Abstract: An instrument to determine, under actual field conditions, which type of VA demand meters are most appropriate for a particular consumer. The type of electrical load that the consumer places on an electrical supply system can introduce harmonics and phase distortion in the system resulting in VA meters producing incorrect readings. These VA meters are intended to measure the true apparent power S which is defined as S=V.sub.rms XI.sub.rms but, are generally designed to operate according to one of the following approximate formula:(1) S'=V.sub.rect XI.sub.rect X(1.11).sup.2 and(2) S"=.sqroot.P.sup.2 +Q.sup.2 where P is the active power and Q the reactive power.The instrument contains circuit to measure the rms values of the voltage and current waveforms from which the true apparent power S can be obtained as well as circuits to determine the value S' and S" according to the two approximate formula from which error signals e.sub.1 and e.sub.

Abstract: A true RMS instrument using a multijunction thermal converter with two beaters automatically determines and displays the RMS value of an unknown voltage using two steps or modes. In an AC mode, the unknown voltage is attenuated and balanced in the thermal converter with a DC voltage. DC voltage is stored in digital form in a first latch. In the DC mode, the unknown attenuated voltage is replaced by a chopped DC voltage which is then balanced with the DC voltage obtained in the AC mode. The unknown voltage is determined by decoding the contents of a second lath which controls the chopped DC voltage.