Quality Test & Inspection: Improve Pressure Decay Leak Testing

Hydrogen testing addresses the limitations of the pressure decay method.

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 injected into a test object, and any decrease in air pressure over time signifies a leak. However, the pressure decay method has limited sensitivity and is unable to determine the location of leaks.

Another technology, hydrogen leak testing, addresses these shortcomings. Depending on the application, hydrogen testing functions as an enhancement to pressure decay systems or as a substitute method. The hydrogen and pressure decay methods are complementary, using similar test procedures and test apparatus.

Varying methods

The hydrogen method employs a robust, self-calibrating and

maintenance-free microelectronic probe that is sensitive and 100% selective to hydrogen. The test gas, a non-flammable mix of hydrogen and nitrogen, is injected in 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.

Before 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 more appropriate for a laboratory than a manufacturing floor.

Helium testing must be performed in a vacuum, requiring the installation and maintenance of a well-engineered vacuum chamber and multiple stages of vacuum pumps. 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 hydrogen-testing instrument is 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 resemble pressure decay.

Locate leaks

p> 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 needed to repair rejected items and to make sure quality assurance can implement the appropriate corrective action in the manufacturing process.

When leak location is determined with hydrogen, a tracer gas charging unit can be incorporated in 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. Leak-location testing also can be manually performed by submerging objects in water or by applying soap bubbles to the exterior, but these wet methods are messy, time-consuming, prone to operator error and may be corrosive to the test objects.

The pressure-decay method measures total leakage, including 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 a hydrogen-charging unit and a hydrogen probe to the pressure-decay system, such defects can be easily pinpointed and corrected, thereby reducing down time.

Sensitivity, reliability and cycle time

The pressure-decay method provides limited sensitivity, but only is viable for rigid objects with a 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 a long cycle time to achieve an adequate level of sensitivity for most applications. For medium-sized objects, sensitivity is limited to the detection of leaks emitting 0.5-1.0 cubic centimeter per minute-10 times less sensitive than current tightness specifications for automotive components containing fuel and several orders of magnitude away from the requirements for components that contain gas such as refrigeration and air conditioning parts.

Pressure-decay testing is susceptible 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 software algorithms and thermometers that compensate for temperature distortion to a limited degree, but it is not possible to fully eliminate this problem.

External temperature variations also can have an affect on the method. For example, the heat from a human hand or a breeze from an open door can throw off the test results and cause false acceptance of an aluminum object.

The pressure decay method is 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.

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 obsolete. Existing pressure decay systems can be upgraded or retrofitted to use hydrogen, or replacement systems can be installed.

A combination approach sometimes is possible for objects with multiple compartments that have differing sizes, materials and tightness specifications. This combination approach also can 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 also is useful when the test object contains elastic or highly plastic sub-components such as hoses.

Compatibility

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 quality standards.

Because the testing process, test apparatus, training requirements and 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 to achieve higher sensitivity, leak location ability, improved reliability and shorter cycle time. Q

Claes Nylander is president of Sensistor Technologies Inc. (North Billerica, MA). For more information, call (978) 439-9200 or e-mail [email protected].