Patent GB-818881-A (GB)
Title: Devices for detecting changes in temperature and temperature sensitive controllers incorporating such devices
Inventor: HIGHAM EDWARD HALL
IPC: G01K7/34
Filed: 1954-11-10
Abstract: 818,881. Automotor temperature-control systems. UNILEVER Ltd. Feb. 9, 1956 [Nov. 10; 1954], No. 32503/54. Class 38 (4).[Also in Group XXXVIII] A device for detecting the changes in temperature comprises a transducer having two electrodes 1, 2 capable of being positioned adjacent opposite sides of a part of the stem of a conducting-liquid-in-glass thermometer 3 and of converting the changes in the position of the meniscus of the liquid in said part of the stem into changes of capacitance and a network 4 of which the transducer forms part, for representing the changes in capacitance as an electrical signal. The electrodes 1, 2 are embedded in an insulator and are arranged so that when the thermometer 3 is inserted in the transducer and clamped in position, its stem passes centrally between the two electrodes. The transducer body is provided with a window through which the meniscus when at the level of the electrodes can be viewed. The electrodes are connected to the other components of network 4 by means of coaxial cables (not shown) and the capacitances of the two cables together with the stray capacitances of the earthed body of the transducer arm shown as C2 and C3. The mutual capacitance of'the electrodes is shown as C1. The liquid in the thermometer is held substantially at earth potential by an earthed guard electrode (not shown) which surrounds the portion of the thermometer stem below the transducer body. Thus since the conducting liquid is earthed, as it rises between the electrodes 1, 2 it reduces their mutual capacitance by shielding the electrodes from each other. In addition to the capacity C1 the network has inductively coupled ratio arms which are in the form of a tapped inductor 5, a balancing capacitor C5 and a resistance network comprising a resistor 10 and a potentiometer 11 for balancing the resistive component. The network is supplied with a 25 kc/sec. alternating current from an oscillator comprising a valve 12 and parallel resonant circuit 13 and the change in voltage across one of the arms of the inductor 5 is amplified by a two-stage tuned amplifier comprising valves 14 and 15. Part of the output of the amplifier is rectified by a germanium diode 17 and the resultant D.C. voltage is applied to a magic eye indicator 16. The whole of the amplifier output is rectified by a valve 18 and the varying D.C. signal voltage is applied to a control circuit. As shown the control circuit comprises a thyratron 22, the anode of which is fed with A.C. from one winding of a transformer 23, one end of the winding being connected to the cathode of the thyratron 22 and the other end being connected to the anode through the solenoid 24 of a mercury switch. The cathode of the thyratron is connected to the cathode of a cathode follower 21 and hence to earth through the cathode resistance of the cathode follower and the cathode follower is fed with a sawtooth potential from a transisition signal generator comprising valves 19 and 20. The D.C. voltage is applied between the control grid of the thyratron and earth. The sawtooth voltage is arranged to have a period of oscillation of approximately 3 seconds and the thyratron is so biased that when the D.C. signal is zero, the sawtooth voltage does not cause it to fire but so that it will fire at some point in the cycle when the D.C. signal is present. The thyratron will fire when the potential difference between the grid and cathode is less than a critical value and provided that the anode is positive. It will be extinguished when the anode voltage goes negative but will fire again on every positive half-cycle of the anode supply voltage until the potential difference between the grid and cathode is brought above the critical value by the sawtooth voltage from the signal generator. Once the thyratron has fired in any period of oscillation of the signal generator the mercury switch in its anode circuit will remain operated since the solenoid is shunted by a condenser which will hold it operated during the negative half-cycles of the anode voltage. Since the time at which the thyratron will fire during the period of oscillation of the sawtooth voltage is determined both by the amplitude of the sawtooth voltage and by the amplitude of the varying D.C. signal the period during which the mercury switch remains operated will depend upon the amplitude of the varying D.C. signal at the instant the thyratron fires for the first time in any period of sawtooth oscillation. The mercury switch controls an electrical heating element which delivers heat energy to the liquid whose temperature is being controlled and as the temperature of the liquid approaches the control value the mercury switch is maintained operated for increasingly shorter periods during the sawtooth voltage cycle resulting in a reduction in the heat input to the system. When the meniscus of the liquid rises above the predetermined value the mercury switch is no longer operated so that the heat input is reduced to zero. In an alternative construction the amplifier output is compared with the output voltage of a potentiometer which is coupled to the shaft of a motorized valve which controls a non- electric heating system, the potentiometer providing a D.C. potential which is a function of the valve opening. The difference between the potentiometer voltage and the amplifier voltage is used to control the motor of the motorized valve. Specification 818,882, [Group XXXVIII], is referred to.