Bubble Leak Testing using an acrylic vacuum chamber

Bubble Leak Testing or Bubble Emission Testing is a procedure where leaks are visually detected and located through the emission of bubbles while the test specimen is submerged in water. In this article, we will talk about using an acrylic vacuum box as a leak detection device. We are also going to talk about the advantages, disadvantages, the internal pressurization, and the vacuum bubble leak emission test procedures.

Please note that not all bubble leak tests are created equal. There are many bubble leak testing methodologies. In this article we will specifically dive into the two most widely adopted bubble emission detection methods which are:

1. ASTM D 3078: Standard Test Method for Determination of Leaks in Flexible Packaging by Bubble Emission
2. ASTM F 2096: Standard Test Method for Detecting Gross Leaks in Packaging by Internal Pressurization (Bubble Test)

We carry a large selection of Bubble Leak Test Systems. We have standard sizes and options to customize your bubble leak testing system.

See our Bubble Emission Leak Testing Systems Catalog

What is a Bubble Leak Test

Bubble Leak Testing using an acrylic vacuum chamber
Bubble Leak Testing using an acrylic vacuum chamber

Also known as a bubble emission test, submersion leak test, underwater immersion leak test, or “dunking test” is a testing method of detecting and locating leaks in the specimen through the emission of bubbles. There are two ways to perform a bubble emission leak test: Pressurized (ASTM F2096) and Vacuum (ASTM D3078). The pressurized method is when the test specimen is pressurized above the ambient pressure, depending on the altitude of the test, the gauge pressure can be as high as the test allows. The specimen may be pressurized through sealing or through the feed through tubes connected to the test specimen while the bubble leak test is performed. The advantage of pressurizing the part is a better leak detection due to the additional pressure which is induced onto the test sample. The disadvantage is that this test is of higher complexity than the Vacuum Method. Another disadvantage of the Pressurized Test is that many test specimen cannot withstand higher pressure and will therefore burst or get damaged by high internal pressurization. However, the pressurized leak testing method may be the way to go due to test requirements. The Vacuum test method on the other hand, does not require any additional pressurizing of the test sample and therefore can be advantageous due to the test simplicity that it offers over the pressurized method.

The bubble leak test works as a result of the pressure difference between the test part and the chamber. When a leak is present, the gas inside the part will flow from a higher pressure toward a lower pressure. Therefore, the submerged part will emit gas bubbles, which will be detectable visually. The larger the leak, the larger and more frequent the gas bubbles will escape from the specimen. The accuracy of the bubble leak test is in the neighborhood of 10-3 atm-cc/sec. A hole size as small as 0.001 inch or 0.025 mm in diameter can be detected with the immersion leak test. Although it should be noted that holes of that small size are seldom perfectly circular and therefore it is almost impossible to come to an agreement on the exact hole diameter and hence size. Holes of this size cannot be detected with the naked eye, they would have to be at least 25 times larger to be seen. This leak detection accuracy is as good or better than pressure decay leak test, air decay leak test, dye penetration test, or flow rate leak test but not better than the high vacuum helium leak test, gas sniffing methods, or the tracer gas leak test.

Bubble Emission Leak Testing Equipment

ASTM D3078 Standard Test Method for Determination of Leaks in Flexible Packaging by Bubble Emission
ASTM D3078 Standard Test Method for Determination of Leaks in Flexible Packaging by Bubble Emission

Integrity Testing utilizing a Vacuum Chamber (ASTM D3078)

The most ideal bubble leak testing equipment is an acrylic vacuum chamber due to the fact that acrylic is transparent which enables a full view of the interior during testing. It is also the most cost effective and the best bang for the buck when it comes to accuracy of detection.

This acrylic chamber has to be coupled with a vacuum pump which is either a rotary vane pump or a venture pump. The Rotary Vane pump is powered by electricity, will pull a higher vacuum (30 in Hg), and does not require a pressurized supply air to run. A Venturi Pump on the other hand, does not require electricity, will pull a good vacuum (28 in Hg), and will require a pressurized air line. Whether you decide to go with a rotary vane pump or a Venturi pump will depend on your utilities and test requirements – there is no right answer here, both are very viable options.

Integrity Testing utilizing an Internal Pressurization System (ASTM F2096)

Another ideal method for detecting gross leaks in a sealed package is to internally pressurize the test specimen through the insertion of a Pitot-Style Static Probe. This system does not require a vacuum pump because it does not utilize a vacuum chamber for detection. This system requires a Pressure System which consists of pressure source, a high accuracy vacuum regulator, a static probe, a Magnehelic Differential Pressure Gage.

Keep in mind that the internal pressurization method does require the static probe to puncture the specimen in order to deliver internal pressurization. However Internal pressurization allows for better pressure control, handling of specimen such as turning or rotating the pouch during test, no additional vacuum pump, no O-Rings, no sealing to worry about.

To recap, the advantages of bubble emission leak testing are:

1. Cost Effective Seal Integrity Testing:

When it comes to product seal or closing integrity, there are no better methods for your wallet than the water immersion leak test. In fact, the next best method is an order or two of magnitudes more expensive.

2. Simple and Easy Leak Test

Placing your test specimen into a water bath, pulling a vacuum, and looking for bubbles is a pretty simple way of detecting and locating leaks. This is advantageous because production line operators without much training can perform this test.

3. Practical Testing of Leaking Pouches and Packages

All that is needed is an acrylic box and a vacuum pump to perform this test. The accuracy of the test is pretty good for most commercial and medical packages.

4. Visual Detection and Location of Package Leaks

Leaks can be visually detected and located within seconds.

5. Quick Sample Prep

Many test samples do not require much preparation meaning that the test process flow can go very smoothly.

