PythonCalculator: problem with division per volume(dataset)


I’m using paraview on linux, I have installed numpy, and I am importing data from openfoam8. I have a scalar field (called T) and a mesh.
On paraview guide it is written that “volume (dataset)” is the cell volume field; what i want to do is divide T per cell volume, but if I write " T/ volume (dataset)" in pythoncalculator it generatse the error “dataset is not defined”, so how could I define the dataset?
T is a scalar field that represent number of particles, so i’m trying to calculate the particle concentrations.
Please tell me if you need more information; I’m not very expert with paraview and I have never programmed in python.