DH Instruments PPCK PLUS, Operation And Maintenance Manual

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PPCK+™ Operation and Maintenance Manual
Page 49
©2001 DH Instruments, Inc .
Gauge Mode, Dynamic Compensation For Atmospheric Pressure
PPCK+ allows gauge pressure measurements with an absolute RPT by calculation,
subtracting the value of atmosphere (tare) from the RPT's absolute measurement to
arrive at a gauge value. The appropriate tare value changes with the natural evolution of
atmospheric pressure at a given location. For this reason, the value of the tare is
redetermined each time the RPT is known to be exposed to atmospheric pressure
(e.g., the PPCK+ is vented and the pressure is stable). However, if atmospheric
pressure changes significantly between opportunities to redetermine the tare, for
example if running an extensive test that does not include returns to a vented condition,
the changes in atmospheric pressure during that time will not be taken into account and a
significant zero offset in the gauge pressure indications could result.
To minimize zero errors due to the evolution of atmospheric pressure between taring
opportunities when using an absolute RPT in
gauge measurement mode, PPCK+
dynamically compensates the atmospheric tare value. PPCK+'s on-board barometer
measures atmospheric pressure independently from the RPT. Between opportunities to
update the tare value (vented conditions), the difference between the on-board
barometer reading at the time of the last tare value and the current on-board barometer
reading is used to compensate the tare value. This value is called ATMOFFSET. This
technique, which relies only on the resolution and short term stability of the on-board
barometer, rather than its absolute measurement uncertainty, allows gauge
measurements with an absolute RPT with an additional uncertainty due to possible
changes in atmospheric pressure limited to
±
2.5 Pa (0.00035 psi).
Recommendations For The Use Of The AutoZ Function
The AutoZ function provides a powerful and easy to use tool for improving the
measurement uncertainty specifications of a PPCK+ and reducing the calibration recall
interval by compensating for zero drift between full recalibrations. The following simple
recommendations will help assure that you use this feature to best advantage.
•
Always leave AutoZ ON when operating in the gauge measurement mode unless
the on-board barometer has malfunctioned (see Section 3.4.4.7).
•
Always leave AutoZ ON when operating in the absolute measurement mode if
ZNATERR was set properly at calibration and the AutoZ routine using a valid
atmospheric reference has been run regularly.
•
Execute the run AutoZ routine only when a reference whose measurement
uncertainty is known to be significantly lower than that of the PPCK+ RPT is
available. Keep range ratios in mind when comparing uncertainties. A
±
0.01 % FS
barometer has roughly 200 times lower measurement uncertainty than a
±
0.02 %
FS 2 000 psi (14 MPa) PPCK+ range because the RPT/barometer range ratio is
10:1.
•
The best day to day reference for running AutoZ in absolute mode is a properly
calibrated, barometric range, DHI PPCK+ interfaced directly to the PPCK+
COM2 port. The on-board barometer can also be used as a stable atmospheric
pressure reference.
•
Allow the PPCK+ to stabilize at atmospheric pressure and ambient temperature for
5 to 10 minutes before running AutoZ in gauge measurement mode.