Computerized Mass Flow Dry Air Leak Testing Speeds Test Cycles

How many things are now done by computers that were normally done by hand or could not be done reasonably at all? I am writing this introduction on my computer; this article came to me on a computer (fax), probably was written on a computer, and describes computer handling of yet another NDT technique. The article is worth reading and computers are worth our learning to use - in many different ways. Frank A. Iddings Tutorial Projects Editor

Tighter quality control requirements, cost control, competition, and improved leak testing technology are joining forces to increase the importance of volumetric leak rate testing for a variety of products or components that must reliably contain a liquid, gas, or vacuum.

Part acceptance specifications may require specific leak rate readings for such attributes as material porosity, seals, assembly deficiencies, fit and function problems, and fastening/joining integrity. Volumetric leak rate data also may be required for both accepted and rejected parts, to serve as input to OEM statistical process control and other quality programs. Such data may be integral to ensuring compliance of final assemblies or products with standards of the Society of Automotive Engineers (SAE), American Society for Test and Materials (ASTM), and others. For these reasons, growing numbers of manufacturers are requiring 100 percent testing to verify integrity of both components and assembled systems.

Historically, leak testing of containment parts has involved some form of wet bubble testing, often carried out in a hostile environment of oil mist, dust, and varying temperatures. Such testing usually has the relatively simple goal of determining whether a part leaks or not. In bubble testing, the operator pressurizes the part, submerges it in a water bath, and then watches for a stream of escaping bubbles signaling a leak.

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Apply tighter leakage specifications without sacrificing accuracy or reliability.

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Although this time honored technique can detect very small leaks and pinpoint their location, it cannot provide an exact measure of the rate of leakage. In addition, it’s a slow process that demands the constant attention of skilled personnel. After testing, the part usually must be dried before it can continue through the manufacturing or shipping process.

Computerized Dry Air Leak Testing
Earlier dry air leak testers likewise were used primarily as go/no-go indicators, sometimes because they could not produce a volumetric leak rate reading, sometimes because the accuracy of their readings was suspect due to uncontrollable variables. However, recent advances in dry air leakmeasurement technology now make possible the use of leak rate readings as a reliable quantitative indicator of both product quality and the supplier’s production process control.

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

With recent advances, including computer control, dry air leak testing provides a way to satisfy today’s more stringent testing requirements without creating a bottleneck in the production process. A fully automated or semi automated dry air leak testing system speeds testing and lessens the need for specially skilled operators. Multiple testing programs can be stored and recalled for quick changeover, allowing the same testing station to readily 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, keyed to product serial numbers, that can be stored for later review or printout. 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 averages and standard deviations as tools for statistical process control analysis.

All data kept or generated by the unit can be printed out or downloaded to another computer in a form compatible with standard spreadsheet software for further analysis. Cost justification for such systems typically is achieved easily through improved efficiency, productivity, and quality.

Two Basic Methods
Two basic methods of dry air leak testing are available for production line applications. One measures the rate of pressure decay, and the other directly measures leakage rate in terms of mass flow.

In the pressure decay method (Figure 1), the test part is pressurized and then isolated from the pressure source. Any change in the part’s gage 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, known as the differential pressure method (Figure 2), pressurizes a reference volume along with the test part, and a transducer reads the pressure differential between the nonleaking reference and the test item, over time. Again, calculations convert this pressure reading into a measure of leakage. With both methods, adverse conditions such as ambient temperature changes, drafts, test part deformity, or seal creep can cause problems.

In the mass flow method (Figure 3) the test part is pressurized and any leakage is compensated for 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 controlled by a regulator. In either case, the amount of air that flows in to replace leakage flow is measured directly in standard cubic centimeters per second.

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 earlier. Automated mass flow testing stations using this technique now can provide accurate, reliable, and fast leak detection in the most challenging production line installations.

Figure 1 - Pressure decay/gage pressure: test item is pressurized from air supply line, then isolated by closing valve V1; pressure tranducer X1 reads loss in pressure from isolated item; pressure gage change is converted by calculation to provide measure of leak rate.

 

Figure 2 - Pressure decay/differential pressure; test item and nonleaking reference volume are pressurized from the same air supply line, then both are isolated by closing valve V1; reference volume is isolated from the test item by closing valve V2; pressure tranducer X2P reads pressure differential between reference volume and test item; pressure differential is converted by calculations to provide measure of leak rate.

