Patent GB-882984-A (GB)
Title: Apparatus for determining a property of a substance by capacitive means
IPC: G01B7/06
Filed: 1958-04-14
Abstract: 882,984. Determining physical properties by electrical tests; capacitors; capacitance measurement. FORTE-FAIBBAIRN Inc. April 14, 1958 [Oct. 21, 1957], No. 11739/58. Class 37. The physical properties of a material, particularly its moisture content or dimensions, are measured by inserting the material between the plates of a capacitor to vary the frequency of an associated oscillator. The frequency variations are measured by mixing the output of the oscillator with that of a second substantially identical oscillator having a standard capacitor in its frequency determining circuit. The resulting difference signal is limited and fed to a non- resonant device to give a D.C. signal recorded on a meter which varies in dependence upon the difference frequency and thus indicates the moisture content &c. of the material under test. Fig. 1 shows an apparatus in which a first oscillator 13 of frequency f2 has associated with its frequency-determining circuit a reference capacitor 12a and a second oscillator 14 of frequency f1 having a capacitor 11. The oscillators 13, 14 are substantially identical and may for example be of the conventional tuned anode, tuned grid types, Fig. 3 (not shown). The outputs of the two oscillators are added in a resistive adding circuit 15, Fig. 4 (not shown), and applied to a detector limiter circuit 16, Fig. 5 (not shown), to produce the difference frequency of f1 and f2 as shown at 25 and at 26 in pulse form. The frequency of waveform 26 is measured in a cup and bucket type pulse counting circuit 17, Fig. 6 (not shown) to give an indication on D.C. meter 12 proportional to the pulse repetition frequency and thus to the difference frequency f1-f2 and therefore to the physical properties of the material inserted in the test capacitor 11 of oscillator 14. The capacitive test cell 11 is arranged so that the projection of the cross-sectional dimension of the sample on the plates of the capacitor is smaller than their area so as to maintain the flux substantially uniform throughout the sample and the cell is completely enclosed in an earthed shield to reduce external disturbances. As shown in Figs. 7, 8 the capacitive test cell 11 comprises tubular metal parts 82, 82a, end plates 83 and cover 84 forming together the earthed shield and one plate of the capacitor. The other plate being a metallic electrode 87 embedded in insulating material 85 covered with insulated disc 86. The sample 81 tested is placed in the space between plate 87 and face 92 of cover 84. The bottom part 84a of part 82a is spaced from plate 87 so that the capacity per unit area between 87 and 84a is approximately equal to that between plate 87 and 92. An alternative construction of the test cell 11 which enables the moisture content of whole bags of wool tops to be measured comprises as shown in Figs. 9, 10 a hollow cylindrical metal container 100 having an unearthed capacitor plate 112 in the shape of a hollow inverted cone the whole being covered by a hinged lid 104 forming with the metallic bottom and sides of the container the second electrode of the capacitor. The hinged lid 104 of the cell may be raised and whole bags 114 of wool tops to be measured may be inserted. Specification 882,985 is referred to.