Leak Testing: Moving Beyond the Most Popular Methods
By Claes Nylander, President, Sensistor Technologies, Linkoping, Sweden
Leak testing is an essential element in product quality testing for a wide range of industries. From the automotive industry to HVAC manufacturers, 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% 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.
While leak testing is almost always a matter of exactly measuring the leak rate of a manufactured part or product, n othing is ever absolutely tight—but leak testing ensures that the product has been tested for maximum allowable leakage.
Leak testing can be done for several reasons—all extremely important. It can ensure that the outer environment is protected, ensuring that flammable, toxic or corrosive substances remain within an object’s walls. It can also ensure that a liquid or gas that is essential to the function of a system—such as an air conditioning unit or hydraulic valves—is contained within that system for the necessary period of time.
Bursting the bubble
There’s no doubt that leak testing is important, but the question remains which methods are the most practical and effective. 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 each of these methods 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.
But water dunking provides minimal quality assurance. A very small leak might make a few or miniscule bubbles. If the leak is within a recess, air from the leak may collect inside the recess and stay there. 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, you can pull down air bubbles that mask bubbles from a small leak. The operator needs to wait until the object has cleared itself of non-leak related bubbles. Also, the operator’s perspective can be limited; he or she may not be able to see a small leak if it is 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. One certainly doesn’t want to employ a leak testing method that itself can be the cause of product failure.
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 in that it can be easily automated and it is dry.
But pressure decay testing is not 100% accurate at all time depending on a number of variables. For one thing, it is an incomplete method, as it cannot 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. One’s ability to measure extremely small leaks hinges on the internal volume of the object and whether the object is made of a rigid or flexible material. This form of test also relies heavily on temperature; temperatures rise as the air is compressed inside the object and the pressure will not stabilize until the temperature has stabilized.
Unfortunately, temperature can also be affected by elements outside the test. For example, if one is testing an aluminum object, the heat from a human hand or a breeze from an open door can completely throw off the test results and be cause for false acceptance.
If you need to test a large object, such as a gasoline tank, pressure decay testing is not going to be your best choice. Objects that are large in volume demand too long of a cycle time to get results from this method. Pressure decay testing really only works 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
In recent years, more complex methods such as the use of tracer gases like helium and hydrogen have proven to be effective methods for detecting evidence of a leak and measuring that leak.
Helium has been the most commonly used tracer gas for leak testing. This is simply due to the fact that 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, and recently gaining some notoriety for their Martian mineral-testing application, have also been developed for leak testing applications. Set up to detect helium as it dissipates from an object, these mass spectrometers that use helium 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 to has drawbacks. A mass spectrometer is a delicate piece of equipment and is very expensive to maintain. Plus the machine’s pumps need to be regularly checked and serviced.
In addition, helium itself can be the cause of problems. Helium is a highly viscous gas, and should it spill, can be very difficult to clear from the testing equipment. It also has a tendency to cling to surfaces. Helium is expensive to buy, and as it is a non-renewable natural resource, the price continues to rise .
The lightest gas
Perhaps the most misunderstood gas, at least in terms of its relevance to leak testing, hydrogen is actually the superior option for many reasons. Often ruled out by experts because of its perceived volatility, hydrogen actually can be completely safe when handled in the right form.
By using pre-diluted hydrogen with nitrogen, one can completely avoid this 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% hydrogen/95% nitrogen. According to the international standard ISO10156 any hydrogen/nitrogen mixture containing less than 5.7% hydrogen is classified as non-flammable. Hydrogen can therefore be safely employed for leak testing. And it should be.
The lightest element in the universe, hydrogen has half the viscosity of air or helium, so it spreads easily throughout the test object, penetrates leaks more readily, and vents away much easier than any other tracer gas.
Hydrogen is also much less expensive than helium, in addition to being environmentally friendly. And hydrogen detectors cost much less than the price of 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 are also robust enough for industrial use, allowing the easy detection of leaks down to 5x10-7 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 that increases in frequency pitch the closer you get to the leak location. There is no need to create a vacuum, which means the user can save a lot of testing time.
Hydrogen leak testing can also be automated. Especially in Japan, the automotive component manufacturers are beginning to run more and more chamber type, automated hydrogen testing systems on their components, thus saving cycle time and money.
For more information on leak testing, go to www.sensistor.se .