DECAY LEAK TESTING:
There are many different methods to measure air pressure decay.
The basic principle is simple, if the component under test
leaks, the air pressure is going to drop if you stop pressurizing
There is a variation of the same principle that uses vacuum
air extraction instead of air pressure, but the principle
is the same: the relative vacuum drops if it leaks.
Air pressure decay is widely used in the industry as a simple,
unexpensive and clean way to test for leaks on a production
An air pressure decay tester can also test for a specification
expressed in air flow at a given pressure in standard conditions
of atmospheric pressure and temperature , like SCCM
(standard cubic centimeters per minute). In that case , the
pressure decay instrument performs a simple calculation to
convert a pressure drop into a flow reading,
using the volume of the component under test. That is the
main flaw of pressure decay technology: it is volume dependent,
because the same leak will create a larger pressure drop if
there is less volume.
Another flaw is its sensitivity to air temperature changes
and to movements of the test fixture. Boyle 's law PV=nRT
makes that any change in temperature in a closed volume will
a corresponding change in pressure. Equally a volume change
even minute will create a pressurue change. Air mass flow
leak testing is also affected by temperature and volume change,
don't listen to unscrupulous individuals who claim that flow
testing technology is immune to temperature or volume changes.
Wether you measure air flow or air pressure changes,
Boyle's law PV=nRT still applies and affetcs the instruments
If the principle is that simple, why can't I buy a gauge,
some valves and make my own? If you have a lot of time for
testing, if you do not need a conversion to flow units, if
your part is a ambient temperature,if your own developement
time is free and if you don't mind spending a great lot of
time servicing weak components in your contraption, that's
probably the best solution for you.
Otherwise, ATEQ sells a basic dependable pressure decay instrument
at US$2990 , designed by people who know leak testing, and
a lot more dependable and sophisticated instruments,
designed for the real world of production line manufacturing,
where automation, test speed, electromagnetic immunity and
dependability are paramount.
There are various ways to make a pressure decay leak tester
1) keep the internal volume of the instrument small
2) use sensitive pressure sensors whose range is adapted to
the pressure changes you are measuring
3) learn the adiabatic stabilisation of a part under test
4) balance the adiabatic effects as much as possible
5) fill fast
6) no volume changes
It sounds obvious when you read that, but a lot of commercial
"industrial" pressure decay leak testers do not meet those
6 basic rules
There are various ways to make a leak tester more dependable.
1)Use dependable valves that won't leak after a lot of cycles.
2)Do not use heat generating components (solenoid valves or
other)in the tester
3)Use sensors that resist to overpressures and of the adequate
range for what you are measuring
4)Balance the temperature effects as much as possible
5) Use temperature compensation features when necessary
6) Design your electronics to be insensitive to electromagnetic
It sounds obvious but a lot of commercial "industrial" leak
testers use of the shelf valves like you would if you made
your own. Some pretend they can read
incredibly small changes in pressure , their "resolution",
in a dependable way out of a sensor that measures tens of
PSI. Some propose architectures with
sensors left unprotected outside a box with wires hanging
like antenas, like a student's science project.Those are not
real industrial testers, just industrial looking.
Too many unscrupulous companies sell the "bells and whistles",
the color screen, the curves, the key chain, the computer
features. They try to make you forget what is the real purpose
of buying an industrial air pressure decay leak tester: fast
and clean measurement, durability.
Fast repairs and support, you have to consider the proximity
of repair and calibration service offices, the proximity and
qualification of applications engineers,
, the certifications (ISO,CE), the internationnal reputation..
ATEQ uses 2 basic configurations for pressure/vacuum decay
leak testing . They all have advantages and disadvantages
depending on applications.