Mass conservation in COMSOL
Hello,
I am a user of COMSOL 3.5 and have realized something a little bit strange with mass conservation during my simulations. Therefore, I decided to build the easiest model I could in order to study this issue. I created a hexahedral pipe through which oxygen and water (gas phase) circulate. NavierStokes and MaxwellStefan equations are used to describe momentum and mass transport. The lower boundary is supposed to be a reactive surface and therefore an oxygen flux is set as boundary condition. If I calculate the oxygen mass flow (kg/s) that is going into the system (using the boundary integration tool, integrating the flux) and subtract the oxygen mass flow that is going out, I approximately obtain the same amount of oxygen that is disappearing through the reactive boundary, which means that mass balance for oxygen meets satisfactorily. However, if I do the same to check the water mass balance, the result is really not good. Calculating the difference between the water mass flux at the inlet and at the outlet, I obtain a noticeably different amount than the water that is being generated on the reactive boundary, which is the same (in mass terms) as the oxygen that is being consumed. The minimum mesh element quality is 1.000, so I guess that I cannot blame this on truncation errors or similar. Can you give me an explanation for this? Thank you very much for your help. 
Hi, I suspect that you need to compensate the total mass balance for the reacting species. It is all described here:
http://www.comsol.com/support/knowledgebase/1100/ Niklas Rom, COMSOL Quote:

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