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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