Leak detection has become a necessary quality task for some time now. The methods of detecting leaks have changed in recent history. Figure 1 shows those common leak test methods and what range of leak tightness can be achieved and measured. Within this article, we will discuss the questions that need to be answered and the choices that need to be made for your application. The choices will often be more than one at first, but will eventually be one of level or leak tightness (specification), time allowed and method to check for leak tightness (throughput desired) and of course budget for equipment and manpower.
The leak tightness specification usually comes from the end user (customer). He has taken into consideration what his “acceptable leak rate” is and what life expectancy may be placed on that part that needs to be leak tight. Whether the part will keep out some external influence or keep in some gas or liquid really doesn’t matter for leak detection. We will test for both conditions.
A few rules to consider:
1. Try to leak check the part as it will be used; for example, the vessel that holds a gas or liquid should be tested pressurized from the inside out. A vessel that needs to keep gas or liquid out should be evacuated and “bombed” (sprayed) with a tracer gas (i.e. helium) from the outside. There are always exceptions that may need some other technique. The reason: stresses may induce a leak in one direction and not the other.
2. Helium gas, for example, in atmospheric pressure rises, therefore leak check from the top down spraying method.
3. Once a leak is found, attempt to fix the leak temporarily. Use of Apiezon Q (putty) or simple vinyl electrical tape should stop a leak and allow you to continue looking for more leaks. There are more examples of technique but the technician can determine what is most appropriate.
As a general rule: The shorter the time to find a leak or confirm leak tightness, the more expensive the method will be. Figures 3, 4 and 5 show some of these methods. They are the classic methods of leak checking with a helium mass spectrometer leak detector. Some methods involve a vacuum chamber that adds cost but will reduce cycle time for detection of leaks.
The external pressure or spraying method involves pumping air out of the test object to create a vacuum, spraying helium gas onto the outside of the test object and then detecting if helium gas enters the work piece with a helium leak detector, thus isolating the area where a leak may be. Figure 3 depicts the operating schematic for this mode of operation.
Overpressure and vacuum method is the opposite of spraying. The helium gas is injected into or pressurizing the test object. Then the charged or pressurized part is placed in a vacuum chamber and chamber is evacuated. This is a very fast method as there is no searching across the test object you will get a fast overall test of the part. Figure 4 depicts the operating schematic for this mode of operation.
Once a test object is known to leak, from the method depicted in Figure 4, the leak can be isolated by locating that leaking spot with a “sniffer” probe. The sniffer method involves pressurizing with helium gas inside the test object and detecting the presence of helium gas from outside the test object. Figure 5 depicts the operating schematic for this mode of operation.
Most helium leak detectors can be configured for any method mentioned above.
Next to determine is how long you are willing to wait for the leak detection method, time to test. A term called “response time” is to calculate your speed for detecting a leak. When you were young and watched for the air bubbles to appear from that leaky bicycle inner tube, time was not important. Today with mass production demands this wait time has to be minimized, not to mention the cost just waiting for the bubble to appear or the helium to be detected.
Okay, so you bought a leak detector, bought a helium tank, made a chamber perhaps and are ready to leak check. A designated room or area for leak detecting needs to be set up. This area should be relatively clean, not a “classed” clean room, but clean. Anytime you must use and make vacuum tight joints for testing, care needs to be taken to eliminate dirt and dust and chemical and oils from the work area. You recall the part about helium rising, the ceiling of this area should be high. Allow the helium to rise and not interfere with the testing. A better method is to install a “welders-hood” above the leak test station and vent any stray or leaking helium gas away for the test object and equipment. Never have a ceiling fan blowing down to the work surface. This will re-circulate the rising helium where you don’t want it to be.
Leak checking is a bit of an art. A slow methodical approach to isolating that leaky area is the rule of the day. Check that suspected leak two or three times, count the seconds it takes for your test equipment to sense the leak, the shortest time to respond means you are now closest to the actual leak.
The last part of leak detecting is to think about where you cannot leak check. Barriers, valves, fittings, piping, closed volumes of air, multiple seals etc. mean you cannot allow helium to flow through the leak and be sensed by the detector.
Leak detection is an art form that requires, in some cases, special equipment, training and patience. Leak specifications are the starting point for selecting the appropriate leak test method. From bubble testing, slow and frustrating, to overpressure and vacuum method, fast and accurate, you have choices. Those choices and questions are part of the decision process to get you the best equipment based on your needs and budget. Other methods for high volume leak testing are available that would require training and different techniques that are not mentioned in this article.
