Conceptually a volume is a sink and asource. It therefore defines the level of inertial pressure r in a closed loop and acts as aloop breaker.
This simple volume will become singular for incompressible fluids and close to singular for fluids with very low compressibility. For both cases we advise to use VolumeFlex or a Reservoir instead.
Since there is no formula to compute density_derp_h for this volume, an upper bound has to be set in the parameter density_derp_h_set. Alternativeley the derivative can be taken from the media model for all the media that implement the corresponding formula by setting density_derp_h_from_media=true (default:false).