Temperature measurement with three lead resistance thermometers

Abstract

Measurement with a three-lead resistance thermometer uses two current sources to set up a current I in the thermometer element and each of its potential leads while a current of 2I is established in the current lead with all currents being in a direction such that the currents in the potential leads are in the same direction with respect to the thermometer element. A standard resistor is connected in series with one potential lead and a voltage measurement is made across the potential leads, including the standard resistor, to provide a measure of the temperature being sensed by the resistance thermometer. The current sources each utilizes a voltage divider connected between one terminal of a constant potential source and another terminal along with a follower circuit connected to receive an input from an intermediate point on the voltage divider. The follower circuit has a gain of magnitude corresponding with the reciprocal of the fraction of the potential used from the voltage divider and produces an output into a calibrating resistor through which a specific current is to be maintained. Each current source also includes a second voltage follower circuit for producing the potential at the other terminal of the voltage divider circuit as required to control the current in the calibrating resistor.