INTRODUCTION
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PRECISION or REPEATABILITY is the variation in readings obtained when repeating
exactly the same measurement. The precision of an instrument is the ability to repeat a
series of measurements on the same piece and obtain the same results for each measured
value. The variation in the measured values can be expressed in terms of a standard
deviation of the measuring error. The instrument will be more precise if the standard
deviation is smaller.
Accuracy versus Precision: Confusion often exists between the terms accuracy and
precision because the terms are often interchanged in their usage, but they are two
different concepts. The accuracy of an instrument can be improved by recalibrating to
reduce its error, but recalibrating generally does not improve an instrument’s precision.
R.M.S. (ROOT MEAN SQUARE) is the square root of the mean of the instantaneous
values squared.
R.M.S. VOLTAGE is the effective value of a varying or alternating voltage. The
effective value is that value which would produce the same power loss as if a continuous
voltage were applied to a pure resistance. In sine wave voltages, the R.M.S. voltage is
equal to .707 times the peak voltage.
SENSITIVITY is the impedance through which a tester will detect a fault. Sensitivity is
usually expressed in Ohms. One of the most common examples is the UL 120K ohm
minimum sensitivity requirement.
VOLT AMPERE (VA) is the product of the R.M.S. voltage applied to a circuit and the
R.M.S. current, in amperes, flowing through it.
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