Low-resistance thermometry

A system of resistance thermometry is described which uses thermometer elements of the order of a fraction of an ohm. The advantages obtained include a reduction in the size of the thermometer and the ability to expose it directly to a fluid. The measurement method uses a transformer-coupled potentiometer circuit with a very large ratio between the standard resistor and the thermometer resistance. The measurement method may be applied to other resistive transducers for which a low resistance is advantageous. Substantial lengths of twisted-pair cable may be used. An analysis of the effects of transformer and cable loss is presented, and the question of optimum transformer design is considered in detail. It is shown that, for deflection output, the deviation of the performance of the overall circuit from ideal performance can be kept within 1% this deviation can be taken up in calibration. For null-balance output, the deviation is much smaller.