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Claes Nylander

Metrology

Leak Testing: Moving Beyond the Most Popular Methods

From pressure decay to the lightest gas, each method has its advantages and drawbacks.

Published: Wednesday, May 11, 2005 - 21:00

Leak testing is an essential element in product quality testing for a wide range of industries. From the automotive industry to heating, ventilation and air conditioning manufacturing, countless products and parts have to be tested for tightness in order to meet specifications and be granted the positive end of the accept/reject option. In fact, for many suppliers to the automotive industry, leak testing is an integrated part of production: 100 percent of their parts are tested against a leak standard to meet quality requirements that are as important as the price or design of the product. Although no system is ever absolutely tight, leak testing strives for measuring exactly the leak rate of a manufactured part or product, ensuring that the product has been tested for maximum allowable leakage.

Leak testing may be performed for several reasons, all extremely important. It ensures that the outer environment is protected, keeping flammable, toxic or corrosive substances within an object’s walls. It can also ensure that a liquid or gas essential to the function of a system such as brakes, air conditioning units or hydraulic valves is contained within that system for the necessary period of time.

Bursting the bubble of water-dunk testing.There’s no argument that leak testing is important, but the question of which leak testing methods are the most practical and effective remains. For years, the simpler methods have been the most popular. Methods prevalent in every industry include water-dunk testing and pressure decay. Each offers the advantage of minimal investment, but they both also have major drawbacks.

Dunking an object into water can be an effective method for determining if and where an object has a leak. Theoretically, bubbles form at the source of the leak as a result of air pressure, and the amount of bubbles per minute can signify the size of the leak.

However, water dunking provides minimal quality assurance. A very small leak might make few and/or miniscule bubbles. If the leak is within a recess, air from the leak may collect inside the recess and stay there, never to be detected. Whether air bubbles rise to the surface or stick to the test object depends on surface tension.

Also, water dunking is extremely dependent on the operator’s involvement. When manually dipping an object into the water, the operator can pull down air bubbles that mask bubbles from a small leak. The operator needs to wait until the object has cleared itself of bubbles that are not leak-related. Also, the operator’s perspective can be limited: He or she may not be able to see a small leak if it’s on the reverse side of the object.

One final drawback to the water-dunking method is tied to the water itself. Water can have a number of debilitating effects on products, the most harmful being corrosion. Organizations won’t want to employ a leak testing method that can itself be the cause of product failure.

Pressure-decay methods feel the heat
The most common method for leak testing, pressure decay, measures the decrease in pressure in an object. A drop in pressure signifies a leak: The greater the pressure drop, the larger the leak. This method is convenient because it can be easily automated and it’s dry.

But pressure-decay testing isn’t 100-percent accurate at all times because it depends on a number of variables. For example, it’s an incomplete method, as it can’t be used to pinpoint the location of a leak. Finding the source of the leak requires additional tests.

The success of pressure-decay testing is also highly dependent on materials and temperatures. The ability to measure extremely small leaks hinges on the internal volume of the object and whether the object is made of rigid or flexible materials. This type of test also relies heavily on temperature: Temperatures rise as the air is compressed inside the object, and the pressure won’t stabilize until the temperature has stabilized. Temperature can also be affected by elements outside the test. For example, if an aluminum object is being tested, the heat from an operator’s hand or a breeze from an open door can completely throw off the test results and be the cause for false acceptance.

When there’s a need to test a large object, such as a gasoline tank, pressure-decay testing isn’t -the best choice. Objects that are large in volume demand too long of a cycle time to gather results from this method. Pressure-decay testing really only works well for small-volume objects.

Finally, there can be issues with efficiently testing flexible plastic bottles and rubber parts using pressure decay, as they counteract the pressure decay by reducing their volume.

Testing with tracer gases is evidently better
In recent years, more complex methods, such as the use of tracer gases including helium and hydrogen, have proven to be the most effective methods for detecting evidence of a leak and for measuring that leak.

Helium has been the most commonly used tracer gas for leak testing. This is because helium is the lightest of the inert gases and helium mass spectrometers are extremely sensitive to trace amounts of this gas. Mass spectrometers, commonly used to analyze unknown gases, have recently been gaining some notoriety for their Martian mineral-testing application, and they’ve also been developed for leak testing applications. Set up to detect helium as it dissipates from an object, these helium mass spectrometers typically have an external pump that create a vacuum outside the object, which allows for the relatively easy detection of helium atoms.

But the helium method, too, has its drawbacks. A mass spectrometer is a delicate piece of equipment and is very expensive to maintain. In addition, the machine’s pumps need to be regularly checked and serviced.

Also, the helium itself can be the cause of problems. Helium is a highly viscous gas and, should it spill, it can be very difficult to clear from the testing equipment. Helium has a tendency to cling to surfaces. It’s expensive to buy, and as it’s a nonrenewable natural resource, the price is always on the rise .

Seeing the light with the lightest gas
Perhaps the most misunderstood gas, at least in terms of its relevance to leak testing, is hydrogen. Hydrogen testing is actually the best option for many reasons. Often ruled out by experts because of its perceived volatility, hydrogen can be completely safe when properly handled.

By using pre-diluted hydrogen with nitrogen, it’s possible to completely avoid the flammable concentration range. In fact, standard hydrogen-nitrogen mixtures are commonly used as shielding gases for welding. A suitable concentration for leak testing, available in industrial grade from most gas suppliers, is 5 percent of hydrogen to 95 percent of nitrogen. According to the international ISO 10156 standard, any hydrogen-nitrogen mixture containing less than 5.7 percent of hydrogen is classified as nonflammable. Therefore, hydrogen can—and should—be safely employed for leak testing.

An engine is leak tested using the hydrogen method following a pressure test.
 

As the lightest element in the universe, hydrogen has half the viscosity of air or helium—spreading easily throughout the test object, readily penetrating more leaks and venting away much easier than any other tracer gas.

In addition to being environmentally friendly, hydrogen is also much less expensive than helium. Hydrogen detectors cost much less than most mass spectrometers.

The advent of a new type of hydrogen detectors has overcome another perceived hydrogen leak testing obstacle. Based on microelectronic hydrogen sensors, these new hydrogen detectors are unique in their high sensitivity and high selectivity to hydrogen. They’re robust enough for industrial use, allowing the easy detection of leaks down to 5 x 10-7 cc/s.

Hydrogen leak detectors are easy for a non-engineer to operate. The test gas is simply injected into the test object and a hand probe connected to the hydrogen detector is used to search for leaks. The detector indicates with an audio signal (which increases in frequency pitch the closer the operator gets to the leak location). There is no need to create a vacuum, which means the user can save significant testing time.

To facilitate things, hydrogen leak testing can be automated. Particularly in Japan, automotive-component manufacturers are beginning to run more and more “chamber type” automated hydrogen testing systems on their components, saving cycle time and money.

Extremely effective and efficient, easy-to-use, cost-effective and safe, hydrogen leak detection is now being used in a number of industries including: automotive, refrigeration and air conditioning, medical, water and telecommunications. Hydrogen testing has been getting extremely positive references from industry experts and academicians alike.

It’s clear that there are many options available for leak detection and testing. But if you’re looking for a leak testing method that isn’t full of holes, hydrogen testing is clearly your best choice.

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About The Author

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Claes Nylander

Claes Nylander is president and co-founder of Sensistor Technologies and associate professor in applied physics. He received a doctorate in applied physics at Sweden’s Linköping Institute of Technology in 1983, and has conducted research in biosensors and microelectronic sensor technology and published a number of scientific articles on this subject.