Tracer gas leak testing

The term "tracer gas leak testing" describes a group of test methods to detect and measure a tracer gas flowing through a leak. These techniques differ for the tracer gas used and for the realization technology. The most commonly used tracer gases are halogen gases (e.g., CFC, HCFC, and HFC refrigerant), helium, and a mixture of nitrogen and hydrogen (95% / 5%). Helium has been used successfully as a tracer gas for a long time because of its physical properties (12, 13). It is neither toxic nor flammable and is does not react with other compounds. Helium has low viscosity and relatively low molecular mass, so it easily passes through cracks. In the same environmental conditions, it flows through orifices 2.7 times faster than air. Since its concentration in air is low (i.e., 5 ppm), it is easy to detect an increment of helium concentration. It is important to remember that background concentration in air is a limiting factor for any tracer gas detector.

There are two ways to carry out leak testing with tracer gas: internal detection of tracer gas entering from leaks (i.e., outside-in method) and external detection of tracer gas escaping from leaks of a filled unit (i.e., inside-out method). The inside-out method can be executed with atmospheric sniffing or with vacuum chamber detection, while the outside-in method is generally implemented by putting the unit to be tested in a room containing the tracer gas or, very rarely, spraying the tracer gas on the unit surface.

Sniffing, an inside-out technique, involves a probe or wand being moved over the test part. The probe detects the leak as it passes over the leak. The speed, distance from the part, and the probe sensitivity determine the accuracy of leak detection. However, sniffing has the distinct advantage of being able to locate a leak on the test part, unlike the other methods described, and has the ability to sense leaks as small as 10^-7 mbar∙l/s, depending on the tracer gas. Sniffing is not recommended in a high volume production environment, other than for locating leaks for repair. Disadvantages include a high chance of missing leaks due to operator dependency, fragile equipment in rugged environments, and the rejection of good parts because of the inability to quantify the leak. The minimum leak rate measurable by a sniffer is the concentration of the tracer gas in the working area, a value known as background level. This level may change during the production cycle and increases because of leaking units. Relating to the tracer gas used, in case of a big leak in the part under test, a lot of tracer gas escapes from it and may remain for a long time in the working area, strongly affecting the subsequent tests causing rejection of good parts.

Vacuum chamber inside-out leak testing is the most complex system of leak detection, but it is theoretically suitable for finding very small leaks, using the proper tracer gas. This method involves placing the test bag inside a vacuum chamber connected to the integrated helium leak detection system. The vacuum chamber encapsulates the entire test bag, ensuring that any helium molecules escaping from the bag are captured and directed towards the sensor.

Depending on the vacuum chamber dimensions, the evacuation group could call for a high pumping speed, which would introduce high level of particles in the test bag. In the case of a large leak in the part under test, a large amount of tracer gas escapes relative to the amount of tracer gas used. A long pumping time could be required to lower the tracer gas in the detector to an acceptable level compatible with system function, during which the system is unusable.

Although use of helium gas as a tracer gas has been well documented in the literature, its use for detecting defects in flexible bags used in the biopharmaceutical industry has been limited because of the operational and functional issues listed below:

1. Long down times due to inefficient process: Helium gas disperses slowly into the atmosphere, so, in the case of large leaks, its high concentration will contaminate the area for a long time, even hours, effectively making it inefficient for production purposes.

2. Raw material/equipment cost: Conventional belief that highly pressurized systems provide high sensitivity to method detection results in large quantities of helium being used and hence, high costs. The most suitable helium detector is based on a mass spectrometer, which is an expensive and delicate apparatus requiring much care and maintenance. There are high costs due to loss of tracer gas for the test.

3. Test time: To be able to detect smaller leaks, the test time can be long which affects production cycle time and hence overall cost of the product.

4. Product performance effect: The testing method can impact product cleanliness performance and may increase bio-burden level of test bag.

5. Nature of flexible bags: Traditional hard vacuum leak testing is often difficult to perform on flexible wall parts. The pressures resulting from a full vacuum can damage the part.

To address issues discussed above, ATMI developed the helium integrity test (HIT) method and apparatus which allows for fine leak testing of parts using a helium mass spectrometer (MS) leak detector. The technology is based on the inside-out testing principle, however, it addresses issues that prevented its use in leak testing of flexible bags. This method can achieve desired results at lower part differential test pressures and with faster cycle times compared with traditional helium accumulation methods.