OHAUS AV114C, Service Manual

Page: 65 / 108

4-3
CHAPTER 4 TESTING
4.2.1 Repeatability Test
Repeatability is a word used in balance specifications meaning the Standard Deviation of a set of
similar weight readings. To determine whether a balance meets the calculated Standard Deviation
value in the Specification Table 1-2, perform the following test:
Test
1. Tare the balance. The reading on the display should be 0g.
2. Select a mass weighing near the maximum capacity of the balance and place the mass on
the center of the Pan. Observe and record the reading.
3. Remove the mass. Reading should return to 0g +1 count.
4. Repeat this test for ten readings. If the standard deviation of the readings is less than +1
count, the balance passes the Repeatability Test.
Adjustment
If the deviation for any set of readings (using the same mass placed on the center of the Pan) is greater
than +1 count, the balance does not meet the Repeatability Test specification. Inspect and correct the
following areas:
1. Check for mechanical obstructions. Any foreign object touching any part of the moving Pan
can cause a balance to fail the Repeatability Test. Inspect and correct as necessary.
2. An error in the Off-Center Load Test can affect the results of the Repeatability Test. Inspect
and correct if necessary. See Off-Center Load Test.
3. Foreign material or debris located in the balance between the Pan, Pan Support and
the Top Cover can cause the balance to fail the test.
4. Environmental influences such as vibrations, drafts or a non-level surface can also cause
failures.
5. If the balance fails the test, refer to table 2-1 Diagnostic Guide for assistance.
4.2.2 Off-Center Load Test
The Off-Center Load Test is used to determine whether displayed weight values are affected by
moving the sample to different areas of the Pan.
Test
1. Place 1/2 of the balance capacity in the center of the Pan.
2. Note the reading.
3. Move the mass halfway (between the center and the edge) to the front of the Pan. Note
any differences in the displayed weight reading.
4. Repeat this test for the back, left, and right positions of the Pan.
5. Maximum allowable change in displayed weight readings is + 1 count for each of the four
positions. If this reading is exceeded, it usually indicates a defective Load Cell.