GB1227613A - - Google Patents

Abstract

1,227,613. Measuring moisture content. Measuring capacitive parameters. INDUSTRIAL NUCLEONICS CORP. April 9, 1968 [April 27, 1967], No.17079/68. Headings G1N and G1U. In apparatus for measuring the dielectric constant and hence the moisture content of a plurality of paper webs using a plurality of capacitive probes having a common control station, adjustment of each of the measuring circuits to give zero output in the absence of a web is facilitated by the provision of a respective voltage controlled variable semi-conductor capacitor connected in series with the probe capacitor and two 180 degrees out of phase A. C . voltage, the variable semi-conductor capacitor being controlled by an adjustable D.C. source at the control station. As shown in Fig.2, a probe 12 is connected to the control station 19 by a cable 18. The measuring circuit comprises a capacitive probe 37 having two active electrodes 36, 39, the fringe field of which depends on the moisture content of the web 11. Electrode 36 is energized with A.C. from two oscillators 31, 32 of different frequencies. Electrode 39 is connected via output lead 47 to band pass filter 48, 49, detectors 51, 52 and a ratio forming circuit 53. The capacitance of varactor diode 42 is controlled by adjusting the position of slider 23. Because the capacitance of varactor diode 42 is usually about five times the capacitance between plates 36, 39, capacitine voltage divider circuits may be employed to reduce the effective capacitance of the varactor diode, Figs.3 and 5 (not shown). To reduce the effect of the alternating voltage on the capacitance of the varactor diode, two varactor diodes may be connected in series back to back relation with respect to the A.C. sources and in parallel relation with respect to the D.C. source, Fig.4 (not shown). A transformer may be used instead of inverter 41, Fig.5 (not shown). The probes may be driven to and fro to scan the width of the webs.