Leak testing is a broad term that includes a multitude of technologies. For the purposes of this article, different leak test methods will be referenced but not described in full detail. This article defines the broad term leak test and takes a closer look at the pressure decay leak test method. Furthermore, this article will describe how a pressure decay test works, considerations for the pressure decay method, and how recent technological advancements have impacted manufacturing environments.
What is Leak Testing?
A leak test is a procedure used to determine if an object, product, or system functions within a specified leak limit. A leak occurs when a gas or liquid flows through an object via an imperfection or manufacturing defect such as a hole, crack or weak seal. These imperfections create high- and low-pressure zones within a product, forcing the gas or liquid to flow from the high-pressure area to the low-pressure area. The primary leak test method discussed in this article uses pressurized air to identify leaks.
Leak Testing Methods
There are many different types of leak test methods, which have different detectable leak rate limits. This article will primarily discuss the pressure decay leak test method. A pressure decay test identifies if a part is leaking or not within a predetermined leak rate limit. The smallest detectable leak rate for the pressure decay method is 10-4 mbar*l/s or 0.0059 SCCM.
The benefits of pressure decay leak testing include:
- Fast (depending on the internal test volume of a part)
- Easy to set up
- Only requires compressed clean,
- dry air
- No pre or post processing required
Determining a leak rate is vital to selecting the best leak test method.
Typical industries for pressure decay leak test methods include:
- Sealed Electronics
- Medical Devices
- Consumer Goods
An object with one port or opening, such as a catheter, can be directly connected to the front port of a leak tester without a fixture.
How a pressure decay test works
During a pressure decay test, a product is attached to a leak test system and filled with air. Once pressurized, the air source is closed off and the pressure is allowed to settle. During the test any decrease in air pressure over time signifies a leak.
Variations of pressure decay methods
- Pressure Decay – Measures the pressure change of an object under positive pressure
- Vacuum Decay – Measures the pressure change of an object under negative pressure
- Occlusion – Checks for a blockage in the gas flow path of an object
- Burst – A destructive or nondestructive ramping pressure test that measures the point at which the device opens or has a catastrophic event (rupture).
- Crack – Typically performed on check valves to detect weeping prior to reaching the opening pressure. A downstream sensor monitors for weeping.
- Chamber – Finds leaks in sealed packaging or devices that do not include an opening for filling.
Depending on the functional use of an object or part, any of the above tests may be required.
With the implementation of EtherNet/IP and the Internet of Things (IoT), leak test data collection and monitoring can be stored and shared remotely.
Considerations for leak testing:
What is the intended use of the part?
What medium is being constrained inside or outside of a part? A medical device manufacturer designing an IV set may try to keep saline inside the IV set. An automotive manufacturer may have designed their manifold to prevent exhaust gases from escaping the exhaust manifold. An acceptable hole or porosity in these parts is contingent upon the application by which a leak limit will be determined.
An oil molecule is larger than a water molecule and a water molecule is larger than an air molecule. If an eight micron hole is subjected to 45 psi of pressure, air will create a noticeable leak, whereas water will only create a droplet and oil may not leak at all.
During a pressure decay test, a product is attached to a leak test system and filled with air. Once pressurized, the air source is closed off and the pressure is allowed to settle.
What pressure range is a part subjected to during use? What safety factor is required? Finding the appropriate pressure range for an application is vital. If the test pressure is too low, quality may be affected, and faulty parts may pass the leak test and go on to fail during use. Conversely, selecting a pressure range that is too high will extend the time it takes to complete the test and possibly damage the part.
The test volume plays a significant role in creating a repeatable and sensitive test. Reducing the overall volume enables shorter test times with greater sensitivity. Parts that cannot have a volume reduction will benefit from pneumatic and sensor assemblies appropriately sized to meet test cycle expectations.
Acceptable Leak Rate
Everything leaks but what leak rate is acceptable for an application? Leak rates are most often specified by regulatory requirements specific to an industry. Often when testing a new product some trial and error is involved in finding what leak rate is acceptable for the product’s application.
The material of the part under test affects test time. Compliance influences both the fill and settle steps in a leak test. If a malleable object is subjected to pressure and is not given enough time to settle before a test begins, the part may be expanding or contracting during the leak test, yielding inconsistent results.
During the design and development of a product it is important to consider the test criteria to ensure a part has been manufactured correctly. Design criteria such as test pressures, access ports, shared walls or vessels, internal test volumes, and leak rate specifications all have an impact on the complexity and time required for a leak test. How the object connects to a leak test system helps to determine what kind of leak test will be run on the object. A fully enclosed object with no port to fill the item, such as a waterproof enclosed electronic device, requires a chamber test which includes a sealing fixture. An object with one port or opening, such as a catheter, can be directly connected to the front port of a leak tester without a fixture.
A leak test is a procedure used to determine if an object, product, or system functions within a specified leak limit.
Technology inside leak testing systems has grown in capability and sensitivity. As previously mentioned, leak test time is highly dependent upon the specifications of the object under test. Technological advances in valves, sensors, chips and regulators, enable faster and more accurate leak tests. This allows manufacturers to improve throughput and quality.
With the implementation of EtherNet/IP and the Internet of Things (IoT), leak test data collection and monitoring can be stored and shared remotely. A production manager or a manufacturing engineer can evaluate their leak test results conveniently in their office. In addition, with appropriately sized valves, faster chips and more accurate sensors, manufacturers can benefit from improved quality and accuracy.