Patent GB-813907-A (GB)
Title: Improvements in or relating to capacitance measuring instruments
IPC: G01N27/22
Filed: 1955-09-07
Abstract: 813,907. Capacitance; moisture content; thickness measurement. SANGAMO WESTON Ltd. Sept. 7, 1955 [Sept. 9, 1954], No. 25670/55. Class 37. A direct-reading capacitance-measuring instrument, applicable to determining moisture content of material forming the dielectric of a test capacitance, comprises a low-impedance source of constant alternating voltage, connected through direct-current blocking means of low reactance to parallel paths respectively comprising a half-wave rectifier in series with a variable capacitance and an oppositely poled half-wave rectifier in series with the test capacitance ; impedances being connected in series between the rectifier-capacitance junctions and a direct-current instrument indicating the test capacitance value being connected to the junction of the impedances to respond to the difference in the voltages appearing at the capacitance rectifier junctions. In basic circuits (Figs. 1, 2, not shown) a constant voltage radiofrequency generator charges a capacitance on negative half-cycles through a germanium diode and on positive half cycles the capacitance discharges through another such diode and a D.C. instrument indicating capacitance; alternatively a capacitance is charged on positive half cycles from the generator through a blocking capacitance and a series inductance coil, while discharging directly through another inductance coil. A D.C. instrument indicating capacitance may be connected in series with either inductance coil. In a preferred form, Fig. 3, parallel paths comprising diode 10 in series with variable capacitance C 2  and oppositely poled diode 11 in series with test capacitance C 1  are energized from constant-voltage R.F. generator 13 through blocking capacitance 17. Choke coils 18, 19 are connected in series between the junctions of the rectifiers and capacitances and a D.C. measuring instrument 12 is connected between the earth return and the junction of the chokes. Initially capacitance C 2  is adjusted to equality with test capacitance C 1  without dielectric therein, as shown by zero indication of the instrument 12 in response to equal and opposite discharge currents of the capacitances; and calibration may be checked by connecting a known capacitance across C 1  and noting the meter reading. It is shown mathematically (Fig. 4, not shown) that the average rectified current i indicated by the instrument is given by where # is the frequency of the R.F. source, E its peak voltage, and C the capacitance to be measured; and for the apparatus of Fig. 3, that the differential average rectified current in the meter, I m , is given by so that after initial balance the meter reading is proportional to the incremental capacitance of C 1  due to the introduction of dielectric material, and hence to the moisture content thereof, in quantity of which the meter may be calibrated. In Fig. 7 (not shown) the choke coil 18 and variable balancing capacitance C 2  are respectively replaced by a high resistance bypassed by a large capacitance and by a fixed capacitance, whereby drift due to variations of diode resistance with temperature is reduced and the meter reading is still proportional to the variations of the capacitance C 1 . Various modifications of the circuit of Fig. 3 are set forth (Fig. 9 to 13, not shown). The test capacitance may be coupled to the remainder of the apparatus over a connecting cable, and it is shown that, by cutting the cable to a critical length and tuning the input with a variable capacitance, it is made resonant at the working frequency and presents to the measuring apparatus an impedance equivalent to that of the capacitance variations of the test capacitance (Fig. 8, not shown). In a moisture-sensitive capacitance cell testing device (Fig. 14) a constant amplitude R.F. voltage is derived across resistance-shunted coupling loop 24 from the tank circuit 22 of a crystal-controlled tetrode Colpitts oscillator excited from voltage-regulated power supply 27 over variable resistance 37 controlling the R.F. voltage. Balancing capacitance C and diode 10 are connected in series across the coupling coil together with oppositely poled diode 11 in series with blocking capacitance 45 and terminals T, to which there is connected over a coaxial cable 28 the test capacitance 26 shunted by inductance 44, which cancels a proportion of the standing capacitance. Capacitances 30, 40, 41, 42 are selectively connectible by switch 36 across the coaxial cable input to determine the range of measurable capacitance variation, while between the junctions of the rectifiers and associated capacitances are connected inductance 18, variable resistance 30<SP>1</SP> and inductance 19. Between the junction of the latter two elements and the earth return there is connected a D.C. measuring instrument 12 whose zero is set by resistance 30<SP>1</SP> and whose sensitivity is adjustable by shunts 32 to 35 selectively engageable by switch 31. Any increase in capacitance of the test capacitance produces a corresponding increase in the instrument current, which may be calibrated in terms of cell capacitance of moisture content of dielectric. Calibration is checked by connecting a known capacitance in place of test capacitance C and adjusting resistance 37 for full-scale deflection, after which the test capacitance is replaced and the appropriate capacitance 30, 40-42 is inserted for balance. The apparatus is applicable to the measurement of film and layer thicknesses and the like, and for strain measurement. The direct voltage appearing across the measuring instrument may be utilized to operate a control system.