3. OPERATING PRINCIPLES
Page 39
© 2009 DH Instruments, a Fluke Company
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. For higher range Q-RPTs, this is easily accomplished with a variety of digital
barometers or with a lower range Q-RPT in the PPC4 system. For lower range Q-RPTs, a piston
gauge or other standard may be required to provide
P
std,0
with low enough uncertainty.
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 and negative gauge measurement modes with an Axxx Q-RPT,
dynamic compensation for atmospheric pressure
Q-RPTs with the designation Axxx are intrinsically absolute but they can also be used in gauge and
negative gauge measurement modes (difference from atmosphere) (see Section 3.2.11). 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 modes on an Axxx Q-RPT, 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 modes, the AutoZ routine is run automatically whenever
the PPC4 is in the vented condition. This ensures 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 3.2.3).
The measured gauge pressure is calculated using
ΔP
atm
to correct the value of
P
offset.G
.
P
gauge
= P
u
- P
offset.G
-
ΔP
atm
In gauge measurement mode with an Axxx RPT, execution of the AutoZ function to update
the value of
P
atm,0
, occurs automatically whenever PPC4 is vented. Updating
ΔP
atm
occurs
automatically continuously during unvented operation.
AutoZ in gauge measurement mode with a Gxxx or BGxxx Q-RPT
Q-RPTs with the designation Gxxx or BGxxx are intrinsically gauge. They have an
atmospheric reference port and measure gauge pressure (difference from atmosphere).
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 PPC4 is in the
vented condition. This ensures the regular automated updating of the
P
offset,G
value.
When using a Gxxx Q-RPT in gauge measurement mode, with AutoZ ON, gauge pressure is
calculated as:
P
gauge
= P
u
- P
offset,G
AutoZ ON/OFF
The AutoZ function can be turned ON and OFF, separately for gauge and absolute
measurement modes, see Table 7.
AutoZ ON and OFF
summarizes the effect of AutoZ ON
and OFF for different Q-RPT types in absolute and gauge measurement modes.
Summary of Contents for PPC4
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Page 194: ...PPC4 OPERATION AND MAINTENANCE MANUAL 2009 DH Instruments a Fluke Company Page 182 Notes...
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