In this write up, we are going to cover everything that needs to be covered when it comes to understanding and performing a Vacuum Bubble Leak Test. We have been in business since 2008 developing, designing, building, and assembling vacuum chambers and leak testing equipment. At the time of writing this, we have worked across 117 different industries (not companies, but industries). We have been involved with countless small and large-scale projects. One interesting pattern that emerged was that we noticed that one industry type was using a specific leak or quality testing method than another. The food industry tested leaks one way while the medical industry tested leaks another way. It seemed like everyone was doing what everyone else was doing within their specific industry. There was no cross pollination between industries when it came to leak testing. The resources online were fragmented at best. And we noticed answering the same questions about leak testing when asked by our clients. This is when we decided to write a comprehensive guide on vacuum bubble leak testing.
Keep in mind that even after close to two decades of working in this field, we do not know everything (we sure do try), and we welcome your comments and engagement and the opportunity to learn something from you as well.
If you are looking for a more compressed and summarized version of this write up, feel free to head over to Bubble Leak Testing Using an Acrylic Vacuum Chamber for a quicker read. If you want to further your understanding of vacuum bubble leak testing, keep reading.
See our Bubble Emission Leak Testing Systems Catalog
Vacuum Bubble Leak Testing
The leak test is accomplished through the placement of the specimen inside a vacuum chamber. The vacuum chamber must be transparent which is why Acrylic Vacuum Chambers are used. After the lid is closed, a vacuum pump is utilized to pull a vacuum. The pressure differential will start inflating a flexible package; therefore, a package restraining mesh is used to keep the specimen from floating up. The operator looks for bubbles emitting from the test specimen indicating a leak.
To generate a Vacuum, and hence pressure differential, three various vacuum pump options are used. The Rotary Vane Vacuum Pump, a Piston Pump, or a Venturi Pump. Each of these vacuum pumps have its advantages and disadvantages. To touch on these advantages:
1. Rotary Vane Vacuum Pump
A Rotary Vane Vacuum Pump is the most powerful and will pull the highest (29.9 inHg) and fastest vacuum at a reasonable cost. Must have an oil mist eliminator filter installed at exhaust. Depending on how often the vacuum bubble leak test is performed, it may not be ideal for indoor use unless it is used sparsely or the room is well ventilated or an appropriate exhaust oil mist eliminator filter is used.
2. Oil Free Piston Pump
2. The Piston Pump pulls about 27 inHg. A piston pump is maintenance free and does not require vacuum pump oil to run and therefore will not emit oil mist. This is the ideal option if the leak testing system is used often indoors.
3. Venturi Vacuum Pump
3. A Venturi Vacuum Pump will pull about 26 inHg of vacuum. The advantages are that it does not require electricity to run, does not require maintenance, and the vacuum is generated via pressurized air. The pump cost is lowest and the vacuum performance is also the lowest out of the three options.
| Vacuum Pump | Rotary Vane | Piston Pump | Venturi |
| Powered By | Electricity | Electricity | Air |
| Max Vacuum (inHg) | 29.9 | 27 | 26 |
| Pumpdown Time | Fast | Medium | Slow |
| Cost | Medium | High | Low |
| Advantage | Powerful, Fast, Max Vacuum |
Oil Free, Maintenance Free, No Exhaust Fumes |
No Electricity, Maintenance Free, No Exhaust Fumes |
| Disadvantage | Oil Mist Exhaust Fumes | Cost | Slow and Underpowered |
If you are interested to find out more about customizing an acrylic vacuum chamber, please check out our article on Customizing Your Acrylic Vacuum Chamber
Overview of Vacuum Bubble Leak Test System
A Vacuum Bubble Leak testing system also consists of a transparent vacuum chamber. The size of the vacuum chamber is dependent on the size of the test specimen. If a small test specimen such as a package that is 1 inch thick, 1 inch high, and 4 inch long is used, then a smaller vacuum chamber (such as 10 inch cube inside dimensions) is sufficient. On the other hand, if a larger bag is used, then a larger vacuum chamber is needed. Keep in mind that at a bare minimum, 1 inch of clearance should be used on each bag size. It is recommended that the height should at least be 80% more than the thickness. For example, if your test specimen bag is 26 inch Wide, 14 inch Deep, and 8-inch High, a Vacuum chamber the size of 28 inch Wide, 16 inch Deep, and at least 14-inch High should be used.
