integral constraint

Hello,

I am solving a PDE using the math module, and I want to impose the mass conservation in its integral form:

integral[c*dx*dy*dz]=C0

I used the global constraint and inserted the above constraint, but it does not do anything (dose not force the mass conservation) and I get the same results as a case without constraint; am I inserting the integral in a wrong format? is there any other way to force this constraint /integral?


Thanks,
Mohsen