GB1063515A - Measuring instruments for determining a physical property of a material - Google Patents

Abstract

1,063,515. Determining moisture content of material, electrically. INDUSTRIAL NUCLEONICS CORPORATION. Oct. 29, 1963, No. 42685/63. Heading G1N. Relates to a dual frequency capacitor probe apparatus for determining the variable moisture content q of a material irrespective of the variable filling factor p of the material (i. mass/unit volume or unit area). The probe C 1 (Fig. 8) is included in a capacitance bridge, which is balanced when no material is in the probe, and is energized at two different frequencies f1, f2 from voltage sources 40, 42. The unbalance bridge output signal due to material in the probe is amplified in a high gain amplifier 68 and is separated into its components in filters 46, 48, and fed to detector circuits 50, 52 to provide two signals SH, SL which are functions of the filling factor p also of KH and AL the effective dielectric constants at high and low frequency respectively. In the unbalance condition a periodically varying net charge is induced on commonly connected plates of the probe capacitor C 1 and a balancing capacitor C 0 which is virtually removed instantaneously by a feedback circuit across the amplifier including capacitor C 1 64. This feedback arrangement together with suitable design of the probe geometry is said to ensure that the functions SH, SL are linear with respect to the filling factor p which is eliminated in simultaneous equations given for SH and SL, by calculation of the ratio SL/SH in a ratio computer 70. The result is an output signal g (K) from the computer which is entirely independent of p and is determined only by the dielectric constant K which is a function of the moisture content q. The output g(K) may be utilized in a suitable instrument 58 calibrated in percentage moisture content units. The material to be measured may be placed in the spray or fringe field of the probe adjacent the plates as Fig. 11 (not shown) and in addition a known earthed guard electrode may be interposed between the probe plates, Fig. 12 (not shown), so that substantially all the aqtual field between the probe plates passes outwardly as spray field which interacts with the material. Fig. 13 (not shown) illustrates the equivalent circuit of the latter arrangement. A mathematical explanation of the invention is given.