down to the fundamentals - as featured in MDT Magazine.
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.
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
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
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.
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.
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
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.
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 other.
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.
technique introduces a flow-measurement device into the
Gas tracer systems have the ability to detect smaller leakages,
but tend to require higher capital investment.
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.
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.
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
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