DH Instruments PPCK PLUS, Operation And Maintenance Manual

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PPCK+™ Operation and Maintenance Manual
©2001 DH Instruments, Inc .
Page 20
3.1.2.3 PRESSURE CONTROL OPERATING PRINCIPLES
Numerical references in this section refer to Figure 5.
PPCK+ is designed to precisely measures and control gas pressures into a volume
connected to its TEST port (14). Pressure is measured by a Reference Pressure
Transducer (RPT)(17).
Pressure is controlled in two stages.
The first stage of pressure control is valve control. Valve control is used for all
aspects of pressure control except the final stage of dynamic pressure control in
which pressure is maintained at the target value (see Section 3.1.2.4). Valve control
is accomplished by high pressure inlet and exhaust valves (3, 11). The HP inlet
valve (3) controls the admission of pressure from the SUPPLY port (1). The HP
exhaust valve (11) controls the exhaust of pressure out of the VENT port (13).
The HP inlet and HP exhaust valves are dome-loaded needle valves. The valve is
normally closed by a Belleville spring. The valve is opened and its needle position is
controlled by the application of drive pressure onto the dome in opposing force to the
Belleville spring. The drive pressure is supplied from the DRIVE IN port (6) and
controlled by the drive air solenoid valves (2, 4, 10, 12).
When a command is given to set a pressure, the high pressure inlet (3) or
exhaust (11) valve must be opened to admit or exhaust pressure. The opening
procedure takes 5 to 10 seconds during which PPCK+ may appear to be idle.
To notify the operator that the valve opening procedure is in process, the main run
screen pressure control characters FLASH
<VLV> during the valve opening routine
(see Section 3.1.1).
The second stage of pressure control is thermal pressure control (TPC). TPC is
used in the final stage of dynamic pressure control when very fine control is needed
to maintain pressure at the target value (see Section 3.1.2.4). TPC, uses a
pressurized volume in which there is a heating element and around which there is a
forced air cooling system (16). Heating and cooling the gas in the volume causes it
to expand or contract, increasing or decreasing pressure. This patented, pressure
control system allows extremely fine pressure control without hysteresis or wear of
mechanical parts.
The TPC system response is dependent on the volume connected to the TEST port.
Whenever TPC response indicates that the test volume has changed, PPCK+ runs
an automated routine to determine the new test volume. This routine is initiated
automatically when needed. The execution of the volume determination routine may
occur occasionally while TPC is active in dynamic control mode. This normal
behavior appears as a delay in pressure control during which the pressure may vary
by up to 700 kPa (100 psi). The volume determination configuration can also be
launched by the user (see Section 3.4.4.1).