Patent ID: 8012766

Claim:
A method of aspirating a liquid, comprising: providing an aspirating probe which comprises a probe tip and a piston pump, wherein the probe tip and piston pump are in fluid communication; measuring an initial gas pressure (P A/Dinitial ) in the tip of the aspirating probe prior to the tip entering the liquid to be aspirated; moving the tip into the liquid, whereby a volume of gas is located between the top of the liquid and the piston of the piston pump; moving the piston of the piston pump a predetermined distance which corresponds to a selected volume of liquid to be aspirated, whereby a column of liquid is aspirated into the tip; measuring the gas pressure in the volume of gas when the piston stops moving and the column of liquid aspirated into the tip has equilibrated (P A/Dfinal ); obtaining Volume/ADC, wherein ADC is pressure as expressed in analog/digital counts and Volume/ADC is the change in the volume of the gas per unit ADC; determining the piston volume created by the movement of the piston; and after aspiration determining the volume of the column of liquid aspirated by the following formula: V liquid aspirated =V piston volume −(( P A/Dinitial −P A/Dfinal )*Volume/ ADC ), wherein P A/Dinitial is initial gas pressure in ADC before the liquid enters the tip, and P A/Dfinal is final pressure of the volume of gas in ADC.