Not too long ago, when you wanted a product to be leak-proof, you simply put it under water, made sure it didn’t bubble, and thereby concluded there were no leaks. Such “bubble testing” takes time and depends on the operator’s ability, making it totally inappropriate for the modern production environment. Also, it doesn’t generate the quantitative measurements that are the lifeblood of quality assurance engineering.
Dry-air leak testing methods—some of which can detect leaks as small as 0.01 standard cubic centimeters per minute (sccm)—are the methods most commonly used today by a wide range of industries—from medical devices, to automotive, to appliances, and aerospace, among others. These dry air methods enable quality managers to define leaks quantitatively.
“No leaks allowed” standards are concepts of the past. There are a variety of dry air leak test methods and best-practice techniques for each type of method, which will enable compliance to ISO 9001 and comparable quality management standards to be achieved. Generally speaking, these dry air leak test methods include;
- Pressure testing
- Differential pressure-decay testing
- Mass flow leak testing
In addition, tracer gas testing and especially helium mass spectrometer leak testing, are used in more demanding applications where leaks as small as 10-5 standard cubic centimeters per second (sccs) must be detected in a production environment. If one truly understands leak testing application requirements and best practice techniques for these various leak test methods, the selection of which type of testing to perform is a rather straightforward matter.
The first step in designing a leak testing solution is to correctly define what the leak limits are. Leak testing applications laboratories begin with an engineering analyses of a specific application to determine and quantify how much a product or component can leak. Often, correlation studies are performed to verify if it is possible to use dry-air test methods instead of hydraulic fluids. Sample parts are tested as part of an initial engineering analysis. These determine the production requirements and leak standards to be achieved so that quality engineering of test solutions can begin. The first step in this process is to select the leak testing method that is the best match to application requirements.
In this “Leak Testing 101” series we will discuss the various dry air leak testing methods and the issues and techniques that affect testing costs and gauge repeatability and reproducibility (GR&R).
First, let’s take a look at the pros and cons of pressure-decay testing.
The big plus of pressure-decay testing—or at least the thought behind it—is that the leak detectors for pressure-decay leak testing have the lowest upfront cost. It is probably for this reason that the method is still in use, although in many applications the real costs of pressure-decay testing are actually much higher than many realize.
In the pressure-decay method for leak testing (see figure 1), a part is pressurized, the test circuit is isolated, and the pressure drop associated with a leak is measured. A transducer reads the pressure change. Calculations then convert these time/pressure readings into a measure of leakage rate. The higher costs of pressure-decay testing stem from the difficulties inherent in the test methodology. Pressure-decay leak testing is relatively difficult because measurements are highly vulnerable to changes in testing conditions such as drafts or temperature and there are often difficulties in determining the volume of test parts and test circuits, which must be known in order to calculate results.
Also, pressure-decay leak testing requires two measurements of pressure with sufficient elapsed time between measurements. When speed of testing is an issue, this built-in delay makes the pressure-decay method less desirable. More important, the two measurements and the time lapse significantly increase the potential for measurement error. The amount of time you need to wait between measurements varies. Sometimes, long intervals between measurements can make for extreme accuracy, but these long wait times are typically not practical. The larger the part volume, the longer it takes to measure the pressure drop. Moreover, very large flows are also impractical with pressure decay, because when pressure drops very fast, it will probably not be measured accurately.
Thus, although pressure-decay leak testing instruments have a relatively low upfront cost, the extra time it takes to perform testing (if the results are reliable enough for the given application) is another expense that needs to be factored in to overall cost. It can still be the best leak test method for a specific application, but the trend lines are in the other direction. Most applications now require tighter GR&R even for very low leak rates, often with large volume parts, and with a desire to keep test cycle times to the bare minimum to cut overall testing costs.When you factor all these considerations in, it often leads one to use other leak test methods instead.
In the upcoming issues of this “Leak Testing 101” series I will discuss differential pressure-decay testing, mass-flow leak testing, temperature compensation issues, and many other topics. By the conclusion of Leak Testing 101, my goal is to bring all quality managers up to speed on the real factors that affect leak testing cycle times, costs, and reproducibility.
If you would like your specific questions on best practices for leak testing (and other testing topics) to be discussed in future articles, please leave your comments and suggestions in the Comments area below.
ABOUT THE AUTHOR
Jacques Hoffmann is founder and president of InterTech Development Co., a world leader in test-centric assembly specializing in automated leak and functional testing with mass flow, hydraulic, helium, or pressure decay technology (ISO-17025 accredited). InterTech Development Co.-engineered solutions are used by hundreds of quality management, product design teams, and manufacturers worldwide and the company’s worldwide support organization maintains offices in North America, Asia, and Europe.
Note: The above article has been reproduced from an article written by the author for Quality Digest