Comment (by Leonardo Rosa Werneck):

In this case, the issue is not with VolumeIntegrals, but with IllinoisGRMHD. VolumeIntegrals expects HydroBase variables, and when using IllinoisGRMHD, those variables are only set correctly if one explicitly calls the routine that converts from IllinoisGRMHD's native variables to HydroBase.

This is therefore not a diagnostic issue, but rather a consequence of IllinoisGRMHD's choice of gridfunctions. For that reason, I think it is reasonable to expect IllinoisGRMHD to handle this conversion, rather than placing that responsibility on the diagnostic thorn.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2929/grhaylet-illinoisgrmhd