Patent ID: 8100007

Claim:
A method for detecting contact of the tip of a pipetting needle with a liquid contained in a vessel, said detecting being effected within a time interval during which the pipetting needle is moved towards said liquid in said vessel for performing a pipetting operation, said method comprising: applying ultrasonic pulses to the pipetting needle by means of a electromechanical transducer which is mechanically connected to a part of the needle which is located at a predetermined distance from the tip of the pipetting needle, wherein each of the ultrasonic pulses has a time interval of less than about 250 μs; transmitting through the pipetting needle mechanical pulses generated by the application of said ultrasonic pulses to said pipetting needle; said transmitting of mechanical pulses including transmission of mechanical pulses generated by said transducer towards the tip of said pipetting needle and also transmission in the opposite sense of mechanical pulses reflected at said tip; receiving said reflected mechanical pulses with said electromechanical transducer, said electromechanical transducer thereby generating a corresponding electrical output signal; and monitoring said electrical output signal, selecting at least one specific component thereof by means of a time-of-flight or signal phase analysis, and by evaluating the variation with time of predetermined characteristics of a parameter of said at least one selected component of said output signal in order to detect the position of the needle at which the pipetting needle contacts the free surface of said liquid contained in said vessel and for providing a resulting signal representative of the result of said evaluation.