Linearization of Sensor Response: Bondable Resistance Temperature Sensors

(...continued)

Linearization of Sensor Response
The readout methods shown previously can be quite accurate, but are somewhat awkward to use in that they read sensor resistance directly, and the nonlinear characteristic of sensor resistance-versus-temperature requires the use of tabulated data to convert resistance values to the equivalent temperatures. A very simple method exists for converting the nonlinear sensor to a linear resistance change with temperature with good practical accuracy. This is accomplished by shunting the sensor with a fixed resistor equal in value to 3.75 times the +75.0° F (+23.9° C) value of sensor resistance, or 187.5 ohms for standard TG sensors . The resultant resistance change with temperature has a lower slope, but is quite linear as shown earlier . A plot of deviation from linearity for this circuit is shown below, and provides much higher error readability.

Deviation from linearity for shunted TG sensor.

A useful application of shunt linearization is to provide an asymmetrical bridge circuit that has a linear output voltage with temperature. This circuit can be used with a digital voltmeter for direct temperature readings (expressed in mV) or to drive one axis of an X-Y recorder for directly plotting temperature against another variable. A simplified version of this circuit is shown below.

Linearization circuit.

This linearization circuit requires a constant-voltage excitation source, and this can be conveniently provided by a single silver-oxide battery. The output factor is 0.5 mV/V/° F (0.9 mV/V/° C), which can be scaled down by use of a high-resistance voltage divider at the output terminals. A balance potentiometer is incorporated for balancing out the tolerance on the nominal sensor resistance and the offset caused by leadwire resistance. (When the Celsius temperature scale is used, it is more convenient to balance at +24° C, rather than +23.9° C, so that the readings are in round numbers.) Calibration can be checked by substituting a precision decade box for the TG sensor and dialing in resistances equivalent to various temperatures from the resistance-versus-temperature tables.

Because of the asymmetrical bridge, a three-wire lead system cannot be used effectively with the linearization circuit shown above to eliminate the errors from a temperature-induced leadwire resistance change. However, it is possible to use the three-wire method for the more limited purpose of compensating the initial offset error caused by the leadwire resistance. This is accomplished by adding the third wire (shown dashed) and breaking the connection at the point marked X.

Page 4 of 15