Best Practices for Selecting a Gas Leakage Test Method

Engineer reviewed by:

• Mathew French, Technical Marketing Manager – Aerospace & Energy @ The Lee Company

In the world of manufacturing, reliability in the field depends on the seamless integration and performance of every component in an assembly. To maintain system integrity, testing is typically required to confirm that each individual unit is properly built and is functioning as expected. Leakage testing is a critical quality control measure in this process – and one of the most important tests to perform. Whether you’re designing a hydraulic or pneumatic system, leakage testing plays a vital role in helping you:

• Verify that system performance meets specifications
• Identify defective components early.
• Reduce total manufacturing cost and operational risk.
• Ensure compliance with environmental and safety regulations.
• Minimize non-conformance and enhance customer satisfaction.

Various methods are available to test the leakage rate of a component or system, each offering unique advantages and disadvantages. In this article, we’ll examine four common gas leakage testing methods and share tips to help you identify the optimal test method for your application.

Bubble Test  •  Pressure Decay Test • Mass Flow Test • Helium Leak Test

Verifying Leakage Requirements

To find a test that aligns with your specific needs, begin by defining the leakage requirements of your component or system. Understanding the best practices associated with common leakage testing methods will help further guide your selection.

In theory, the most logical way to verify leakage performance is to test the component with the fluid actually used in the system and establish the acceptable specification limits accordingly. In practice, however, leak testing is typically conducted using referee fluid (such as dry gases like air or nitrogen). This approach is preferred because dry gases provide a cleaner, safer, economic, and more sensitive method for leak testing. Gas testing is also used in lieu of testing on certain toxic fuels or other media that may be harmful.

The table below provides an overview of commonly used fluids in leakage testing, highlighting key characteristics that influence their selection across various applications.

To quantify acceptable leakage limits, engineers must conduct studies using the actual system fluid, then correlate those findings with leakage measurements using dry gases as part of the engineering analysis. Leakage studies can be performed by an outside laboratory, or internally with the proper resources and experience. Defining these limits makes it easier to choose a test method that is most appropriate for your specific application.

Other factors can affect leakage, including pressure, temperature, fluid viscosity, sealing area, material selection, and geometry. While these elements are beyond the scope of this discussion, they should be understood when establishing acceptable leakage testing limits.

The following sections describe four distinct testing methods for detecting leakage.

Bubble Test

One of the simplest methods for detecting leakage is the bubble test. Using this method, the component or test unit is pressurized with gas, then submerged in water for a specific period of time. The setup is then observed to check for the presence of bubbles, which indicate leakage.

A bubble test can be performed in two ways:

• Submerged Method: The entire component is pressurized and submerged in water. Using this method, the leak path can be visually observed no matter where on the component it is located. This added visibility can help in determining appropriate corrective action to stop or prevent leakage for this component – and future components – quickly and effectively.
• Outlet Submersion Method: The component is connected to test tooling where only the plumbed outlet tube is submerged in water. This allows observation of the bubble rate without fully submerging the entire component.

A controlled soapy water solution may be used to help visualize external leaks on installed equipment.

Cost-effective and versatile, this method is easy to implement and provides the operator with straightforward “pass” or “fail” acceptance criteria based on visual confirmation of the presence or absence of bubbles.

Although effective, bubble testing is less precise for very small leaks and may not be suitable for all applications. Since it relies on a more subjective visual inspection, it can be difficult for the operator to quantify the specific amount of leakage present with high resolution. It is possible to observe the number of bubbles over a period of time, however, this requires standardized test equipment and procedures to account for variation in bubble size and volume.

Due to its simple setup, bubble testing can be used for a wide range of production volumes. It is best suited for applications with higher leak tolerances or in scenarios where the extreme sensitivity of helium leak testing would be excessive and/or provide little added value. An example of this would be zero leak aerospace components, where bubble testing can be used to accurately determine leakage performance. It is also useful for rapid functional checks, preliminary testing before using helium-based methods, or in applications where helium leak requirements are greater than 10^-3 SCCS.

Pressure Decay Test

Pressure decay testing is another common method used to measure leakage. With this test, the component or system is pressurized to a specified level, and pressure loss is subsequently measured over time. The difference in pressure is called pressure decay.  When this difference is measured over a known period of time, it is referred to as the “pressure decay rate,” which can be used to determine if the system is leaking at an acceptable rate.

Pressure decay tests require a simple set up and low-cost equipment, making them an efficient and economical choice for leak detection. They output a sensitive leakage result without the need for expensive test gases. Additionally, these tests can be automated to reduce the burden on your internal teams. This method also provides flexibility in test setup, making it advantageous in manufacturing environments.

