Measuring and testing – Fluid pressure gauge – Diaphragm
Reexamination Certificate
2000-05-26
2002-11-05
McCall, Eric S. (Department: 2855)
Measuring and testing
Fluid pressure gauge
Diaphragm
C073S861470
Reexamination Certificate
active
06474168
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention pertains to a pressure sensor for measuring absolute dynamic pressure, and more particularly, a sensor comprising a frame, a diaphragm arranged in the frame and attached thereto along parts of the outer edge of the diaphragm, where the diaphragm has a measurement side toward the surroundings and a rear side, and further a reference chamber behind the diaphragm rear side and a restriction connecting the reference chamber to the surroundings, as well as a signal-providing element arranged to detect mechanical stress in an attachment part in the outer diaphragm edge. The invention also pertains to a photoacoustical gas detection sensor, a microphone and a hydrophone based upon use of the pressure sensor. Further, the invention pertains to a method for manufacturing a pressure sensor for dynamic absolute pressure, and a method for manufacturing a photoacoustical gas detection sensor.
Pressure sensors based on movement/flexure of a diaphragm are as a starting point only able to measure differential pressures. E.g. in silicon pressure sensors the sensor element consists of a diaphragm which on respective sides thereof is in contact with a fluid under pressure, and which diaphragm flexes dependent on the pressure difference between the fluids.
In order to manufacture an absolute pressure sensor with a differential sensor as a starting point, one has to provide a substantially closed chamber with a reference fluid on one side of the diaphragm. The volume of this chamber is called the reference volume. The reference volume, and consequently the reference pressure, will vary with temperature. Hence, temperature changes will introduce errors in the measurement system.
Another aspect that must be taken into consideration in connection with the reference volume, is that when the diaphragm flexes toward the reference chamber, the fluid in the reference volume will be compressed, and thereby form a pressure, depending on the compressibility of the fluid. This phenomenon is termed pressure feedback, and such pressure feedback affects the sensitivity of the system. In order to minimize the pressure feedback, the reference volume must be large, so that the diaphragm flexing volume constitutes only a small fraction of the reference volume. (Alternatively, the diaphragm must be rigid, and this means at the same time low sensitivity.)
One way of overcoming the two above mentioned problems of temperature errors and pressure feedback, is to arrange a reference chamber with a vacuum behind the diaphragm. A temperature increase will then not give any increase of pressure in the reference chamber, and pressure feedback cannot arise. However, such an embodiment with a vacuum in the reference volume, will lead to limitations in the pressure values that can be measured. If the static pressure is approximately 1 bar, as will normally be the case in the surroundings, the diaphragm must be dimensioned to withstand a pressure difference of 1 bar if a vacuum is used in the reference volume, and such a diaphragm will be rather poorly suited for measuring very small pressures, in particular dynamic pressures e.g. in connection with acoustical oscillations. When these small, dynamic pressures are to be measured, the diaphragms must be dimensioned in relation thereto, and a static pressure difference of e.g. 1 bar will then possibly lead to destruction of such a diaphragm. In other words, in connection with measuring dynamic pressures having small pressure values, e.g. in the
1
pascal range, one must use a reference volume that contains a fluid.
The sensitivity of the measurement system will also depend on the volume displacement of the diaphragm. The volume displacement is flexure volume per pressure unit, i.e. the volume occupied by the flexed diaphragm in the reference chamber, divided by the pressure. In general, a good sensitivity implies a large flexure/volume displacement. This can easily be appreciated by considering a very thin diaphragm that flexes easily when a pressure differential is present. The disadvantage of a large volume displacement is that the pressure feedback increases, and the sensitivity of the sensor is reduced.
If it is desirable to measure rapidly changing, dynamic pressures, typically in connection with sound oscillations, it is possible to make a small channel into the reference chamber, thus letting the surrounding medium into the chamber. At the outset this will make the diaphragm more robust to exposure to atmospheric pressure, temperature changes, or to handling. Such a channel or opening that connects the reference volume to the sensor surroundings, or more generally to the pressure input of the sensor, is called a restriction. This is because the opening/channel is so narrow that it will take a long time until the pressure inside the reference volume is equalized in relation to the external pressure. The restriction has the effect of a negative feedback regarding slow changes, i.e. low frequencies in the pressure oscillations. The cross section area and the length of the restriction represent a flow resistance, equivalent to an electrical resistance, and the reference volume multiplied by the fluid compressibility, represents a reservoir, equivalent to an electrical capacity, and together the restriction and the reference chamber then operate as a first order lowpass filter for the pressure oscillations, since the low frequencies have sufficient time to get through the restriction and thereby influence both sides of the diaphragm, while the high frequencies will merely affect the diaphragm side facing the surroundings/the pressure input, and therefore will be measurable. In other words, the sensor will be sensitive to frequencies higher than the corner frequency of this filter. By shaping the restriction and the reference chamber in a suitable manner, it is possible to control the corner frequency of the filter, and in this way the measuring range of the sensor. If it is desirable to achieve an extended measuring range down toward low frequencies, then a large reference volume will be advantageous, since this provides a low corner frequency.
However, size is often an important criterion in manufacturing a sensor that fulfils requirements set by an application. In order to make a small sensor, it is of course important that the reference volume is made as small as possible, but with a small reference volume, and when it is desirable to measure relatively low frequencies, one must prepare a very narrow restriction, and the diaphragm must give an extremely small volume displacement. Also if it is desirable to manufacture pressure sensors in planar technology, i.e. in batches, the size of the reference volume should be restricted. The advantage of this type of manufacturing is that the sensors can then be manufactured at a reasonable cost.
The most common application regarding measurement of dynamic pressures, is in sound measurement, which comprises dynamic pressures all the way down into the &mgr;Pa range. When it is desirable to make measurements in these pressure ranges, static pressure variations are quite destructive. Variations due to high and low pressures may amount to several tens of kilopascals (1 kPa=10 mBar). If one wishes to measure dynamic pressures under water at various depths, the static pressure changes may be even much larger. When dynamic pressures shall be measured, one will often use sensor elements that are sensitive only to dynamic pressure, and the most common example of such a sensor element is a piezoelectric crystal. Such crystals have many good characteristics, inter alia a low price, a high natural frequency and a low sensitivity to acceleration. However, it is a disadvantage that such piezoelectric crystals have a limited stability over time, as well as poor low frequency characteristics (in comparison with monocrystalline piezoresistive structures).
Therefore, one has lately to an increasing degree changed to using diaphragm sensors made by silicon, which material exhibits better stability.
McCall Eric S.
Presens AS
Wenderoth , Lind & Ponack, L.L.P.
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