WIKA CPB3800 - Handbuch (Seite 30)

30 WIKA operating instructions model CPB3800
14062232.03 12/2013 GB/D
GB
6. Commissioning, operation
6.6.2 During calibration
When the pressure balance is correctly set up and there are no leaks the piston
should "float" for many minutes without it being necessary to touch the spindle pump
handwheel. On the initial setting up, however, there may be some air trapped in the base
of the piston/cylinder unit. As this leaks past the piston the weights may fall slightly but it
will only be for a matter of a few minutes until the air has escaped. If the piston continues
to fall, check the connections for leaks.
During calibration, the weights should be rotated by hand. It is desirable that the weights
should only be rotated when approximately the correct pressure is obtained. Weights
should not be brought to rest by fully releasing the pressure and allowing the piston
head to rotate against its stop under the full load of the weight pile.
It is essential that the weights spin freely during readings. The piston stops moving when
the pressure is too high or too low. At the lowest pressures the weights will not spin
for more than a few seconds unless a very thin oil is used, but providing the weight is
rotated by hand before taking a reading and is obviously "floating" an accurate reading
will be given.
CAUTION!
Care should be exercised at all times when rotating the weights. Failure
to do so may cause damage to the actual piston unit, or possibly injury to
the operator.
Therefore, the rotational motion should be stopped by hand. Only then
new masses for further test points can be placed or the pressure can be
released completely.
6.6.3 Datum levels
When testing gauges on liquid it is occasionally necessary to take into account heads
of liquid since a height difference of 10 mm corresponds to approximately 1 mbar. The
datum levels of the models CPB3800 piston units are marked with a groove on the
outer diameter of the piston unit. It should be noted that when the pressure balance is
re-calibrated by a laboratory other than DH-Budenberg/WIKA, the datum level at which
the tests have been carried out may differ from this standard and therefore allowance
should be made for any variation.
The drawing shows the head effect that may have to be compensated for when high
accuracy calibration is desired. The following formula will enable the head correction to
be calculated.