Despite the low upfront cost, pressure decay tests requires longer test cycles compared to other leak detection methods. This additional time should be factored into the total cost of the test method in your decision-making process. Pressure decay testing is also inherently vulnerable to testing and measurement errors. Variations in test conditions, like temperature, volume, or other external forces, may significantly affect results. These errors arise in part because test  calculations are dependent on the volume of the parts and test circuit. The volume of each part will change slightly within its own tolerances, which can lead to further accuracy errors.

Additionally, pressure decay testing requires reliably taking two pressure measurements over a defined time interval. This process can introduce added measurement error into the leakage reading. Such errors can lead to higher gauge repeatability and reproducibility (GR&R) scores, which are undesirable. Since pressure decay test results are independent of the leak path location, it is important to implement robust error-proofing practices and setup checks in production environments. Shut-off valves used during the testing process can themselves become potential leak paths if they are faulty or improperly sealed.

Pressure decay testing is particularly well-suited for applications with high production volumes or where leak rates exceed 1 SCCM. In cases where test speed is critical or where detecting very low leak rates is required – especially in components with high internal volume – alternative methods may yield more accurate and cost-effective results.

Mass Flow Test

Mass flow testing offers a fast and accurate method that works across a wider range of leak conditions, at a cost similar to pressure decay testing. This test is performed by the component or system with a fixture, then pressurizing it along with a supply volume. A mass flow meter is used to directly measure the volume of gas that flows from the supply volume into the component in order to replace any volume lost due to leakage. Loss is typically measured in mass per unit time, such as lbm/min (pound mass per minute) or kg/min (kilograms per minute). Because mass flow depends on mass, and not just volume, the density of the fluid (mass per unit volume) directly affects the leakage measurement.

Helium Leak Test

Helium leak testing is considered the most stringent method for detecting leakage, capable of identifying leaks as small as 1E^–09 SCCS at the molecular level. This exceptional sensitivity is largely due to helium’s small atomic size – roughly half that of nitrogen – which allows it to pass through extremely fine openings.

There are several different ways a helium leak test can be performed:

Sniffer Probe: In this technique, the test component is pressurized with helium and a sniffer probe is used to detect escaped gas. An operator manually scans all external surfaces by moving the sniffer probe near any suspected leak paths. Sniffer probes are used to confirm that leakage is below a set maximum amount. They are not designed to provide exact measurements or data points. For applications where test sensitivity and cost are more important than speed or repeatability, this method proves especially useful. Sniffer probes are one of the lower cost helium leakage tests and allow the operator to pinpoint the source of any localized leaks and obtain a leakage value.

The primary disadvantages of this method are its slow cycle time and poor measurement repeatability. Because the operator must manually scan all external surfaces with the sniffer probe, the process is time-intensive. Atmospheric conditions – even those as minor as a small gust of wind – can impact the reading. The observed leakage value also tends to be inconsistent, leading to poor GR&R performance.

Accumulation: Using the accumulation method, the test component is placed in a chamber and pressurized with helium. The part remains in the chamber for a set period of time and a sniffer probe is used to detect the presence of helium within the chamber. Accumulation testing can detect leak paths originating from any location within the part.

This method is inexpensive to set up and removes the operator as a potential source of error. It is important to note that residual gas from prior tests can provide false positive results. To prevent this, the setup will require adequate circulation in the chamber as well as long test cycle times. This can make it difficult to provide firm, quantitative test results unless the test is performed over an extended period.

Vacuum: In vacuum testing – like the accumulation method – the component is placed in a chamber and pressurized with helium. The chamber is evacuated by vacuum to eliminate any residual helium from previous tests and minimize interference caused by stagnant or trapped gasses in the chamber. A mass spectrometer is then used to identify and measure the presence of helium leaking into the chamber.

Vacuum testing offers significantly higher sensitivity, making it the most appropriate leak test method for applications with extremely low leakage requirements. It’s important to note that this method is not ideal for very low pressure applications. Since the test relies on creating a pressure differential by pulling a vacuum, the test pressure must be above the differential generated by the vacuum related to the inlet, which is typically 14.7 psid. It is also the most expensive method due to the equipment, maintenance, setup, and testing times required.

The cost of helium gas is a notable drawback for all of these testing methods. Helium is significantly more expensive than alternatives like compressed air or nitrogen, and its price is subject to frequent fluctuations. These factors contribute to higher operational costs when performing helium-based leakage tests. As a result, helium leak testing is often considered a method of last resort.

For high precision aerospace applications where the smallest leak can be devastating, utilizing helium as a test fluid remains the gold standard. However, to manage costs effectively, engineers often employ referee gases and alternative methods selectively, using high precision techniques only when necessary.