Performance of Single Junction Thermal Voltage Converter (SJTVC) at 1 MHz via Equivalent Electrical Circuit Simulation
The physical structure of the SJTVC can be represented via simulation using an equivalent RLC circuit to investigate the performance and estimate the error sources when using this device as a precise tool for AC voltage and current measurements. This paper describes instruments and techniques that apply on 0.6 V thermal converters to achieve this target. The technique was performed, theoretically and practically, to investigate the frequency-dependence of the SJTVC up to 1 MHz. The LTspice/SwCAD III simulator was used to analyze the effect of the physical structure of SJTVC in an effort to better understand the origin of errors at 100 kHz and 1 MHz. The practical results were firstly measured at National Institute for Standard and Technology (NIST), US to validate the simulation model at 100 kHz. The model then used to estimate the AC-DC transfer error of the SJTVC at 1 MHz. Some of the dominated error sources such as the changes of the real part of the heater impedance, the effect of thermocouple grounding and the power consumed in the internal resistance of the thermocouple are estimated at 100 kHz and 1 MHz. The expanded uncertainty assigned to the AC-DC automated calibration system of NIS is also described.
Mathematical model of high-precision reproduction of alternating current
The optimal solution to the problem of metrological assurance in the field of production and operation of the precision measuring instruments of alternating current in Ukraine is to create a high-precision measurement standard. The theoretical principles of creating such measurement standards were outlined. A method of comparing alternating current with equivalent direct current using the precision thermal converter and shunts was selected to construct the measurement standard. Since the reproduction of alternating current unit must be realized by comparison with equivalent direct current, the main sources of measurement uncertainty of this unit is a combination of measurement uncertainty of both a thermoelectric converter and a precision shunt. As the main result of this work, a mathematical model was obtained for the AC reproduction process, taking into account the contribution of precision meters of both the output voltage of the resistance measure and the output signal of the thermal converter.
Errors in AC-DC Transfer Arising from a DC Reversal Difference
Ac-dc transfer is examined in the presence of a dc reversal difference. It is shown that the definition of ac-dc difference normally used by standards laboratories leads to a systematic error which is of concern in establishing primary standards. An expression is derived for this error and it is shown that it can be eliminated by use of an alternative definition. Using the derived expression, errors for single-junction thermal converters are investigated. Calculations, based on data given by one manufacturer, for currents over the range 20% to 120% of rated, indicate that the systematic error may be in excess of 4 ppm for a reversal difference of about 0.05% and possibly as high as 70 ppm for a reversal difference of 0.2%. Experimental data for reversal difference as a function of current are presented for eleven thermal converters, some with Evanohm and some with Nichrome heaters, and with current ratings (Ir) ranging from 2.5 to 15 mA. Systematic errors arising from the definition are evaluated for each converter. For those converters in the group with reversal differences of less than 0.05% at rated current the systematic errors are found to be less than 0.1 ppm for 0.2 ? I/Ir ? 1.2.