Testing 1, 2, 3: A review of leak-testing methods
January 25, 2006 by Jacques Hoffmann
The requirement of some products to be “leak-proof” can become quite burdensome for manufacturing engineers without access to adequate leak testing expertise. Far too often, leak testing technology is poorly integrated into assembly operations as an afterthought to the assembly line design.
As a result, those assemblies rarely meet Repeatability & Reproducibility (Gauge R&R) requirements for leak detection, a concept to ensure stable measurements where a tester gets the same results each time they measure. Poorly integrated technology also slows manufacturing operations down considerably, with significant, though often unrecognized, bottom line impact. The following is a review of the basic approaches to leak testing, including the pros and cons of each method.
Whenever there is a need to segregate highly noxious or otherwise hazardous substances, the costs of leak testing become a secondary consideration to the exacting standards to be achieved by the testing process. Many aerospace components, for example, contain highly flammable liquids and gases, and their manufacturing operations need to verify that each and every part meets the tightest tolerances for leaks. In applications where the leaks to be detected are below 0.001 standard cubic centimetres per second (sccs), the use of a helium mass spectrometer is often advised. Helium testing usually involves 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 to detect ionized helium.
The biggest plus of this method, and usually the one that compels the use of this technology, is its accuracy and reliability. Helium mass spectrometers can measure leakage as slow as 10-6 sccs. This accuracy is largely due to the sensitivity of the mass spectrometer sensor itself, which operates under hard vacuum conditions and is essentially counting ions. These units offer reliable and consistent leak detection for leak rates less than 10-3 sccs.
There is, however, a downside to using this method. Helium testing is quite costly, often double the cost of other methods. Test chamber and test circuit components are very expensive, especially for testing large part volumes. Compared to plain air, of course, the helium itself adds an expense. These costs are usually prohibitive for any application that does not require tests for leaks less than 0.01 sccm.
Pressure decay testing
Another leak testing option is a method built upon measurement of pressure decay rates. In this method, a reference volume is pressurized along with the part to be tested, and a transducer reads the pressure differential between the non-leaking reference and the test part over a period of time. There are no direct
leakage rate detections with this method. Rather, time/pressure readings are converted into leak rate calculations.
The biggest plus of pressure decay technology, especially when compared to helium testing, is its significantly lower cost. For one thing, the cost of using helium gas is eliminated. However, the lower costs of pressure decay instruments can be extremely misleading, as the overall testing time may cause a drag on assembly line speed. This is because a pressure decay method inherently requires two measurements and elapsed time between these measurements. In typical assembly operations, pressure decay testing can increase per part assembly time by 20 to 40 per cent. In addition, the time lapse between measurements in a pressure decay test can be far more troublesome for reasons beyond assembly speed considerations. A two-step measurement process coupled with the time lapse needed between measurements significantly increases the potential for measurement error. Because measurements are highly vulnerable to changes in plant conditions such as drafts or temperature fluctuations, there are difficulties in determining the actual volume of test parts and test circuits, both of which must be known in order to calculate results. For similar reasons, pressure decay methods are impractical for leak testing parts with very large flow leaks. If the pressure drops too rapidly it cannot be measured accurately.
Mass flow testing
The downsides of the aforementioned methods makes their selection an unlikely choice in the lion’s share of test-centric assemblies where cost, accuracy and speed are paramount concerns, and issues of liquid/gas toxicity are not present. Mass flow sensing for leak testing offers the most accurate, reliable and cost-effective method for nearly all applications where leaks greater than 0.5 sccm must be detected, as well as many
applications with tolerances in the 0.3-0.5 sccm range.
Unlike pressure decays methods, mass flow methods use single-step direct measurements of heat transfer of a flowing gas from 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 leak rate measurement. In the mass flow method, a part is pressurized and any leakage is compensated naturally by air flowing into the part from the source, which can be a reference volume reservoir pressurized along with the part or an air-supply line with pressure controlled by a regulator.
Knowing which method to use is the first step in building optimized test-centric assemblies. However, the specific way one implements the selected testing methods is also critical. It is quite common, for example, to select leak testing instruments with generic sensing devices that are not customized to an application. This approach is inherently flawed. One needs to remember that it is not the cost of the testing instrument that is important, but rather the cost of the overall testing process. Gauge R&R of testing instruments is not an especially meaningful measurement because it is the actual Gauge R&R of the testing during real assembly that counts. Fixturing on parts being tested is usually of equal or greater importance than the leak testing instrument itself. For that reason, reputable manufacturers of leak testing technology will provide in-house engineering expertise that can customize instruments and optimize tooling and fixturing design for test-intensive operations.
Perhaps the most important consideration in building test-centric assemblies is in sourcing design and test engineers with proven expertise in leak testing methods. Engineering firms that specialize in leak testing will usually provide free application evaluations with recommendations for best assembly and test methods in terms of speed, accuracy and cost. It is not uncommon for re-engineered test-centric assemblies to cut testing time by orders of magnitude. This potential for throughput gains cannot be ignored.
Jacques Hoffmann is founder and president of InterTech Development Company, which specializes in automated leak and functional testing. Jacques can be reached at 847-679-3377 or firstname.lastname@example.org.