Measuring and testing – Fluid pressure gauge – With protective separator
Reexamination Certificate
1997-12-15
2001-04-03
Oen, William (Department: 2855)
Measuring and testing
Fluid pressure gauge
With protective separator
Reexamination Certificate
active
06209397
ABSTRACT:
A membrane pressure sensor is used when a device responding to pressure should, for specific reasons, not come into contact with the substance to be measured. This may, for example, be the case when the substance to be measured is corrosive and would attack the instrument in the case of direct contact therewith, when the substance is highly viscous and thus the forwarding of the pressure in the dead spaces of the instrument would be hindered, or when the substance tends to crystallize or to polymerize and would thus clog connections with the instrument. This list of applications of a membrane pressure sensor is not complete and includes only a few possible examples.
The term device responding to pressure refers in the majority of applications to a pressure gage to measure and display the excess pressure of the substance to be measured. It may, however, also refer to a pressure transducer, a pressure switch, a pressure sensor, or a differential pressure measurement device or the like. In the following, the term pressure gage refers to an example of one of the possible devices responding to pressure, which is connected to the membrane pressure sensor and to which the pressure of the substance to be measured is transmitted by means of the membrane pressure sensor.
A prior art membrane pressure sensor has a base with a shallow recess, which is surrounded by an annular joining face. A usually round membrane, which has an annular attachment face, is attached to the annular fluid-leakproof joining face by soldering or by some other means. Thus, the walls of the recess and the membrane form the boundaries of a fluid chamber which is filled, during operation, with a suitable filling fluid. The fluid chamber is connected with the pressure gage via a line such that the pressure of the filling fluid in the fluid chamber is measured and possibly displayed by the pressure gage. On the side of the fluid chamber turned away from the membrane a measurement substance space, which is filled with the substance to be measured, is formed in the membrane pressure sensor, but is isolated from the fluid chamber by the membrane.
The pressure acting on the elastic membrane from the measurement substance space deflects the membrane. If the membrane pressure sensor were not solidly attached to the pressure gage, a volume of the fluid corresponding to the deflection of the membrane would escape from the fluid chamber. This volume of fluid is usually referred to as the working volume of the membrane pressure sensor.
The characteristic &Dgr;v(p), which reflects the dependency of the working volume &Dgr;v on the pressure p in the measurement substance space, is used to indicate the mode of operation of the membrane pressure sensor. It is required of this characteristic that it have, in the region of the pressure to be measured, the most uniform slope possible, i.e., run as linear as possible, such that the system-induced measurement error is kept as small as possible. When low pressures must be measured with high accuracy, i.e., in regions with low nominal pressure, the slope of the characteristic must be comparatively great in its linear region so a large working volume is associated with low pressures. The slope of the characteristic of a membrane pressure sensor in its linear region is usually referred to as its K-factor.
The curve of the characteristic of a membrane pressure sensor is determined essentially by its membrane. Although a flat membrane permits a characteristic with a steep slope at low pressures, it is not suitable for membrane pressure sensors since the characteristic is not linear.
This prior art membrane has on its edge an annular attachment face, in its center a flat central face, and a non-flat, annular connection face between the attachment face and the central face. It should be pointed out that the terminology conventional with flat membranes is used here, according to which the various regions or sections of a membrane are referred to as “faces”, although the sections or regions of the membrane are bodies with a volume which does not equal zero. The name “faces” is justified since the thickness of the membranes, measured perpendicular to the plane of the membrane, is very small in comparison with the membrane dimensions in the plane of the membrane. In the prior art membrane, the connection face consists of annular concentric waves such that the connection face has a waved profile in its radial cross-section. This design as a so-called wave membrane permits high linearity of the characteristic, whereby it has been demonstrated that, to the extent that linearity has been improved by increasing the number and height of the waves, the hardness of the membrane is simultaneously increased and, accordingly, the K-factor or the slope of the characteristic is reduced.
The object of the invention is to improve the generic membrane such that it permits a characteristic with greater linearity and a greater slope, i.e., a higher K-factor. At the same time, a membrane pressure sensor with a correspondingly improved membrane is provided.
With the membrane according to the invention, provision is made that the connection face is formed by at least one annular step which consists of one flat annular face and one truncated-cone-shaped step face, which connects radially inward to the annular face, whereby the annular face and the central face lie in planes parallel to each other and whereby the height of each step face measured perpendicular to the plane of the central face is very small compared to the width of the step. Here, very small height means a height which is essentially in the range from 0.2% to 2.5% of the width of the step.
The truncated-cone-shaped design of the step faces does not rule out that the transition between the respective step faces and the flat faces adjacent thereto is rounded and that the truncated-cone shape is overlaid by a slight arch.
With the membrane according to the invention, the entire active face, i.e., the face enclosed by the attachment face, thus consists of flat faces disposed in various planes parallel to each other as well as one or a plurality of step faces of a very small height connecting the flat faces to each other. The diameter of the step face connecting to the central face becomes advantageously larger with an increasing distance from the plane of the central face. The same is true for the remaining step faces when more than one step is provided.
It has been demonstrated that the design of the membrane according to the invention yields a characteristic with greater linearity and also a steep slope in the linear region, i.e., a large K-factor. The design according to the invention permits a K-factor which is, for example, 10 times larger than that of an otherwise comparable wave membrane. Furthermore, the membrane according to the invention has a stable neutral position, because the at least one step causes stabilization, in contrast to the flat membrane.
The design of the membrane makes it possible that the fluid chamber of the membrane pressure sensor can be designed with a small volume such that the transmitting fluid volume enclosed in the fluid chamber is small. This in turn results in a reduction of temperature-induced measurement errors.
In an advantageous form of the membrane according to the invention, provision may be made that the number of annular steps is at least two and preferably at least three. Three steps have proved to be particularly advantageous.
When two or more annular steps are provided, the membrane is designed such that the distances between the planes of the central face and the planes of the annular faces are increasingly larger radially outward. It has also proved advantageous that the heights of the step faces be essentially equal.
In an advantageous form of the invention, provision may also be made that in each radial cross-section of the membrane the points on the circumference of the central face and the radially outward points of the circumference of all annular faces all lie on a circumscribing circle. It has been d
Heller Heinz
Neubeck Kurt
Schwagerl Robert
Oen William
Roth & Goldman
Wika Alexander Wiegand GmbH & Co.
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