Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
2002-04-16
2003-12-09
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
C141S059000, C073S295000, C073S049200, C340S605000, C340S626000, C361S037000, C361S120000
Reexamination Certificate
active
06662122
ABSTRACT:
The invention relates to a method for the controlled proportioning of liquids by means of a positive-displacement device to dislocate a gas cushion and a reception volume connected thereto which has an aperture to the environment for the reception and discharge of a liquid by dislocating the gas cushion by means of the positive-displacement device.
The proportioning of liquids while dislocating a gas cushion is performed by means of manual or electric pipettes or automatic proportioning devices. The gas cushion is an air cushion, as a rule. The positive-displacement device is mostly designed as a cylinder having a slidable piston therein. The liquid is mostly received into a pipette tip which can be exchanged. Pipette tips are mostly conceived as disposable articles which can be discarded after a single use. They are made of plastic material, as a rule. The suction and expulsion of the liquid by means of the gas cushion has the advantage that this avoids contaminations of the positive-displacement device and the other re-used components of the proportioning device. A gradual contamination by aerosols or any faulty handling of the proportioning device may be counteracted by filters which are preferably integrated in the pipette tip.
The electric air-cushion pipettes and automatic proportioning devices which are known still are very susceptible with regard to the correctness of the liquid volume discharged and the error in proportioning. They do not even detect major deviations from the desired value of the liquid volume to be discharged, which are caused by a leakage in the proportioning system. Errors in proportioning may also arise while liquids are proportioned under a high vapour pressure. Deviations in the temperature, density, viscosity, and surface tension of the liquid to be proportioned may also cause major errors in proportioning which will not be detected. Errors in handling, e.g. air intake because of too early a withdrawal from the liquid during reception or too small a feed volume, also lead to errors in proportioning.
From U.S. Pat. No. 5,895,838, a method has been known for correcting a temperature-dependent dispensing error in dispensing liquids such as in pipetting, and an apparatus for dispensing liquids such as a pipette which has a higher degree of accuracy. According to the method, dispensing is performed by means of two chambers which are connected to each other via a gas passage. The first chamber, in addition to communicating with the gas passage, communicates with the liquid to be dispensed and the second chamber is gas-tight except for the gas passage. To receive a liquid in the dispensing apparatus, the volume of the second chamber is enlarged, which causes an entry of gas into it from the first chamber and, in turn, an inflow of the liquid to be dispensed into the first chamber until a pressure balance is reached between the chambers. A measurement is made of the change of temperature of the gas flowing from the first chamber into the second chamber and the change in volume which is caused in the second chamber is corrected on the basis of the change of temperature as measured so that the desired amount of liquid is received into the first chamber. A temperature sensor is installed in the second chamber in the vicinity of the gas passage to measure the temperature and a sensor which is mounted in the first chamber may be used in addition. This method is only suitable for the correction of temperature-related errors in proportioning.
From DE 35 31 241 A1, a device has been known for the controlled dispensing of liquids which has a cannula to receive the liquid and an axially movable pressure transducer at one end of the cannula where a connection remaining constant specifically in its volume is disposed between the cannula and the pressure transducer. At least one pressure sensor is disposed between the pressure transducer and the cannula, specifically within the connection, and a regulating connection is provided between the pressure sensor and a driving device for the pressure transducer.
Accordingly, it is the object of the invention to provide an improved method for the controlled proportioning of liquids by means of a gas cushion.
The object is attained by a method having the features of claim
1
. It further is attained by a method having the features of claim
10
. Advantageous aspects of the methods are set forth in the sub-claims.
The point to start from for the invention is that the error in proportioning which is caused by error sources such as deviations of the vapour pressure, temperature, density, viscosity, and surface tension of the liquid to be proportioned, modifications to the geometry of the pipette tip, leakages of the system, and errors in handling will influence the pressure which prevails in the gas cushion while it is dislocated. The invention involves a measurement of the pressure in the gas cushion to comprise all of the aforementioned error sources.
According to the first approach, in a method for the controlled proportioning of liquids by means of a positive-displacement device to dislocate a gas cushion and a reception volume connected to the positive-displacement device which has an aperture to the environment for receiving and discharging a liquid by dislocating the gas cushion by means of the positive-displacement device,
the pressure p
ab
in the gas cushion is measured at the time of a complete discharge of a liquid volume,
the discharge time t
ab
for a complete discharge of the liquid volume is measured, and
the time demand &Dgr;t to compress the gas cushion by the positive-displacement device is determined by means of the pressure p
ab
, the dead volume V
o
of the gas cushion, a gas condition equation, and the volumetric gas flow Q discharged by the positive-displacement device,
a corrected discharge time t
oab
is determined as a proportioning control magnitude by a deduction of the time demand &Dgr;t from the discharge time t
ab
measured, and
the deviation of the corrected discharge time t
oab
from a desired discharge time t
soll
is output and/or is saved and/or is resorted to for regulating the discharge of the liquid volume.
The discharge time t
ab
is that time which the positive-displacement device needs from the beginning to the end of gas cushion dislocation and, hence, to the time at which the liquid volume is completely discharged. The dead volume V
o
of the gas cushion is that volume which the gas cushion has at the beginning of liquid reception. The positive-displacement device simultaneously causes the gas cushion to undergo a dislocation and a compression during the discharge time t
ab
. The dis-location of the gas cushion is matched by the liquid volume discharged. The above error sources have an impact on the discharge time and the extent to which the gas cushion is compressed. As a result, the error in proportioning may be characterized by the time demand &Dgr;t for the compression of the gas cushion. Thus, the corrected discharge time t
oab
which results from the reduction of the discharge time t
ab
by the time demand &Dgr;t is a control magnitude suited for the proportioning of liquids which allows to discover errors in proportioning. To this effect, the deviation of the corrected discharge time t
oab
from a desired discharge time t
soll
may be output, e.g. by signalling the degree of the deviation or, if a tolerable deviation is exceeded, by giving a warning message (e.g. acoustically, optically or in another perceivable form). In addition or instead, the deviation may be saved, e.g. to furnish a proof on how precise a proportioning was. In addition or instead, the deviation may be re-sorted to in order to regulate the discharge of a liquid volume, e.g. by enhancing the accuracy of proportioning from discharge to discharge if the same liquid volume is discharged several times or step by step.
The error in volume (change in volume) &Dgr;t V may be determined by multiplying the time demand &Dgr;t by the volumetric gas flow Q.
The beginning of the discharge time t
ab
may be favourably
Barlow John
Bhat Aditya
Eppendrof AG
Sidley Austin Brown & Wood LLP
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