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What is a pressure/vacuum decay test?
Pressure decay 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.
Guiding Equations and Relationships
The following fundamental equations are essential to measure leakage and work with pressure decay testing.
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.
All leak rate units are at standard atmosphere conditions (70 F, 14.7 psia)
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.)
Units of Measure:
Keep in 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 and InH2O.
Pressure Decay Leak Test Cycles
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.
Load 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.
Mass Flow Testing for Leakage and Obstructions
Mass 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 test.
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 nor obstructions.
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Other types of occlusion testing include measurement of back pressure or pressure drop.
Statistical Procedures on Pressure Decay Test Results
Pressure 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 ways, including:
Control 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:
Control charts for the mean (X-bar) can help the manufacturer in several ways:
Control 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.
Calibration vs. Decay Measurements in Pressure Decay Leak Testers
The 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. 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.
Determination 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. 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).
Calibration, 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 Technology (NIST).
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.
APPENDIX A: A Survey of Applications for the TME Pressure/Vacuum Decay Leak Testers
Automotive Components Industry:
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.
Medical Device Industry: Testing Multi-Lumen Catheters
This system consists of a pneumatically actuated radial sealing 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, labelled 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.
Food Industry:
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.
APPENDIX B: Technical Help &#; Relationship between Altitude and Pressure