iET PRS-300 Series, Руководство по эксплуатации, Страница: 35 / 50

MAINTENANCE
PRS-300 Series
25
Chapter 8
MAINTENANCE
8.1 Verifi cation of Performance
8.1.1 Calibration Interval
The PRS Series instruments should be
veri fi ed for performance at a calibration in-
terval 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.
8.1.2 General Considerations
It is important, whenever testing the PRS
Series Decade units, to be very aware of the ca-
pabilities and limitations of the test instruments
used. Such instruments have to be signi
fi cantly
more accurate than the speci fi ed accuracy for
all applicable ranges in order to perform this
task, allowing for a band of uncertainty of the
instrument itself, the test setup and the environ-
ment; consult IET Labs for information.
It is important to allow both the testing
instrument and the PRS-300 to stabilize for
a number of hours at the nominal operating
temperature of 23
0
C, and at nominal labora-
tory conditions of humidity. There should be
no temperature gradients across the unit under
test.
In the case of the PRS-300 a Kelvin type
4-wire test terminals should be used to obtain
accurate low-resistance readings.
Steps should be taken to minimize
thermal-emf effects, by maintaining an even
temperature and by using only low-emf con-
nectors. Use of meters with a “True Ohm”
function is recommended.
Proper metrology practices should be fol-
lowed in performing this veri fi cation.
8.1.3 Calibration Procedure
1. A DMM is used to measure the resistance
of the PRS-300. The DMM should have a
measurement accuracy signi
fi cantly more
accurate than the ±(70 ppm + 1 m Ω ) for all
resistance values.
2. Determine the allowable upper and lower
limits for each resistance setting as would
be done for a decade box from 0.1
Ω to 20
M Ω s based on the speci fi ed accuracy of the
PRS-300.
3. Con fi rm that the resistances fall within these
limits
4. If any resistances fall outside these limits, See
section 8.1.3 Adjustment of internal resistors.
8.1.4 Adjustment of Internal Resistors
During the adjustment process a DMM
such as the Fluke 8508A or Keysight 3458A
using 7 ½ digits of resolution or better is con-
nected to the binding posts of the PRS-300
using a 4 terminal Kelvin connection.
The gold shorting links must be removed
on the binding posts of the PRS-300.