Using the Right Technology to Measure Leak Rates Cuts Time to Market
"Does it leak?" That seemingly simple
question can be a challenge for today's design engineers to answer.
Mass flow leak testing, combined with the right software, is often the
solution.
Clean air regulations, coupled with a desire for higher performance engines, for example, have led automobile manufacturers to specify much tighter tolerances than in the past for a wide range of components, including fuel injectors, valves, and filters. In medical applications, concern for containing infectious agents and various testing and treatment technologies have created a demand for leak rates below 5 standard cubic centimeters per minute (sccm). Consumers want cleaner appliances, from generators to lawn mowers, and they expect waterproof cameras and printer cartridges to be leak-free. Demand for products that reliably contain a liquid, gas, or vacuum compel product designers to revamp designs and also reliably measure leak rates in prototypes and finished products.
A decade ago, wet bubble testing was commonly used to test the design and production of leak-sensitive parts using. In this method, parts are pressurized and submerged in a water bath. If a stream of bubbles is visible, the product leaks. In the absence of such bubbling, a part is considered to be sufficiently leak-free. By today's standards, this methodology is grossly inadequate.
Today, three methods of dry air testing have replaced the wet bubble test: (1) pressure decay testing, (2) mass flow detection, and (3) helium-based technology. For the vast majority of prototyping challenges, mass flow leak testing, when combined with statistical software to track test results, can speed test times by as much as 70 percent with time-to-market reductions of plus or minus 50 percent, depending on the lag time that leak testing creates in the prototyping process.
Pressure Decay Testing
In the pressure decay method of leak testing a reference volume is
pressurized along with the prototype part, and a transducer reads the
pressure differential between the non-leaking reference and the test
prototype over time. Calculations then convert these time/pressure
readings into a measure of leakage rate. This is a relatively difficult
method to use because measurements are highly vulnerable to changes in
testing laboratory conditions, such as drafts or temperature, and it
can be difficult to determine the volume of test parts and test
circuits, which must be known in order to calculate results.
The pressure decay process requires two measurements of prototype pressure be taken, allowing for sufficient elapsed time between measurements. When speed of prototype testing is an issue, this built-in delay makes the pressure decay method less desirable. More importantly, the two measurements and the time lapse significantly increase the potential for measurement error. The amount of time necessary 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.
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) are another cost that need to be factored in to overall cost.
Mass Flow Testing
In contrast, mass flow methods use single-step measurements that are
accurate to a level of 0.3 sccm or less, depending on the application.
In the mass flow method, the prototype is pressurized and any leakage
is compensated naturally by air flowing into the prototype from the
source, which can be a reference volume reservoir pressurized along
with the part or an air-supply line whose pressure is controlled by a
regulator. The amount of air that flows in to replace leakage flow is
measure directly in sccm.
Mass flow sensing is done by measuring heat transfer to a flowing gas from the leakage flow directed across a heated element. Temperature sensitive resistors measure the temperature of the incoming and outgoing flow, and the transducer creates an output voltage proportional to the mass flow creating the leakage-rate measurement.
Helium Testing
When applications require testing for leaks below 0.001 sccs (standard
cubic centimeter per second), helium mass spectrometers that can
measure leakage as slow as 0.000001 sccs are used. For example,
aerospace components using hazardous gases generally require such leak
standards and helium testing technology. Helium testing usually
proceeds by pressuring a test part with helium or a helium/air mixture
inside a test chamber. The chamber is then evacuated and a mass
spectrometer samples the vacuum chamber, ionizes helium present such
that it can be detectable. Because helium is not affected by
temperature variations or part volumes, it is the most reliable and
consistent leak detection system for leak rates less than 10-3sccs.
Test chamber and test circuit components are very expensive and
increase dramatically for larger part volumes. This relative expense
therefore makes it overkill for nearly all prototype leak testing
challenges unless the ultimate product is handling noxious or even
lethal gases.
Product design teams will usually benefit by enlisting advice from engineers who are expert in all testing technologies. It is important, however, to find engineers that are familiar with all available methods such that you don't attempt to jerry rig an inadequate method for the task or needlessly add testing time when faster methods could be sourced and made available.
The Test of Choice--Mass Flow
Mass flow leak technology is the method of choice for most prototype
testing because it is fast, provides a direct measurement of leakage,
is relatively affordable and sufficient accurate for applications with
a sccm greater than 0.5 sccm and for many in the 0.1 - 0.5 sccm range.
Not all mass flow leak testing technology is alike, however, and how
one selects and adapts mass flow leak testing to the prototype
development has great bearing on the time-to-market reductions.
For one thing, the sensors used in mass flow leak testers vary in quality. Some manufacturers merely take standardized sensors and fit them into one or more instrument models. However, if the sensors are customized components in instruments, and selected for functioning in the ranges that are most important to the specific prototype function, they can improve speed and accuracy.
A second consideration is how the instruments are calibrated and how their standardization is validated. If an instrument relies on mechanical devices for calibration they are subject to much greater variation in testing results when compared to calibration and validation using solid state technology. Solid state technology also minimizes equipment breakdown or need for repairs.
Thirdly, mass flow sensors also vary in terms of how they compensate for temperature variations. This is particularly important when testing newly cast prototypes that may still be warm from a casting process. In these instances, either time must be added to allow the parts to cool to room temperature, or the mass flow has to be calculated taking the temperature variation into consideration. Some mass flow instruments automate temperature compensation and some do not.
Finally, the software and computing capabilities used with the mass flow instruments have significant impact on the speed of prototyping. Typically in highly leak-sensitive parts and products, one needs to test prototypes thousands of times, e.g., at different vacuum levels or at different temperatures. One also needs to recall many test data points in order to make an analysis, usually by feeding these data points into offline software such as Microsoft Excel or storing them in database software such as Microsoft Access.
Better-quality mass flow instruments store test parameters for dozens of delta points (i.e., preset vacuum levels), along with correction to atmospheric pressure and temperature per product design specifications. The PC used is then able to combine real-time results on the order of millions of test records for all delta point tests into a single continuous graph showing changes in vacuum level and airflow over time against each point's upper and lower limits (i.e., the accept/reject standard). PCs used with mass flow leak testers must be able to recall stored test data, either in total or by date, accept/reject results (e.g. high or low pressure). In this way, data tables and curves for multiple samples of the same design can be merged to compare average flow performance versus high/low extremes for the various prototype sample populations. For prototyping, it is essential that this all happen in real-time.
Often, the same leak testers used in prototyping are the ones that are best suited for test-intensive assembly operations. Products with tight tolerances for leaks typically must be tested one-by-one during final assembly operations. The speed of assembly is in part related to the quality of the leak testing instrument and the way in which the assembly handles the testing stations. Test-centric assemblies using customized mass flow sensors can reduce cycle times up to 70 percent compared to assembly operations using pressure decay leak testers and/or those where testing is added as an afterthought to the assembly line design.
In summary, enlisting the most experienced design and test engineers with proven expertise in all leak and functional testing methods (mass flow, pressure decay, and helium testing) to help you customize prototype leak testing is usually a first step in cutting time-to-market for leak-sensitive products. One needs to source best-in-class mass flow leak testing technology and related processing software with an eye toward shortening time-to-market. Moreover, the best-in-class mass flow technology that provides the most cost-effective solution for rapid prototyping is typically also the best technology for later stages where fast test-intensive assembly operations are needed.
Jacques Hoffmann, founder and president of InterTech Development Company (Skokie, IL). Visit the Web site at intertechdevelopment.com.
