[horstbort]

There is always microgravity, i.e. residual acceleration due to drag and thrusters, and especially vibrations. Worst offenders are actually the astronauts, especially when they work out on one of their exercise devices right next to your experiment

[pfdietz]

There are also tidal accelerations due to not being in the center of mass of the space station.

Robert Forward had a scheme for nulling these over a larger volume using a heavy ring, a technology related to the one he used in the novel Dragon's Egg (which used SFnal superdense matter to enable a space station to exist in close orbit to a neutron star without the tidal forces killing the occupants.)