Measuring and testing – Gas content of a liquid or a solid – Of a liquid
Reexamination Certificate
1998-10-07
2001-05-22
Williams, Hezron (Department: 2856)
Measuring and testing
Gas content of a liquid or a solid
Of a liquid
C073S019040, C073S019100, C073S030030, C073S064470, C073S061730, C073S061470
Reexamination Certificate
active
06234004
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for the measurement of diffusivity or flow velocity. The invention also relates to a microsensor for measurement of diffusivity or flow velocity, which microsensor has a reservoir provided with a passage- or transport area and at least one transducer.
Several principles are known for the measurement of flow velocity in fluids. For example the induction principle (Faraday-principle), the skin friction/hot wire principle and the Laser-Doppler-principle. All these principles make use of measuring devices with considerable dimensions. Consequently, the medium where the measurement takes place have to fulfill specific requirements such as sufficient flow and volume.
The U.S. Pat. No. 5,339,694 describes a sensor for measurement of flow in a matrix, more precisely measuring migration of groundwater. The sensor consists of a circular pipe having a liquid permeable diaphragm. Outside the cylinder are transducers for measurement of conductivity. The cylinder contains a saline solution with a conductivity that differs from that of groundwater. When the sensor is placed in a water saturated stratum, salt ions from the cylinder will diffuse through the permeable diaphragm into the stratum with groundwater. A change in the conductivity in an area outside the cylinder is registered by the sensors outside the cylinder, whereby the flow velocity can be determined.
This sensor makes use of a suitable principle for the determination of small flow velocities through bigger volumes of a matrix. However, due to the dimensions of the sensor, it is not possible to use it in smaller volumes such as layer transitions in fluids or in vessels in vegetable or animal tissues, in filters or biofilm.
SUMMARY OF THE INVENTION
Thus, the aim of the present invention is to provide a method and a sensor, which makes use of a better spatial resolution and provides a lower limit of measurement than described in prior art, and which can measure transport coefficients like diffusivity or flow velocity in fluids such as liquids or gasses, or measure in porous matrixes or close to surfaces.
In particular, it is an aim to provide a sensor which can measure in systems where transport of substance or matter takes place as a combination of diffusivity and advection as e.g. close to surfaces and in porous matrixes.
This is achieved with a method where the sensor is placed in an area or volumen of a medium, that a gas or a dissolved substance is diffused into this medium hereby functioning as a tracer material, that the diffusivity or the flow velocity is determined by measuring either the partial pressure of the gas or a concentration of the gas or liquid dissolved substance, whereby the measurement is carried out in the passage- or transport area on the gas or liquid dissolved substance.
Acccording to the invention first microsensor is characterized in that the transducer has a sensoric tip, that is placed in the passage- or transport area, and that the sensoric tip of the transducer is placed in the mouth of the passage- or transport area.
A second embodiment of a microsensor according to the invention is characterized in that the transducer or measuring device is surrounded by the reservoir, which contains a gas or liquid dissolved substance. Preferably, the passage- or transport area is equipped with a permeable membrane or insert, open for diffusion, which is placed in and in sealing contact to the mouth, that the transducer tip is placed in the membrane or insert, and that the reservoir contains a gas or a liquid-dissolved substance intended for diffusion from the reservoir through the membrane or insert to an area or volume outside the mouth.
The transducer can either be placed longitudinally, with its axis parallel with the axis of the passage- or transport area and with the sensoric tip placed in the membrane, or with its axis and sensoric tip mainly vertical to the axis of the passage- or transport area. Especially the longitudinal solution allows a space saving sensor design.
Microsensors with these features can be miniaturized to such a degree that kinetic transport or diffusion coefficients as well as very low flow velocities can be measured. Measurement by means of the inventive microsensor is a true in-situ measurement, and the placing of the microsensor in the medium will not cause substantial changes in transport coefficients. The use of the microsensor does not require a given minimum diffusion coefficient or flow velocity.
The term “fluid” is to be broadly understood as liquids or gasses, and also incorporates gasses with a mist of liquid in the gas or a liquid with a gas contained in the liquid.
The transducer is mainly an electrochemical microsensor, for example a Clark type oxygen micro-electrode, but other types of microsensors like sensors based on optical fibres, so-called optrodes can be used too.
The membrane or insert described in the second embodiment of a microsensor according to the invention is placed at the mouth of a container and is preferably made of silicone.
However, materials that are diffusible or permeable for dissolved non-ionized substances can also be used. The measuring principle for the microsensor can hereby be extended in a way that it comprises not only diffusion of gasses but also diffusion of liquid-dissolved substances like dissolved non-ionized substances from a liquid in the reservoir. The microsensor according to the invention can perform measurements in both gas and liquid media.
The sensor may have a circular mouth at the end of the container, the mouth having a diameter of between 2 &mgr;m and 3 mm, preferably between 2 &mgr;m and 100 &mgr;m. For the measurement of flow velocity 20 &mgr;m is preferred whereas 100 &mgr;m is ideal for measurement of diffusion coefficients. The axial length of the membrane or insert is between 2 &mgr;m and 10 &mgr;m, preferably 20 &mgr;m for measurement of the flow velocity and 100 &mgr;m for measurement of diffusion coefficients.
Using this microsensor it is possible to reach a lower limit for measurement of flow velocity which is less than 5 &mgr;m/s—that is a factor 100 lower than known from prior art. These low values can be measured by the microsensor, where measurement of a partial pressure or a concentration takes place over a very small length inside the sensor. Due to the small dimensions the volume of the medium where the measurement takes place may be reduced considerably.
The sensor is among other uses intended for measurement of flow velocities in fluids with little flow, or in gasses and liquids close to surfaces, but can advantageously be used in other areas, like in human vessels. Furthermore, the microsensor is intended for measurement of molecular or ionic kinetic transport conditions in the form of combined diffusion and flow in porous matrixes in for example biofilm in bioreactors or in animal tissues like brain or liver. Also, measurement in sediments of the sea bed is possible. Hitherto, measurement has mainly been done through use of tracer elements fed into the medium with measurements carried out in the medium itself. With the present invention it is now possible to measure in-situ the transport of miscellaneous substances in vessels, as for example in human tissues.
Consequently, the sensor can be used in many different applications within medical and scientific research as well as within industrial branches like medical and biotechnological industry, where the exchange of a substance between solid surfaces and liquids is an interesting parameter.
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Gundersen Jens Kristian
Nielsen Lars Peter
Pedersen Ole
Revsbech Niels Peter
Lee, Mann, Smith, McWilliam Sweeney & Ohlson
Unisense APS.
Wiggins David J.
Williams Hezron
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