Automation 101: Pressure Decay Leak Testing

What do shampoo bottles, vehicle gas tanks and Pacemaker housings all have in common? These industrial products all primarily function as containers, holding their contents inside in a “leak free” manner. Countless industrial products such as these are relied upon to resist leakage either flowing out or flowing in. These products are rigorously designed and tested to assure that they live up to this expectation. Industrial manufacturers are highly motivated to test their products for leakage before they’re sent off to customers. Most often they turn to pressure decay leak testing solutions as their test method of choice. Here is more on automated pressure decay leak test technology, methods and benefits.

The Pressure Decay Leak Test in Industrial Manufacturing: Defined

In our last Automation 101 article on vacuum leak testing, we introduced the concept that there is no such thing as completely “leak free” products available on the consumer or industrial markets today. We suggested that manufacturers must define a leak specification as the starting point for developing a leak testing battery for their products. This leak specification states how much technical leakage is allowable in a given product and under given test conditions.

A pressure decay leak test is a form of pneumatic leak testing that uses compressed air or inert gas to accurately measure miniscule leak rates across a product with very high precision, assuring manufacturers that a product will resist leakage as intended. Leak testing is a critical manufacturing step, performed for reasons such as:

• User safety. Above all else, leak-free products must be assured to provide safe, harmless use to customers.
• Quality assurance, which provides proactive management of manufacturing chains producing leak-free products.
• Quality control, which involves setting up systems that will consistently catch and redirect leak-related issues on the line.
• Risk mitigation, which aids in avoiding liability associated with poorly performing leak-free products.
• Regulatory compliance. USDA, FDA, CFR, OSHA, SAE, ASME and various other agencies require certain products to be tested and documented as leak-free.
• Consumer confidence, which secures a company’s reputation for reliable leak-free products.
• Promoting sustainability by reducing rejects, rework and the associated energy and waste in both the manufacturing and use of leak-free products.

Pressure Decay Leak Testing Methodology

Compared to other leak testing methods such as vacuum and mass flow, pressure decay leak testing is largely seen as the least expensive, simplest and easiest to implement test method to use in industrial manufacturing.

The pressure decay method consists of charging the test part with a prescribed pressure of compressible gas and then monitoring any drop in pressure within the part over a fixed period of time. If the pressure measured inside of the part decreases during the test, a leak to the atmosphere would be suspected. Parts are specified with a certain amount of leakage as acceptable, and so the difference between initial and final pressure readings is used to determine pass/fail against this acceptable amount.

Pressure decay testing can be performed in two separate ways:

1. Out-flow pressure decay

As described above, the object being tested is pressurized internally using a compressible gas and precise measurements of this internal pressure are taken over a testing period of time. This method tests the object’s ability to retain its contents from leaking out.

2. In-flow pressure decay

This method places the object being tested inside of a test chamber, where the interior of the object is emptied and sealed and compressible gas is pressured around the object’s exterior. Precise measurements are taken of this external pressure over a testing period of time. This method tests the object’s ability to resist internalizing foreign materials.

For advanced applications needing extremely high precision, pressure decay testing can be combined with mass flow methods where the volume, mass and pressure of the test media is measured across the testing period. This type of test battery is especially useful for quantifying both leak rates and leak volumes, under varying conditions and over long periods of time.

When a Pressure Decay Leak Test Isn’t an Option

While pressure decay testing is the most widely applied method to manufactured products of all types, there are some objects on which it should not be used. Incompatible features include those that are:

• Sensitive, fragile, delicate or otherwise unable to hold pressure.
• Non-rigid, elastic or compressible / expandable materials.
• Parts that are not able to be held or supported firmly in a test fixture.
• Parts lacking an opening that can be used as a test port to connect pressure instruments to (or that cannot have one added).
• Requiring extremely high, low or precise test values, outside of the range of available pressure transducers.
• Requiring extremely long test times (where environmental changes such as natural ambient temperature changes) may impact results.
• Composed of parts with properties that are not constant or that can independently change (such as inconsistent part volumes or those that generate heat due to internal batteries).

How to Automate Pressure Decay Leak Testing

Pressure decay leak testing is very prominent across all manufacturing sectors. It is particularly well entrenched in food and beverage, cosmetic, pharmaceutical, automotive, consumer goods and related industries using molded and blow molded containers. Today’s automated test systems can be installed right on production lines, testing containers and devices in-situ, no longer requiring the traditional remove-test-replace sequence of decades past.

Blow mold lines for beverage manufacturing make a great example: Bottle inserts are molded right on the filling line, making prior testing not possible. As the molded inserts are conveyed to the filler, they can be tested right inline in a matter of seconds for each. Bottles that fail inline leak testing are rejected before they’re filled, saving the waste, leaks and customer dissatisfaction that could result without testing.

Setting up a  pressure decay leak test in industrial manufacturing is fairly straightforward. Manual, semi- automated and fully automated operation are all entirely possible, quite practically regardless of industry and application. In essence, the pressure test itself requires no more than a way to pressurize either the interior or the exterior of a given test object, and an instrument that can precisely read small pressure changes in the pressurized space. These components can be built into an overall system, complete with safety, internal diagnostics, automatic documentation and design change adaptability features included.

Such systems can be provided in many configurations and scales to fit your needs. 

Manual Pressure Decay Testing

Usually delivered as a benchtop system such as AMS LT-201 Leak Test System, a manual pressure decay test station consists of a compressed gas media source and a precision gas measurement instrument. The part to be tested is set on the bench, open ends sealed, and the gas media supply connected to a test fitting ported to the part’s interior. Pressure is applied to the part up to a given test pressure, an initial pressure measurement is taken and then pressure is held for a timed test period (sometimes several seconds to several minutes). 

At the end of the test time, the pressure is measured again and the test values are then reviewed to determine if the part passes or fails. The part is manually disconnected and removed from the test station, results are recorded and it’s ready for the next test. (This describes an out-flow pressure decay system; an in-flow system can be easily provided as well.)

Semi-Automated Pressure Decay Testing

More advanced pressure decay leak test systems, such as AMS LT-401, offer semi-automated, feature-rich test processes that allow for higher volumes, faster test cycles, robust safety features and integrated quality control benefits. Semi-automated systems accept parts placed by operators into their test fixtures and take it from there. Poka-Yoke functions assure that the part is assembled correctly, actuated fixture mandibles orient and seal the part ends, and a PLC-driven test sequence is executed once or across multiple cycles. A part is deemed to pass or fail via the system’s automatic computational comparison of test media values from start to finish, and the results are documented in regulatory-compliant, local or networked QA data storage systems.

Fully Automated Pressure Decay Testing

Testing that occurs within batch or continuous manufacturing lines, without operator part handling, is considered fully automated pressure decay testing. Envision the core of the above semi-automated system placed in line with other production equipment, using robotics or conveyance mechanisms to handle the part, while all other testing functions remain the same. Acceptable parts are then mechanically conveyed downstream to packaging, while failing parts are rejected to accumulating tables awaiting rework or repair. Fully automated systems offer the highest volumes and least amount of human labor, and given their cost, usually are only selected by the largest manufacturers.

Questions About a pressure decay leak test? 

Automated Machine Systems (AMS) is an Industrial Automation Integrator in Cincinnati, Ohio. We specialize in advanced automation solutions for Medical, Plastic Processing, Consumer Goods, and Transportation industry manufacturers. With 99.5% on-time delivery, 97% customer satisfaction, 24/7 service, and over 20 years of earning our customer’s trust, AMS is your partner for industrial automation. To chat, reach us at (513) 771-3525, or by email at info@amsmachinesinc.com.