MAINTENANCE
PRS Series
17
Chapter 8
MAINTENANCE
8.1
Verification of Performance
8.1.1 Calibration Interval
The PRS Series instruments should be verified for
performance at a calibration interval of twelve (12)
months. This procedure may be carried out by the
user, if a calibration capability is available, by IET
Labs, or by a certified calibration laboratory. If the
user should choose to perform this procedure, then
the considerations below should be observed.
8.1.2 General Considerations
It is important, whenever testing the PRS Series De-
cade units, to be very aware of the capabilities and
limitations of the test instruments used. Such instru-
ments have to be significantly more accurate than
the specified accuracy for all applicable ranges in order
to perform this task, allowing for a band of uncer-
tainty of the instrument itself, the test setup and the
environment; consult IET Labs for information.
It is important to allow both the testing instrument
and the PRS Substituter to stabilize for a number of
hours at the nominal operating temperature of 23
0
C,
and at nominal laboratory conditions of humidity.
There should be no temperature gradients across the
unit under test.
In the case of a resistance substituter, substantial
Kelvin type 4-wire test terminals should be used to
obtain accurate low-resistance readings. It is conve-
nient, once the zero-resistance has been determined,
to subtract it from the remaining measurements. This
can be done automatically in many instruments which
have an offset subtraction capability.
Steps should be taken to minimize thermal-emf ef-
fects, by maintaining an even temperature and by us-
ing only low-emf connectors. Use of meters with a
“True Ohm” function is recommended.
Proper metrology practices should be followed in per-
forming this verification.