Another driving factor in determining the Vacuum Chamber size is the number of test specimen that will be simultaneously tested. The easiest way to increase the testing thruput of your test specimen is to test multiple at once. There is a downside, since the vacuum bubble leak test is a subjective test where it is heavily reliant on the operator, the more test samples are tested at once, the easier it becomes to miss a leak since the operator must look for bubble emission of each sample individually. Hence the more test samples are bubble leak tested at once, the less reliable the results become.
What would be a good multi-sample bubble leak test?
The best bubble leak test is to test a single sample at once. An operator can comfortably test up to 8 test samples at once. That also depends on how large the test samples are. A good rule of thumb is to test up to 8 samples inside a vacuum chamber that has a 20-inch Wide by 14-inch-Deep footprint. A good practice is that at least one inch of separation between each test sample should be followed. If the test specimen is larger than 20 inches, it is recommended to test a single test specimen per bubble leak tests.
The vacuum bubble leak testing system should also have a package restraining mesh, preferably connected to the lid. This mesh is required to press the buoyant test specimen below the water level. If the mesh is not present, the test specimen will not be submersed under water and air bubbles will not be visible in case of a leak. In some cases when the vacuum is pulled, the specimen will expand and float upwards, the package restraining mesh will hold them under water. Keep in mind, if your test specimen float, a package restraining mesh is critical. There are instances, where the test specimen is heavy enough to stay fully submersed under water during test. If that is the case, the package restraining mesh may be removed from the lid in order to improve visibility during test.
If a Rotary vane vacuum pump or a Piston Pump is used on the System, a Vacuum Pump Protection Valve must be installed upstream from the pump – between the vacuum chamber valve and the vacuum pump. This Vacuum Pump protection valve consists of a reservoir and a floating ball (like a ping pong ball). If water is pulled into the vacuum line, it will start filling up the pump protection valve reservoir, the floating ball will float upwards and close the vacuum line preventing any water from entering the vacuum pump. One of the reasons is that water may be pulled into the vacuum line is when the vacuum chamber is overloaded with specimen that will expand when vacuum is pulled. If the test specimen expands, the water level will be pushed upwards. If the water level is pushed to the maximum height of the vacuum chamber, it will start getting pulled into the vacuum pump line. This is why a Vacuum Pump Protection Valve is critical; because water may permanently damage the vacuum pump.
See our Bubble Emission Leak Testing Systems Catalog
A Vacuum Bubble leak testing system must also have a decent vacuum control system. The most cost effective is the manual system. The manual system consists of a venting valve, vacuum valve, vacuum adjustment valve, and a vacuum gauge. The Venting Valve is used to vent the chamber in order to release vacuum from the vacuum chamber. The Vacuum Valve is connected to the vacuum pump and is used to close off the vacuum pump if a hold is required. The Vacuum Adjustment Valve, is a bleed valve, that is used to control for the vacuum level. The most common test level is 50% vacuum which is about 15 inHg / 7.5 psi. The vacuum adjustment valve can be set to a target vacuum and locked into place. After that, the target vacuum is set, and when the test is initiated, the vacuum will be held constant until the hold and dwell time has elapsed. The vacuum gauge, is a device which measures and outputs the vacuum level inside the vacuum chamber. It is critical to know what the vacuum level is during the test. The most common is the dial vacuum gauge that will show within 2% of accuracy the vacuum level of the chamber. Another vacuum gauge is also the digital vacuum gauge which outputs the vacuum levels with much greater precision and accuracy, especially in the lower vacuum levels below 10 Torr absolute.
An automatic option is also available for the vacuum bubble leak test. This system consists of a manual venting valve, it is always good practice to have a manual vacuum release option even if the system is fully automated. Furthermore, the automatic system consists of two solenoid valves and a digital vacuum gauge. Both the solenoid valves and the digital vacuum gauge are connected to the vacuum controller. The vacuum controller constantly monitors the vacuum levels inside the chamber and will open and close each solenoid valve accordingly. The advantages of the automated system are that user can set the target vacuum and the dwell time and press start. The vacuum will be pulled towards the target vacuum. Once the target vacuum is reached, the controller will hold it for the set dwell time. After the dwell time has elapsed, the vacuum will be released and the test is complete. This system is preferred in a high thruput and qualified/validated environment where tweaking is not permitted after the bubble leak testing process has been established, validated, and qualified. It is also a preferred method as the operator can solely focus on looking for bubbles after the “Start” button has been pressed and the controller will perform the test protocol without operator intervention.
