Enhancing pressure decay leak testing with hydrogen trace gas
Pressure decay has been the most widely used method of leak testing in manufacturing production lines for decades. The process is uncomplicated, inexpensive and easily automated. Air is simply injected into a test object, and any decrease in air pressure over time signifies a leak. However, the pressure decay method has significant shortcomings, such as limited sensitivity and the inability to determine the location of leaks.
A relatively new technology, hydrogen leak testing, addresses these shortcomings. Depending on the application, hydrogen testing can be an enhancement or a substitute to pressure decay methods. The hydrogen and pressure decay methods are complementary, using similar test procedures and test apparatus.
The hydrogen method uses a robust, self-calibrating and maintenance-free microelectronic probe that is extremely sensitive and 100% selective to hydrogen. The test gas (a non-flammable mix of hydrogen and nitrogen) is injected into the test object, and leakage is detected in a variety of ways. The test object can be enclosed in an accumulation chamber, where the presence of hydrogen is measured over a certain time interval to determine the total leakage. Alternatively, a hydrogen probe can scan the object’s exterior, either manually or robotically, to pinpoint the location of leaks.
Prior to the invention of hydrogen leak testing probes, helium was the only tracer-gas method used for automated leak testing. Unfortunately, helium proved cost-prohibitive for many leak-testing applications. Helium testing offers high sensitivity, but it uses a mass spectrometer — an expensive and delicate apparatus that requires regular maintenance depending on the contamination levels to which it is exposed. This makes helium testing more appropriate for a laboratory than a manufacturing floor.
Moreover, helium testing is typically performed within a vacuum, requiring the installation and maintenance of a well-engineered vacuum chamber and multiple stages of vacuum pumps. Perhaps most significantly, the helium gas itself is an expensive, scarce natural resource.
Hydrogen testing offers equivalently high sensitivity without the high cost and complexity of the helium method. The price of the hydrogen gas mixture is significantly less than the price of helium. The hydrogen-testing instrument is far less expensive to purchase and maintain than mass spectrometers and the process does not require a vacuum chamber. In general, the process, testing apparatus, training requirements, and cost of the hydrogen method more closely resembles pressure decay.
Finding the leak location
Pressure decay is an “integral” test, meaning that it measures the total leakage from an entire object. It does not locate the specific source or sources of leakage. Determining the location of leaks is necessary if rejected items are to be repaired. It is also important for quality assurance to implement the appropriate corrective action in the manufacturing process.
Leak location is easily done with hydrogen. A tracer-gas charging-unit can be incorporated into a pressure decay test system. When the pressure decay system detects a leak, hydrogen is injected into the object and the hydrogen probe scans the exterior of the object, manually or robotically, to quickly and accurately pinpoint the location.
Alternatively, objects rejected by the pressure decay system can be set aside and subsequently tested offline by a separate hydrogen-based leak detection system. It is also possible to manually perform leak-location testing by submerging objects in water or applying soap bubbles to the exterior, but these “wet” methods are messy, very time-consuming, prone to operator error, and possibly corrosive to the test objects.
Maintaining the testing apparatus
The pressure decay method measures total leakage including any leaks in the test equipment itself, as well as possible leaks in seals and connections to the test object. Several false rejections can occur before this problem is suspected. Detecting the location of the defect in the testing system can be difficult and time-consuming.
By adding the hydrogen charging unit and hydrogen probe to the pressure decay system, it is possible to easily pinpoint and correct such defects, thereby significantly reducing downtime.
The pressure decay method provides limited sensitivity, and it is really only viable for rigid objects with a relatively small internal volume. For pressure decay, sensitivity is a function of the object’s size and the time interval of the test. Medium and large objects require an unacceptably long cycle time to achieve an adequate level of sensitivity for most applications. For medium-size objects, sensitivity is limited to the detection of leaks emitting 0.5 cc/min to 1.0 cc/min — ten times less sensitive than the new tightness specifications for automotive components containing fuel and several orders of magnitude away from the requirements for components containing gas, such as refrigeration and air conditioning parts.
Improving reliability and cycle time
Another problem with pressure decay testing is the susceptibility to distortion by changes in the temperature of the air inside the test object. Temperature rises as air is compressed and the test processes must wait until the temperature stabilizes. Some pressure decay systems now employ algorithms and thermometers that can compensate for temperature distortion to a limited degree, but it is not possible to fully eliminate this problem.
External temperature variations can also have an effect. For example, the heat from a human hand or a breeze from an open door can affect the test results and cause the false acceptance of an aluminum object.
The pressure decay method is also ill suited to testing elastic or plastic materials. Elasticity counteracts the pressure decay and plasticity may give the opposite effect if material gives way under pressure.
Substituting when necessary
It is not possible to completely fix these sensitivity, reliability and cycle time limitations of the pressure decay method. Instead, hydrogen testing is introduced as a substitute method when one or more of these issues render pressure decay less effective or not suitable. Existing pressure decay systems can be upgraded or retrofitted to use hydrogen or replacement systems can be installed.
A combination approach is sometimes possible for objects with multiple compartments that have differing sizes, materials and tightness specifications. Some systems use the pressure decay method to test one compartment or section of a test object such as a gearbox, and use the hydrogen method to test a different compartment of the same object. The compartment tested with hydrogen must meet a more stringent tightness standard because it will operate at a higher pressure.
This combination approach can also be used for objects with a single compartment. Pressure decay can be employed as an integral test for the object as a whole, and hydrogen testing can be conducted with a local enclosure applied to particularly sensitive locations. A similar approach is also useful when the test object contains elastic or highly plastic sub-components such as hoses.
Enhancing pressure decay with hydrogen
Hydrogen and pressure decay are compatible methods that can be deployed interchangeably or in conjunction with each other depending on the requirements of the manufacturing process and the quality standards.
Because the testing process, the test apparatus, the training requirements and the cost of the hydrogen method closely match the characteristics of pressure decay, it is easy for manufacturers to incorporate hydrogen into their existing pressure decay systems or replace them with hydrogen systems in order to achieve higher sensitivity, leak location ability, improved reliability, and shorter cycle time.