Will it leak?

Today's Improved Test Methods Tell If Parts Leak and How Fast

Computerized mass-flow tests meet new demands for leakage-rate data on complex injection molded medical and automotive parts.

It's not just for containers any more. Volumetric leak testing has steadily gained importance outside the packaging realm for products such as complex injection molded medical devices and automotive components that must reliably contain a liquid, gas, or vacuum.

Part acceptance specifications may require specific leakage-rate readings for attributes such as material porosity, seals, assembly deficiencies, fit-and-function problems, and fastening/joining integrity. In addition, volumetric leak-rate data may be required as input to SPC and other quality-control programs. Such data may be integral to ensuring compliance of final assemblies or products with SAE, ASTM and other standards. As a result, a growing number of manufacturers are requiring 100% testing to verify integrity of both components and assembled systems.

WET VERSUS DRY TESTING

Historically, leak testing has involved some form of "wet bubble" testing with the simple goal of determining whether or not a part leaks. In bubble testing, the operator pressurizes the part, submerges it in a water bath, and then watches for a stream of escaping bubbles that signal a leak.

Although this technique can detect very small leaks and pinpoint their location, it cannot provide an exact measure of the rate of leakage. It is also a slow process that demands the constant attention of skilled personnel and requires parts to be dried off after testing.

Recent advances in dry-air leak measurement now make possible the use of leak-rate readings as a reliable quantitative indicator of both product quality and the manufacturer's production process control.

The ever-growing need for shorter test times has tended to conflict with the reliability and precision of dry-air leak readings and this method's ability to detect very small leaks. However, with proper choice of dry-air leak sensor, it is now possible to obtain faster test cycles and apply tighter leakage specs without sacrificing accuracy or reliability.

A fully automated or semi-automated dry-air leak-testing system with computer control speeds testing and lessens the need for specially skilled operators. Multiple testing programs can be stored and recalled for quick changes, allowing the same testing station to accommodate a variety of different parts.

The computerized system can provide graphic real-time monitoring of test conditions and test-cycle dynamics, and produce unambiguous readings that are keyed to product serial numbers and that can be stored for later review or print-out. It can keep running counts of accept/reject results and causes of rejects (e.g., high or low pressure, gross leaks, restarts), along with date and time of each test. It can perform calculations on stored data to prepare statistical records such as averages and standard deviations as a tool for SPC analysis.

All data kept or generated by the test instrument can be printed out or downloaded to another computer in a form compatible with standard spreadsheet software for further analysis.

TWO BASIC METHODS

Dry-air leak testing for production-line applications takes two forms. One measures the rate of pressure decay, and the other directly measures leakage rate in terms of mass flow.

In the pressure-decay method [ILLUSTRATION FOR FIGURE 1 OMITTED], the test part is pressurized and then isolated from the pressure source. Any change in the part's gauge pressure over time must be converted by calculations into a measure of leakage rate.

A faster and more accurate version of the pressure-decay type is known as the differential-pressure method [ILLUSTRATION FOR FIGURE 2 OMITTED]. A reference volume is pressurized along with the test part, and a transducer reads the pressure differential between the non-leaking reference and the test item over time. Again, calculations are needed to convert this time/pressure trend into a measure of leakage rate.

Both of these methods are vulnerable to interferences from adverse conditions in the plant, such as ambient temperature changes, drafts, test-part deformity, or seal creep. The mass-flow method offers greater reliability in the face of such conditions.

In the mass-flow method [ILLUSTRATION FOR FIGURE 3 OMITTED], the test part is pressurized and any leakage is compensated naturally by air flowing into the test part 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. In either case, the amount of air that flows in to replace leakage flow is measured directly in sccm (standard cubic centimeters per minute).

In the past, direct mass-flow leakage measurement was considered slower and less reliable than pressure-decay measurement. Recently, improved mass-flow sensor technology, coupled with the use of microprocessor-based electronics and control reservoirs, has dramatically raised the performance of these leakage measurement systems. The reservoir allows the test to be isolated from the air-supply line by serving as an alternative source that is more stable than the conventional supply-line pressure regulators used in the past. Automated mass-flow testing stations using this technique now can provide accurate, reliable, and fast leak detection in the most challenging production-line installations.

WHY MASS FLOW IS BETTER

One reason why today's mass-flow systems are faster is that pressure-decay systems require two measurements of test-part pressure with sufficient elapsed time between measurements. This doubles the opportunities for measurement error to result in an equivalent or larger error in the subsequent leak-rate calculations. Also, due to other variables external to the test process (i.e., ambient temperature variations), the probability of measurement error increases directly with the length of the interval between measurements.

