Ohaus Corporation www.ohaus.com
15
Defender
™ Series Bases Service Manual
CHAPTER 4 TESTING
4.1
Testing
Before and after servicing a Defender series Base, conduct Load Cell Resistance Checks, Calibration using an
external Indicator, and Performance Tests to confirm that the base meets specifications. Allow time for the
base to stabilize after moving it from an area which is at a different temperature than the area where it is to be
operated. Allow one hour for each 5°F (2.7°C) temperature change before using the base.
The following tests require a Digital Multimeter, an Indicator and hand tools. The Digital DC Voltmeter should
have a reading range from 1 mV to 50 V full-scale. It should be capable of reading differences of one micro-
volt per increment. The indicator can be either the Ohaus
Defender 3000 series, or an equivalent commercial indicator.
ARTTENTION:
Make sure the test area is free from drafts and that the base rests on a level
and vibration-free surface.
4.2
Load Cell Resistance Checks
Use an ohm-meter to measure across each pair of wires in turn, and compare the results with Table 4-1. The
Load Cell must be completely disconnected from the Indicator and at no load when these tests are made.
In addition to the four resistance elements which make up the Wheatstone Bridge, there are commonly one or
two resistors in the excitation lines. The resistance across the excitation wires is usually the highest resistance
measured across any two wires.
If the resistance readings are in the range specified, skip to the next section. If they are incorrect
– for
example, outside the expected range, open circuit or short-circuit across any two wires
– the likely causes are
a damaged or faulty Load Cell or incorrect or faulty wiring. If the Load Cell is defective, replace it. (See
Chapter 3.)
TABLE 4-1. LOAD CELL RESISTANCE READINGS (in Ohms)
LC Models
E+ to E
–
S+ to S
–
PW15AH
381 ± 4
350 ± 1
SSH
380 ± 15
350 ± 10
