Analog linearization of transfer function of resistive temperature transducers

Different measuring transducers are applied for temperature measurement and control. Among them Platinum RTDs are the most widely used sensors because of their accuracy, stability over a long time period, wide temperature range -260 to 850 °C, easy to fabricate and availability at a reasonable cost. However, the disadvantage of platinum RTD is a significant second-order nonlinearity (within 3.7% over the range -100 to 800 ° C) which should be compensated.
In order to linearize the transfer function of Pt RTD the digital methods are often applied. The advantage of analogue methods is their ability to be implemented for different types of the secondary measuring devices. There are analogue methods of the linearization that minimize the nonlinearity up to 0.11% over the range -100..800°С assuming the calibration at the midpoint and at the endpoint of the range. The improvement in the linearity up to 0.05 ° C is possible using analogue methods in a narrow range of temperature measurement 50…125 ° C.
In the case of the calibration of transfer function of Pt RTD performed at certain points, the absolute nonlinearity errors of the temperature-voltage conversion are equal ± 4 ° C over the range 0 ... 400 ° C and ± 1 ° C over the range 0 ... 200 ° C
For converting the resistance of the RTDs into the voltage, the unbalanced resistive bridge circuits are employed. These circuits constitute the additional nonlinearity dependent on the ratio of the resistances of bridge circuit resistors. The increase of this ratio leads to the reduction of error of nonlinearity of transfer function of resistive bridge circuit and sensitivity of the output signal.
It is proposed to compensate the error of nonlinearity of transfer function of resistive bridge circuit by the change of supply voltage of bridge circuit depending on the measuring temperature. The output voltage of the bridge circuit that is summed with the supply voltage using the certain coefficient was used to form the compensating supply voltage. In this case, the error of nonlinearity does not exceed 1.5°С over the temperature measurement range from 0°С to 800°С, and 0.1°С over the temperature measurement range from 0°С to 400°С. In order to reduce the error of nonlinearity of transfer function of resistive bridge circuit over the temperature range from 400°С to 800°С it is necessary to form the additional components of the compensating supply voltage of the bridge circuit, proportional to the change of the output voltage of the circuit.
In the case of the use of one additional component of the compensating supply voltage of the bridge circuit, the error of nonlinearity does not exceed ± 0.25 ° C over the temperature measuring range of 0…800 ° C. The error of nonlinearity is less (does not exceed ± 0.1 ° C) if we use two additional components of the compensating supply voltage of the bridge circuit
The use of the additional voltage of supply voltage of the bridge circuit proportional to the measuring temperature allows to improve the linearity of transfer function of the resistive temperature transducers and to increase the accuracy of temperature measurement.