[vasava]

I am simulating fluid+solid flow with multiPhaseEulerFoam solver and monitoring the solid mass flow across patches using the following:

    solids1 {
        type patchExpression;
	aliases {alphasolids alpha.solids;
		 Usolid U.solids; }
        patches (
	water_inlet
	outlet1
	outlet2
	outlet3
        );
        verbose true;
        expression "mag(sum(normal()&Usolid*alphasolids*area()*998.2))";
        accumulations (
            average
        );
	outputControl timeStep;
        outputInterval 10;
    }

However when I check the mass balance (the same way as I would do in Fluent) I get 5-10% error. This expression is from swak4Foam tutorial example.

Am I doing something wrong?