DH Instruments PPCH-G, Operation And Maintenance Manual

Page: 39 / 160

3. OPERATION
Page 29 © 2007 DH Instruments, a Fluke Company
3.2.3 PRESSURE READY/NOT READY
There is a Ready/Not Ready indication LED on the PPCH-G front panel. Ready/Not Ready
status is also available remotely. This indication is intended to provide the operator with a
clear and objective criterion for determining when a valid pressure measurement can be made.
The Ready/Not Ready LED indications are:
<Green, no flash>
Pressure Ready , control idle. PPCH-G is NOT actively controlling
the pressure. This indicates no control is occurring and the pressure
stability is within the stability limit.
<Green, slow flash>
Pressure Ready , control active. PPCH-G is actively controlling the
pressure and the criteria for a Ready condition to occur in the active
control mode have been met.
<Red >
Pressure Not Ready . Either PPCH-G is idle and pressure does not
meet the stability limit or PPCH-G is actively controlling the pressure
and
Ready conditions have not yet been met. The control
characters in the middle of the second line of the MAIN RUN screen
flash if control is active (see Section 3.1.1).
When pressure control is idle (measure mode):
A Ready condition occurs any time the
pressure rate of change is inside the stability limit. The stability limit is defined in terms of
rate of change of pressure in current pressure units per second. The stability limit is set by
default (see Table 8) and can be customized (see Section 3.4.6).
When pressure control is active:
The criteria for determining the Ready/Not Ready
condition depend on whether the current control mode is static, dynamic, monotonic or ramp
(see Sections 3.2.2.2, 3.2.2.3, 3.2.2.4, 3.2.2.5). Pressure
Ready/Not Ready parameters are
set by default when AutoRange is used, or a control mode is selected (see Table 8). The
parameters can be customized if desired (see Section 3.4.6).
3.2.4 GAUGE MEASUREMENT MODE, DYNAMIC
COMPENSATION FOR ATMOSPHERIC PRESSURE
PPCH-G utility sensors and Q-RPTs are intrinsically absolute but they are also 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,
P
offset,G
, from
the Q-RPT’s absolute reading using AutoZ (see Section 3.5.1). In gauge measurement
mode, the AutoZ routine that measures
P
offset,G
, is run automatically whenever the PPCH-G 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. PPCH-G
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
PPCH-G’s on board barometer,
P
atm,0
, is also recorded. Later, when no longer vented, the
change in atmospheric pressure,
Δ P
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
:
Δ P
atm
= P
atm
- P
atm,0
Dynamic compensation for atmospheric pressure uses
Δ P
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
- Δ P
atm