5. MAINTENANCE, ADJUSTMENTS AND CALIBRATION
Page 159
© 2009 DH Instruments, a Fluke Company
5.11
LEAK CHECKING
Leaks within the PPCH pressure controller/calibrator or in the test system connected to it can prevent
PPCH from being able to properly set and control pressure. If PPCH operational symptoms indicate the
possibility of a leak, the test system connected to the PPCH should first be leak checked. If no leak is
identified, the PPCH itself should be leak checked separately independently from the test system.
Large leaks in a visible component can usually be identified by an accumulation of test fluid. When
looking for a leak, first examine the test system vessels and interconnect points for any accumulation of
fluid. If fluid is observed, wipe it away and check if it returns while the PPCH is pressurized. Any leak
large enough to cause visible accumulation of fluid in a short time is likely to cause the PPCH to be
unable to operate reliably.
Other than the accumulation of fluid method, the only way to determine if a significant leak is present is
by a pressure decay method. This can be performed using the PPCH to set a pressure and then using
the internal Q-RPT or utility sensor to measure decay. However, to arrive at conclusive results, the leak
test must be performed very carefully. The pressure must be truly stabilized before starting the
measurement and the measurement must be made over a substantial period of time.
Section 5.11.1 describes the pressure decay leak testing method. Section 5.11.2 describes procedures
to localize a leak within the PPCH.
5.11.1
PRESSURE DECAY LEAK TESTING METHOD
PROCEDURE
To determine whether a leak is present in a system by measuring the rate of pressure decay,
a pressure is set, allowed to stabilize and then the pressure change over time is observed.
PRINCIPLE
To measure a leak by the rate of pressure decay method, proceed as follows:
To leak test a system external to the PPCH, connect the system to the PPCH
TEST
port
and purge and prime it (see Section 2.4.4). If there is a PG7302 piston gauge in the
system, it must be isolated from the system as the leak through it’s piston-cylinder can be
confused with a leak in the system.
To leak test the PPCH itself, plug the
TEST
port using a DH500 plug (provided in PPCH
interconnections kit). It is not possible to establish conclusively that the PPCH is leaking
unless no system is connected to the
TEST
port and the port is plugged
Use
[SETUP]
,
<6control>
,
<2mode>
,
<2static>
to set the PPCH to operate in static
control mode (see Section 3.4.6.2). This mode is used because it turns off the thermal
pressure control unit (TPCU) whose action might interfere with the pressure decay
measurement (see Section 3.2.1).
If the PPCH has a Lo Q-RPT, use
[RANGE]
to select the Lo Q-RPT as the active Q-RPT.
Use
[ENTER/SET P]
(see Section 3.3.10.2) or the direct pressure control keys (see
Section 3.1.3) to set a pressure of at least 90% of the active Q-RPT or utility sensor
maximum pressure or of the maximum pressure of the test system connected to the
PPCH.
Once the desired pressure has been set, press
[ESC]
to abort pressure control. This
stop assures PPCH will not attempt to control pressure and interfere with the pressure
decay measurement.
Wait for the effects of the just completed pressure change to dissipate. This requires at
least 5 minutes and may be longer if there is a large test volume connected.