Patent GB-396085-A (GB)
Title: Improvements in automatic electric controlling apparatus
IPC: G05D23/20
Filed: 1931-11-16
Abstract: 396,085. Potentiometers; resistance measurements. HODGSON, J. L., IVANOFF, A.; and HOWARD, G. P. E., Biscot Road Works, Luton. Nov. 16, 1931, No. 31747. [Class 37.] Temperatures which are measured by thermocouples or electric. pyrometers or flows whose rate is made to cause a voltage variation are controlled by arranging the thermo-couple, pyrometer, or resistance in a wheatstone bridge, potentiometer or ratiometer which is balanced only at the desired temperature, clamping the galvanometer or ratiometer pointer intermittently, and causing the clamped pointer to control the temperature or. flow. As shown in Fig. 1, a thermo-couple 2, in series with a galvanometer 1, is connected across a potentiometer resistance 3 at variable tapping points 4, 5. The tapping 4 is adjusted by hand to correspond to the desired temperature, the tapping 5 being adjusted to correct for variation in cold junction temperature. A standard cell 8 and galvanometer 9 are connected across the potentiometer resistance 3 and an adjustable calibrated resistance 10 in series therewith for standardizing the potentiometer by adjusting a resistance 6. This arrangement enables adjustment to be made of the thermo-couple factor by tapping off a different amount of the resistance 10. Fig. 2 shows a modification in which automatic compensation for cold junction temperature is effected. Resistances 3, 11, 13, of negligible temperature coefficient and a resistance 12 of nickel are arranged as shown. With a rise in atmospheric temperature, which causes a drop in the electro-motive force of the thermo-couple, the value of the resistance 12 rises, causing an additional potential drop. Instead of being tapped as shown, the resistance 3 may be divided into a fixed and a variable part. Fig. 4 shows a ratiometer having coils 17, 18 supplied from lines 19, 20 and in series with variable resistances 21, 22. The resistance. 21 is adjusted by hand to a value corresponding to the required value of the temperature, the resistance 22 being varied by the temperature to be controlled. Fig. 5 shows a wheatstone bridge in one arm of which a resistance thermometer 24 is connected. A manually adjustable resistance 25 is included in the second arm. A variable resistance 26 enables the sensitivity of the galvanometer 23 to be varied, or the galvanometer or its magnetic circuit may have a variable shunt. Fig. 6 shows the means for causing the pointer of the galvanometer or ratiometer to control the temperature or flow. A motor-driven shaft 57 carries a cam 56 which intermittently lifts a jaw 55, thereby clamping the pointer 53 against a second jaw 58, and then continuing to lift the clamped pointer until it contacts with a cam arm 61. This is normally loosely clamped by a brake 64, which is released once during each revolution of the shaft 57 by a cam 65. The cam arm 61 carries a shutter 8 which is arranged to intercept, to an extent dependent on its position, a jet of air passing between nozzles 12, 13, this jet operating a diaphragm or piston and thereby controlling a valve, switch, &c., for example the switch mechanism described in Specifications 368,273, [Group XIII), and 368,353, [Group XXXVIII]. The cam arm 61 may also operate a pointer or recording pen.