TM Electronics, Inc.
Family of Pressure/Vacuum Decay Leak Testers
DECAY LEAK TESTING
is a pressure/vacuum decay test?
testing measures the change in pressure between atmospheric
pressure and your pressurized test sample. Unlike some other
current methods of leak testing, this test method yields
quantitative information, hard data points that can be recorded
and upon which decisions can be made. This removes the dependency
upon the operator and allows specific accept/reject criteria
to be set, and this method is quite simple to use. It is
fast; 2-4 second cycles are achievable, keeping in mind that
test time is volume dependent. Pressure decay testing is
as sensitive as the time available for the test. Pressure
decay may include vacuum testing since a "vacuum"
is merely a pressure below atmosphere. All references to
"pressure decay" hereafter are equally applicable
to "vacuum decay" testing.
Equations and Relationships
The following fundamental
equations are essential to measure leakage and work with
pressure decay testing.
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).
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
leak rate units are at standard atmosphere conditions (70
F, 14.7 psia)
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,
Units of Measure:
mind that Leak Rate units of measure are expressed in flow
rates (for example, sccm, cc/sec) and pressure decay units
of measure are expressed in units of pressure, such as psig
Pressure Decay Leak Test
The Pressure Decay leak test cycle consists
of three distinct phases, not counting the load and unload
times. The following diagram illustrates the relationship
between these phases.
and Unload are the times it takes to engage and disengage
your part or package from the pressurizing and pressure decay
measuring instrument. Although not technically part of the
actual test cycle time, these periods must be taken into
account in order to realistically project the time needed
to test individual items. Charge is the period of
time in which the part is being pressurized to the predetermined
test pressure (or slightly above this pressure, so any stability
changes can be taken into account). Settle is the
period allowed for the volume of the pressurized part or
package to change and stabilize due to the stresses introduced
by pressurization. This is particularly crucial in the case
of flexible materials whose volume may change substantially
with pressurization. If this is an issue for your product,
you will want to review the discussion on restraining plate
fixtures in a later unit of this course. The Settle period
also allows time for the adiabatic temperature rise (the
heat generated through compression of a gas) to stabilize.
Test is the data taking period in which the measurement
of the decay of pressure is taken.
In the pressure
decay leak test illustrated above, the graph of the drop
in pressure (Y axis) over time (x axis) is called the decay
curve. TME uses the decay curve in its "Test Plot"
graphic display and in its "Memory Reference Curve"
technology, in which the decay curve for an acceptable test
part is determined and reject decisions are made by the test
instrument by comparing the test decay curve to the acceptable
"memory reference curve" for the test part.
Flow Testing for Leakage and Obstructions
flow testing uses intrinsic properties of air to directly
measure the amount of air escaping a closed system. While
pressure decay testing is often the method of choice for
leak testing, mass flow testing has several advantages, including
speed of test (1-2 seconds, rather than the longer time required
for the pressure decay cycle described above). In addition
the mass flow test is not dependent on temperature or air
density, which may be a difficulty for the pressure decay
Mass flow testing is a better choice for identifying
obstructions in an open-ended test part. Unlike mass flow
testing for leakage, mass flow testing for obstructions uses
a continual flow model to calculate the blockage in an open-ended
device, such as a medical catheter or refrigeration tubing.
Any obstructions in the part will restrict the flow of air
through the device, thereby causing the part to fail the
test. TME leak and flow testers can be programmed to perform
both a pressure decay leak test and a flow test consecutively
on a test part, confirming that the part has neither leaks
Other types of occlusion testing
include measurement of back pressure or pressure drop.
Procedures on Pressure Decay Test Results
decay testing provides test results that are quantifiable,
variable statistical data. Quantification of information
allows the data obtained in the tests to be used in several
- Documentation that your product
meets the required specifications for leakage;
of your testing protocol, particularly important in the medical
device manufacturing/packaging industry;
charts are commonly used to aid in manufacturing process
control. The objective of control charts is to monitor the
process in real time so if something goes wrong, it can be
noted and corrected with the minimum of lost product. With
regard to a manufactured product you are performing a pressure
decay leak test on, for example, the concept behind control
charts is as follows:
- A manufacturing process
"in control" will result in end-of-test pressure
decay values that fall consistently in a predictable range
around the average. In addition, the average value will not
change appreciably over time when the process is "in
- A single end-of-test pressure
decay value is referred to as "x". The average
of values over a period of time is referred to as the mean,
or "X-bar". The range of t test values is the difference
between the maximum and the minimum values.
processes always vary slightly due to manufacturing and material
variations, "good" product test values will go
up and down within a range around the mean value. That range
can be statistically predicted using the mean test value
plus and minus three standard deviations (a measure of the
variation inherent in the process). The "acceptable"
range is the set of pressure decay end-of-test values that
fall between the upper and lower control limits. These control
limits are automatically calculated in the TME test instrument
from the previous test results in the Datalog.
data points on the control chart consist of subgroups of
test results. These subgroups can be as small as two tests,
or as many as 20 tests. Subgroups are used to minimize the
effect of a testing error or a single bad part.
