[Paraview] Unable to select: Append attributes

[George Gerber]

Hi Paraviewers,

I would like to do a mesh refinement analysis using the predefined filters
of paraview (I can already do such an analysis with numpy and pylab).
My objective is to calculate the difference between the pressure fields of a
fine grid simulation and a coarse grid simulation.

I have two OpenFoam simulations: A fine grid simulation and a coarse grid
simulation (attached tarball).
I have imported the results of both simulations into paraview.
Then I have resampled the fine grid solution as follows:
Select fine grid source >Resample with dataset > Source>Coarse grid
Then I apply a calculator on the resampled solution and basically rename the
point pressure variable to 'presampled'
I also apply a calculator on the coarse solution and rename the point
pressure variable to 'pcoarse'

The next step should be:
Select both calculators and apply the 'Append attributes' filter.
However, this filter is not selectable at this stage (grayed out)
I can confirm that both calculators have the same number of points and
cells.

Can anyone tell me what I am doing wrong or whether there is a better way of
performing a grid sensitivity study in paraview?

Thank you,
George
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