Patent GB-823844-A (GB)
Title: Improvements in or relating to voltmeters, temperature indicators, and temperature controllers
Inventor: LAND THOMAS
IPC: G01K1/02
Filed: 1955-04-06
Abstract: 823,844. Electric indicating systems. LAND PYROMETERS Ltd. March 28, 1956 [April 6, 1955], No. 10009/55. Class 40 (1). [Also in Groups XXXV and XXXVI] A voltage measuring circuit responsive, e.g. to a thermocouple output, applicable to temperature control, and consuming no current comprises a Wheatstone bridge wherein an indicating meter is shunted by first and second resistors in series with the voltage source and by third and fourth resistors separated by a tapped fifth resistance; an excitation voltage derived from a variable potentiometer being connected between the junction of the first and second resistors and the resistance tapping, and the potentiometer being adjustable to zeroize the current in the bridge arm containing the voltage source, so that the meter indication represents the voltage value independently of the source voltage. Fig. 1 shows a Wheatstone bridge whose branches comprise comparatively low resistors r 1 , r 2  in series with a thermocouple e and comparatively high series resistors R 1 , R 2  separated by a tapped potentiometer R 3 , across a meter G having a series calibration resistor r 3 . Excitation from a variable potentiometer P energized from a voltage source over a variable resistor Q is applied between the junction of r 1 , r 2  and the slider of potentiometer R 3 ; the control knob of potentiometer P adjusting a pointer moving over the meter scale coaxially with the meter pointer (Figs. 2, 3, not shown). By replacing the thermocouple by a variable voltage source of resistance equal to the most probable thermocouple resistance value, balancing the bridge by adjusting R 3  for zero voltage input, calibrating the meter for several values of input voltage with zero bridge excitation by adjusting r 3 , and adjusting Q for the movements of the meter and potentiometer pointers to agree over the scale with the voltage input open circuited, the device consumes zero current from the thermocouple and is thus independent of variations in the resistance thereof. In Fig. 4 the meter pointer 20 and a follow-up pointer 19 are provided with contiguous capacitance vanes operating in known manner to control a servo amplifier A and motor M geared to drive pointer 19 in follow-up relation to pointer 20, adjust the bridge excitation potentiometer P to zeroize the current consumed from the measurement source, and drive an indicating pointer 13 over visible scale 12. In Fig. 5 the meter pointer 28 and a reference pointer 31 move over an edgewise scale calibrated in temperature values of e.g. a furnace in response to the thermocouple voltage, and a magnetic vane 29 on pointer 28 is movable between coils 30 on the pointer 31 to couple the output of oscillator O to energize a relay at coincidence to switch-off or reduce the furnace heater current. Reference pointer 31 is adjustable, together with the bridge excitation potentiometer P, from the panel over linkage 39 to preset and indicate a required furnace temperature, and the actual temperature is regulated to maintain pointer 28 in coincidence therewith independently of variations in thermocouple resistance. By presetting potentiometer P so that at all values of thermocouple voltage the excitation voltage is slightly in excess of that required to zeroize the input current, an open-circuit thermocouple is indicated by maximum deflection of the meter.