Mass Flow vs. Pressure Decay Leak Testing

Mass flow and pressure decay are both pressure leak testing technologies, and it can be a challenge to determine which option is best for a specific application. The two methods work in different ways. Pressure decay testing, for example, tests leaks by measuring changes in pressure either inside or outside of a part. It pressurizes the test part and isolates it from the pressure source, measuring a pressure drop over time compared against a set pressure loss to identify good or rejected parts. Additionally, this measurement maybe identified as a volumetric flow rate through a calibration process.

Mass flow testing, however, pressurizes the part to a set pressure and holds constant pressure on it. It measures the rate of flow through a flow sensor as the part remains at constant pressure. The result is instantaneous, based on a measurement of make-up air into the part matching the flow of a leak. The critical factor with mass flow testing is maintaining a steady pressure to yield a reliable result. Once the flow pressure has been stabilized, the pressure is sent through the flow sensor during the test. For accurate testing, it is important that the test cycle time (e.g., fill time and test time) is long enough to accommodate stable pressure within the largest volume part being tested. Only then is it possible to achieve stable pressure and test the flow rate with repeatability. 

Choosing the Right Leak Test

Mass flow testing and pressure decay testing both have benefits. Pressure decay testing, for example, is accurate, easy to automate, and highly sensitive to small leaks. It also tends to be simple and inexpensive. It is a flexible solution that works with virtually any type of part. However, it requires careful calibration against an accurate leak standard. Additionally, the calibration can be affected by the volume of the test part. This makes it necessary to set up multiple test parameters in an instrument when testing parts of varied sizes with the same leak test instrument.
Mass flow testing, on the other hand, is unfazed by changes to part volume and doesn’t require calibration against a leak standard at all. Instead, the leak is measured by the flowmeter factory calibration. Mass flow testing requires a consistent pressure, like that created by a compressor. Mass flow testing works well with applications featuring variation in volume between parts, or where parts are different sizes. This test method is sensitive to fluctuations in air supply pressure, which makes it harder to achieve accurate measurements at low flow rates. Mass flow testing is a challenge when testing applications with small reject rates, or with a high vacuum or low-pressure requirements where there is less air available to measure flow. It also tends to be more expensive, due to the fact it requires a pressure sensor, a flowmeter, and additional valves and filters.  

Comparison of Mass Flow Testing and Pressure Decay Testing

Mass Flow Testing  Pressure Decay Testing 

Offers a direct measurement of volumetric flow 

Requires a leak standard to convert to volumetric flow 

Measures rate of flow through a flow sensor 

Measures changes in pressure with a pressure transducer 

Requires steady pressure 

Tests using either air pressure or a vacuum 

Works with parts that require flow and have volume or size variations 

Works with virtually any part, but not ideal for parts with variable sizes 

Doesn’t need calibration 

Measures pressure drop only without calibration. Requires consistent volume for comparable data 

More expensive, but ideal for lab testing due to its repeatability 

Fast and inexpensive – ideal for large high-volume production 

Instantaneous measurement after stable pressure is achieved 

Cumulative reading over a fixed time 

Talk to a CTS engineer to see which leak test method is right for your part and application. 

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