Thermal converters as AC-DC transfer standards for current and voltage measurements at audio frequencies

Abstract

Thermal con verters and associated equipment that are used as ac-dc transfer standards at t he National Bureau of Standards for the precise measurement of current and voltage at power and audio frequencies are described. The tandards and t h e equipment are primarily used to standardize a-c ammeters and voltmeters submitted to the Bureau for certification. Th e ac-dc transfer may be made with t hese thermal converters at currents from 1 milliampere to 50 amperes, voltages of 0.2 to 750 volts, with an accuracy of 0.01 percent at frequencies from 25 to 20,000 cycles per second.The special tests to ins ure t he required accuracy of the transfer standards are described, and the results are presented. A number of factors that limit the transfer accuracy of thermal con verters have been discovered, and the results of special tests and theoretical work to evaluate these factors are discussed. The solu t ions, by an approximation method, of certain pertinent nonlinear differential eq uations governing the heating of a conductor by an electric cun'ent arc given.

Advanced thermal sensors for precision AC voltage metrology

“…The most accurate method of measuring ac voltage at frequencies from 10 Hz to 100 MHz is by comparing the heating effects of an unknown ac signal to that of a known dc reference using thermal converters [1,2]. Traditional single junction thermal converters (SJTCs) consist of a thin heater wire suspended between posts, with a thermocouple placed at the midpoint of the heater.…”

The practical uses of AC-DC transfer instruments

“…They are largely due to Peltier heating and cooling at the ends of the heater, and to Thomson heating in the two halves of the heater [6,7]. Both of these effects cause asymmetry in the temperature rise of the heater.…”

“…Thermoelements, as well as TVCs and TCCs, sometimes have a significant ac-dc difference, 6, particularly at higher frequencies, 6 = (V^-V,)/V, .…”

RMS Voltage Measurements by Thermal Converter with Temperature Controlled

“…Thermal converters (TCs) are widely used as a device to measure the rms value of an ac voltage with the highest accuracy (1). The most common form of the TC consists of a heater wire to which a voltage is applied, and a thermocouple which senses the temperature of the heater.…”

“…1, it is clear that the thermocouple emf Vth is controlled to be constant by VT, r, so that the rms value of Vx can be expressed by the variation of Vh,r, which is the measured output. By the minor loop, it is controlled so (1) where A and B are coefficients inherent to the materials and dimensions of the heater and the thermocouple, and vary with ambient temperature. The existence of BVh2 in (1) indicates that the TC does not exhibit simple square characteristics, and the values of BV1,2 range from 0.02 to greater than 0.1 at rated heater voltage (4).…”