| General principles:
The
leak detection in a package is done
by applying a differential pressure
between inside and outside of container
and by tracing this variation in time.
the differential pressure can be carried
out:
- by making vacuum
around the packing (sealed hermetically)
of which the internal pressure is
different of vacuum; the difference
in pressure to which the walls of
the container are subjected is then
equal to: Pressure in the package
(generally, equalizes with atmospheric
pressure) - Pressure of vacuum setting
(external; included between –200
and -950mbar relative);
- by applying a
pressure of "inflation”
in the package, placed in ambient
atmospher; the difference in pressure
to which the walls of the container
are subjected is then equal to: Inflation
pressure (intern) - Atmospheric pressure
(external);
Principle
of the leak test by vacuum setting:
Implementation
material:
The
package to be tested is placed in
a hermetic enclosure, connected to
a vacuum circuit during the phase
of vacuum setting.
This enclosure is of rigid type (bell
machined to adjust the forms outsides
of the package) in the solid packaging
case (bottles out of glass sealed,
metal drums...); it is of flexible
type in the nonrigid packaging case
(plastic sachets, flexible blisters...)
and comes to be plated on package
at the time of vacuum setting, allowing
the atmospheric pressure to press
against package (without flexible
deformation of package).
In the 2 enclosure types, the required
objective is to reduce volume subjected
to vacuum
P=f(t)
Curve:
The
typical variation curve of pressure
in enclosure subjected to vacuum according
to time, during a leak test cycle,
is as follows:
Zone
1
= period of vacuum setting during
which the enclosure is emptied;
Zone 2
= instability of pressure after closing
of the vacuum circuit, due to the
inertia phenomena and pressure front;
Zone 3=
zone of steeply sloping stabilization
of pressure increase, due to thermodynamic
phenomena and pressure front decrease;
Zone 4
= zone of stabilization to weak slope
of pressure increase (damping of the
phenomena of zone 3);
Zone 5
= complete stabilization of pressure
in the enclosure (pressure = constant)
Leakage
survey:
The
research of leak in the enclosure
consists in detecting a stronger slope
in zones 3, 4, or 5 that the typical
slope of a tight package; indeed,
a air leakage (from the package towards
the enclosure) will generate a pressure
increase in enclosure volume.
Comparison
typical curved /curve with leakage:
The
measurement of
slope is done by measurement P2-P1
during a period t2-t1;
differentiation between typical slope
and slope with leakage is more detectable
at the approach of zone 5;
thus, for example, a leak corresponding
to a slope of +2mbar/s will be detectable
with more difficulty in zone 3 having
a typical slope of +18mbar/s (the
leak accounting for only 10% of the
total slope observed) that in zone
4 having a typical slope of +6mbar/s
(the leak accounting for 25% of the
total slope); the same leak of +2mbar/s
will account for 100% of the slope
in zone 5.
The detection zone chosen, in industrial
process, is zone 4 (inaccuracy in
zone 3, production frequency incompatible
with waiting for zone 5).
Case
of the very important leaks:
Then
air leaks very quickly, pressure in
enclosure is quickly equal to the
pressure in package, as of time zone
4; a null slope then is measured as
if packaging were tight;
the solution to detect these large
leaks consists in measuring a minimum
vacuum threshold present in enclosure,
in time zone 4.
Principle
of the leak test by setting under
pressure:
Implementation
material:
After
possible obturation of its openings,
the container to be tested is pressurized
(inflation) by air introduction (from
50 mbars to 10 bars).
P=f(t)
Curve:
The
typical variation curve of pressure
in product to be controlled, during
a leak test cycle, is as follows:
Zone
1
= period of inflation during which
the packaging is subjected to the
inflation pressure;
Zone 2
= instability of pressure after closing
of the inflation circuit, due to the
inertia phenomena and pressure front;
Zone 3=
zone of steeply sloping stabilization
of pressure decay, due to thermodynamic
phenomena and pressure front decrease;
Zone 4
= zone of stabilization to weak slope
of pressure decay (damping of the
phenomena of zone 3);
Zone 5
= complete stabilization of the pressure
in packaging (pressure = constant);
Leakage
survey:
The
leakage research in package consists
in detecting a stronger slope in zones
3, 4, or 5 that the typical slope
from tight package; indeed, a air
leak will generate a pressure decay
in tested volume, according to the
"law of perfect gases":
P x V = Constant (at constant temperature).
Comparison
typical curved /curve with leakage:
The
measurement of slope is done by measurement
P2-P1 during a period t2-t1;
differentiation between typical slope
and slope with leak is more detectable
at the approach of zone 5;
thus, for example, an air leakage
corresponding to a slope of -2mbar/s
will be detectable with more difficulty
in zone 3 having a typical slope of
-18mbar/s (the escape accounting for
only 10% of the total observed slope)
that in zone 4 having a typical slope
of -6mbar/s (the escape accounting
for 25% of the total slope); the same
leak of -2mbar/s will account for
100% of the slope in zone 5.
The detection zone chosen, in industrial
process, is zone 4 (inaccuracy in
zone 3, production frequency incompatible
with waiting for zone 5).
Case
of very important leaks:
Then
air leaks very quickly, the pressure
in package is quickly equal to atmospheric
pressure, as of time zone 4; a null
slope then is measured as if package
were tight;
the solution to detect these large
leaks consists in measuring a minimum
pressure threshold present in the
package in zone 4.
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