Leak detection, in which the value of the leakage
rate is generally nonquantifiable, and leak measurement, in which techniques are used to
record the rate of leakage.
The common feature of methods used to detect leaks is that they are almost always
operator-dependent, require strict supervision and are often very messy. The methods
listed below will indicate a leak condition, but they will not accurately quantify the
degree of leakage.
Observation. Components are
filled with pressurised gas and immersed in a liquid. Typically they are filled with air
and immersed in water, but nitrogen under acetone is also used. The most common
observation technique is to look for a bubble stream. A variation of the bubble-stream
method is to use sealed components submerged in a liquid within an enclosed volume; a
vacuum is created at the surface of the liquid to draw the fluid into the closed device.
This method is often used to soak-test parts over a prolonged period and then to visually
check for condensation inside transparent parts. It is also used to check the weight of
parts to ensure that product, that is, the powder, gas or liquid inside the device, has
not been forced out and vapour/liquid has not been forced inside.
Chemical trace. Chemicals are
added to the working media within an assembly prior to some form of functional test. If
they leak onto the surface of the component, they can be readily seen when viewed using
Chemical penetration. Chemical
is sprayed onto one side of a component and by capillary action emerges on the opposite
surface. This technique is also referred to as dye penetration. The difference between
chemical trace and dye penetration is that the former is a trace substance in the fluid
whereas the latter is coated onto the surface of the container.
Gas sniffing. Components or
assemblies are filled or injected with an easily identifiable gas to create a pressure
differential. Helium, hydrogen and sulphur hexafluoride (SF6) gases are employed and an
operator searches for concentrations of gas at the surface of the piece being tested.
Helium is the most commonly used.SF6 is generally avoided whenever possible because of its
toxicity, but it is mandatory for some electrical equipment because of its
flame-suppression properties. For sealed assemblies such as blister packs or swallowable
cameras, the item must be closed in an environment containing the tracer gas.
Ultrasonic testing. Because they
are noisy, large gas flows can be found using an ultrasonic detector. This method is
suited to finding large leaks, but it is not recommended for the fine leaks in a
production environment. When using ultrasonic testing, sources of misleading indications
and other problems can include low test pressures and capillary action sealing,
high-pressure situations and component damage, contaminated liquids, and preferential
leeching and rogue surface tensions.
Leak measurement attempts to quantify the flow of the fluid over a period of time.
There are three commonly used fundamental techniques for leak measurement: a pressure
change within a known volume, a direct measurement of flow, or measurements of changes in
Absolute measurement. A
transducer or gauge is used to measure pressure change within the device being tested. In
general, this technique is difficult to apply because small changes in pressure are being
measured. The sensitivity of the transducer as a percentage of the full scale becomes an
issue because the pressure differences can be extremely small even when trying to detect a
Reference comparison or differential pressure
technique. This technique involves locating the test and reference
volumes either side of a transducer diaphragm. Ideally, these volumes should have similar
pneumatic characteristics, and they are arranged to be as stable as possible and at the
same pressure. Subsequent time-related changes in the pressure of the test volume are used
to measure the value of any leakage. A differential pressure transducer is placed across
the two volumes. The sensitivity of the transducer is not related to the actual pressure
in absolute or gauge pressure terms, but to the sensitivity of the differential pressure
transducer at the test pressure. For example, it is possible to detect 0.0005% of the test
pressure using this technique by testing at 1 bar gauge (2 bar absolute).This may be 100
times more sensitive than using the absolute measurement technique mentioned above.
Differential pressure component dosing.
This technique involves investigating sealed components. In this technique, twin
tandem pressurization and reference volumes are used together with the sequential
application of a differential pressure transducer. It is designed to identify gross leaks
in sealed parts. The differential pressure technique alone will not differentiate between
a good part and a gross leaking part; dosing is used with the differential pressure
technique to detect the gross leaking parts.
|A leak testing facility using pressure decay dosing technique
Inter-stream testing. Two test
pressures and three interrelated differential pressure transducers are used to
simultaneously test two adjacent volumes. This technique is used when there are two fluid
volumes in a single part, which could leak to the outside or between each other.
Controlled pressure. This
involves creating a prescribed pressure within the product being tested and then measuring
the gas flow that is needed to maintain that pressure using a flow measurement transducer.
Free flow.This technique
introduces a flow-measurement device into the gas stream.
Gas tracer systems have the ability to detect smaller leakages, but tend to require
higher capital investment.
Helium. This requires enclosing
the test piece within a sealed chamber and evacuating the chamber and/or test piece. After
charging one or the other with helium, the gas loss from the respective volume is
monitored with a mass spectrometer that detects the helium.
Hydrogen. This gas is used in
the same way as helium. It is also used in a technique that employs a sensor to detect and
measure levels of, in this case, hydrogen in a continuous ambient or low pressure gas
flow, which allows the use of lower cost enclosures and ducting. The test gas is a mixture
of 95% nitrogen and 5% hydrogen, which is nonexplosive and relatively inexpensive.
Gas bombing. This involves
exposing a sealed component to a gas-charged environment and subsequent transfer to a test
chamber that is monitored by gas sensing instrumentation. It is essential that the
possibility of a gross leak is eliminated before fine leak measurement is attempted.
Various halogens can be used in all of the above techniques, but they are avoided if
possible. In addition to toxicity, they are heavy gases that tend to accumulate readily in
low recesses. As a result, contamination occurs and the test area is difficult to clear;
this is made worse by the tendency of these gases to adhere to surfaces.All gases will
stick to the tooling surface to some degree, most particularly when flooded from a leaking
component. However, hydrogen and helium are lighter and they are more easily removed or
dispersed by extraction, flushing or natural diffusion.
There are a variety of leak testing techniques in use today.When considering
an item requiring leak testing it is important to ascertain first
whether detection of the leak or measurement of the leakage rate
is more important. Quantifying the leakage rate will dictate the
leak measurement technique that is required.