PPCH™ OPERATION AND MAINTENANCE MANUAL
© 2009 DH Instruments, a Fluke Company
Page 34
3.2.3
PRESSURE READY/NOT READY
There is a
Ready/Not Ready
indication LED on the PPCH 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 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 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 is idle and pressure does not meet
the stability limit or PPCH 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 automated 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 10) 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 10).
The parameters can be customized if desired (see Section 3.4.6).
3.2.4
GAUGE MEASUREMENT MODE, DYNAMIC
COMPENSATION FOR ATMOSPHERIC PRESSURE
PPCH 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 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 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’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
