DH Instruments PPC4, Operation And Maintenance Manual

Page: 33 / 228

3. OPERATING PRINCIPLES
Page 21 © 2009 DH Instruments, a Fluke Company
Figure 7. Ready/Not Ready in Static Control Mode
Ready/Not Ready When Not Actively Controlling
When NO automated pressure control is active, Ready is indicated whenever the rate of change
of pressure is less than the current stability limit (see Section 4.4.5.1 for Basic Interface and
Section 5.7.8.2.1 for Advanced Interface).
3.2.4 Gauge and Negative Gauge Modes with an Axxx
(Absolute) Q-RPT, Dynamic Compensation for
Atmospheric Pressure
Q-RPTs with the designation Axxx are intrinsically absolute but they are also 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,
P
offset,G
, from
the Q-RPT’s absolute reading using AutoZero (AutoZ, see Section 3.2.22). In gauge or
negative gauge measurement mode, the AutoZ routine that measures
P
offset,G
, is run
automatically whenever the PPC4 is in the vented condition. This ensures the continuous
automated updating of the
P
offset,G
value corresponding to atmospheric pressure. Gauge pressure
is the measured absolute pressure, P
u
, minus the atmospheric offset.
P
gauge
= P
u
- P
offset,G
However, atmospheric pressure can change between opportunities to run AutoZ and update
the value of
P
offset,G
, for example when running an extended test without venting. PPC4 uses
dynamic compensation of atmospheric pressure to correct for these changes in
atmospheric pressure. When AutoZ executes, and P
offset,G
is determined, the reading of
PPC4’s on board barometer, P
atm,0
, is also recorded. Later, when no longer vented, the
change in atmospheric pressure,
ΔP
atm
since P
offset,G
was updated is the difference between
the current barometer reading, P
atm
, and the barometer reading at the time of AutoZ
execution, P
atm,0
:
ΔP
atm
= P
atm
- P
atm,0
Dynamic compensation for atmospheric pressure uses
ΔP
atm
to correct the value of P
offset,G
,
thus always compensating real time for changes in atmospheric pressure:
P
gauge
= P
u
- P
offset,G
-
ΔP
atm
Gauge pressure measurement on an Axxx (absolute) Q-RPT allows instantaneous switching
between gauge and absolute measurements modes. Any additional uncertainty in gauge
pressure mode due to the dynamic compensation for atmospheric pressure technique is a