Applications
AVTM24-1J REV A April 2006
29
INTERFERENCE
Alternating current interference in the sample under test
can cause several digits fluctuation in a displayed reading.
Interference can also be due to pickup in long test leads,
especially in the vicinity of strong electric or magnetic
fields. In these cases, fluctuation of the reading can
sometimes be reduced by twisting the pairs of leads
together.
If interference is causing fluctuation in the display, the
correct reading is the average of the highest and lowest
readings.
A DC voltage in the item under test, however small, will
produce an error in the reading. Such a voltage can be
caused by ground currents in grounded items such as rails,
pipes, etc., or by chemical or thermo—electric EMF in
items made of dissimilar materials. If DC voltage
interference is reasonably steady, this effect can be
eliminated by taking a second reading with leads
interchanged (C1 with C2 and P1 with P2) and averaging
the first and second readings. Actually, only the C leads
need be moved; the P interchange can be made by using
the forward-reverse switch on the instrument.
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