charts for the mean (X-bar) can help the manufacturer in
- If, for example, a temperature
problem in the manufacturing equipment is causing weaker
than usual closures, the downward trend in pressure decay
test values will be obvious on the control chart even before
the product reaches the point of failures. This gives the
machine operator an opportunity to correct the temperature
problem with little or no loss of product.
data points outside of the control limits may give the machine
operator an indication that an instability is developing
in the process that needs to be investigated before a large
quantity of bad product is produced.
charts for range (the difference between the maximum burst
value and the minimum burst value within a subgroup) also
have a place in identifying when the manufacturing process
is becoming erratic and inconsistent.
vs. Decay Measurements in Pressure Decay Leak Testers
question has been raised as to the relationship between "calibration"
(accuracy of the pressure decay leak tester) and "resolution"
(ability of the tester to detect a pressure difference -
"decay" - that occurs during the leak test. Specifically,
confusion exists regarding how the "resolution"
of the leak test system can be 0.0001 psi, even though the
accuracy of the instrument's "calibration" (accuracy)
may be significantly greater, such as +/- 0.075 psi. The
answer to the question lies in the definition of these two
parameters of pressure decay leak testers, and the understanding
of what is happening during a pressure decay leak test.
of leaks in a device using the "pressure decay"
method involves pressurizing the device to a predetermined
pressure, locking out the supply source of the pressure and
sensing a pressure change ("decay") during a predetermined
test time. Modern pressure decay measuring instruments such
as the TM Electronics Solution Leak Tester are capable
of detecting pressure changes as small as 0.0001 psi. This
ability to detect pressure change during the test time is
defined as the instrument's "resolution".
This is not related to the starting pressure of the
test (except for the special circumstance in which specified
leakage rate is used as the test criterion).
on the other hand, refers to the accuracy of the leak tester's
reproduction of the desired test pressure for the leak test.
For example, if you are performing a pressure decay leak
test using 50 psi as your test pressure, accurate calibration
ensures that your test instrument will consistently reproduce
that test pressure, in a specific set of units of measure,
in a standard set of conditions, every time you test, within
a range of variation that is defined by instrument accuracy.
TM Electronics leak test instruments are calibrated with
reference to a known "standard" gauge, controlled
by a standards body such as National Institute of Standards
To summarize, the resolution
of the leak test system is defined by the smallest pressure
difference (decay) the instrument can consistently detect
during a predetermined test time, regardless of the starting
pressure of the leak test. The calibration of the
instrument is defined as the accuracy of the test pressure
applied at the beginning of the leak test. For all practical
purposes, these two items are not related, but totally separate
characteristics of the leak test instrument.
A: A Survey of Applications for the TME Pressure/Vacuum Decay
This TME Solution Leak Test
System is designed to perform two pressure decay leak tests
in sequence at two different pressures on an electronic connector
housing. The system is comprised of a one-port TME Solution
Leak Tester and a custom test fixture with their associated
air and sensor lines. When a test item is properly placed
into the test fixture, the system will automatically perform
a leak test of the connector cavity with the part small bottom
vent membrane sealed closed, and then a second lower pressure
decay test is LINKED to the original test. The lower pressure
test will test the part with the vent membrane open to atmosphere.
Device Industry: Testing Multi-Lumen CathetersIndustrial
Products Industry: Vacuum Leak Test for Pipe Fitting
system consists of a pneumatically actuated radial sealing
fixture fixture comprised of three pneumatic seal clamps.
The seal clamps are intended to close around the outside
diameter of the catheter and seal any leak paths from the
catheter distal tip and proximal side ports during pressure
decay leak. The seal clamps can have their individual positions
adjusted along the length of the catheter to seal the side
ports. The individual seal clamps, labeled Seal 1, Seal 2
and Seal 3, can be individually actuated in any order using
the "Ports and Seals" menu. The Radial Sealing
Fixture is customized to accommodate several catheter lengths.
This application involves performing a vacuum
decay leak test on a very large pipe fitting open on both
ends. The MDT-50-VAC leak tester and the test assembly comprise
a benchtop system. The test part is engaged on the benchtop,
and the top of the fitting is sealed with a plastic plug.
A vacuum is then created inside the fitting, which effectively
enhances the seal on the open end of the test part, and a
vacuum decay test is performed to evaluate the seams of the
The customer produces thermoformed trays
that will hold food products. The need was to confirm the
package quality after thermoforming, and to detect any cracks,
holes or defects in the tray before filling. The empty tray
fits over a raised rectangle on the fixture base and the
edges are sealed to the fixture by means of manual clamps.
A direct leak test is then performed by introducing pressure
through the raised rectangle into the air space inside the
tray. The purpose of the raised rectangle onto which the
tray is fitted is to minimize the air space to pressurize,
thus increasing the sensitivity and shortening the time for
the test. This fixture could be customized to be applicable
to any rigid, non-porous package form or product that can
be sealed to the flat surface of the fixture.
B: Technical Help - Relationship between Altitude and Pressure