DH Instruments PPC2+, Manual

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PPC2+™ Operation and Maintenance Manual
Page 57
©2001 DH Instruments, Inc.
2. ZCURERR: The difference between ZSTD and the RPT indication at the autozero
pressure at some time after the calibration of the RPT
(ZCURERR = RPT w/out
AutoZ - ZSTD).
3. ZNATERR: The difference between ZSTD and the RPT indication at the autozero
pressure just after the RPT has been calibrated . This is referred to as the natural
error at the autozero pressure. Because no RPT is perfectly linear and ZSTD is not
perfectly accurate, the disagreement between the RPT reading and ZSTD at the
autozero pressure is unlikely to ever be zero (except for gauge RPTs for which the
zero point is forced at calibration).
4. ZOFFSET: ZCURERR corrected for ZNATERR, represents the drift of the RPT
relative to the reference (ZOFFSET = ZCURERR - ZNATERR). The current RPT
reading, adjusted by ZOFFSET, is the autozeroed RPT reading (e.g., the RPT
reading corrected for zero drift since it was calibrated). For an absolute RPT used in
gauge mode, ZOFFSET also includes the value of atmospheric pressure that is being
subtracted to arrive at gauge pressure.
The AutoZ function manages the determination, storage and application of ZNATERR and
ZOFFSET individually for each PPC2+ range and measurement mode.
AutoZ ON/OFF
The AutoZ function can be turned ON and OFF. When AutoZ is ON, the ZOFFSET is
always applied to the pressure measured by PPC2+. When AutoZ is OFF, ZOFFSET is
not applied (except for an absolute RPT in gauge mode for which ZOFFSET is the current
value of atmospheric pressure which is always subtracted to achieve gauge pressure).
Gauge Mode with an Absolute RPT, Dynamic Compensation for
Atmospheric Pressure
PPC2+ 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 PPC2+ 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, PPC2+
dynamically compensates the atmospheric tare value. PPC2+’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 accuracy, allows gauge measurements with an absolute RPT with
an additional uncertainty due to possible changes in atmospheric pressure limited to
± 0.001 psi (8 Pa).