InterTech’s Testing Technology & Methodology Offering
Mass Flow Technology
Mass flow leak testing provides a quick and direct measurement of leakage independent of pressure. InterTech’s patented mass flow transducers test volumes from less than 1 cc to over 10 liters with an accuracy of up to 0.01 sccm. The key difference between this and the differential pressure method is that the mass-flow transducer reads the flow of air moving from the reference volume into the leak test item. The result is a direct reading of the leakage rate.
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.
There is actually a wide range in the capability of mass flow leak detectors one finds in the marketplace. 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 leak testing application, 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.
InterTech mass flow instruments automate temperature compensation while many of our competitors do not.
Downstream Testing Method
Failsafe leak testing where one never misclassifies a bad part as a good part is doable. Better yet, good parts are never misclassified as bad parts and yield is maximized. However, this standard of perfection is achievable only if one understands and manages all the factors that can undermine leak testing integrity.
InterTech has developed and patented the bias leak flow testing method to allow true test part leakage to be measured more precisely in less time and is especially helpful in testing low leak limits. This bias leak testing technique couples downstream (bell jar) leak testing with a known preliminary airflow that confirms a leak-tight bell jar seal or sets a known leak reference.
Downstream mass flow leak testing is used for difficult applications where there is a requirement for testing at pressures in excess of 150 psig, short test times, measurement of small leaks, very accurate and repeatable results, and/or precise temperature compensation.
The downstream method has the advantage of providing faster test times because the volume affecting the leak system response and the related stabilization time required is minimized. The best-in-class mass flow sensors are custom tuned for maximum stability in relevant ranges, and as a result can accurately measure very small test part leaks.
When failsafe leak testing is the goal, the promise of greater accuracy usually makes downstream testing the method of choice. Sometimes the high pressures under which products or components function and where they are leak tested also mandate that the downstream testing method is used, despite the added costs for downstream test fixturing.
Helium Testing – Mass Spectrometry
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 considered the most reliable and consistent leak detection system for leak rates less than 10-4sccs.
Equipment costs, maintenance costs, extra time required to evacuate helium from test fixturing in between test cycles, and ever rising helium costs often makes this method an expensive option. The Test chamber and test circuit components can be expensive and costs can increase dramatically for larger part volumes. This relative expense requires users to really understand the application as the method can be viewed as overkill for many leak testing challenges.
Comparing helium mass spectrometer leak detectors to generic mass flow leak detectors can lead one to the erroneous conclusion that using only expensive helium testing will work for applications requiring identification of leaks under 0.5 sccm. InterTech combines customized mass flow sensors with a variety of techniques for 0.01 sccm mass flow leak testing with 10% gauge R&R, bypassing the need for helium and the associated costs and complications of helium testing.
However in instances where Helium testing is required InterTech has the expertise and knowledge to build systems (including Helium reclaim) capable of meeting the greater sensitivity the application calls for.
Pressure Decay / Differential Pressure Decay
In pressure decay testing a measurement of change in pressure is compensated for test time and part volume to prove a reading of leakage in sccm. Pressure decay testing is usually used to leak test small parts. Higher resolutions are achieved by differential pressure decay transducers. Differential pressure decay testing is typically used for test pressures in excess of 150 psig (10 bars).
With valves 1 and 2 open, the test item and reference volume are pressurized and then isolated by closing valve 1. The reference volume is then isolated from the test item by closing valve 2. The transducer reads the pressure differential between the reference volume and test item twice over time. A down side to this method is that the larger the volume, the smaller the change in pressure for a given leak rate, resulting in longer test times as test volume increases.
The indirect nature of the test process and the time needed to track pressure changes and take two measurements create inherently longer testing times. This means a greater probability of measurement error exists than for methods that require only one measurement. The probability of measurement error is directly related to the interval length between the two measurements. These factors are what lead many engineers to consider and use mass-flow leak testing a preferable and more reliable option.