Aspirating a liquid, includes: providing an aspirating probe comprising probe tip and piston pump, wherein the probe tip and piston pump are in fluid communication; measuring an initial gas pressure in the probe prior to liquid entering the liquid being aspirated; moving the tip into the liquid; moving the piston a distance corresponding to the volume of liquid being aspirated; measuring gas pressure in the volume of gas when the piston stops moving and the liquid pulled into the tip has equilibrated; determining piston volume created by movement of the piston; and determining volume of liquid aspirated by the following formula: Vliquid aspirated=Vpiston volume−((Pinitial−Pfinal)*Volume/unit pressure), wherein Pinitial is initial gas pressure before liquid enters the tip, Pfinal is the final pressure of volume of gas after the liquid has equilibrated, and Volume/unit pressure is the change of gas volume for each change of unit pressure.