Chemical apparatus and process disinfecting – deodorizing – preser – With indicating – signalling – recording – sampling – or...
Reexamination Certificate
2000-01-05
2001-12-11
Kim, John (Department: 1723)
Chemical apparatus and process disinfecting, deodorizing, preser
With indicating, signalling, recording, sampling, or...
C422S050000, C422S062000, C422S068100, C422S082010, C422S082020, C422S105000, C324S658000, C324S679000, C324S691000, C324S705000, C324S709000, C324S722000, C324S724000
Reexamination Certificate
active
06328934
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a device for detection of when a test probe of conducting material gets into contact with a liquid surface.
PRIOR ART
During, for example, pipetting it is very important to rapidly and safely detect when the tip of the pipette gets into contact with the liquid surface. An erroneous detection may lead to that the pipetting is not performed and an absent or delayed detection may result in contamination of the tip of the pipette.
The level detection procedures used by tradition today are either conductive, capacitive, hygroscopic, optic or acoustic.
Conductive level detectors operates in such a way that the conductivity (ability to conduct) between two measuring points are compared with a pre-set value. If the conductivity is higher (or lower) than the pre-set value, this represents a signal that a liquid or, where appropriate, a septum is found. The conductive detection procedures are, however, impaired by the disadvantages
that they demand two test probes,
that there is a risk that moisture will shorten the test probes,
that solutions with low ion concentration, e.g. distilled water, can not be detected,
that there is a risk for electrolysis of the detected solution and
that it is impossible to distinguish between whether the test probes have got into contact with a septum or the liquid, in the case this is covered by a septum.
Capacitive level detectors operate by capacitance changes. A freely suspended test probe, in the shape of a conductor, has a certain capacitance relative to the surrounding and this capacitance increases when such a test probe gets into contact with the liquid or, where appropriate, with a septum. The disadvantage with the capacitive detection procedures is
that it is impossible to distinguish between whether the test probe has got into contact with a septum or the liquid, in the case this is covered by a septum.
Hygroscopic detection procedures operate with pressure variations during aspiration in progress. A slow aspiration is started, whereupon the pipette is lowered towards the liquid surface. When the tip of the pipette touches the liquid surface, or where appropriate, with a septum, a pressure wave appears in the tube system, which pressure wave is detected. The disadvantages with the hygroscopic detection procedures are
that it is not possible to distinguish between whether the tip of the pipette has got into contact with a septum or the liquid,
that the aspiration has to be performed during the detection,
that considerable demands are put on tubes and tube arrangement and
that such procedures are slow.
Optical level detectors search for the liquid surface by detecting reflected light. The disadvantages with optical detection procedures are
that the detector is easily contaminated,
the optical detectors can not distinguish between foam and liquid and
that special, light conducting test probes are required.
Acoustical detectors operate in the same way as a sonar. Such detectors emit sound which bounces on the liquid surface or, where appropriate, a septum. The detector measures in this connection the time elapsing for the sound to travel from the detector, bounce on the liquid surface and travel back to the detector. If the measured time is less than a predetermined limit, this may be interpreted as an indication of that the liquid surface/septum is reached. The acoustic detection procedures have the disadvantages
that they demand special, sound conducting test probes and
that they are not able to distinguish between a septum and a liquid.
SUMMARY OF THE INVENTION
The aim of the present invention is therefore to provide a device, which do not present the above mentioned disadvantages, for detection of when a test probe gets into contact with a liquid surface, independent of whether this is covered by a septum or not.
This is achieved by the device according to the invention, by means of that the test probe is electrically screened by a screen, isolated from the test probe, with a part of the test probe projecting in front of the screen, that the test probe on one hand is connected to a first alternating voltage source via a first impedance and on the other to one of the inputs of a differential amplifier, while the screen on one hand is connected to a second alternating voltage source via a second impedance and on the other to the other input of the differential amplifier, that the output of the differential amplifier on one hand is connected to one of the inputs of a first multiplier, whose other input is connected to a third alternating voltage source, and on the other to one of the inputs of a second multiplicator, whose other input is connected to a fourth alternating voltage source, and that a signal processing unit is connected to the outputs of the multiplicators and disposed to based on the output signals o f the multiplicators derive on one hand a first signal, representing the resistive load of the test probe, and on the other a second signal, representing the capacitive load of the test probe, and to provide a signal when the resistive load is low contemporarily as the capacitive load is high as an indication of that the test probe has got into contact with the liquid surface.
REFERENCES:
patent: 4010650 (1977-03-01), Piatkowski, Jr.
patent: 4968946 (1990-11-01), Maier
patent: 5304347 (1994-04-01), Mann et al.
patent: 5512838 (1996-04-01), Roach
patent: 0288215 (1988-10-01), None
patent: 0338400 (1989-10-01), None
Kivineeme Juhan
Ljung Arne
Dinsmore & Shohl LLP
Kim John
Pharmacia AB
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