A LOW-FREQUENCY RELATIVE PERMITTIVITY METER FOR ELECTRICALLY CONDUCTING LIQUIDS

A new technique is described for measuring relative permittivity of conducting liquids (of phase angles 10(-6) to 0.3 rad) in the frequency domain below 50kHz. The low frequency limit of the method, extending down from 20 Hz, is lower, the smaller the conductivity of the solutions, and is determined by the values of the phase angles to be measured and the magnitude of electrode and other parasitic effects that cannot be controlled effectively at very low frequencies. The technique is based on the four-terminal cell principle, but differs from other existing methods in two ways: (1) the amplified potential between the two probes and the voltage converted current through the cell are clipped at practically equal heights before being subtracted and fed into a phase sensitive detector; (2) measurement of the unknown phase angle of the solution investigated is made relative to the known phase angle of a reference solution of the same conductivity. The former procedure eliminates the need to know the amplitudes of the two voltages being compared, and makes the method faster and more accurate than the existing ones. The latter makes it possible to avoid errors due to electrode polarization effects, provided it is assumed that these are determined essentially by the conductance of the systems investigated.