I deal with incompressible laminar flow (SIMPLE scheme).

For calculating mass conservation I add up the product A*V where A is the cross sectional area of a cell and V is it's velocity along a pipe. I perform the summation all over a cross sectional cut in a wind pipe.

Is there any other technique to verify mass conservation? What are reasonable mass conservation values? Always 100%?

Thank you.

For an incompressible flow, the mass conservation condition reduces to a divergence free velocity condition. Hence you can check the divergence of velocity in your domain and see if it is zero. It won't be exactly zero. It should be small however, depending on the order of accuracy and boundary treatment.