The ac-dc transfer difference of the multijunction thermal converter (MJTC) developed at Physikalisch-Technische Bundesanstalt (PTB) has been evaluated for various heater resistances between 27 and 700 {}. By optimizing the heater resistance for voltage transfer to 190 {} and for current transfer to 27 0, transfer differences of afew parts in 10 7 are achieved up to 100 kHz. Transfer differences caused by thermoelectric effects could be shown to be smaller than 10-8 , due to the nearly ideal periodic structure of the converter and the special construction of the intermediate leads between heater and vacuum throughleads.An increase of the transfer difference at low frequencies is caused partly by the change in the real part of the heater impedance, which can be compensated by adding series or parallel resistors to the heater. Another part is found to be a current transfer difference which can only be diminished to a few parts in 10 7 by increasing the number of thermocouples or decreasing the input power.
This report describes key comparison CCEM-K6.a of ac-dc voltage transfer standards at the lowest level of uncertainty, held under the auspices of the Consultative Committee for Electricity and Magnetism (CCEM) and completed in May 1996. Twenty-two national metrology institutes (NMIs) took part in this comparison, which concentrated on the basic voltage level of 3 V and a frequency of 1 kHz. Results for additional optional frequencies, such as 20 kHz, 100 kHz and 1 MHz, were also reported by most of the NMIs. Multijunction thermal converters (three-dimensional MJTCs and a planar MJTC) manufactured at the Physikalisch-Technische Bundesanstalt (pilot laboratory) were used as travelling standards. All the results show agreement with the reference value within the expanded uncertainty (k = 2) given by each participant. The expanded uncertainties associated with the reference value are 0.4 × 10 −6 at 1 kHz, 0.5 × 10 −6 at 20 kHz, 1.0 × 10 −6 at 100 kHz and 6.7 × 10 −6 at 1 MHz. Degrees of equivalence with the reference value are given.
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