The values of electrical DC voltages and resistances can be reproduced with a relative uncertainty of 10–9 by means of the Josephson and quantum Hall effects. For AC voltages and alternating currents (which are just as important in metrology), no precise realizations exist. For this reason, the alternating quantities are traced to equivalent direct quantities (AC/DC transfer) by comparing the temperature increase caused by an alternating current in a filament with the temperature increase caused by a direct current of a known rate.
For this task, thermal converters are used for which the temperature increase of the filament is measured by means of numerous series-connected thermocouples. Earlier customary thermal converters had a complicated three-dimensional structure. Because it was necessary to construct them by hand under a microscope, they were unsuitable for the mass production necessary to meet the demand of calibration services and national metrology institutes. On the other hand, at PTB, Manfred Klonz and Thomas Weimann developed a thermal converter that could be manufactured via thin-film techniques, as used in the semiconductor industry. For this work, they were awarded the 1990 Helmholtz Prize in the field of “Precision Measurement of Physical Quantities”; the prize at that time was endowed with 10,000 German marks.
The planar multi-junction thermal converter rested on a 2 x 3 mm2 silicon oxide window that was 3 µm thick; this window had been anisotropically etched into a 5 x 8 mm2 silicon chip. A heater and 108 thermocouples made of copper and constantan (a copper-nickel alloy) were deposited under vacuum onto the window. The current-carrying heater heated the window, which was a poor heat transmitter. The temperature increase caused in this way was measured by the thermocouples. It was possible to produce up to 100 thermal converters on one wafer at the same time. This allowed the new measuring element to be produced in large quantities and with high precision. Since the properties of the planar multi-junction thermal converter were calculated as a function of the size and shape of its components, it was possible to optimize them via computer simulation.
In this way, Klonz and Weimann manufactured a planar multi-junction thermal converter able to measure AC voltages at 2 V for frequencies from 10 Hz to 1 MHz with an uncertainty of less than 10–6. Since then, the multi-junction thermal converter has been developed further, yielding measurement uncertainties of some 10–7 for very low frequencies in the mHz range and 10–6 for high frequencies up to 100 MHz.
This has allowed PTB to reinforce its position as a leading institution in AC/DC transfer.