A method for removing supernatant from a sample tube. The method includes providing a sample tube having a pellet at a bottom of the sample tube and a supernatant liquid above the pellet, visually inspecting the sample tube to determine one or more geometric properties of the pellet, and determining an expected height of a top surface of the pellet based on the one or more geometric properties determined in the visual inspection step. The method also includes inserting an aspirator into the supernatant liquid, moving the aspirator downwards towards the expected height of the top surface of the pellet, and aspirating the supernatant liquid through the aspirator.
