Closed
products or packages can be tested for leaks in a variety
of ways. These include force decay, displacement decay, trace
gas leak detection, mass spectrometry, and surrogate chamber
pressure decay testers or vacuum decay testers. This article
will concentrate on the latter. Non-destructive
pressure or vacuum decay chamber testing is a method for
checking a sealed, non-porous package or product for leaks
while maintaining the quality of the part or package. It
is necessary that the test item contains some air or other
gas inside - this is called the "head space". The
package or product is enclosed in a surrogate chamber that
provides an interstitial air space around the test item.
This air space is then pressurized (or, in case of a vacuum
test, evacuated) and stabilized, and decay of the pressure
or vacuum in this air space (indicating air leaking into
the head space of the package or product) is measured. The
test chamber used in non-destructive chamber leak testing
is called a "surrogate chamber" because the actual
pressure or vacuum decay leak test is done on the air contents
of the chamber surrounding the test item rather than on the
test item itself.  In
addition to leak testing closed or sealed products, this
non-destructive method of testing will detect leaks from
pinholes, cracks, seal and channel leaks in the walls or
seals of common package materials such as films, foils and
laminates. Packages can be pre-formed lidded trays, blister
packs, or pouches.
| If
you have questions or would like more detailed information
on pressure or vacuum decay leak testing, click
here to review the basics. |
Surrogate
Chamber Testing with Pressure or Vacuum Decay If
your product or package is closed or sealed so it cannot
be pressurized from an external source, an alternative method
of pressure decay leak testing involves creating a closed
space around the test item (a surrogate chamber) and pressurizing
(or evacuating) it.
A pressure differential can thus be created across the non-porous
product or package walls or seal. Once stabilized, air movement
from the higher pressure to the lower will indicate the presence
of a leak path, providing a quantitative measure of package
or product leak integrity without disrupting the package
seals. Because air moves in or out of
the package or closed product in the presence of a leak,
the air volume around the test object must be adequate to
create a detectable change in the chamber pressure. Keep
in mind, though, that when you minimize the interstitial
volume (the area around the package, which will be subject
to pressure or vacuum during the test) and maximize the instrument
resolution (within reason), about 10-4 sccs is an achievable
sensitivity. The method is quantitative; your test results
are amenable to SQC analysis for process control.
Understand
your test item
1. What materials comprise
your test item or package?
Materials
(package walls and seals, closed product surfaces etc.) must
be non-porous to air movement, so paper and Tyvek® are
not suitable for this type of testing, as they are porous
to air movement. Examples of closed products suitable for
chamber testing include closed ended extrusions, vials, bottles,
and welded housings.
2.
If you are testing a package, KNOW YOUR SEAL STRENGTH.
When
leak testing a package, the leak test pressure cannot approach
the burst seal strength. TME engineers recommend that the
leak test pressure not exceed 25% of the package burst seal
strength. Seal strength can be quantitatively determined
by using burst, creep and creep-to-failure testing. These
tests require pressurizing the entire package and measuring
the peak rupture pressure (burst test). Inflation testing
provides a whole-package minimum seal strength and also indicates
the weakest seal area, and is equally applicable to peelable
and non-peelable seals. Keep in mind that inflation seal
strength testing is destructive.
Designing
your Surrogate Chamber The
chamber must be sealed from the atmosphere. This creates
the vacant space that will be pressurized or vacated to perform
the test. Care must be taken that the seal is strong enough
to prevent air leakage when pressurized or vacated. When
the test pressure in the chamber space has been stabilized,
you will measure leakage by pressure change in vacant chamber
space over a predetermined period of time as pressure leaks
into (or out of, in case of vacuum test) the test item or
package. Minimize the interstitial
volume of the chamber (the vacant chamber space surrounding
the test item or package). The smaller the interstitial volume,
the more sensitive the test. TME design engineers fabricate
your test chamber to minimize the interstitial volume around
your particular test item.
Typical
chamber fixture to accommodate pre-formed, filled and lidded
trays How it Works:
The
tray is inserted into the test chamber and the cover is locked
down. The airspace in the chamber is then pressurized (or
evacuated), stabilized and tested for pressure (vacuum) decay.
No decay, no leaks; if the seal leaks, there will be measurable
pressure or vacuum decay. Fixture
for non-destructive testing of induction welded bottle seals  How
it Works:
The fixture head is lowered onto the bottle
shoulder where a seal is made. The airspace in the chamber
thus created is pressurized (or evacuated), stabilized and
tested for pressure (vacuum) decay. No decay, no leaks; if
the induction welded seal leaks, there will be measurable
pressure (vacuum) decay.
 Adequate
"head space" is necessary. The nature of the pressure
or vacuum decay test requires a minimum headspace inside
the closed product or package. "Head space" refers
to the amount of air enclosed in the test item or surrounding
the product inside a package.
 Issues
Related to Test Sensitivity
High resolution
instrumentation can detect pressure changes as small as 0.0001
psi in the interstitial space (the space surrounding the
closed product or package). In order to detect this pressure
change in the interstitial space, there must be sufficient
air in the head space relative to the air in the interstitial
space to create this much of a change in pressure. TM Electronics
engineers evaluate each potential application of chamber
testing technology to assure that this condition is met.
If
you recall the Leak Rate equation (see Appendix A) and substitute
the interstitial volume Vc for the system volume
V, you can see the chamber leak effect:
APPENDIX
A Pressure Decay Leak Testing Equations:
where
V is the volume of the medium exiting or entering and t is
the time period during which you are measuring the change
in volume. This is the basic Gas Law, on which all inflation
leak testing is based. Leak rates are expressed in various
units of measure which will generally reflect whether you
are measuring a relatively high leak rate (for example, 10
cc/min) or a low leak rate (for example, 1 x 10 -3 cc/sec). 
Where
Q is the flow rate through the orifice, d is the orifice
diameter, P1 and P2 are the pressure on either side of the
orifice, is the specific density of the medium, k is a dimensional
constant and T is the temperature of the system. To get consistent
measurements of leak rate, the temperature must be constant,
and the gas in a state where it is incompressible. Of course,
because matter can flow through an orifice in either direction,
in general, leak rates can be assessed using either pressure
or vacuum. 
where
P is the pressure in the test system, V is the internal system
volume, and t is the test time. The units of measure chosen
will determine the appropriate leakage rate output (sccm,
sccs etc.) |
