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.
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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. SCCM.
The benefits of pressure decay leak testing include:
- Economical
- Fast (depending on the internal test volume of a part)
- Easy to set up
- Only requires compressed clean,
- dry air
- Quantitative
- Nondestructive
- 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:
- Automotive
- Sealed Electronics
- Medical Devices
- Packaging
- 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
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Pressure Decay &#; Measures the pressure change of an object under positive pressure
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Vacuum Decay &#; Measures the pressure change of an object under negative pressure
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Occlusion &#; Checks for a blockage in the gas flow path of an object
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Burst &#; A destructive or nondestructive ramping pressure test that measures the point at which the device opens or has a catastrophic event (rupture).
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Crack &#; Typically performed on check valves to detect weeping prior to reaching the opening pressure. A downstream sensor monitors for weeping.
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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.
Medium
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.
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Pressure Range
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.
Internal Volume
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.
Material
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.
Design
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.
Technological Advances
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.
As with almost every facet of vacuum systems, there is no single method which fulfils every situation and every criterion. This is certainly the case with leak detection, with four main methods being employed: the bubble test; pressure decay test; pressure rise test; and helium sniffer mode/helium vacuum mode tests. These four tests roughly correspond to the &#;simplistic&#; bubble test (for low-vacuum pressures), through to the &#;high-tech&#; helium tests (for high-vacuum pressures).
The Bubble test is best illustrated by placing a punctured bicycle tube under water and marking where the bubbles come from or placing washing-up liquid around the joint of an active water/gas pipe and observing whether the liquid forms a froth. Both are reliable ways of detecting a low-pressure leak. The bubble test is employed up to vacuums of 10-4 mbar.
The pump-down test is conducted by evacuating a closed vacuum vessel until a certain pressure is obtained, then closing the pump&#;s inlet valve. After a pre-determined period of time, the inlet valve is again opened, and the time is recorded for the pump to return the vacuum to the original evacuated level. This process is repeated a number of times. If the time to return the vacuum to the original level remains constant, then a leak is present. If this time period decreases, this indicates reduced gas liberation (outgassing) on the inside of the system (i.e. a &#;virtual&#; leak), however, it does not exclude a leak from also being present.
Alternatively, the pressure rise test is made by plotting the vacuum level against the time after a vacuum level has been achieved, and after isolating the system, the curve will be a straight line if a leak is present. However, if the pressure rise is due to gas liberation from the system walls, the rise will gradually taper off to reach a final, stable value.
In most instances both phenomena occur simultaneously, which makes separating one from the other almost impossible. If the volume of the chamber or item under test is known, then the leak rate can be calculated (i.e. the volume x (measured rise in pressure)/time taken).
The pressure drop test is not dissimilar to the pressure rise test. It is only rarely used to check leaks in vacuum systems, and only when the (positive) gauge pressure does not exceed 1 bar, since the flange connections used in vacuum technology will not tolerate higher pressures.
However, the pressure drop test is frequently employed in tank engineering. Pressure drop tests allow leak rate measurements to 10-4 mbar*l/s but results can be distorted if condensation occurs. As one can see, the differential pressure decay test is fraught with caveats, but if employed under laboratory conditions, it is a good tool at determining both leaks and leak rates.
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