Improvements in or relating to thermistor circuits for thermometry

摘要

706,368. Temperature measuring bridges. STANDARD TELEPHONES & CABLES, Ltd. Dec. 14, 1951. No. 29347/51. Class 37 [Also in Group XXXVIII] A thermistor bridge calibrated against a first thermistor is so designed and arranged as to accommodate replacement thermistors of the same type without recalibration. As shown a first thermistor r15 is permanently connected with its own shunt resistance R to form one arm which is permanently connected with a resistance R1 forming an adjacent arm and each of the stock or replacement thermistors is similarly associated with its own characteristic resistances R, R1 so that to substitute another thermistor for r15 an entire half or branch of the bridge is replaced as at 7-9. The value of R for each thermistor is selected in accordance with a formula given in the specification so that over a required temperature range e.g. 0-40‹C. the resistance value P of the parallel combination varies linearly with temperature. The value of R1 for each thermistor is made equal to the value P at the mean temperature of the range covered. In the permanent half of the bridge the arm 1-2 comprising the temperature calibrated rheostat R3 also comprises a fixed resistance R2 and with R3 at the setting corresponding to the mean temperature of the range covered the fixed resistance R4 in the adjacent arm 2-3 equals R2 + R3 and the ratios R2 + R3/R4 and P/R1 both equal unity and the bridge is balanced for the first thermistor r15 and any replacement thermistor. The maximum value of R3 is made equal to or near the value p which corresponds to the total amount of resistance change of the parallel combination R + r15 over the chosen temperature range and R2 has a value equal to or k times the minimum resistance P of R + r15 over the chosen temperature range where k is the ratio of the maximum value of the R3 rheostat actually used to the value p. The value k differs normally from unity since the R3 rheostat actually used is normally a commercial variable resistor having an upper value' as near as possible to the value p. The ratio of P to R1 for each thermistor is substantially a constant at each point of the temperature range covered so that e.g. a 0-40‹C. scale on R3 against the first thermistor unit or bridgehalf r15 + R + R1 also serves for any replacement combination. ' Each thermistor unit or bridge-half may be mounted on a conventional base for a thermionic valve with three pins connected to the resistances R1, R2 as at 7, 8, 9. The main apparatus comprises the permanent bridge-half and galvanometer ' 6 connected to corresponding sockets in a valve holder from which may thus be readily detached any thermistor unit or bridge-half to be replaced. Specification 558,299 is referred to.