What is Air Leakage?
Air leakage is the unwanted flow of air through the wall of a vessel or sealed container. Pressure variation from the inside of the vessel to the atmosphere outside the vessel cause the air to flow from higher pressure to lower pressure areas. Air leakage generally occurs from the inside out—from inside an object (higher pressure) to outside the object (lower pressure)—but can also happen from the outside in when ambient atmosphere flows into an evacuated (vacuum) vessel.
What is Air Leak Testing?
Air leak testing is an effective method of determining if a product has been manufactured to meet leak-tight specifications. Microscopic holes, failed seals, and countless other defects can cause air leaks that can lead to significant problems. Everything from automotive radiators to medical devices to sprinkler system components requires air leak testing to ensure proper and safe operation.
In general, air leak testing is clean, non-destructive, and highly repeatable. Air leak tests can be performed on any product that requires air-tight or water-tight construction. It is used not only to detect leaks, but also to provide non-subjective data on leakage rates for further analysis. Air leak testing helps OEMs ensure that their products meet the required leak-tight specifications and helps them find and eliminate design and/or manufacturing flaws.
Advantages of Air Leak Testing
Compared to other leak test processes, air leak testing provides significant advantages in three key areas: time/work saving, quality improvement, and cost reduction.
Time Saving/Work Saving
- Many air leak testing processes can be fully or partially automated
- Most air leak testing equipment is relatively easy to use, with no special training required
- Maintenance is generally easy, and requirements are minimal
- Leak standards can be quantified
- Product defect rate can be greatly reduced
- Leak measurement data can be used in statistical processing/analysis for future product improvements
- Product yield rates are improved as rejects (and claims) are reduced
- Test time is reduced
- Operational costs are relatively low for most air leak testing processes
Air Leak Testing Methods
Bubble Test/Submersion TestPerhaps the most basic air leak testing method, bubble testing involves pressurizing the part or vessel, submerging it in water, and watching for bubbles caused by escaping air/gas. The size of the leak is directly proportional to the intensity and size of the bubbles created. While submersion testing is effective for locating leaks, it is not effective for accurately measuring or quantifying a leak.
Ultrasonic MeasurementEvery leak creates some sort of sound, and the frequency of that sound can be higher or lower depending on the size of the leak. The smaller the leak, the higher the frequency—in many cases, fine leaks emit sound at a frequency that is too high for the human ear to hear. Ultrasonic leak detectors are, most often, handheld devices that are paired with headphones, an adjustable meter, and accompanying computer software; other accessories may be used for certain applications. Ultrasonic measurement can generally detect leaks up to 10-2 scc/s, making them best suited to finding “larger” small leaks, but not very fine leaks. Additionally, these devices cannot actually measure the leak rate, but only provide an estimate based on the frequency of the sound.
Pressure Decay TestingPressure decay air leak testing monitors the internal pressure of a sealed part or vessel. The vessel is first pressurized to a predetermined level, usually with dry air or nitrogen, then monitored for set amount of time; any decrease in pressure indicates a leak. The sensitivity of pressure decay testing depends on the total volume of the part being tested, as well as the test time and the resolution of the pressure transducer. In general, this air leak testing method is easier to use than many other processes for leak rates as low as 10-2 scc/s. However, it can be susceptible to environmental and mechanical instability.
Vacuum Decay TestingVacuum decay testing is essentially the opposite of pressure decay testing. Instead of pressurizing the part or vessel, it is evacuated and monitored for any increase in pressure caused by test media entering the part. Vacuum decay air leak testing is more sensitive and precise than pressure decay testing, and is less affected by temperature changes or other environmental factors. However, the pressure difference applied to the part during test cannot be greater than 14.7 psiv. Vacuum decay testing may also be susceptible to liquid evaporation and surface outgassing, which can affect and limit sensitivity as traces of humidity/evaporation may increase internal pressure and create a false leak.
Helium Sniff TestingIn helium sniff testing, the part or vessel is pressurized with helium to a predetermined level, then scanned across its surface with a helium sniffer device. Any leak in the vessel will cause an increase in helium concentration at the point of the leak, which will be detected by the sniffer. Helium sniff testing not only locates the leak point, it is also highly sensitive (down to 10-5 scc/s). However, as it generally requires manual operation, it offers slower throughput than some other air leak testing methods, and its effectiveness is limited in environments where background air flow reduces the concentration of tracer gas or in areas where there is a high concentration of helium near the test site.
Helium Spray TestingFor helium spray testing, the part or vessel is evacuated using a pumping system with an integrated helium detector. Any gross leak will prevent the vessel from reaching vacuum. If vacuum is reached successfully, the part is then sprayed with helium; any fine leak will pull helium into the evacuated part. The helium detector will sense the helium and measure the leak rate. Helium spray air leak testing can accurately locate a leak and is highly sensitive (down to 10-6 scc/s). However, multiple leaks may be “masked” if they are located too close to each other, and throughput is generally low.
Helium Accumulation TestingThis air leak testing method starts with the part or vessel inside a sealed enclosure and connected to a pump (or pumps) and a helium sensing device. The vessel is evacuated; if vacuum cannot be reached, a gross leak is present. Once vacuum is achieved, the part is pressurized with helium to a predetermined level. The ambient air within the chamber is continuously mixed via blowers/fans to guarantee uniform air concentration. After a set length of time, air samples are pumped through the helium sensor, which monitors the helium pressure to detect any increase from the initial atmospheric level—any pressure change indicates a leak in the part. Helium accumulation air leak testing offers good sensitivity (down to 10-3 mbar l/s, depending on enclosure size), high reliability, and good repeatability, and is not susceptible to temperature.
