Measuring and testing – Fluid pressure gauge – Diaphragm
Reexamination Certificate
1998-12-23
2001-04-10
Oen, William (Department: 2855)
Measuring and testing
Fluid pressure gauge
Diaphragm
Reexamination Certificate
active
06212956
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a detector for detecting the presence or absence of a liquid or the presence or absence of a gas in a microfluidics system, and in particular to a capacitative gas/liquid detector that provides a large change of capacitance per unit area.
BACKGROUND OF THE INVENTION
In microfluidics systems, it in often necessary to detect the presence or absence of a liquid or to detect the presence or absence of a gas at a predetermined location in such systems. For example, U.S. patent application Ser. No. 09/114,978, the entire disclosure of which is incorporated herein by reference, assigned to the assignee of this disclosure, discloses bubble valve-based pressure regulators that operate in response to pressure sensors that use a capacitative detector to detect the level of liquid in a capillary array. U.S. patent application Ser. No. 09/116,427, the entire disclosure of which is incorporated herein by reference, and also assigned to the assignee of this disclosure, discloses pressure regulators that include an active primary pressure regulator and a passive secondary pressure regulator. An array of capillaries is used as the secondary pressure regulator. The primary pressure regulator is controlled by a controller that operates in response to a capacitative pressure sensor coupled to the capillary array. Some embodiments of the capacitative pressure sensor detect the presence or absence of liquid in ones the capillaries constituting the capillary array. Other embodiments detect the liquid level in the capillary array.
In a United States patent application simultaneously filed with this disclosure and entitled Gas Extraction Device for Extracting Gas from a Microfluidics System, the entire disclosure of which is incorporated herein by reference, two of the inventors and others disclose a device for use in a microfluidics system that extracts from the system gas released from the liquid in the system. Some of the embodiments disclosed employ an active control system that operates in response to a detector that detects when a bubble of gas that accumulates at a location in the system has grown to a size that justifies extraction.
Conventional capacitative detectors for detecting the presence of a liquid typically include a pair of opposed conductive plates and a capacitance detector circuit that measures the capacitance between the plates. Interleaved conductive fingers located on one surface may be used instead of the opposed conductive plates, and the term plates will be understood to encompass such interleaved fingers. The capacitance detector circuit applies an alternating signal having an amplitude of several volts between the plates to measure the capacitance. When measured in this way, the capacitance between the conductive plates is proportional to the area of the plates and the dielectric constant of the medium separating the plates, and is inversely proportional to the distance between the plates. Since liquids generally have a greater dielectric constant than gases, the capacitance measured between the plates when the plates are in contact with a liquid is greater than that measured when the plates are in contact with a gas such as air. However, the change in capacitance is relatively small, and can be masked by the stray capacitances between the conductive plates and other elements of the microfluidics system. Consequently, it is often difficult to detect whether the plates are in contact with the liquid or not. This is especially true when a simple, low-cost capacitance detector circuit is used. The detection reliability can be increased by increasing the area of the plates, but restraints imposed by the small dimensions of the locations where the plates are positioned to detect the presence or absence of the liquid often prevent the area of the plates from being increased sufficiently to provide the sought-for detection reliability.
Thus, what is needed is a capacitative gas/liquid detector that can reliably and easily detect a change in capacitance caused by a liquid contacting a sensor. What is also needed is a capacitative gas/liquid detector that can easily be fabricated using the same micromachining techniques used to fabricate the other major elements of the microfluidics system.
SUMMARY OF THE INVENTION
The invention provides a gas or liquid detector, abbreviated below as gas/liquid detector, for an ionic liquid. The detector includes a capacitative sensor and a capacitance detector. The capacitative sensor has a capacitance and includes a pair of electrodes separated from one another. The electrodes are positioned to be capable of contact with the ionic liquid. Contact between one of the electrodes and the ionic liquid forms a Debye element in the ionic liquid. The Debye element includes a capacitative element and an associated shunt conductor. The shunt conductor has an exponentially-increasing conductance-versus-voltage characteristic. The capacitance detector is connected to the capacitative sensor and measures the capacitance of the capacitance sensor by applying an alternating voltage between the electrodes. The alternating voltage has a voltage amplitude less than the voltage amplitude at which the Debye element ceases to be predominantly capacitative.
The invention also provides a gas extraction device for removing gas from an ionic liquid. The gas extraction device comprises a bubble capture chamber, a gas/liquid detector according to the invention and a bubble removal system. The bubble capture chamber is structured to accumulate a bubble of gas. At least one of the electrodes constituting the capacitative sensor of the gas/liquid detector is located in the bubble capture chamber and is shaped to substantially follow the perimeter of the bubble capture chamber. The capacitance detector of the gas/liquid detector generates an output signal that changes state when the bubble of gas accumulated in the bubble capture chamber has grown to a predetermined size and overlaps at least part of the capacitative sensor. The bubble removal system is coupled to the bubble capture chamber and operates in response to the output signal of the capacitance detector to extract the bubble of gas from the bubble capture chamber.
The invention also provides a pressure sensor for determining the pressure of an ionic liquid. The pressure sensor comprises a pressure-to-position converter and a gas/liquid detector according to the invention. The pressure-to-position converter is in pressure-communication with the liquid and is configured to establish a liquid surface whose position depends on the pressure of the liquid. At least one of the electrodes of the capacitative sensor of the gas/liquid detector is located to contact the liquid in the pressure-to-position converter. The fraction of the area of at the least one of the electrodes of the capacitative sensor that is in contact with the liquid depends on the position of the liquid surface.
Finally, the invention provides a gas/liquid detector for an ionic liquid. The gas/liquid detector comprises a capacitative sensor and a capacitance detector. The capacitative sensor includes a first electrode, a second electrode and Debye elements extant in the liquid adjacent such portions of the electrodes that are in contact with the liquid. The Debye elements each include a Debye capacitor with an associated shunt conductor. The shunt conductor has an exponentially-increasing conductance versus voltage characteristic. The Debye elements adjacent the first and second electrodes are connected in series by conduction through the liquid. The Debye element adjacent at least the first electrode has a substantially greater capacitance than the capacitance between the electrodes absent the Debye elements. The capacitance detector is connected to the capacitative sensor and measures the capacitance of the capacitative sensor by applying an alternating voltage between the electrodes. The alternating voltage has a voltage amplitude less than the voltage amplitude at which the Debye element extant adjacent
Barth Phillip W.
Donald David K.
Field Leslie A.
Hoen Storrs T.
Agilent Technologie,s Inc.
Hardcastle Ian
Oen William
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