Measuring and testing – Fluid pressure gauge – Diaphragm
Reexamination Certificate
1999-12-22
2001-07-24
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Fluid pressure gauge
Diaphragm
Reexamination Certificate
active
06263739
ABSTRACT:
The invention concerns a membrane pressure sensor according to the generic portion of claim
1
.
A membrane pressure sensor is used to transfer a pressure of a test substance to be measured to a pressure measurement device when the latter should not come directly into contact with the substance to be measured for specific reasons.
A prior art membrane pressure sensor has, in principle, two spaces or chambers separated from each other by a membrane, one of which is designed to accept the test substance and which can be impinged upon by the pressure of the test substance. The other chamber on the other side of the membrane is filled with a filling fluid and is connected to a pressure measurement arrangement. A pressure exerted on the test substance is transferred to the fluid by a corresponding deviation of membrane such that the pressure in the test substance can be detected without the test substance coming directly into contact with pressure measurement arrangement.
To guard against rupture of the membrane and ensure monitoring of its condition, it is further known to use a two-layer membrane, i.e., two substantially parallel membranes, whereby the space between them forms an air evacuation space.
Prior art membrane pressure sensors are usually made of metal or metal alloys with corresponding corrosion resistance. Usually, elastic membranes are used whose working volume, i.e., the volume displaced by the displacement of the membrane, should be high. The reason for this, in particular in mechanical pressure measurement arrangements, is that for the deformation of a measurement element which is detected and displayed as the value representative of the pressure, a minimum working volume must always be displaced by the membrane and pushed into the pressure measurement arrangement. This is true in particular when the membrane pressure sensor is to be connected to the actual pressure measurement arrangement by a remote line which also has elasticity.
Metal membranes are also known, which, also to achieve a higher working volume, are designed with concentric waves, i.e., these membranes have a sine-wave shape in the radial cross-section.
In most cases, the problem in the choice of the appropriate materials for the membrane pressure sensor consists in that corrosion of the pressure sensor must be avoided. As a result, there are currently membrane pressure sensors made completely or partially of plastics for special applications.
These known pressure sensors made of plastic are only useful to limited extent for some special applications, in particular in the semiconductor industry, in plastics manufacture, or in the production of technical glass, since in these cases an introduction of foreign metal ions into the test substance, i.e., into the process media or materials must strictly be avoided in the aforementioned applications.
The known membrane pressure sensors made of plastic use elastomers for the membranes. However, metal ions, which contaminate the test substance, are leached out of these highly resistant elastomers, for example, vinylidene fluoride-hexafluoropropylene rubber (FKM). It has been proposed to coat these elastomer membranes with polytetrafluoroethylene (PTFE) to reduce the aforementioned leaching effect.
To prevent any leaching of metal ions from elastomers, a plastic pressure sensor is known which has a spherical-segment-shaped membrane made of solid polytetrafluoroethylene (PTFE). This membrane is, however, very stiff such that slight pressure changes can no longer be measured with reasonable accuracy. Besides that, the aforementioned material (PTFE) tends to wrinkle and, consequently, strong hysteretic phenomena distort the measurement result. Moreover, such a membrane has temperature responses which also distort the measurement result.
Plastic pressure sensors with a plate-shaped membrane are also made of perfluoroalkoxy copolymer (PFA), which because of the high stiffness of the membrane, have, however, only small working volume and poor response behavior.
These membrane pressure sensors made as single membranes also have the disadvantage that diffusion occurs through the membrane. Metal ions dissolved in the fluid, which come from the pressure measurement arrangement made at least partially of metal, can diffuse into the filling fluid and thus negatively affect the test substance. The diffusion effect is present with elastomers to a considerable extent.
To combat diffusion through the membrane, a pressure sensor of two spherical-segment-shaped membranes made of solid polytetrafluoroethylene (PTFE) arranged one behind the other is known. Between the membranes arranged with a large distance between them, an annular element, also made of solid polytetrafluoroethylene (PTFE), is also inserted between the membranes to transfer force. The membranes are sealed relative to their respective chambers by means of O-rings. The intermediate space formed by the two membranes is aerated such that diffusion is prevented. Moreover, with this system the double design of the membranes results in increased process safety, since the rupture of a membrane does not inevitably result in contamination of the process medium with metal ions.
This known pressure sensor has however the disadvantage that the double membrane is particularly stiff, has a pronounced flow behavior (hysteretic effects) because of the material, is subject to temperature influences, and also the weight of the annular element transferring the force greatly impairs the response behavior of the pressure sensor such that reasonable measurement accuracy cannot be obtained until the high pressure range (starting from approximately 2.5 bar). In addition, the chamber on the test substance side is sealed on the membrane on the test substance side by an elastomer seal (O-ring) such that leaching effects of metal ions from the elastomer also occur here and can contaminate the test substance.
Compared to this prior art, the object of the invention is to propose a membrane pressure sensor which permits high measurement accuracy at low pressures in test substances in which any contamination by foreign metal ions must be strictly avoided.
The object is accomplished with a membrane pressure sensor with the characteristics of claim
1
.
According to the invention, the membrane pressure sensor has a hollow space formed between a housing and a cover which is subdivided by two membranes parallel to each other into a pressure sensing space or chamber for the impingement of a test substance with pressure to be measured, into an air evacuation space between the membranes, and into a pressure output space or chamber which can be filled with a fluid to forward the pressure to a pressure measurement arrangement, whereby the membranes are made of a perfluoroalkoxy copolymer (PFA) and are designed congruent with the interposition of a plastic fiber insert and with shaping. The pressure sensing side or test substance side of the membrane is in direct contact with the cover, and an O-ring is provided to seal the air evacuation space between the housing and cover. The housing and cover are made of a fluoropolymer.
Through the use according to the invention of shaped, e.g., waved, in particular concentrically waved perfluoroalkoxy copolymer (PFA) membranes, the positive material properties of the chemically neutral, metal-free, and heat formable PFA material can be used effectively in a membrane. Through the shaping of the membrane, it is advantageously deformable in the direction of its primary plane such that a large working volume may be achieved with low response pressures.
Because of the low E-modulus of plastics, their elastic deformation range is usually small. By means of the shaping, for example, in the form of sine-wave-shaped concentric corrugation, the material expansion required with flat membranes for their deflection is converted into a bending load at the points of curvature. With an appropriate number of a bends in the shaping, i.e., a comparatively great length of the membrane in the direction of the primary plane of the me
Gareus Joachim
Hörning Udo
Seefried Thorsten
Aw-Musse Abdullahi
Fuller Benjamin R.
Roth & Goldman
Wika Alexander Wiegand GmbH & Co.
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