THE STATE STANDARD FOR ALTERNATING CURRENT IN THE FREQUENCY RANGE 20 to 1·10 6 Hz STATE STANDARDS
O. P. Galakhova, G. P. Telitchenko · 2008
Alternating current is a major physical quantity whose measurement is required in most industrial areas. This explains the high demand for measuring instruments (MI) for alternating current and the variety of them. The particular measuring tasks determine the strength of the current and the frequency, as well as the errors of the MI. Currents in the range 0.1‐25 A may be taken as basic, where the detector in the MI is directly connected in the circuit of the measurement object, while lower and higher values of measurements are provided by scale converters. The unit of current in the range 1·10 ‐8 to 25 A is transferred at frequencies of 20‐10 6 Hz in accordance with the state test scheme (STS) for alternating-current measuring instruments [1]. This STS lays down methods and means of transmitting the unit of alternating current from the state special standard (SSS) to working MI through a system of working standards of corresponding level. The scheme provides for centralized reproduction of the unit of alternating current [2‐4] in the range 1·10 ‐3 ‐20 A at frequencies of 20‐1·10 6 Hz. The SSS design is based on comparison of alternating current with direct current from their identical effects on the measuring converter. It is important to measure the root-mean square (RMS) value for alternating current, so the SSS uses type SKZ comparing converters: thermoelectric converters (TC). A thermoelectric converter consists of a metal heater and a thermocouple having thermal contact with the heater but electrically isolated from it. When the heater is included in a current circuit, it converts the electrical energy into thermal energy, and a thermo-EMF arises at the output from the thermocouple, which is proportional to the square of the input current [5, 6]. The standard uses time-separated comparison of the alternating current with direct current, where the TC in turn is connected in the circuit for the alternating current and then in a direct-current circuit. The effective alternating current is considered as equal to the direct current when there are identical output thermo-EMFs for each connection of the TC. The alternating current is determined from the measured value of the direct current. The standard consists of a set of measuring instruments that include standard thermoelectric converters (TEC), highly stabilized sources of direct and alternating current, and also auxiliary equipment for the operation of the standard. It is a group standard represented by five sets of seven TC each. For nominal currents of 1, 3, 5, 10, 30, 50, 100 mA, there are two sets: one made from multielement air-cooled TC and the second made with vacuum one-element units. Each TC in any set is designed for one value of current in the above series in the frequency range 20‐10 6 Hz.