Device for measuring the density of gases and liquids

摘要

The density is measured by measuring the frequency changes, occurring with changing density, of an electromechanical oscillating system exposed to the mass forces of the substance to be determined. This oscillating system is excited into resonant oscillations with the aid of an electronic amplifier, the oscillating system together with a transmit transducer and a receive transducer forming the feedback branch of an oscillator. The electronic amplifier is constructed as a phase-locked loop which consists of a phase detector at the input followed by a low-pass filter and of a voltage-controlled oscillator at the output. The phase-locked loop is closed with an internal feedback from the output of the voltage-controlled oscillator to one input of the phase detector and the second input of the phase detector is used as amplifier input. The oscillating frequency of the oscillator is equal to the resonant frequency of the mechanical oscillating system which depends on the density. It is regulated in such a manner that a constant phase angle of the receive signal with reference to the transmit signal occurs at the receive transducer.