Lawnmower motors, espresso machines, home generators and more-- designs for appliances with valves, filters and similar applications often require leak testing to demonstrate readiness for market release. Usually, the same leak testing methods that are used in prototype designs are also the best ones for assembly processes. In this article we compare four methods for leak testing (bubble tests, pressure decay, mass flow sensors, and helium mass spectrometers) and talk about their pros and cons for various applications and how they can affect time-to-market and product quality.

Bubble Testing

When an appliance needs to be “leak-proof”, the specifications for acceptable leak rates can vary. As a general rule, however, the leak standards for consumer products in past decades were far looser than what is expected today. For that reason, so-called bubble testing is more or less obsolete as a leak detection method, since it is a relatively crude method for measuring leaks.

In the bubble testing method, a product is pressurized and put in water. Any bubbling is indicative of leaks, and in fact, even minor leaks can be detected by this method. But whether one is prototyping or testing during full production, bubble testing is impractical. For one thing, products need to be dried, which can be difficult, especially at production levels. This is a slow process and requires a bit of skill by operators. Perhaps the most significant drawback at the prototype development stage is that such wet leak testing methods provide no quantification of leaks, such that later design iterations do not truly benefit from the leak identification.

Pressure Decay Testing

One dry method for testing leaks is pressure decay, in which a part is pressurized, then removed from the pressure source, and monitored for pressure changes over time. The data on pressure changes is then converted into measures of leakage rates.

Differential pressure testing is a more accurate variant of the pressure decay method. With this type test a reference volume is pressurized along with a test part, and the pressure differential between the non-leaking reference volume and the test item over time is measured by a transducer. This is similarly an indirect method of measuring leakage rate, as the time/pressure data must be converted into leakage rate first.

Some mistakenly think that pressure decay testing is the least expensive dry testing method available. Actually, the difficulties of controlling test environments from drafts, seal creep, ambient temperature changes, or test part deformities often translate into increased testing costs to get the required accuracies.

Further, the indirect nature of the test process and the time needed to track pressure changes create inherently longer testing times. This process requires two measurements, not one, with sufficient elapsed time between measurements. If a prototype development project requires testing many test items this can add up to significant delays.

Mass Flow Testing

A dry, direct leak measurement method that only requires one reading and is less susceptible to adiabatic effects is called mass flow testing. In mass flow tests, a part is pressurized along with a reference volume (or an air supply line with regulator is used), and the amount of air that flow in to replace a leakage flow is measured directly in sccm (standard cubic centimeters per minute). Mass flow testing thus involves one direct measurement, which takes less time and is likely to be more accurate than two measurements separated by time.

Mass flow sensors use temperature transducers. Leakage flow is directed across a heated element such that heat is transferred to the flowing gas. Temperature-sensitive resistors measure the temperature of the incoming and outgoing flow. The temperature transducer bridge is balanced when both resistors are exposed to the same temperature; when they are imbalanced, there is an output voltage proportional to the mass flow, providing the direct leakage rate measurement.

Note: Simple pressure decay instruments are inexpensive but slow and inaccurate. Differential pressure decay and mass flow instruments cost the same but mass flow testing is faster and more accurate.

Helium Testing

In other industries where it is essential to accurately measure leaks below 0.001 sccs (standard cubic centimeter per second), helium mass spectrometers are used. An appliance prototyping application that requires the significantly more expensive helium testing method is quite difficult to imagine. Rather, this method is more typical in aerospace or other applications that include lethal gases.

In the helium testing method, test parts are pressurized with helium and leakage flow is detected with mass spectrometry sampling of the testing chambers. Equipment costs, maintenance and helium costs makes this method gross overkill for any application where there is no need to fully monitor leak rates less than 10-3 Sccs. Thus, helium testing is typically appropriate for only the most demanding applications.

Optimizing Testing Methodologies - Mass Flow Sensors

For any leak test-intensive prototype development project, the care one takes to customize the mass flow testing method to the application will have great bearing on time to market. For example, if one is testing newly cast prototypes that are still warm from the casting process one either has to have a means to automatically compensate for temperature variations or wait until parts cool to normal room temperatures. The latter approach of waiting for parts to cool is inherently fraught with error, since testing environments are constantly changing depending on current conditions. This is one reason why the cheapest mass flow leak testing instruments typically cost less, i.e. they don’t have the ability to automate temperature compensation.

Similarly, cheaper mass flow instruments use inferior methods for calibration that likewise introduce room for error. Instruments that use mechanical devices for calibration are far more variable than those that use solid state technology for calibration. Solid state calibrated machines also are far less likely to break down or need maintenance.

One also needs to source mass flow sensors that are customized to function optimally in the critical ranges for an application. The cheaper mass flow instruments have only one type of sensor, which may lack the functionality for scaling to the needed testing requirements.

Prototyping speed and ultimate time-to-market is also significantly impacted by the processing power accompanying the mass flow system. Typically, in highly leak-sensitive parts and products, one needs to test prototypes thousands of times, at different temperatures, different vacuum levels, and other varied conditions. Analyses usually require the ability to recall ALL test data points and to feed data into offline databases and analytical software. The PCs used with the better mass flow instruments combine millions of real-time test records for all delta points into a single continuous graphs, and allow one to store and later recall test data in detail for in-depth statistical analysis. If a system doesn’t allow real-time processing of results it not only is cumbersome but also translates into delays in time-to-market as engineers grapple with the inadequate data at hand.

Testing Expertise

Pinpointing the location of leaks is typically a straightforward matter requiring little technical expertise. One would simply dunk a part under water to pinpoint the leak location, or pressurize the part with helium and literally sniff leaks out. The more challenging aspect of prototype development involving leak testing is in quantifying the leaks such that design iterations are truly tested. For that process, consulting with proven experts in leak testing is a recommended step for all product designers of appliances with valves, filters, or other components that need to be “leak-proof”. Such consultations have repeatedly helped product designers save up to 10% in product development costs and likewise significantly cut time-to-market by up to 15%.

Typically at issue is how to design test fixturing and how to integrate software and hardware at the system level. Competence in hardware design for fixturing that will prevent seal creep, manage temperature-related effects, minimize testing volumes for quick response, eliminate part distortion during testing, customizing software to work in real time and experience in handling comparable challenges can make or break the success of prototype leak testing.

Leak-testing processes that are optimized for prototype development typically have great bearing on the best way to create competitive test-centric assembly operations for full production. Efficient test-centric assembly processes are especially critical to the production of valves, actuators, sensors and other similarly sophisticated products that can be manufactured competitively in the United States. In the face of fierce foreign competition, partnering with leak testing experts in order to build and maintain a niche in such products is more important than ever before.

Jacques Hoffmann is founder and President of InterTech Development Company (www.intertechdevelopment.com), a world leader in test-centric assembly specializing in automated leak and functional testing with mass flow, hydraulic, helium, or pressure decay technology (ISO-9001-2000 International Standards for Quality Management). InterTech Development Company-engineered solutions are used by hundreds of product design teams and manufacturers worldwide and the company’s worldwide support organization maintains offices in North America, Asia, and Europe. For more information, contact Jacques E. Hoffmann, +847 – 679 – 3377, FAX +847 – 679 – 3391, info@intertechdevelopment.com, InterTech Development Company, 7401 North Linder Avenue, Skokie, IL 60077 – 3220.