Patent GB-591147-A (GB)
Title: Improvements relating to apparatus for measuring such physical states of a material as affect the dielectric losses therein
IPC: G01N27/22
Filed: 1944-09-19
Abstract: 591,147. Automatic control systems. FERRANTI, Ltd., TOOTAL BROADHURST LEE CO., Ltd., TAYLOR, M. K., and NIELD, L. C. Sept. 19, 1944, Nos. 17903 and 17904. [Class 38 (iv)] [Also in Group XXXVI] Moisture content.-Apparatus for measuring a physical state such as moisture content of a material comprises a condenser between the plates of which the material is disposed, which condenser is incorporated in the tuned circuit of an amplifier, a tunable oscillator circuit for providing an input to the amplifier, automatic means in one of the circuits adapted to vary the tuning thereof cyclically and repeatedly through a frequency band including the resonant frequencies resulting from all likely changes in the physical state, and means for deriving the value of the resonant amplitude of oscillation in the tuned circuit at different degrees of amplitude occasioned by different dielectric losses due to difference in the physical state. In the apparatus shown in Fig. 1, an oscillator comprises a valve 10, fixed condenser 11, tunable condenser 12, and a tuning coil 13 connected through a condenser 15 to the valve grid and through a resistance 16 to earth. The tuning coil 13 forms the primary winding of a transformer, the secondary winding 18 of which is connected between the control grid of a pentode amplifier 20 and, earth. The anode of the pentode 20 is connected to the positive H.T. through a load inductance 22 and to earth through two condensers 24, 25 in series. The condenser 25 consists of two spaced plate electrodes between which the web 26 of the material to be tested is passed so as to form the dielectric of the condenser. The anode of the valve 20 is also connected through a condenser 28 to the anode of a diode rectifier 29. The anode of this valve is connected to earth through a load resistance 30 and also to the control grid of a triode D.C. amplifier 31 through a resistance 32. The anode of the valve 31 is connected to the H.T. supply through a load resistance 34 and also through a milliammeter 35 to the sliding contact of a potentiometer 36. The condenser 28 and inductance 30 and the series connected condensers 24, 25 form two branches of an A.C. parallel tuned circuit. Oscillations generated by the oscillator are amplified by the pentode 20 and rectified by the diode 29. The D.C. potential developed across the resistance 30 is applied to the control grid of the valve 31, the reading of the milliammeter 35 being proportional to the current through this valve. A sample of absorbent material of known moisture content is placed between the plates of the condenser 25 and the circuit tuned to resonance. The minimum reading of the milliammeter 35 is noted, or the milliammeter may be adjusted to zero by varying the potentiometer 36. The milliammeter may thus be calibrated in terms of moisture content. The sample to be tested is then placed between the plates of the condenser 25 and the frequency is varied automatically through a frequency band which includes the resonant frequency at which the oscillator and the pentode amplifier with the tuned circuit are in tune, giving the minimum reading of the milliammeter for that particular sample. The tuning condenser 12 is driven by a small motor so that a band of frequencies is repeatedly swept by the oscillator. The output of the pentode 20 then consists of a short pulse of oscillations each time the generated oscillations sweep through the resonant frequency of the tuned circuit. If the time constant of the diode circuit is long compared with the band-sweeping time, the diode output is D.C., and the D.C. amplifier 31 and milliammeter 35 operate as with manual tuning. Alternatively, the time constant of the diode circuit although large compared with the oscillation period, may be small compared with the time taken by the oscillator to sweep the frequency band. In this case, the rectifier output consists of D.C. pulses recurrent at the band-sweep frequency. A second pentode is employed in place of the triode 31, the milliammeter 35 being connected to the secondary of a transformer, the primary of which is in the anode circuit of the second pentode. The grid of the second pentode is fed through a low-pass filter circuit. The oscillator frequency is automatically varied through the selector frequency band repeatedly in the same direction at predetermined intervals of time by the apparatus shown in Fig. 6. The tuning condenser comprises a series of fixed quadrantal plates 61 and a series of movable plates 62 of similar shape, which are earthed by a slip-ring. The movable plates 62 are arranged to form two diametrically opposite sets and are fixed on a spindle 63 driven by a motor 64. A rotary switch 65 is fitted on the spindle and is arranged to connect the condenser in circuit only when the frequency of the generated oscillations is decreasing from a maximum to a minimum value. A moving web may be arranged to pass between the condenser plates and a servo-mechanism may be controlled so as to restore the moisture content when a change occurs, as by varying the rate of heating or the speed of the web. Specifications 275,741, 337,785, [both in Class 37], 396,098, [Group XXXVI], and 558,491 are referred to.