FCO770 Users Guide Issue 1 Page 55
PREFILL PRESSURE
• This defines the nominal pressure for the reference volume and is used to compensate
the test pressure for variations in the applied prefill pressure.
• The prefill pressure is used in conjunction with the prefill tolerance for tolerance
checking and as the setpoint when an automatic regulator is fitted.
• A prefill output is provided to connect the pressure source to the reference volume.
This is active during standby, and turned off after the jig delay and prefill timers.
Traditionally, the jig or vent output is used to isolate the pressure source from the
reference volume, but the FCO770’s prefill output provides a more flexible solution.
PREFILL TOLERANCE
• This is used to ensure that the prefill pressure is valid for the test.
• At standby, if the pressure is out of tolerance then test can not be started. This check
may be disabled on a test by test basis for situations where the pressure is not applied
at standby, e.g. vacuum tests with air saving.
• A tolerance check is also made just before the pressure dump occurs at the fill stage.
If the pressure is out of tolerance here then a Pressure Gross result is given.
• Set to 100% to completely disable the tolerance check.
PREFILL TIME
• This is normally only used for vacuum tests with air saving to allow time for the
reference volume to be filled. It is usual for positive pressure tests to fill the reference
volume in standby so no additional prefill time is required.
STAB. PREFILL TIME
• The test requires a small period of time for the isolation valve to operate before the fill
stage is started. Systems with long pipes or slow valves may need more time for the
valve to operate; other systems may require time for the pressure to stabilise after
isolation.
• There is a preset minimum delay of 0.2s after the prefill output is turned off. The
delay may be increased with the Stab Prefill Timer. Note that the delay will be the
greater of 0.2s and the stab prefill time.
TEST PRESSURE
• Set the nominal test pressure which will depend upon the reference pressure,
reference volume, and test chamber volume. This is used in conjunction with the
pressure tolerance setting to check for gross leaks. Note that the test pressure reading
is compensated for variations in actual prefill pressure from nominal prefill pressure
to achieve a more sensitive test. The test pressure displayed is not the actual pressure,
but the compensated pressure.
PRESSURE TOLERANCE
• Sets the pressure tolerance for the level at which gross fail will be indicated during
stabilisation.
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