GB1114157A - Improvements in or relating to the determination of moisture content - Google Patents

Abstract

1,114,157. Determining moisture content. ASSOCIATED ELECTRICAL INDUSTRIES Ltd . Sept. 20, 1965 [Sept. 21, 1964], No.38380/64. Heading G1N. Apparatus for determining the moisture content of a material by measuring the attenuation of a beam of modulated microwaves directed through the specimen is compensated for temperature variations by means of a circuit including a temperature sensitive resistor. Oscillations at 10000 Mc/s modulated at 3200 c/s are passed through the specimen and a variable attenuator the output from which is detected to produce an audio frequency signal. This is amplified and rectified and the resulting D. C. voltage is compared with a reference voltage. Any error signal is amplified and applied to a reversible motor which adjusts the variable attenuator and also the slider of a potentiometer to give an output voltage Fig. 1 (not shown). The sample and variable attenuator may be replaced by a standard attenuator for calibration. The temperature of the specimen is detected by a thermistor TH1 connected in a bridge circuit with Zener diode MR1. At balance transistor VT1 passes a constant current through potentiometer 33 the slider of which is positioned by the motor driving the variable attenuator. Variation of temperature unbalances the bridge to vary the current through 33. The gangs of a two-ganged potentiometer may be connected one at each end of potentiometer 33 so that the circuit may be adjusted for specimens having a different value of A o in the relationship where W is the wetness of the specimen f (T) is a function of temperature A is the attenuation and Potentiometer RV3 is adjusted in accordance with k 2 and variable resistor RV2 is adjusted in accordance with k 1 . Adjustment for different values of W 0 may be made by adding a constant voltage from an adjustable source Fig. 5 (not shown) to the potential 1 difference across the output potentiometer, or by passing a constant current from an adjustable source Fig. 6 (not shown) through the output potentiometer, and adjusting the sources in accordance with A 0 . Alternatively the potential difference across the output potentiometer may be applied to a buffer amplifier (e. g. of the long tailed-pair type) and the output adjustably biased. The apparatus may be modified by comparing the output from detector 19A (Fig. 7) with the output from a detector 19B connected to reference attenuator 11. The amplitude and phase sense of the output from transformer 55 is indicative of variations in attenuation in the specimen and the output is applied to a phase-sensitive demodulator controlling servomotor 31 to compensate for such variations. The transformer may be dispensed with if the polarity of one detector 19B is reversed.