PPCH™ OPERATION AND MAINTENANCE MANUAL
© 2009 DH Instruments, a Fluke Company
Page 26
admitted to the test system from the hydropneumatic pump to the test system. An outlet
valve(11) of the same type as the inlet valve precisely controls liquid exhaust.
The inlet and exhaust valve can adjust and maintain pressure rates of change very precisely.
When conditions are appropriate, a second stage of pressure control is used to maintain a
target pressure value within very narrow hold limits in dynamic control mode. The second
stage is thermal pressure control (TPC). TPC, uses a pressurized volume in which there is a
heating element and around which there is a forced air cooling system(14). Heating and
cooling the fluid in the volume causes it to expand or contract, increasing or decreasing
pressure. This pressure control system allows extremely fine pressure control without
hysteresis or wear of mechanical parts.
The TPC system response is dependent on the operating liquid and the volume connected to
the
TEST
port. It is minimally effective when there are leaks in the test system or the test
volume is large. PPCH automatically determines whether conditions are appropriate to use
TPC and runs an automated routine to determine the test volume. This routine is initiated
automatically when needed.
