Differential Mass Flow Testing

There are times — especially in cases when a stable, steady source of air isn’t present or when the air pressure has periods of instability— when direct mass flow testing may not be the best leak testing option. When pressure is unsteady, differential mass flow testing ensures stability and repeatability, and reduces variability.

Differential mass flow testing is similar to mass flow testing, except that it uses a reference volume as the pressure source. The reference volume is of similar or exact design or volume of the part under test to help eliminate the measurement of flow characteristics that are not associated with a leak. Examples of this could be in products like washer bottles or catheter tubes, that expand and contract under pressure, or in adiabatic conditions where heat transfer is absent.  

Differential mass flow testing is similar to mass flow testing, except that it uses a reference volume as the pressure source.


Achieving Stable Test Pressures with Differential Mass Flow Testing  

In standard mass flow testing, the test part is pressurized and the pressure is held constant with a regulated source. In a differential mass flow test, both the test part and reference volume are pressurized at the same time, then the external pressure source is isolated from the circuit. 
The image shows a flow chart of differential mass flow testing, where a flow sensor is represented by an orange oval, and several valves are represented by green rectangles. The illustration shows a reference volume that is much larger than the test part. 

With differential mass flow testing, the values are open between the reference volume and the flow sensor, allowing the pre-pressurized volume or air to hold pressure constant on the part. As the part leaks, air from the reference volume is measured as it flows through the flow meter into the part.  

Differential mass flow testing only works as a calibrated mass flow measurement. The reason for this is that a pressure change is always occurring in the reference part, as flow occurs the reference volume pressure is constantly dropping. Since the pressure differential is constantly changing across the flow meter, it’s necessary to calibrate for that rate of change.

As a test method, differential mass flow testing allows for stable test pressure of makeup air during a test on large flexible parts and typically offers improved repeatability and less variation, depending on the application test pressure and reject rate.

Talk to a Leak Test Engineer about Differential Mass Flow Testing

With over 40 years of providing leak and flow test solutions, we have helped many manufacturers like you. Whether you want to learn more about this test method or require assistance to determine the best approach to your leak test, reach and out tell us about your application.