Improvement of two-conductor thermoresistive converters with commutative inverting of sample currents

The improved method of compensation the influence of the lead wire resistance for two-wire resistance temperature detectors (RTDs) on the basis of commutation inverting of the reference measuring currents is proposed. For the averaging of signals generated as a result of commutation inverting of reference measuring currents, predominantly low-pass filters are used. Their usage limits the speed of temperature measurement. In order to increase the speed and reduce the impact of transient switching processes, the analogue memory devices at the inputs of the lead wires were used instead of the low-pass filters. The schematic and principle diagrams of the thermoresistive transducer have been developed. In this case, the primary resistance transducer additionally comprises a diode switch and the standard resistor which are constructively located on the output contacts of the RTD. To compensate the initial value of the RTD resistance, the value of the standard resistor is chosen to be equal to the initial value of the RTD resistance. To reduce the effect of voltage difference on diodes on the accuracy of temperature measurement, a transistor pair with normalized voltage values of the base-emitter is used. To simplify the switching device, the reference current source of one polarity is continuously connected to the input of the lead wire, and the source of the reference current of the opposite polarity – is connected to the lead wire by a diode switch on two diodes. Accordingly, the reference current of one polarity passes through the RTD, and the opposite polarity – through the standard resistor. The passage of the pulse measuring current through the RTD allows increasing its value, which improves the sensitivity of converting the RTD resistance into the voltage. The influence of the error of the reference measuring currents formation on the transfer function of a thermoresistive transducer and the compensation of the influence of the lead wires’ resistance have been analyzed. With a relative error of the reference currents of 0.01, the relaxation factor of the lead wires resistance is equal to 34 dB and increases with a decrease in a relative error. Research of the principle electric circuit of a two-wire thermoresistive transducer was carried out by computer simulation. The developed model completely reproduces all functional transformations of the developed principle electric circuit. The research was carried out for a copper 100 Ohm (Cu100) RTD. The research of the influence of nonidentity 
of transistor switches of a primary thermoresistive transducer and the errors of the reference measuring currents formation was carried out. When commutation transistors and sources of reference currents with metrological characteristics that provide the required accuracy of temperature measurement are chosen, the effect of the change in resistance of the lead wires within the range 0.. 30 ohms is completely compensated. It was found that the error of the non-linearity of the transformation function within the range 0…20 °C does not exceed 0.1 °С. As a result of the study of influence of a discharge time constant of analogue memory devices, a choice of optimal values of the frequency of a clock generator and the values of parameters of the analogue memory device elements was conducted.