SECTION F: ROUTINE MAINTENANCE
F-1.
General
This section of the manual contains routine
maintenance information regarding the
AMPTEC RESEARCH Model 620A-4
Fail Safe Ohmmeters. Calibration should
be performed on a regular basis to ensure
continued instrument accuracy. The
recommended calibration interval is
1
year
.
The AMPTEC 620A-4 is a four wire Kelvin
ohmmeter.
The 620A-4 must be calibrated
using four wire Kelvin connections to the
resistance standard
in order to eliminate
lead resistance and contact resistance errors.
The alligator lead set (option “290") that is
supplied with the 620A-4 accessory package
is a 2 wire lead set and should not be used
for the 620A-4 calibration. (All U.S.A.F.
Tech Order’s that involve the 620A-4 with
the 2 wire alligator lead set require the user
to record the zero offset and manually
correct for in-series lead or connection
resistance errors when measuring squib
resistance).
F-2.
Required Test Equipment
The following list of the standard resistors
required to calibrate the AMPTEC 620A-4.
Calibration of versions other than the
AMPTEC 620A-4 use the same basic
procedure, however different standard
resistor values may apply for different
resistance ranges.
Precision Resistors:
0.1 ohm ± 0.01% Accuracy
10 ohms ± 0.005% Accuracy
100 ohms ± 0.005% Accuracy
1000 ohms ± 0.005% Accuracy
10000 ohms ± 0.005% Accuracy
Test Leads:
4-wire lead set
(
AMPTEC Option "300" or "301C")
F-3.
Calibration Procedure
The 620A-4 should be calibrated with fully
charged batteries and should be allowed to
warmup for a minimum of 15 minutes
before beginning the procedure. The
adjustments are accessed by removing the
four feet screws, then lifting off the top
cover only. The locations of the adjustments
are shown on drawing number 620A-600 at
the back of this manual.
Note: RV201 is factory set
and should never be adjusted in the field.
F-3-1. Zero Offset Adjustment
1.
Select the 20 ohm range. Connect
the Kelvin clips to the 0.1 ohm
standard resistor.
2.
Adjust potentiometer RV2 for a
display indication of 0.100.
F-3-2. Full Scale Adjustment
1.
Select the 200 ohm range. Connect
the Kelvin clips to the 100 ohm
standard resistor.
2.
Adjust RV1 for a display indication
of 100.00
.
3.
Check all remaining ranges with the
appropriate standard resistors. All
ranges must be within the
specifications outlined in Section B.
