Readout Methods: Bondable Resistance Temperature Sensors

Readout Methods

One method of reading temperature with TG temperature sensors is to connect the sensor to a Wheatstone resistance bridge, and convert the resistance readings to equivalent temperatures with the tables given in this publication. But, the leadwires can cause two different errors in a Wheatstone bridge. First, the resistance of the leads, which can be appreciable with remote gages, produces an initial offset error, and desensitizes the arm of the bridge containing the temperature sensor. The second error is the result of resistance change in the leads caused by temperature variations. Except under unusual conditions, errors of this type are very small. In cases where long leadwires are necessary, special calibration techniques or compensation systems can be used (Ref. 1 and 2 ).

A variation of the above method is capable of providing accurate compensation for leadwire resistances with a three-wire system. The circuit is shown below, in which a precision decade box is used in place of one resistor in the Wheatstone bridge.

Three-wire null-balance circuit for TG sensor.

The decade box in this circuit is varied to keep the output at null, and the indicated decade resistance is therefore the same as the sensor resistance. Provided leads 1 and 3 are of the same length and size, resistance changes in the leadwire circuit caused by temperature changes common to all wires will not create errors in the reading. Three-wire compensation is effective in this case because this is a true null-balance system in which the bridge arm adjacent to the sensor (the decade box) is always set to the sensor resistance at the time of readout.

Excitation power can be either dc or ac, depending on the null indicator chosen. Excessive excitation can create errors due to self-heating in the sensor, but this error is easily avoided or corrected as discussed later .

A more sophisticated readout system, which eliminates the need for manual rebalance, is shown below. This arrangement eliminates leadwire errors by use of a four-wire system.

Four-wire circuit for TG sensor.

If the digital voltmeter has a high enough input impedance, the readings will be a known function of sensor resistance, regardless of resistance change in any of the leadwires. A current level of one mA will allow the voltmeter to read sensor resistance in terms of mV (50.0 ohms reads as 50.0 mV).

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