PPCH-G™ OPERATION AND MAINTENANCE MANUAL
© 2007 DH Instruments, a Fluke Company
Page 68
<7intern>
Access internal functions including control configurations, on-board barometer viewing,
pump and reservoir functions (see Section 3.5.7).
<8cal>
View and adjust RPM3 utility sensor, Q-RPT and barometer calibration coefficients (see
Sections 3.5.8 and 5.2).
<9log>
View the PPCH-G incident log (see Section 3.5.9).
3.5.1
<1A
UTO
Z>
PURPOSE
To offset the PPCH-G system Q-RPTs relative to a reference value in order to compensate
for possible changes in Q-RPT zero between full recalibrations.
PRINCIPLE
AutoZ Purpose and Principle
The main component of the change over time of the PPCH-G Q-RPTs is change in zero or
offset, independent of span. Offsetting or “rezeroing” PPCH-G Q-RPTs relative to a
reference between recalibrations allows measurement uncertainty specifications to be
maintained with less frequent full calibrations. The PPCH-G AutoZero function (AutoZ)
provides full on-board support for the rezeroing process to simplify its application by the user.
The AutoZero function uses three values:
1.
P
std,0
:
The pressure value indicated by the AutoZ reference, the device that is acting as
the reference relative to which to offset the Q-RPT.
In
absolute measurement mode
, the pressure at which AutoZ is performed is normally
atmospheric pressure and the
P
std,0
value can be supplied by manual entry.
In
gauge measurement
mode
,
P
std,0
is always zero (atmospheric pressure) which is
supplied by definition when the Q-RPT is vented to atmosphere.
2.
P
u,0
:
The pressure reading of the Q-RPT, with no AutoZ offset, at the time AutoZ is
performed.
3.
P
offset,G
and
P
offset,A
:
The difference between the reading of the Q-RPT with no AutoZ
offset (
P
u,0
) and the indication of the AutoZ reference (
P
std,0
) for gauge (
G
) or absolute
measurement mode (
A
):
P
offset
= P
u,0
- P
std,0
P
offset
represents the change in zero of the Q-RPT relative to the AutoZ standard (
P
std,0
).
The AutoZ function manages the determination, storage and application of
P
offset
for
PPCH-G system Q-RPTs in absolute and gauge measurement modes.
When the Q-RPT is calibrated,
P
offset
is set to zero.
P
offset
is then redetermined at regular
intervals using the AutoZ function. The most recent value of
P
offset
is applied to the Q-RPT
reading to correct for change in zero over time.
The AutoZ function can be turned ON and OFF. Table 9 summarizes the effect of AutoZ ON
and OFF for PPCH-G Q-RPTs in different measurement modes.
AutoZ in absolute measurement mode
Q-RPTs with the designation Axxx are intrinsically absolute. They have an evacuated and
sealed reference. Axxx Q-RPTs can measure absolute pressure (difference from vacuum).
All PPCH-G Q-RPTs are of the Axxx type.
In absolute measurement mode, the source of
P
std,0
must be an absolute pressure, nominally
atmospheric pressure, with uncertainty significantly better than that of the Q-RPT that is
being AutoZeroed. Given the high range of PPCH-G Q-RPTs, this is easily accomplished
with a variety of digital barometers, including PPCH-G’s on-board barometer (see Section
3.5.7.2).
