Liquid transfer device for an analysis unit

A liquid transfer device for an analysis unit, the liquid transfer device comprising:

• a liquid transfer needle having a tip;
• a capacitive liquid level detector for detecting a dipping of the tip of the liquid transfer needle into an analysis liquid, the analysis liquid being contained in a vessel, said capacitive liquid level detector comprising
• a coaxial electrode arrangement including said liquid transfer needle and at least two coaxial electrodes coaxially surrounding said liquid transfer needle with the exception of the tip which projects therefrom, said at least two coaxial electrodes being electrically insulated from said liquid transfer needle and from each other, wherein liquid transfer needle and said coaxial electrodes of said coaxial electrode arrangement are coupled to a detection circuit comprising isolation amplifier circuit means for producing an output signal which corresponds to the input signal in phase and level, wherein an output side internal resistance of said isolation amplifier circuit means is lower than an input resistance thereof;
• said liquid level detector further comprising
• an a.c. voltage source connected to an input of said isolation amplifier circuit means and a first one of said at least two coaxial electrodes, said first one of said at least two coaxial electrodes forming a signal electrode,
• wherein the other of said at least two coaxial electrodes is coupled to an output of said isolation amplifier circuit means and forms a compensating electrode.

A method for detecting a level of liquid in a vessel of an analysis unit, said method comprising the steps of:

• providing a liquid transfer device for an analysis unit, the liquid transfer device comprising a liquid transfer needle having a tip thereupon, and a capacitive liquid level detector for detecting a dipping of the tip of the liquid transfer needle into an analysis liquid, the analysis liquid being contained in a vessel, wherein the capacitive liquid level detector comprises a coaxial electrode arrangement including the liquid transfer needle and a first coaxial electrode coaxially surrounding the liquid transfer needle with the exception of the tip which projects therefrom, said first coaxial electrode being electrically insulated from said liquid transfer needle, wherein said liquid transfer needle and said first coaxial electrode of said coaxial electrode arrangement are coupled to a detection circuit comprising isolation amplifier circuit means for producing an output signal which corresponds to the input signal in phase and level, wherein an output side internal resistance of the isolation amplifier circuit means is lower than an input resistance thereof, with the liquid level detector further comprising an a.c. voltage source connected to an input of the isolation amplifier circuit means and one of the first coaxial electrode and the liquid transfer needle, with the one of the liquid transfer needle and the coaxial electrode forming a signal electrode, wherein the other of the liquid transfer needle and the first coaxial electrode is coupled to an output of the isolation amplifier circuit means, and forms a compensating electrode;
• said method further comprising the steps of
• applying an input signal to said detection circuit and one of said liquid transfer needle and said first coaxial electrode;
• applying an output signal of said detection circuit to another of said liquid transfer needle and said first coaxial electrode, thereby forming a stray field therebetween, said stray field forming a useful capacitance;
• actively compensating other capacitance of the coaxial electrode arrangement;
• and
• measuring a change in voltage at an input of the isolation amplifier circuit means, said change in voltage being measured as a useful signal which is indicative of contacting of said liquid transfer needle to said analysis liquid.