3. OPERATION
Page 23 © 2002-2007 DH Instruments, a Fluke Company
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
function of the resolution and short term stability of the on-board barometer, not its absolute
measurement uncertainty. This additional uncertainty is ± 1 Pa (0.00015 psi).
3.2.4 MULTIPLE INTERNAL AND EXTERNAL Q-RPTS
A base PPC3 pressure controller includes a high precision, utility pressure sensor whose
range is equal to the controller’s range. The utility sensor is not intended to serve as a
pressure reference for low uncertainty measurement. It is for indication, pressure control,
system safety and maintenance functions only.
To obtain low uncertainty, traceable pressure measurement, PPC3 uses Quartz Reference
Pressure Transducers (Q-RPT). Up to six Q-RPTs can be included in a PPC3 pressure
controller/calibrator system. One or two Q-RPTs can be built into the PPC3, up to four may be
located outside the PPC3 enclosure, in RPM4 reference pressure monitors (one or two per
RPM4). Once the Q-RPTs available to a PPC3 system have been set up and identified, their
use is managed by PPC3 transparently to the operator.
The use of multiple Q-RPTs, combined with PPC3’s infinite ranging capability and
AutoRange feature (see Section 3.3.4), make it possible for a single PPC3 pressure
controller/calibrator system to cover an extremely wide range of test ranges and
measurement modes with low uncertainty in each range.
Communication between RPM4s and a PPC3 controller is by daisy chained RS232
connections from PPC3’s COM2 port. One common pressure connection can be made
between the PPC3 and RPM4 pressure ports (see Section 2.3.7 for information on setting up
a PPC3 with external RPM4s). RPM4 Q-RPT modules’ SDS function protects RPM4
Q-RPTs from overpressure when they are not in use.
The PPC3 RPT function (see Section 3.3.5) is used to identify Q-RPTs available to the
PPC3. The PPC3 then manages the internal and external Q-RPTs transparently to the
operator, selecting the appropriate Q-RPT for the range of operation and operating valves to
connect and disconnect them as needed.
Each Q-RPT has a default range, which is its maximum range. It can also be downranged
using PPC3’s AutoRange function.
The PPC3 internal pneumatic layout for handling its utility sensor and one or two Q-RPTs
depends on the number of Q-RPTs and their type (Axxx absolute, Gxxx gauge, BGxxx bi-
directional gauge). Figure 13 provides pneumatic schematics of the different PPC3 internal
utility sensor and Q-RPT configurations with a chart of valve status for various operating conditions.
Before operating PPC3 with external Q-RPTs, check carefully that the RPM4’s Q-RPT
TEST(+) port(s) is/are connected to the PPC3 TEST(+) port. If an external Q-RPT is NOT
connected to the PPC3 TEST(+) port, it will NOT measure the pressure generated by PPC3 and
PPC3 may reach a pressure higher than the target pressure before its watchdog function
recognizes that the external Q-RPT is not connected (see Section 3.4.4.1).
The PPC3 watchdog function monitors differences in the change in pressure indicated by
an active, external Q-RPT and the PPC3 Hi utility sensor or Q-RPT (watchdog). If the
difference becomes excessive, control is aborted, an audible warning sounds and a
warning is displayed. If this occurs, make sure the PPC3 and RPM4 Q-RPT TEST(+) ports
are connected together and try again.
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