Featured in Medical Device Technology
May 2002 Leak testing
can be separated into two fundamental disciplines.
down to the fundamentals of leak detection & measurement
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
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 ultraviolet light.
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.
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 concentration.
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 large leak.
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.
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
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.
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.
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.
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