In contrast, the mass-flow method uses a single-point measurement, which is generally more accurate and is completed in much less time (typically less than one second), minimizing the impact of uncontrollable variables.

Mass-flow sensing commonly employs the principle of heat transfer [ILLUSTRATION FOR FIGURE 4 OMITTED]. Leakage flow is directed across a heated element so that some of the heat is transferred to the flowing gas. Temperature-sensitive resistors (R1 and R2) 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 the flow crossing R2 is hotter, the bridge becomes unbalanced, resulting in an output voltage proportional to mass flow, which provides the leakage-rate measurement.

This technology is less affected by ambient temperature variations than is pressure-decay testing. When temperature variations are severe, special computer software can compensate for the effects of temperature changes.

Mass-flow sensing can provide highly accurate leakage readings (to 0.05 sccm) over a much wider range of leak/volume ratios and testing conditions than differential-pressure systems, and at about the same cost. (Both types of equipment are priced in the $10,00015,000 range, while simpler microprocessor-based pressure-decay testers sell for about $6000-8000.)

Mass-flow sensing is particularly well suited to rapid measurement of small leaks, or leaks in larger-volume cavities of 20 liters or more. With computerized control, it can also simultaneously test different passages within the same part. The improved performance of new mass-flow leak sensors enables them to be used as true measurement instruments capable of being evaluated as precision gauges. Repeatability and reproducibility (R&R) studies in typical applications yield consistent R&R percentages of less than 10% in most applications.

MASS FLOW GAINS GROUND

Although pressure-decay leak testers are currently predominant in the market, mass-flow leak-measurement systems are beginning to gain ground, particularly in the automotive sector as well as the more cost-sensitive medical-device market in applications where precision testing rather than simple gross leak detection is sought.

Generally, if price rather than performance is the driving force - as may be the case when testing smaller and simpler devices - most manufacturers opt for the basic pressure-decay tester. While differential-pressure systems have overcome past problems of repeatability, they are still sensitive to temperature and volume variations of parts being tested.

In automotive, the transition to mass-flow leakage-rate testing in the last few years was aided by the miniaturization and lower cost of computers and driven by new complex injection molded applications. These include enclosures for electronics, engine gas-recirculating valves, fittings for fuel lines; fuel injectors, and valves for automatic brake systems.

In the medical-device arena, mass-flow leak testing is gaining favor in some very complex injection molded parts. One example is a blood collection system that takes donated blood and runs it through a plastic device where it is separated into different components. Still other new mass-flow leak-testing applications include printer cartridges and throw-away waterproof cameras.

THE 'ULTIMATE' LEAK TEST

Under ideal conditions, dry-air technology can be pushed to detect leak rates as low as 0.001 sccs. But certain critical devices must be tested for much smaller rates. Such devices include valves, fittings, or instrument housings developed for use with hazardous gases or for certain aerospace and automotive components. For these applications, automated testing systems using helium mass spectrometers can measure leakage as slow as 0.000001 sccs.

Helium testing can detect such very low leakage rates due to the small molecular structure of helium, which allows the gas to pass easily through pores that would block larger molecules of gases such as oxygen or nitrogen.

The most common helium test method involves pressurizing the test part with helium or a helium/air mixture inside a test chamber [ILLUSTRATION FOR FIGURE 5 OMITTED]. The chamber is evacuated, inviting the helium to pass through any leakage points into the surrounding vacuum. A mass spectrometer then samples the vacuum chamber, ionizing any helium present in the sample to make even very small amounts of helium readily detectable.

Because the helium method is insensitive to temperature variations, it can deliver more reliable and consistent leakage data in shorter cycle times than might be possible with a dry-air system. However, the higher cost of the helium method (base equipment price is around $30,000), usually restricts it to cases where its advantages are essential.

Helium techniques can be especially useful in situations where a sealed product must be tested for leakage. Here, pressure enclosed within the product cannot be accessed to measure any change. This challenge can often be met with helium testing, whereby the product is first submitted to a pressurized helium environment, often referred to as "bombing," and then to a vacuum environment. If any helium invaded the product during the bombing process, it will be drawn out again into a vacuum chamber, where its presence will be detected.

Jacques Hoffmann is president of InterTech Development Co., a manufacturer of custom leak-test equipment in Skokie, Ill.