Leak
Test Comparison: Carrier Gas Mode
vs. Pressure Decay
Advantages of the
new T‑Guard™ Leak Detection Sensor
There are several
methods of leak testing. In this article,
we compare (differential) pressure decay,
a fairly widespread method, with a new integral helium testing method
at atmospheric pressure in carrier gas mode.
The pressure decay
method is easy to understand and to carry out, with relatively reliable
results. However, there are recognized shortcomings with respect to
temperature, size, humidity, surface and flexibility of the parts being
tested:
- Temperature
variation: As pressure is closely related
to temperature, every variation in temperature
will simulate a leak or suppress a true leak.
- Test
object size: It is a well-known fact that a small leak in
a small part will result in a higher pressure difference than a small
leak in a big part. That’s why pressure decay is limited to parts
well below 5 liters (1.3 gallons) in volume.. But even with very small
parts, the tested leak rate cannot be below 1x10-3 mbar l/s. With
a part of 1 liter inner volume, the tested leak rate might not
even be in the 10-1 mbar l/s range, making testing for water tightness
unfeasible.
- Humidity:
Humidity is problematic for pressure decay because every fluid
generates a constant pressure of its own (vapor pressure), also influencing
temperature.
- Test
speed: The (inner) surface of the test
object influences the measurement speed
because it changes the time needed for
the internal pressure to stabilize.
- Flexible
parts: These cannot be tested at all
because flexible parts try to maintain
a constant internal pressure.
One of the two
automatic measurement modes in the new
INFICON T‑Guard Leak Detection Sensor
features a carrier gas mode, which is
outlined below. The test object is pressurized
with (diluted) helium, which will escape
through any leak. By putting the test
object in a chamber and generating a
constant gas stream around the object,
T‑Guard can detect the smallest amounts
of helium in the gas stream. Through intelligent design and enhanced
software, the T‑Guard Leak Detection Sensor is able to identify the
total leak rate of the test object. The test specifications include:
- Leak detection
in the range of 10-5 mbar l/s in less
than one minute for small parts and chambers
- Leak
detection in the range of 10-3 mbar l/s
in less than one minute for larger parts
with a volume of several liters (not
possible with pressure decay)
Leak
detection measurement with T‑Guard Leak Detection
Sensor provides the following advantages:
- Temperature
independence
- Scalability to
any size, together with the leak rate
- Unaffected by
humidity
- Inner surface
of the test object is irrelevant
- Flexible objects
can be tested
- Higher
sensitivity (100x more sensitive than
differential pressure decay)
- No
maintenance needed for the measurement
device
How the Carrier
Gas Method Works
In carrier gas mode,
a suitably sized stream of air passes
around the test object. This stream transports
helium from any given leak to the T‑Guard™
Leak Detection Sensor, which measures the helium concentration in the gas stream
and reports the actual leak rate to the user.
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