IET Labs HARS-X, User And Service Manual

Page: 13 / 15

7
HARS-X and X2
7 Operation
Chapter 4
MAINTENANCE
4.1 Verifi cation of Performance
4.1.1 Calibration Interval
The HARS Series instruments should be veri fi ed 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 certi
fi ed calibration laboratory.
If the user should choose to perform this procedure,
then the considerations below should be observed.
4.1.2 General Considerations
It is important, whenever testing the HARS Series
Decade Units, to be very aware of the capabili-
ties and limitations of the test instruments used. A
resistance bridge may be employed, and there are
direct-reading resistance meters or digital multimeters
available that can verify the accuracy of these units,
especially when used in conjunction with standards
that can serve to con
fi rm or improve the accuracy of
the testing instrument
Such test instruments must be signi fi cantly more accu-
rate than ±(2 m
Ω
) for all applicable ranges,
allowing for a band of uncertainty of the instrument
itself. A number of commercial bridges and meters
exist that can perform this task; consult IET Labs.
It is important to allow both the testing instrument
and the
HARS Substituter to stabilize for a number of
hours at the nominal operating temperature of 23
O
C,
and at nominal laboratory conditions of humidity.
There should be no temperature gradients across the
unit under test.
Substantial Kelvin-type 4-wire test terminals should
be used to obtain accurate low-resistance read-
ings. It is convenient, once the zero resistance has
been determined, to subtract it from the remaining
measurements. This can be automatically done in
many instruments which have an offset subtraction
capability.
4.1.3 Calibration Procedure
1. Con fi rm the zero resistance of the unit.
2. Determine the allowable upper and lower limits
for each resistance setting of each decade based
on the speci
fi ed accuracy.
For the HARS series, these limits for any
resistance “R” are [R±(0.0001 R + 2
m
Ω )].
3. Con fi rm that the resistances fall within these
limits after subtraction of the zero resistance.
4. If any resistances fall outside these limits, the
associated switch assembly may require service
or replacement.