Measuring and testing – Gas content of a liquid or a solid
Reexamination Certificate
2001-04-11
2004-01-20
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Gas content of a liquid or a solid
C073S019050, C073S019100, C073S019120
Reexamination Certificate
active
06679096
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns a method for the measurement of the concentration or the partial pressures of gases, particularly oxygen, in fluids as well as a gas sensor for its execution. The measurement of the concentration of oxygen or other gases is of major significance in various fields of engineering, such as in chemical processing, environmental and medical technology etc.
DESCRIPTION OF THE PRIOR ART
Traditionally, dissolved oxygen is measured galvanically, polarographically or potentiometrically in electrochemical measuring cells according to the Clark principle, meaning, the current is measured as a function of the time or of the potential or the potential in the zero current state in an electrolyte between a cathode and an anode.
In order to conduct oxygen into the measuring cell inner zone, a membrane is located which is permeable to oxygen and to gas, respectively, and this membrane separates the electrolyte in the measuring cell from the fluid to be measured.
The measurement presupposes the equilibrium setting of the partial pressures outside and inside the cell, and/or the concentrations corresponding with it by way of the Henry constant. As the electrochemical reaction consumes oxygen, a constant forward flow must be produced by stirring the measuring cell in the fluid. For this reason, many manufacturers offer mountable stirring mechanisms. Further methods are also known which compensate the oxygen consumption and, therefore, are only suitable for the measurement of low concentrations or/and in small volumes. The measuring values must be pressure-compensated, and corrected with regard to salinity and temperature.
The measurement is locally defined at the location of the sensor. Problems easily occur as a result of contamination at the relatively small membrane as well as air inclusions during the change of membranes.
There are other possibilities of oxygen measurement, for example, in the evaluation of the oxygen-concentration-dependent modulation of the phase radiant light received.
For the concentration measurement of other gases, gas-specific methods are known, insofar as recourse is not made to gas chromatography.
All methods mentioned require a complicated and sensitive sensor technology. For this reason appropriate measuring equipment is expensive and has only a low service life and/or requires high expenditure with regard to maintenance and calibration. For measurements under rough conditions, for example the measurement of oxygen partial pressure in low stratums, they are completely unsuitable.
In deeper stratums, such as in the ground water, gas measurement has not been possible up to the present time, the reason being that a measurement could only be carried out by means of a sample extraction. As the water is under a correspondingly high pressure and has a special temperature, degasification would occur at sample extraction and measurement under atmospheric conditions and no measuring values allocatable to the location of measurement would be obtained. Moreover, local measurement would seem problematic because it is not possible to estimate which disturbance affects the system during sample extraction.
BRIEF SUMMARY OF THE INVENTION
The invention is based on the task assignment of stating a method and a suitable gas sensor of the category described in the introduction, which work in an uncomplicated manner and with simple means, which are sturdy and which lead to representative results for the structure under examination.
It was discovered with surprise that the properties of oxygen-permeable synthetic materials such as e.g. PTFE or other gas-specific permeable synthetic materials can be used for building up an oxygen and gas sensor, respectively, which allows the determination of the partial pressure or the concentration of the oxygen or other gases of a gas mixture by means of direct pressure measurement because the partial pressure to be measured within a random fluid causes an equivalent pressure in an enclosed vessel whose wall consists at least partially of such a synthetic material permeable to oxygen and/or gas.
PTFE (polytetrafluoroethylene) and PTFE-similar products such as Teflon, a copolymer of PTFE and <5 mol-% PAVE (perfluoro alkyl vinyl ether), have a relatively high permeability for oxygen. At the same time, PTFE has an outstanding significance among the synthetic materials because of its enormous resistance and stability opposite random influences from its ambient surroundings. Adhesion and cohesion of the material are very low. Subsequently, the frictional forces also are reduced to a correspondingly low level. The surfaces remain clean.
PTFE has a further remarkable property. The permeability of the synthetic material, e.g. opposite the good water-soluble carbon dioxide and the nitrogen mainly prevailing in the atmosphere, lies two orders of magnitude below that of other synthetic materials such as, for example, HDPE (high density polyethylene).
The utilization of the properties
relatively high oxygen permeability and
negligible permeability opposite other liquids and gases
with the gas sensor built up according to the invention leads to the result that the partial pressure of the oxygen in a mixture of substances causes an equivalent partial pressure in a closed Teflon vessel and that this, independent of the absolute pressure existing in the vessel and only dependent on the oxygen partial pressure prevailing beforehand in the vessel, causes a change of the absolute pressure. This means that the measurement of the oxygen concentration can be reduced to a simple pressure measurement.
The method can be applied to any random fluid media.
The pressure is measured by means of, for example, measuring sensors which have longtime and overload stability but which are also almost to a randomly small degree dimensionable and are linear in a wide range, such sensors being on the basis of monocrystal silicium and having an accuracy of approximately 0.4%. With this, the possibility of an inexpensive and precise oxygen partial pressure measurement can be assumed where the concentration is determined, depending on the peripheral and initial conditions of the measurement, both from the rising speed of the pressure with known material properties of the applied synthetic material as well as from the geometry of the measuring cell or—without this knowledge—as a function of the absolute pressure difference.
A further variant exists in the measurement by means of differential pressure transmitters against a reference vessel which is subject to known conditions, e.g. atmospheric conditions, and which is otherwise configured as similar as possible to the measuring system, meaning, in particular with reference to material selection of the measuring vessel and its dimensions.
Water vapor also diffuses through the wall of the vessel, as well as further gases albeit with larger time constants. For this reason, a calibration is performed before a measurement is carried out again where the vessel is purged with nitrogen or a gas mixture with a known composition and/or air. With a tempering of the purging gas, a certain measuring temperature can, if required, be set simultaneously in the vessel.
The measuring cell structural arrangement and the measuring principle can be adapted to the individual measuring problem in question. In the simplest case, the gas sensor consists of a hollow body whose wall is at least partially formed by means of an oxygen-permeable membrane and in which a pressure probe is arranged. In addition to this point-type measurement (e.g., also in spherical-shaped geometry of the measuring cell), and with the assistance of a hose-type measuring arrangement, the integrating measurement along a line or, with the assistance of a hose network, the integrating measurement of a surface is also possible at all times. With this, an essential objective within monitoring task assignments in the environment—the determination of oxygen in the ground air or in the ground water of heterogeneous systems—is obtained in a simple and rep
Geistlinger Helmut
Lazik Detlef
Birch & Stewart Kolasch & Birch, LLP
Cygan Michael
Lefkowitz Edward
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