INFICON T-Guard Leak Detection Sensor - Manual (Page 47)

Operation
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Tech
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Notice If the trigger value is higher than 1x10
-3
mbar l/s per litre of the net volume,
both test samples will be measured in GROSS-mode. Then you must use
outside air to purge the chamber. The helium background on the reference
line must be really stable for GROSS-mode. If this is not possible, a bigger
chamber or diluted helium has to be used to measure in FINE-mode.
Notice If the T-Guard™stops the measurement due to a too high signal in G ROSS
mode, it reports a leakage rate of a hundred times the highest trigger level.
If it stops in F
INE mode for the same reason, it reports five times the highest
trigger level.
In both cases, the PLC output "GROSSLEAK" is active.
After each accumulation measurement and waiting for a certain time, the chamber
will be purged. You can generally turn this automatic purge process off; it must then
be initiated with a control command every time.
5.3.1.1 Hints for a good measurement:
• Use fans inside the chamber to have a perfect mixture of helium and air
everywhere inside the chamber! The gas flow of the fans should be at least once
the chamber volume per second!
• Purge the test chamber before measurement!
An insufficient chamber purge is indicated with the warning W45.
• For best and repeatable results you must apply outside air to the reference inlet.
In order to have a more stable background on the reference inlet, put it loosely
into a tightly closed buffer volume. This buffer volume reduces the helium
background variations.
• Apply external air on the measurement line during the purge time in order to clean
it.
• You should purge the chamber after every measurement, then purge the
measurement line for at least 4 s, then wait for 2 s before performing the next
measurement. This removes the residual helium, which could affect the following
measurements, from the measurement system.
• The chamber must have neither slots nor holes. It must be sealed with rubber
gaskets. Only then are the measurements free from external influences.
Fig. 6-8 Display after finished measurement in Accumulation mode