Chemical apparatus and process disinfecting – deodorizing – preser – Control element responsive to a sensed operating condition
Reexamination Certificate
2000-04-27
2003-04-22
Warden, Jill (Department: 1743)
Chemical apparatus and process disinfecting, deodorizing, preser
Control element responsive to a sensed operating condition
C422S105000, C422S106000, C422S108000, C422S068100, C422S063000, C422S067000, C435S180000, C073S863220, C073S864000, C073S864010, C073S864110, C073S864310, C073S30400R, C116S109000, C116S227000
Reexamination Certificate
active
06551558
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns a liquid transfer device for an analysis unit having a liquid transfer cannula and a capacitive liquid level detector for detecting the dipping of the liquid transfer cannula into an analysis liquid contained in a vessel, wherein the liquid level detector has a signal electrode, a counter electrode and a detection circuit for detecting a change in capacitance between the signal electrode and the counter electrode. The invention also concerns an associated method for controlling the intake of analysis liquid into a liquid transfer cannula and a suitably designed liquid transfer cannula.
BACKGROUND OF THE INVENTION
In analysis apparatus used for analyzing body fluids, in particular of blood, liquid transfer devices are required in order to transfer analysis liquids, in particular liquid samples or reagents. Common liquid transfer devices are, for example, pipettes which are used for suction samples or reagents out of a first vessel and to expel them into a second vessel as well as dispensers with which the liquid transfer cannula is connected via a hose to a greater stock of the liquid which may be discharged through the cannulas by means of a pump device. Dispensers usually also perform the same function as a pipette.
In association with the present invention, the designation liquid transfer device generally refers to any device facilitating dipping into an analysis liquid in an analysis apparatus to effect any kind of liquid transfer operation (suctioning up and/or expulsion of liquid) using a liquid transfer cannula. The liquid transfer cannula is for example a hollow needle which normally consists of a thin tube made from metal or plastic. For reasons of simplicity this is subsequently also referred to as a cannulau. Other known forms are disposable dosing tips which are thrown away after use and replaced with new ones. They can have a tubular or tapered form, if necessary with a nozzle-type varying cross-section, can be made of metal or plastic, for example a conductive plastic, and are also described in the following as “cannula”.
When the needle is immersed deeply into the analysis liquid, a relatively large amount of excess liquid remains on its outer side. As a result not only can the precision of the dosage be decreased but especially problematic is that the excess liquid contaminates another liquid into which the needle is subsequently submerged (so-called “carry-over”) or an uneconomic large amount of washing agent is required.
In order to be able to better monitor the submersion depth, liquid transfer devices are provided with a sensing device for the detection of the dipping of the cannula into the analysis fluid, usually designated liquid level detectors or LLD. The liquid level detector is connected to the vertical drive used to submerge the cannula into the analysis liquid in order to stop the submersion motion when the tip of the needle has dipped a few millimeters into the analysis liquid. In addition to preventing the problem of carry-over, one must simultaneously ensure that air is not suctioned up which could lead to measurement errors affecting the diagnosis. For this reason, a minimum submersion depth must be maintained, which can be approximately between 0.1 mm and 5 mm.
The vertical position of the cannula simultaneously provides indication of the level of the liquid in the respective vessel. For this reason, the liquid level detector simultaneously facilitates monitoring of the amount of liquid in the respective vessel to issue a signal for example when the supply of a reagent liquid is used up and the reagent bottle must therefore be exchanged.
A conventional principle of construction for the liquid level detector is based on the measurement of the electrical resistance between the cannula and an electrode disposed on the cannula tip. The cannula and the electrode are electrically insulated with respect to each other so that the electrical resistance between them is very high in the dry state. When the cannula and the electrode are submerged, the sample liquid provides a conductive connection so that the electrical resistance changes abruptly. This signal can be reliably detected using simple electronics. This method has the disadvantage, however, that both the cannula and an electrode must dip into the liquid, on which unavoidable amounts of excess liquid necessarily remain. The above mentioned problem with respect to carry-over and associated reduced precision is thereby exacerbated, apart from when disposable dosing tips are used.
In this respect capacitive liquid level detectors are superior. The detection signal for dipping of the cannula into the liquid is thereby given by the change in electrical capacitance between two sensor electrodes via an electronic detection circuit including an alternating voltage source. The first electrode is thereby normally the cannula itself (which is made from metal or from an electrically conducting (metallized) plastic) and is connected to the hot terminal of the alternating voltage source (signal electrode). The counter electrode, which is usually at ground, is disposed on the outer side of the liquid container of the conventional devices (beneath its bottom and partially around the side walls of the container). This electrode is normally part of the container support. When the cannula tip enters into the liquid, the capacitance between the signal electrode and the counter electrode changes due to the electrical conductivity and dielectric properties of the liquid.
These types of liquid level detectors are described in EP-A-0 164 679, U.S. Pat. No. 4,818,492 and EP-A-0 355 791. These publications contain more detailed descriptions, the complete disclosure of which are hereby incorporated by reference.
A basic problem of capacitive liquid level detectors is that the change in capacitance when entering into the fluid is very small compared to other unavoidable capacitances (“interfering capacitances” such as the connecting cable and the input of the amplifier). The ratio between the useful signal and the interfering signal is therefore poor. A particular problem thereby is that a portion of the interfering capacitance is not constant, but can change as a function of time in a relatively rapid manner. This is particularly true for capacitive interference caused by moving objects (parts of the automated analysis system, hands or other body parts of the person using the apparatus). Particularly in fully automatic analysis apparatus having a plurality of moving components, such interferences are, in practice, unavoidable.
EP-A-0 355 791 addresses a particular problem of this kind (interference by a membrane closing the container) by setting a reference signal when the membrane contacts and during the subsequent downward motion of the needle, detecting the difference relative to this fixed reference signal. This method is directed to the particular application. Interfering capacitances which change between the fixing of the reference signal and the detection of the liquid surface lead to errors in detection.
The liquid level detector described in U.S. Pat. No. 4,818,492 passively compensates for the interfering capacitances of the leads with the assistance of a bridge circuit. Other capacitive interferences are not thereby eliminated, however, and could also lead to improper detection in this particular configuration.
A liquid transfer device for analysis apparatus having a liquid level detector with improved resistance to interference and more reliable operation is known from document EP 0555710 A2. This publication proposes a coaxial electrode configuration including the liquid transfer cannula and having an active shield via a compensation electrode connected to a voltage follower circuit. In addition, in an advantageous improvement thereof, an additional screening electrode functions as a counter electrode at constant potential.
Such a coaxial, in particular tri-axial configuration having active shielding and accompanying reference electrode facilitates, without speci
Mann Karl-Heinz
Sattler Stephan
Brinks Hofer Gilson & Lione
Gordon Brian R
Roche Diagnostics GmbH
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