massflow calculating using logical condition
 

Hi to all!
I'd like to calculate mass flow rate of air flux at certain part of surface/plane
As for instance, chop outlet surface in two using XY plane with Z-coord Z=0.1m
I can use cfx-post only, not change geometry.
So, massflow should be:
areaInt(Density*velo)@outlet = areaInt(Density*velo v part1)@outlet + areaInt(Density*velo v part2)@outlet,
where velo v part 1 means the expression:
if (0.1 [m] < Z , Velocity v, 0 [m s^-1] ),
velo v part 2 is expression:
if (0.1 [m] > Z , Velocity v, 0 [m s^-1] ).

Unfortunately, the sum is not equal to usual massFlow()@outlet. So, where is a mistake?