For many years, manufacturers have employed "wet" methods in which an operator watches for bubbles to form at any leaks.

Alternatively, "dry" methods utilize gas detectors, either to detect refrigerant leakage after charging the system with refrigerant, or to detect leakage of a tracer gas - helium or hydrogen - in order to ensure tightness prior to charging the system with refrigerant. Let's take a look at each method.

Wet Testing Methods

Two types of bubble testing have been employed since the earliest days of quality assurance testing.

Water dunk testing is simply filling the component to a specific positive air pressure and submerging it in a well-lit tank filled with clear water.

Soap bubble testing is the observation of a pressurized component that has been sprayed or brushed with a soapy solution.

With both methods, bubbles form at the source of a leak as a result of air pressure, and the amount of bubbles per minute can signify the size of the leak. The process is able to detect very small leaks if the operator patiently and carefully observes the component during the entire waiting time. Typically, 10 to 20 minutes elapse before a bubble appears at leaks the size of 1/4 ounce per year.

The main advantage of the bubble testing method is it requires little capital investment, with no expensive equipment to purchase. Water is also readily available, easily disposable, and essentially cost-free. Water dunking also has the advantage of simultaneously providing integral testing (checking tightness of the component as a whole) and location testing (pinpointing a leak).

With "wet" methods, the operator's perspective can be limited and small leaks may remain hidden on the reverse side of the component or in a recess.

With water dunking, the operator must be careful not to pull down air bubbles that mask bubbles from a small leak or result in false rejection. With soap bubble testing, larger leaks sometimes do not cause the formation of bubbles; instead the compressed air just blows in a manner that can be difficult to observe.

Also, the technician needs to be aware that a capillary force can be extremely strong in the types of holes that need to be detected. The smaller the hole, the stronger the capillary force. The result is that liquid that has been sucked into a micro leak by capillary action may not be forced out with compressed air, and no bubble will appear.

Also be aware that some water can have a corrosive effect on the component being tested. Oxidization of surfaces can make it difficult to braze, solder, and weld repairs. There also needs to be a good cleanup of any soapy residue.

Dry Testing Methods

During the past few decades, dry procedures have been widely used in the manufacturing of products and systems. These methods use specially designed detectors or mass spectrometers to spot the presence of tracer gas or refrigerant passing through a leak or to sense the total accumulation of gas that has escaped from the test object into an enclosed chamber.

Dry methods can be automated on production lines. As a dry process, they are noncorrosive and require no cleaning before rejected items are repaired.

Two general categories of dry testing methodology are available. One category involves detecting the leakage of refrigerant already introduced into the system, and the other category uses tracer gas to ensure tightness prior to the introduction of refrigerant.

Detecting leakage of refrigerant with halogen detectors is a common procedure for service technicians repairing products and systems in the field. A similar process is used during the manufacturing process. After charging the system with refrigerant, manufacturers will sniff valves and joints with sensitive mass spectrometers or probe with simpler halogen detectors. The selectivity of halogen detectors has improved over the past few years and presents a cost-effective alternative to mass spectrometers.

An advantage of charging the product or system with refrigerant prior to testing is that the leak testing and the charging are reduced to a single step. If the object passes the leak test, it is already charged with refrigerant and ready to move to the next step in the manufacturing process.

However, many manufacturers cannot introduce refrigerant into an untested object due to environmental concerns and regulations. Also, rejected objects must be cleared of refrigerant prior to repair and the gas must be recovered, according to government regulations and industry standards. This may not be an easy process. Refrigerants are heavy gases and their slow diffusion rate may make them difficult to clear.

Testing Prior To Charging With Refrigerant

For these reasons, manufacturers are increasingly opting to perform leak testing prior to charging systems with refrigerant. Two tracer gases are commonly used: helium and hydrogen. A third tracer gas - refrigerant diluted in nitrogen - is not used as much because of the cost of the gas, the environmental limitations on refrigerant emissions, and the limited selectivity of the available detectors.

Helium testing utilizes specially designed mass spectrometers, which are extremely sensitive to trace amounts of this gas. The method is said to be capable of detecting very small leaks.

Helium testing is used for integral testing of components, such as coils and tubing. Integral tests with helium require that a helium-filled object be placed in a vacuum chamber. After a series of pumps achieve a vacuum in the chamber, a mass spectrometer is used to analyze the amount of helium in the chamber. The process of creating the vacuum contributes to a lengthy test cycle time.

Leak detection with helium can be costly. A mass spectrometer requires regular maintenance. The vacuum chamber can be costly and must be well constructed to withstand the vacuum. The seals must be well maintained. The two or three stages of vacuum pumps also require maintenance and periodic replacement. Helium gas itself is a natural resource, but is in limited supply.

More recently, hydrogen probes have been developed. Hydrogen is a natural resource that is in greater supply than helium gas. The recommended tracer gas mix (5 percent hydrogen / 95 percent nitrogen) is safe and nonflammable.

Especially in Japan and Germany, vehicle manufacturers are increasingly using automated and semiautomated hydrogen-based testing systems on production lines for air conditioning and other components. The hydrogen method utilizes a microelectronic probe that is sensitive and selective to hydrogen.

Being the lightest element in the universe, hydrogen fills test objects quickly, and spreads throughout the test object, penetrates leaks readily, and vents away quickly. Hydrogen also does not stick to surfaces of the testing apparatus.

With hydrogen, integral testing does not require the creation of a vacuum. Hydrogen tests can be performed at atmospheric pressure in an accumulation chamber. A fan clears the accumulation chamber between tests.

Hydrogen testing is also effective when used to probe specific joints and valves, without an accumulation chamber. For example, a U.S. tractor manufacturer performs air conditioning leak tests by positioning an H2000 hydrogen probe (made by Sensistor Technologies) at seven joints. A touch screen programmable logic controller (PLC) displays graphical images and auditory signals that lead an operator sequentially through a 10-second test of each joint.

It should be noted that when the specification calls for detection levels less than 5 x 10-7 cc/sec, the only resource might be to use a helium mass spectrometer in a high vacuum test chamber where the technique can be particularly effective.

Conclusion

In choosing leak detection methods to invest in, contractors and technicians should consider the types of jobs they deal with on a regular basis and consult with manufacturers of various leak detectors, as well as their local wholesalers.

Claes Nylander is president of Sensitor Technologies. Additional information about the company can be found at www.sensitor.com or by contacting mail@sensitor.com.

Publication date: 07/04/2005