3. OPERATION
Page 69
© 2007 DH Instruments, a Fluke Company
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-G 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-G 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
In gauge measurement mode, execution of the AutoZ function to update the value of
P
atm,0
,
occurs automatically whenever PPCH-G is vented. Updating
Δ
P
atm
occurs automatically
continuously during unvented operation.
Take care to consider fluid heads in gauge mode operation. The fluid head when vented may
be different from the fluid head at other pressures due to the height of the reservoir to which the
Q-RPT is connected when vented (see Section 3.3.7,
PRINCIPLE).
AutoZ ON/OFF
The AutoZ function can be turned ON and OFF, separately for gauge and absolute
measurement modes. Table 9 summarizes the effect of AutoZ ON and OFF for PPCH-G
Q-RPTs in absolute and gauge measurement modes.
Table 9.
AutoZ ON and OFF
MEASUREMENT
MODE
AutoZ
STATUS
P
offset
APPLIED
Δ
P
atm
APPLIED
AutoZ
ROUTINE
RUNS
ON YES
When initiated
by operator
Absolute
OFF NO
Not Applicable
Not available
ON YES
Gauge
OFF
YES
NO
Automatically
when PPCH-G
in VENT
condition
Recommendations for the Use of the AutoZ Function
The AutoZ function provides a powerful and easy to use tool for improving the stability over
time of PPCH-G internal and external Q-RPTs and maximizing the recalibration interval by
compensating for change in zero between full recalibrations. The following simple
recommendations will help assure that you use this feature to best advantage.
Содержание PPCH-G
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