 

Figure 3 - Mass flow: test item and nonleaking reference volume are pressurized from same air supply line, then both are isolated by closing valve V1; reference volume is isolated from test item by closing valve V2; mass flow sensor X3 Flow reads flow of air moving from reference volume into leaking test item, provides direct reading of leak rate in standard cubic centimeters per second.

Mass Flow Faster
One reason why mass flow systems are faster is that pressure decay systems require two measurements of test part pressure, with sufficient elapsed time between measurements. Measuring two times doubles the opportunities for measurement error to result in an equivalent or larger error in the subsequent leak rate calculations. And, due to other variables external to the test process, 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 completed in much less time (typically less than one second), minimizing the impact of uncontrollable variables.

Mass flow sensing employs the principle of heat transfer (Figure 4). Leakage flow is directed across a heated element, where 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 pressure decay testing. Where temperature variations are severe, special computer software can compensate for the effects of temperature changes.

Mass flow sensing can provide highly accurate leak readings (to 1.5 standard cm³/s) over a much wider range of leak/volume ratios and testing conditions than, and at about the same cost as, differential pressure systems. It is particularly well suited to rapid measurement of small leaks, or leaks in larger volume cavities of 20 L (5.2 gal) or more. With computerized control, it also can simultaneously test different passages within the same part.

Improved performance now enables the mass flow leak sensor to be used as a true measurement instrument capable of being evaluated as a precision gage. Repeatability and reproducibility (R&R) studies in typical applications of computerized mass flow leak testing systems, including test fixtures and seals, yield consistent R&R percentages of less than 10 percent in most applications.

Figure 4 - Mass flow sensor: leakage air flows across temperature sensitive resisitor R1, passes over heating element, then exits across temperature sensitive resistor R2. Temperature differential between R1 and R2 unbalances transducer bridge, producing voltage output proportional to mass flow.

 

Helium Mass Spectrometry
Helium mass spectrometry offers certain advantages, but its higher cost usually restricts it to specialized applications where those advantages are essential. For example, while dry air technology can be pushed to detect leak rates as low as 0.001 standard cm³/s under ideal circumstances, 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 aerospace and automotive components that demand ultimate reliability in hostile environments or high-performance service. For these kinds of applications, automated testing systems using helium mass spectrometers can measure leakage as slow as 0.000001 standard cm³/s, while providing the same kinds of data generation now available with dry air techniques.

The ability of helium testing to detect very low leak rates results from the relatively small molecular structure of helium, which allows the gas to pass easily through pores that would block larger molecules of most other air component gases such as oxygen and nitrogen.

The most common helium test method involves pressurizing the test part with helium or a helium/air mixture, within a test chamber (Figure 5). 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.

For a different reason, helium leak detection sometimes offers an alternative for leak testing heat exchange devices such as radiators, condensers, and evaporators found in refrigeration and air conditioning equipment. Such devices, designed to transfer heat efficiently, quickly subject air inside the tested cavity to even minor changes in ambient conditions outside the cavity, such as ventilation air currents or drafts. Because the helium mass spectrometer method is insensitive to such temperature variations, helium systems can sometimes deliver more reliable and consistent leakage data in shorter test times than might be possible with a dry air system.

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. In such cases, this challenge often can be met with helium testing, whereby the product first is submitted to a pressurized helium environment (often termed "bombing" the product), then to a vacuum environment. If any helium invaded the product during the bombing process, it will be drawn out again under vacuum where its presence is detected, signaling leakage through the product’s enclosure.

Figure 5 - Helium mass spectrometry: test item is pressurized with helium or helium/air mixture within a test chamber. The chamber is evacuated; helium inside the test item is drawn out through leakage points into the chamber vacuum. A mass spectrometer then samples the vacuum chamber. Even very small amounts of helium are readily detectable.

 

Conclusion
The increasing need to document volumetric leak rate data on both accepted and rejected parts as input to statistical process control and other programs, plus the continuing drive for faster throughput to achieve lower manufacturing costs, are accelerating the trend toward the advanced capability of mass flow based leaktesting systems. While helium mass spectrometry can detect smaller leak points and better tolerate temperature variations, its higher cost usually restricts it to specialized applications. Mass flow systems, on the other hand, generally can provide fast, highly accurate readings over the widest range of leak/volume ratios and testing conditions, offering the best choice where there is a need for true volumetric measurement of actual part leakage, or shorter test cycles, or both.

* InterTech Development Co., 7401 N. Linder Ave., Skokie, IL 60077; (847) 679-3377; fax (847 679-3391.

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