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Background
information
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Automatic leak testing with
air |
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The various
air leak test methods |
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In automatic leak
tests with air, the workpiece to be tested is subjected to a pressure
difference and it is established whether air is escaping. The escaping air
however, cannot be directly measured, only its effects, i.e. either the
pressure variation which results from air leaving the workpiece - which is
detached from the compressed air source (pressure measurement), or the
quantity of air continuing to flow into the workpiece - which is connected
to the compressed air source (flow rate measurement). There are different
variants for both pressure and flow rate measurement.
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Pressure
Measurement
In industrial leak testing, pressure measurement is the most frequently used
method. With smaller test chambers, leaks can be detected from 0,1 cm³/min.
The relative pressure or absolute pressure method allows a
compact construction and the minimum possible volume of the measuring system
itself. It is also distinguished by high operational reliability and a wide
measuring range. The test signal definition depends on the rest pressure
used.
In the differential pressure method at higher test
pressures, a greater accuracy can be obtained than with the absolute
pressure method, as the test signal definition is independent of the test
pressure.
With measurement of the pressure drop with
overpressurisation of the test piece the usual operating conditions are
simulated.
In measurement of the pressure rise in the partial pressure
test method the disturbing influences due to temperature variations or
volumetric instability of the sealing device or of the workpiece are smaller
than in measurement of the pressure drop. Measurement of the pressure rise
in the excess pressure method (capsule method) manages without a levelling
stage. Moreover, in this case the test pressure used is not limited by the
measuring range of the measuring element, as it is not subjected to the test
pressure.
Flow Rate Measurement
Whereas in pressure measurement, the test signal becomes smaller with
increasing test volume, flow rate measurement delivers a test signal
independent of the quantity of the test volume. This is an advantage in
calibration: the test signal corresponds directly to the air flow rate
flowing through the calibration leak. Here it is no longer necessary to know
exactly the quantity of the test volume connected during the calibration
process and to allow for determining the leakage rate.
As a rule, measurement of the volumetric flow rate (e.g.
pressure drop across a screen) is not used for leak testing, but for flow
control such as for example, in monitoring free passage in gas systems which
are tested by the pressure drop method with the same measuring element
(differential pressure sensor) in a subsequent measurement for leaking.
In measurement of the mass flow rate (thermal measuring
methods) the test signal is not only independent of the size of the test
volume, but also of the degree of test pressure. The test signal here
corresponds directly to the leakage rate in standard cm3/min. The
leakage rate does not - as in pressure measuring methods - have to be
calculated.
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