IET Labs HARS-X Series, User And Service Manual

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7
MAINTENANCE
HARS Series
Chapter 4
MAINTENANCE
4.1 Verification of Performance
4.1.1 Calibration Interval
The HARS 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.
4.1.2 General Considerations
It is important, whenever testing the HARS-X Se-
ries 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 multim-
eters available that can verify the accuracy of these
units, especially when used in conjunction with stan-
dards that can serve to confirm or improve the ac-
curacy of the testing instrument
Such test instruments must be significantly more ac-
curate than ±(2 m
Ω
) for all applicable
ranges, allowing for a band of uncertainty of the in-
strument 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-X Substituter to stabilize for a num-
ber of hours at the nominal operating temperature of
23
O
C, and at nominal laboratory conditions of hu-
midity. 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 readings.
It is convenient, once the zero resistance has been
determined, to subtract it from the remaining mea-
surements. This can be automatically done in many
instruments which have an offset subtraction capa-
bility.
4.1.3 Procedure
1. Confirm the zero resistance of the unit.
2. Determine the allowable upper and lower
limits for each resistance setting of each decade
following the specified accuracy. For the
HARS-X series, these limits for any resistance
“R” are [R±(0.0001 R + 2 m
Ω
)]. For the A,
B, or Q series, see specifications.
3. Confirm 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
replacement.
4.2 Schematic
Refer to Figure 4.1 for a schematic of the HARS
decade unit.