A parts-per-million accuracy instrument for AC voltage measurements

An rms ac instrument is described which, in one measurement cycle, using a differential multijunction thermal converter (MJTC), automatically determines and displays a value of an unknown ac voltage by comparing it to an internal dc reference. The new instrument is as easy to operate as a voltmeter but is capable of measuring rms ac voltages with the accuracy of a thermal transfer standard. It has a response time of less than 10 s. Test results have shown that the instrument maintains parts-per-million (ppm) accuracy for frequencies from 100 Hz to 50 kHz.