Humboldt H-1021 Force Determination Adapter - Manual (Page 6)

What to Expect During an Accuracy Check:
Because of the design and construction of the FD-4A, you may observe slight
differences in the readings for the same weight depending on whether it is being added
or removed from the holder during calibration. Weight added may read a little low.
As weight is removed, the springs will relax, but remaining weight might cause readings
to be a little high. This usually amounts to about ± 0.05 pound in the higher ranges.
As weight is added in the 15 pound and up range you may notice a few hundredths
creep before the display stabilizes-this is normal. Remember that each 0.01 increment
on the display is actually 0.000007" (seven millionths) travel of the FD-4A platform.
After removing all weights, the readout may not return to 00.00. Allow the display to
stabilize and/or adjust the “zero” then check the weights again. If properly adjusted,
the unit should read a little high while removing them with the actual test weight value
in between.
How to Adjust Display:
Use the “zero” adjustment to zero the display as necessary when the unit is at rest.
This applies to all temperature situations whether at room ambient or any controlled
bath environment. Setting the display to zero sets the analog ouput to zero since they
correspond. Use the “English” and “Metric” adjustment to set the display to the correct
reading while the calibrating weight is acting on the FD-4A. It is best to do this with the
heaviest test weight or highest anticipated load you will be using. Change the “English/
Metric” switch to the desired scale and turn the appropriate potentiometer to adjust.
The “English/Metric” adjustment do not change analog output voltage. Likewise the
select switch changes only the display and does not alter the analog output voltage
proportionally for the metric scale. Add and subtract weights and make adjustments
until the readings are within tolerance. The output device or data acquisition equipment
should also be checked to verify that the analog output voltage corresponds.
In most cases, calibration to correct the “zero” adjustment while the unit is at rest before
performing a test. The calibration interval for the English and Metric settings on the
display will depend on laboratory practice and/or specific test requirements.
The phase is adjusted at the factory and should not be changed. The span adjustment
controls the analog output voltage and should not be adjusted either unless you have
an accurate multi meter to check the setting. Normally the analog output voltage should
not need adjustment and can be verified accurately enough on the output device or
data acquisition equipment.
If you suspect that the analog output is not calibrated correctly, you may wish to return
the unit to the factory for a complete recalibration or contact the factory for instructions.
Accuracy:
The FD-4A unit should be approximately ±1% accurate (or better) for the value of the
calibration weight that is being used. The Humboldt digital readout unit has an accuracy
of ±2 counts (hundredths) so this should be allowed for the display itself. Therefore, the
overall accuracy of the apparatus should be ±1% + 2 counts. This applies to the display
and analog output voltage since they are in effect equivalent. The weight holder which
weighs one pound, for example, should read between 0.97 and 1.03 on the display
while the analog output voltage should be equal but only 1/10 of the display value.