A sensitive temperature measuring system

A sensitive temperature measuring system, providing an analog output proportional to sensor temperature, has been designed for use in the wall temperature control system of a cloud simulation chamber. A platinum resistance element is the sensor, and a precision wirewound resistor is the reference in a bridge circuit which uses four low cost operational amplifiers arranged to provide two precision 1000 Hz current sources. Problems with an initial bridge amplifier design were corrected in the final design by the use of a low Q bandpass amplifier for the bridge signal followed by a phase detector and an output low pass filter which reduces noise and allows fine adjustment of overall gain. Although a completed system was never built and calibrated, the breadboarded final design features r.m.s. output noise equivalent to 0.2 m°c, and drift over a period of several hours reducible to less than 3 m°c. System nonlinearity is a maximum of 0.6% when the instrument is calibrated over a 10°C range, and full scale accuracy is limited only by oscillator stability to about 4%. The cost of the system is sufficiently low that the system could be used wherever control or monitoring applications require a continuous signal proportional to sensor temperature difference from a reference value --Abstract, page ii