A good bubble leak testing system will also have a drain valve present at the bottom of the wall. If this valve is not present, the only way to drain the tank is via a bucket or by tilting it (which is absolutely not recommended for larger tanks).
Some systems have a drain valve on the side wall. Although not optimal for reasons which will be mentioned shortly, it should be sufficient for draining purposes. However, the drain valve should be placed as low as possible. Most drain valves which are placed on the side are generally one to two inches above the bottom wall which means that the tank will not fully drain and about 1 to 2 inch of water will be left in the tank. It is always recommended to fully drain the tank when it is not in use or if the tank will not be used for several days.
Some systems have a fill valve installed towards the top of the side wall. This is done in order to improve efficiency during the filling process. Generally, the tank is rolled towards a water hose, the valve is turned on, and the hose is pointed into the tank letting it fill. Sometimes a bucket is used to fill the tank. This is why a fill valve may be ideal because the bubble leak testing system can be placed close to a water source and the water can be directly hooked up to the fill valve. Once ready to test, the fill valve is simply opened and water is let in. As soon as the desired water level is reached, the fill valve is closed.
A lid handle is a good feature that allows the operator to hold, lower, or lift the lid easily. Lid clamps will clamp the lid down for easier vacuum initiation – some systems may not have the lid clamp, which means that the lid must be pressed down manually in order to compress the O-ring and initiate the vacuum. A gas support spring is a cylinder that will apply some force to the lid and hold it fully open without the risk of slamming back. If this is not present, there is a risk of the lid falling back and potentially inuring the operator during tank loading and unloading operation. It is therefore always recommended to use any type of mechanism that will hold the door open. Never consider doors that do not have latches or springs for obvious safety reasons. Furthermore, a support spring will free up both hands of the operator since now they can use both hands to handle test specimen without having to hold the door open with one hand.
Smaller sized vacuum bubble leak testing systems may be placed onto a table top. However, larger chambers are recommended to be placed on a table frame structure with lockable swivel casters. Reason being is that fully filled up larger tanks can weight several hundred pounds and are recommended to be placed separately on a table frame capable of handling the weight. The lockable swivel casters allow for easy transportation of bubble leak testing system around the facility.
See our Bubble Emission Leak Testing Systems Catalog
What Vacuum Levels should be used in a vacuum bubble leak test?
The vacuum levels are product specific. Some individual pouches are not meant to be subjected to high amount of pressure differential and may burst at 5psi. If 5 psi of burst pressure are the sealing specifications, the test is a success. The seal strength is correlated to burst pressure meaning that a seal that failed at higher pressure differential is a stronger seal than a seal that failed at lower pressure differential. Furthermore, a larger pressure differential also increases the test sensitivity and the ability to detect leaks as bubble emission. It is therefore advisable to bubble leak test at as high of a pressure (vacuum) as possible because, as mentioned before, the largest pressure differential ensures the highest seal integrity and the highest leak test sensitivity.
However, there is a limit to bubble leak test. As the vacuum approaches 24 inHg (-12 psi) or about 80% of vacuum, the air bubbles that are trapped inside the water as well as air bubbles sticking to the tank walls will start to increase in size and float towards the top. In other words, the water will begin to degas. If vacuum is increased further, the absolute pressure decrease will cause the water to boil at room temperature. The boiling occurs due to physical properties of water which states that as the absolute pressure decreases the boiling point of water is also decreased and if the absolute pressure is decreased sufficiently enough, it will cause the water to boil even at room temperatures. The boiling and degassing of water will in itself create bubble emission and therefore interfere with the actual bubble leak test where the operator will not be able to distinguish between degassing, water boiling, or sample leak. The signal to noise ratio will begin to drastically degrease as the vacuum is increased beyond 24 inch Hg.
The best suggestion is to start low at 5inHg and to look for bubble emission. If your product does well at 5 inHg of vacuum, you should move to 10 inHg. The industry standard is to vacuum bubble leak test at about 15 inHg (-7.5 psi) of vacuum. This corresponds to 50% of vacuum. Reason being is that 15 inHg is equivalent to 18,000ft (5500m) of altitude above sea level. And if your product passes a vacuum bubble leak test at 15 inHg, you can reasonably conclude that your product is fit for transport via truck across the continental United States since the highest altitude a Road will see in the US is about 12,000 feet above sea level. Moreover, airplanes are pressurized to 8,000 feet equivalent and therefore a product which passes a 15 inHg vacuum bubble leak test, is also fit to be transported via cargo airplane. Keep in mind that this does not ensure that the product is safe from accidental cabin depressurization at higher altitudes, a different test must be performed to ensure package integrity at higher altitudes.