6. Versatile Testing Method

The bubble leak test can be utilized on variously shaped and sized specimen - whether that may be a pouch, tray, box, vial, bottle, etc…



The Disadvantages of bubble emission leak test are:

1. Destructive Package Test

Even though the inside of the package is not damaged or destroyed by water, bubble immersion leak test is considered a destructive test. Keep in mind that the insertion of the pitot-style static probe does create a punctured hole on the package itself.

2. Subjective Leak Detection Method

The bubble emission requires the involvement of a testing person. This induces subjectivity into the testing method. The testing operator must be engaged and involved in the test otherwise the false positives may ensure.

3. Preparation and Handling of Sealed Specimen

The test specimen must be cleaned and prepared for the leak test. Also, some may not like the inconvenience of dealing with wet parts.

4. Bubble Emissions Test Procedures are Specimen Dependent

Products which are sensitive to water, such as electronic equipment, may not be suitable for this testing method. Leaks cannot be quantified, there is no way to know how large or small the leak is, only where it is.

Package Testing using Vacuum vs Internal Pressurization

There is no right answer when it comes to selecting an internal pressurization method vs vacuum method. Instead, you should ask yourself what is the best option for our customers? What is the best option for our testing facility? What is the best option for our current business processes? This is something that we cannot answer for you, this is something that you must decided for yourself.

Method Vacuum Chamber Internal Pressurization
ASTM Standard Guidance Document D3078 F2096
Requirements Vacuum Pressure
Specimen Investigation No Puncturing Required Package must be punctured
Handling During Test Cannot be rotated during leak test Can be rotated during testing
Pressure Differential Accuracy Less precise More precise pressure
System Complexity More Complex Vacuum System Less Complex Pressure System

The Methods vary when it comes to the ASTM Standard Guidance Document. The Vacuum Chamber and the Internal Pressurization are guided by ASTM D3078 and F2096 respectively. The vacuum method requires a vacuum source whether that be a vacuum pump or a pressure driven venturi pump. The Pressure method, will require a pressure source and pressure regulating equipment. The specimen investigation does not require puncturing of specimen during the vacuum leak test; on the other hand, the internal pressurization will require puncturing of sample with a probe to inflate with air. The sample cannot be rotated or handled during the test in a vacuum chamber, this is something that can be done with the Internal Pressurization method. The pressure control is better and more precise with the Internal Pressure method because the pressure can be set much more accurately. The vacuum system is more complex because the internal chamber has be air tight and sealed in order to enable vacuum to be pulled.

Bubble Leak Detection Limitations

There are some limitations when it comes Bubble Leak Detection. The only reason bubbles are emitted is because there is a pressure difference between the inside and the outside the of the specimen. The air is driven from a higher pressure environment into a lower pressure environment.

Minimum detectible leak rates

The consensus among vacuum leak testing experts is that minimum detectable leak rate of a bubble emission test is 10-3 standard cubic centimeter per second. A standard cm3 is the volume equal to three dimes stacked vertically. This means that at leak rate of 10-3 std cm3 per second it will take about 100 seconds for 1 cubic centime to leak.

Bubble leak test on permeable materials

Bubble leak test cannot be performed on materials that are permeable because the moment that the bubble leak test is initiated, hundreds of bubbles with starts emerging from material. This will make the locating and pinpointing of holes almost impossible.

Subjectivity of Visual Detection of Bubbles

The subjectivity of this testing method also comes to question when we look into the frequency and size of air bubbles. Assuming that the smallest air bubble that the naked eye can reasonably see in a bubble leak experiment is about 1 mm in diameter. And assuming a perfect air bubble sphere with 1mm (0.1cm) diameter; hence a spherical volume of 5.24×10-4 std-cc. This means that at leak rate of 10-3 (std-cc/sec) the specimen would emit about 2 air bubbles per second.

Limited Air trapped inside of Specimen for Bubbles Emission

Another limitation is the fact that there is a limited amount of air trapped inside the specimen if not air is supplied to the test specimen. This means that less and less air and hence lower and lower pressure is present inside the specimen meaning that at low air volumes, there will not be enough air emitted from the sample for appropriate detection.

Small Improvement if the Specimen is Already Pressurized

Finally, a vacuum chamber may not be much value add if the test sample is already pressurized to a high pressure. Let’s just assume that the specimen is already pressurized to 200 psi gauge absolute and then submersed into a bubble test tank. As mentioned earlier, bubble emission is cause by a pressure difference and the formula below:

Bubble Emission Leak Rate Formula where leak is driven by pressure difference between inside and outside of specimen
Where:
Q = the leak rate
P1 = Is the pressure inside the test sample
P2 = Is the pressure outside of the test sample
R = is the gas constant
V = Volume
t = time

This simply means that the pressure difference times a constant times the change in volume over time determines the leak rate. Lets just assume that R, V, and t are 1 to make our illustration simpler.

In our case, the part is already pressurized to 200 psi and the ambient pressure is 15 psi our leak rate will be:

200 – 15 = 185 units

If you dip this pressurized part into a tank and draw vacuum, your pressure difference will be 200 psi.

200 – 0 = 200 units

Our leak rate would improve a mere 7.5%. This means that you would see 7.5% more bubbles. If the part is pressurized this way, it would not make much sense to place a part with such internal pressurization into a vacuum bubble leak tank.

On the other hand, if a second part is sealed at ambient pressures 15 psi, one way to create a pressure difference is to place this part into the water tank and draw a vacuum. The higher the vacuum is drawn, the more bubbles will emit.

We carry a large selection of Bubble Leak Test Systems. We have standard sizes and options to customize your bubble leak testing system.

See our Bubble Emission Leak Testing Systems Catalog