GB1192748A - Method and Apparatus for Measuring a Property of Dielectric Material - Google Patents

Abstract

1,192,748. Measuring moisture content; measuring dielectric constant. INDUSTRIAL NUCLEONICS CORP. July 7, 1967 [July 7, 1966(2)], No.31393/67. Headings G1N and G1U. A property of a dielectric material, e.g. the moisture content of a paper web, is measured by subjecting the material to an alternating voltage, measuring the phase of the current with respect to the applied voltage, controlling the frequency of the applied voltage in such a way as to maintain the phase at a predetermined value, and indicating the value of the property (moisture content) as a function of the frequency. In one arrangement, Fig. 2, the phase is measured by two phase-sensitive demodulators (74, 76), indicated schematically by reference 48, receiving respective reference signals in phase and in quadrature with the applied voltage to produce respective D.C. signals of opposite polarity and of magnitudes indicative of the in phase and quadrature components of the current. These signals are summed in a resistance chain 56 and used to control a servomotor 62 to control the frequency of the generator. One of the D. C. signals is fed back to control the gain of an amplifier 38 supplying the alternating voltage to capacitive probe 12. A balance circuit 40, which is preferably a bridge circuit (114), Fig. 4 (not shown), is provided for balancing out the phase shift in the absence of the paper. Feedback through impedance 46 maintains point 42 at a constant potential. As described with reference to Fig. 6 (not shown), the resistance chain 56 comprises three potentiometers in series and a switch (146) for selecting a required one of the three outputs to determine the predetermined value (e.g. 25 degrees, 45 degrees, 75 degrees) at which the phase is maintained, as appropriate for the expected range of moisture content of the paper. As described with reference to Fig. 3 (not shown), the generator is a squarewave oscillator (78) of double the frequency applied to the probe and multivibrators (80, 82, 84) are used in known manner to derive the in phase and quadrature reference signals. The squarewave is converted into a sine wave Fig. 4 (not shown) before being applied to the probe. In another arrangement, Fig. 8, the markspace ratio of the output signal of multi-vibrator 140 is representative of the phase. The output from the multi-vibrator 140 is smoothed to a D.C. and compared with a D.C. reference voltage to provide an error signal for controlling the frequency of oscillator 26. Two oscillators are provided: a 10 Mc/s crystal controlled oscillator 24 and the oscillator 26 which has a frequency variable within the range 9 to 10 Mc/s whereby the output of mixer 28 is variable in frequency from 0 to 1 Mc/s. The frequency measurement may be converted in a function generator to give an output signal linearly related to moisture content. Alternatively, the function generator may be omitted and a non- linear, e.g. logarithmic, scale provided on the measuring instrument. Temperature compensation and selectable resistors to compensate for different grades of paper may be provided, Fig. 9 (not shown). The probe may be traversed across the web.