leak
testing
Getting
down to the fundamentals of leak detection and measurement
Featured
in Medical Device Technology May 2002
Leak
testing can be separated into two fundamental disciplines.
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.
Leak
detection
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
ultraviolet light.
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
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.
Pressure
measurement
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.
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.
Continuous
flow
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
trace
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.
Summary.
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
