Lightweight borehole temperature measuring equipment for resistance thermometers
A. E. Beck · Journal of Scientific Instruments · 1963
Lightweight equipment has been developed for measuring temperature in deep boreholes with an accuracy to less than 0.01 degC even when the ratio of leakage resistance to element resistance is of the order of 10 to 1. The measuring circuit is a modified Wheatstone bridge using four leads to compensate automatically for the series lead resistance, the shunt insulation resistance and the variations of these with temperature; this makes possible the use of very small gauge wires and thin insulation on the cable. The output from the bridge is amplified by a temperature compensated, direct coupled transistor amplifier. The strength required to support the cable in a borehole is now the main factor preventing any further reduction in cable diameter and weight.