What products are good candidates for vacuum bubble leak testing?
Sealed or Closed at Ambient Pressures
If the product was vacuum sealed, it is not a good candidate for vacuum bubble leak testing. Reason being is that a vacuum bubble leak test works because the pressure inside the test specimen is greater than the pressure outside of the test specimen to drive the air through the leak path. A vacuum sealed product will have a lower pressure inside than outside meaning that for this test to work, a large enough vacuum must be created in excess of the vacuum the product was sealed at. Some products are vacuum sealed at 27 inHg of vacuum or greater this means that a vacuum of 27 inHg or higher must be pulled in order to create a pressure differential. At 27 inHg the water will degas and boil before any pressure differential is created.
The only way to test a vacuum sealed product inside a vacuum bubble leak tester is to poke a hole and let the product fill to ambient again. Plug the hole with a tape and test. You are essentially testing the sealing process of your vacuum sealed pouch.
Product Interior and Safety Considerations
What happens if seal failure occurs and the pouch interior comes in contact with water? If there is not risk, then a water submersion test is appropriate. However, if spices, flour, powder, etc is tested and these come in contact with water, it will muddy the test tank and it will have to be drained. More importantly, do not test lithium batteries inside a vacuum water bubble leak test, if seal failure occurs and lithium comes in contact with water, fireworks will ensure.
It is critical to determine what risks are present in case of seal failure and when the product contents come in contact with water.
Too Large of test specimen
If the test specimen is too large, it may be impractical to accommodate it with a vacuum chamber and other test methods may be more economically appropriate. Anything that is beyond 4 feet in length and 2 feet in width is becomes too large because the cost and resources of building a vacuum chamber large enough to accommodate the test specimen exceed what can be done via other testing methods such as internal pressurization bubble leak test.
Other Considerations when it comes to vacuum bubble leak testing
A vacuum bubble leak test is a subjective test and dependent on the operator to be fully engaged and present during the leak testing process. The test will vary highly from operator to operator because during one shift, your operator will be serious and eager to detect bubbles while another operator will be watching their favorite Netflix show as they are performing the bubble leak test. It is of utmost importance to properly train the test operators and instill in them a culture of quality.
Another thing to consider is the sample preparation and handling. The test specimen will be submersed into a water bath and these will be wet afterward. The test is a destructive test meaning that the test specimen must be disposed afterwards. The overall test procedure can become time consuming and depending on the number of samples to be best in a batch, resources will have to be allocated to man the vacuum bubble leak tester unit.
Cleaning of testing equipment is also critical as acrylic may blemish and test specimen interior may bind to the acrylic walls. The chamber must be cleaned with a damp cloth. Do not use solvents, thinners, or alcohols as these will blemish the acrylic. The tank water must be drained if not in use for more than a few days.
Conclusion
A vacuum bubble leak test is one of the best and cost-effective ways to test for seal integrity. The test sensitivity rivals other much more expensive testing methods while at the same time the ease of use and the ease of operator training makes it a first choice for quality control. The acrylic test chamber is very versatile and can be utilized as a water submersion vacuum bubble leak test (wet test) or a vacuum leak test (dry test) by simply draining the water.
In summary, the vacuum bubble leak test is cost effective, easy to use, easy to train, with superior leak detection sensitivity capable of detecting holes of 0.025 inch in diameter or a leak of 10^-3 sccm. These are holes that are not visible with the naked eye.
Test Method Overview for Vacuum Bubble Leak Test
| Test Sensitivity | 10^-3 sccm |
| Locate Leaks | YES |
| Quantify Leaks | NO - You may be able to correlate or estimate the number of bubbles to size of leak |
| Objective Test | NO |
| Cost | Low to Medium - Depends on Specimen Footprint |
We do have another write up on Vacuum Bubble Leak testing you may find interesting at: Bubble Leak Testing Using an Acrylic Vacuum Chamber
See our Bubble Emission Leak Testing Systems Catalog
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