate of, and the speed of sound in, an arbitrary fluid in a pipe, tube or duct. The sound speed measured is that of a stationary fluid even though the measurement is made while the fluid may be flowing. These measurements are absolute in that they require no flow calibration, and they are independent of the composition of the fluid and its temperature. For a flowing, approximately perfect gas or gas mixture, given the average molecular weight and ratio of specific heats, the instrument measures the temperature. Also, given an independent measurement of the pressure, it measures the gas density and mass flowrate. The measurement can be made in a high noise level environment even when the temperature varies rapidly over a large range. The instrument uses sound whose wavelength is much longer than the diameter of the pipe, tube or duct.