Watthour meters are customarily calibrated by determining the percentage registration, that is, the percentage of the energy passed through the meter in a short time interval. This may be done by two methods:

1) By precise timing of a number of revolutions of a meter while holding the watt input constant during the period, or

2) By operating the meter for a preselected number of revolutions simultaneously with a calibrated portable watthour standard of higher accuracy than the meter.

For the first method, the watthours registered in a given time are noted while the average power is simultaneously measured during the same period with a standard wattmeter.

Since the energy represented by one revolution, or the watthour constant, has been marked on the nameplate, the watthours registered by the meter on a given period will be Kh ´ R, where Kh is the watthour constant and R is the number of revolutions.

The accuracy of the gear ratio between the rotating element and the first dial of the register can be determined by count.

The percent registration is then readily computed. Thus,

% registration = (meter watthours/ true watthours) x 100%
= [(kh x 3600 x R)/ s x W] x 100%
Kh = watthour constant
3600 = number of seconds in 1 hour
R = revolutions in the test period of s seconds
W = true average power in watts during the test period as measured with indicating instruments

The last formula is the standard formula used in testing watthour meters. The constant marked on the nameplate by some manufacturers may be other than the watthour constant.

This should always be checked before proceeding with the calibration. Very complete information regarding meter constants and other meter data, may be found in the current edition of the Electrical Metermen’s Handbook [3],7 published by the Edison Electric Institute, Washington, DC.

Additional data on testing may be found in meter manufacturers’ literature and ANSI C12.1-1988, American National Standard Code for Electricity Metering [1].

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