[Diger]

I have two questions:
1.
I have a flow with 2 species A and B.
When I define
n=[A] + [B]
as the total molar concentration and in analogy with the mass weighted average as custom functions
Vx=n*vx
Vy=n*vy
Vz=n*vz
where vx, vy, vz are the calculated velocity fields and I specify as custom vectors under surface integrals these newly defined Vx,Vy,Vz and calculate a value at some crosssection (e.g. inlet) I get the same value for every field variable. In fact it is even negative. How is that possible?

2.
When I define a custom field function and define
mass-fraction_A=M_A*[A]/(M_A*[A] + M_B*[B])
Why is that not exactly the same as the internal mass fraction selection?
It is sort of close but not quite: e.g. 5.83 vs. 5.87
Is it maybe becoz the values are interpolated to the position where I want to evaluate it and if I use different quantities to express the value there is some numerical imprecision?