3. OPERATION
Page 75
© 2009 DH Instruments, a Fluke Company
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 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 11 summarizes the effect of AutoZ
ON and OFF for PPCH 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 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 Q-RPTs, this is easily accomplished with
a variety of digital barometers, including PPCH’s on-board barometer (see Section 3.5.7.2).
When using an Axxx (absolute) Q-RPT in absolute measurement mode, with AutoZ ON,
absolute pressure is calculated as:
P
abs
= P
u
- P
offset,A
In absolute measurement mode, execution of the AutoZ function to update the AutoZ value
(
P
offset,A
), is initiated by the operator.
AutoZ in gauge measurement mode, dynamic compensation for atmospheric pressure
PPCH Q-RPTs are intrinsically absolute but they can also be used in gauge measurement
mode (difference from atmosphere) (see Section 3.3.3,
PRINCIPLE). Gauge
measurement mode is achieved by subtracting the value of atmospheric pressure from the
Q-RPT’s absolute reading using AutoZ and by dynamically compensating for changes in
atmospheric pressure between opportunities for AutoZ to execute (see Section 3.2.4).
In gauge measurement mode, the value of
P
std,0
is always zero gauge pressure. Zero gauge
pressure, by definition, is applied to the Q-RPT when it is vented to atmosphere. In gauge
measurement mode, the AutoZ routine is run automatically whenever the PPCH is in the
vented condition. This assures regular automated updating of the
P
offset,G
value
corresponding to atmospheric pressure. Gauge pressure is the measured absolute pressure,
P
u
, minus
P
offset,G
.
P
gauge
= P
u
- P
offset,G
When AutoZ is ON, dynamic compensation for atmospheric pressure is also applied to
compensate for changes in atmospheric pressure between AutoZ updates (see Section 3.2.4).
The measured gauge pressure is calculated using
Δ
P
atm
to correct the value of
P
offset.G
.
P
gauge
= P
u
- P
offset.G
-
Δ
P
atm