Nitrogen Purge TestingCTS’ patented nitrogen purge testing instrument is a localized, sniffer-style tester with superior background control that reduces the chances of false leak failures. Utilizing low pressure nitrogen, the system creates a helium-free environment for the mass spectrometer gas analyzer to sample and detect micro-leaks. Our nitrogen purge testing technology can detect leaks as low as 10-6 scc/s.
Hard Vacuum TestingIn hard vacuum leak testing, a part or vessel is placed inside a sealed chamber. A vacuum pump removes all the atmosphere from the chamber and the part; the part is then filled with tracer gas, such as helium. The mass spectrometer then samples the air inside the test chamber for tracer gas escaping the part. Vacuum testing is extremely sensitive (down to 10-8 scc/s), highly repeatable, and provides reliable and quantifiable results. It can be fully automated and integrated into a production line for fast throughput. This is, however, perhaps the most expensive air leak testing method available.
How to Choose an Air Leak Tester
The most important factor to consider when choosing an air leak testing device is the acceptable leak rate of the tested part or vessel. The largest influence on the acceptable leak rate is the media that must be retained—is it liquid/fluid or gaseous? Air, for example, will pass through a leak path up to 300 times faster than water will travel that same path. Standard non-water (air or gas) leak rates are generally between 1-5 scc/min (standard cubic centimeters per minute), though sometimes as high as 10 scc/min.
The other key factor in selecting an air leak tester is that the device must be capable of supplying at least 100 times greater resolution than the leak rate being measured.
What’s the Difference Between Air Leak Testing & Pressure Testing?
- Air leak testing is intended to detect and locate leaks in sealed, pressurized part or vessel. A wide variety of air leak detection procedures may be used (see above).
- Pressure testing, or proof-pressure testing, is a form of stress testing used to verify that a component can withstand pressures above its intended operating range without incurring permanent damage. Most pressure testing is done via hydrostatic or pneumatic testing.
Air Leak Testing for Medical Devices
Air leak testing is extremely important for medical devices to ensure proper operation and patient health. Many medical devices utilize fluids, gases, or both, thus providing effective barriers for and ensuring the correct passage of these media is critical for the device’s performance and to protect the patient. In general, sample testing is insufficient—100% of parts and components must be tested.
The most common air leak testing methods for medical devices are pressure decay, vacuum decay, and tracer gas testing (see above). Hydrogen is the most commonly used tracer gas, because it tends to spread quickly to penetrate leaks and vents away faster than most other tracer gases. Special considerations must be made for medical device air leak testing, such as testing that is not destructive or corrosive to tested parts, adjustments for flexible or elastic materials, and proper tracer gas removal for sequential testing.
Performing Automotive Air Leak Testing
There are dozens of components and systems in an automobile that may require air leak testing. Common automotive parts that may require air leak testing include brake lines, exhaust components, engine intake manifolds, radiators, and A/C components, among many others.
A number of air leak detection options may be used, depending on where the leak may be originate. From simple soap-and-water spray testing to advanced vacuum or pressure decay testing systems, these tests can give a better look at the air leak problem by pinpointing the location of the leak and measuring its severity.
How to Leak Test Automotive Fuel Rails
High pressure fuel injection is the key to lower fuel consumption in modern automobiles. An improperly calibrated fuel injection system can lead to poor gas mileage, a fire in the engine compartment, or worse. Most vehicles use high pressure direct injection fuel rails, which may operate at pressures as high as 2,000 bar (roughly 150,000 psi).
Fuel rails need to be pressure tested during production to protect against fuel leakage. Most often, the complete subassembly (fuel rail with fuel injectors) is tested; connections and fittings must be tested, as well, after final assembly. The maximum allowable leak rate, in most cases, is 10-4 scc/s.
The most common fuel rail air leak testing method is actually a tracer gas test using helium under vacuum. The fuel rail subassembly is sealed, then enclosed in a chamber; fuel rail and chamber are evacuated simultaneously. The fuel rail is backfilled with helium, and any leakage of helium into the chamber indicates a leak. Clamshell leak testers may also be used—these devices enclose the individual connectors being tested, then use the same general process of evacuation and helium backfill.
Advantages of tracer gas leak testing include accurate and repeatable measurements and high leak sensitivity to detect very fine leaks.
Air Leak Testing Equipment
Cincinnati Test Systems (CTS) offers a full line of air leak testing equipment and accessories. We also offer custom-engineered systems made to match your exact testing specifications. Our air leak test systems include:
- Sentinel™ Blackbelt Pro
- Sentinel™ Blackbelt
- Sentinel™ C20 World Edition
- Sentinel™ C28 World Edition
- Sentinel™ C28 Differential Pressure
- Sentinel™ I28
- TracerMate CS
All CTS air leak testing equipment is calibrated and verified to NIST traceable leak standards. We operate an A2LA lab, accredited to ISO 17025, for the manufacture and recertification of NIST traceable calibrated leak/flow standards.
Contact CTS for Complete Air Leak Testing Solutions
Cincinnati Test Systems is the industry leader in high performance air leak testing equipment. Request a quote on the leak test solution you need, or contact us for more information.