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1. Volume flow
The base of this leakage is the differential pressure measured in a test volume while a test time. To calculate the leakage the "General Gas Law" is used.
Standard units are: [cm3/min], [ml/min], [l/h], [mbar*l/s], [Pa*m3/s], [Pa/s], [mbar/s]
In our country the base of volume flow is the DIN 102. It reflects the reality best of all again.
Definition of the DIN102
2. Mass flow
The base of this leakage is the flow of the gas mass. It will be measured by several measurement systems e.g. thermical massflow sensors. The flow is measured by using a laminar flow element and measuring the gas temperature.
Standard units are: [Ncm3/min], [Nml/min], [Nl/h], [g/h], [kg/h]
To show the different between volume flow and mass flow the first letter of the unit is "N" (=Norm). It will used the norm DIN 1343.
Definition of the DIN1343
The different between both methods is around 10%. If you make a measure with a massflow calibrator on a volume flow test system you will see this different.