DH Instruments PPC3 - Gauge and Negative Gauge Modes with an Axxx (Absolute) Q-Rpt, Dynamic Compensation for Atmospheric Pressure; Figure 9. Ready;Not Ready in Static Control Mode

PPC3™ OPERATION AND MAINTENANCE MANUAL
© 2002-2007 DH Instruments, a Fluke Company Page 22
Static Control Ready/Not Ready
With static pressure control active a Ready condition occurs whenever:
• NO control valve is operating.
• The current measured pressure is at the target value within the hold limit.
• The rate of change of pressure is less than the current stability limit.
Figure 9. 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
3.4.6.1).
3.2.3 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 3.3.3,  PRINCIPLE).
Gauge measurement mode is achieved by subtracting the value of atmospheric pressure,
P
offset,G
, from the Q-RPT’s absolute reading using AutoZ (see Section 3.5.1). In gauge or
negative gauge measurement mode, the AutoZ routine that measures P
offset,G
, is run
automatically whenever the PPC3 is in the vented condition. This assures 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. PPC3 uses
dynamic compensation for atmospheric pressure to correct for these changes in
atmospheric pressure. When AutoZ executes, and P
offset,G
is determined, the reading of
PPC3’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