Capacitive Capsule for Aneroid Pressure Gauge

This invention relates generally to aneroid pressure gauges or barometers and, more particularly, to aneroid pressure gauges wherein changes in pressure are indicated by changes in capacitance resulting from the deflection of the membranes which define the evacuated capsule or cell forming a component of the pressure gauge. Aneroid pressure gauges are known which measure pressure by means of changes in the capacitance of associated components effected by the deflection of one of the membranes of the aneroid capsule forming a component of the pressure gauge. Thus, in one existing aneroid pressure gauge, an aneroid capsule or cell comprising a pair of membranes defining a sealed evacuated space between them has one of these membranes fastened to the chassis or housing of the pressure gauge. A capacitor is provided including one fixed capacitor plate and an opposed movable capacitor plate. The movable membrane of the aneroid capsule is fastened to the movable capacitor plate so that changes in pressure result in a deflection of the capsule membrane which, in turn, results in a corresponding movement of the movable capacitor plate thereby changing the capacitance of the capacitor. Thus, it is the output of the capacitor which is measured in order to obtain pressure measurements. Such capacitance type aneroid pressure gauges are particularly useful in connection with radiosondes which, among other things, measure minute changes in pressure over short intervals of time.

Aneroid pressure gauges of the type described above are not entirely satisfactory, however. Thus, the dielectric coefficient of the air gap between the capacitor plates of the above-described conventional aneroid pressure gauges is influenced by the moisture present in the ambient air as well as the ambient pressure and temperature. The influence of the ambient moisture, pressure and temperature on the dielectric coefficient of the air gap results in a non-linearity between the pressure and capacitance variations thereby resulting in measuring errors.

Accordingly, one object of the present invention is to provide a new and improved aneroid pressure gauge which effects changes in capacitance in accordance with changes in pressure.

Another object of the present invention is to provide such a new and improved aneroid pressure gauge which is mechanically stable and which has extremely good accuracy.

Still another object of the present invention is to provide such a new and improved aneroid pressure gauge which is relatively simple in construction and inexpensive to manufacture.

A further object of the present invention is to provide such a new and improved aneroid pressure gauge which eliminates the deleterious influences caused by the presence of ambient moisture, pressure and temperature.

Briefly, in accordance with the present invention, these and other objects are obtained by providing an aneroid capsule comprising a pair of membranes sealed to each other along respective peripherally extending side edge regions to define a sealed interior space therebetween. A capacitor plate is disposed within the sealed interior space and is affixed to one of the membranes by an electrically conductive supporting member which extends through that membrane but which is electrically insulated therefrom. The other membrane is provided with a charge retaining region located in opposed relationship to the capacitor plate. The capacitance of the device comprising the capacitor plate and the charge retaining region of the membrane is proportional to the ambient pressure by virtue of the movement of the capacitor plate located within the sealed interior space which corresponds to the movement of the membrane to which it is affixed. Since the space between the capacitor plate and the charge retaining region of the membrane is partially evacuated, the ambient moisture, pressure and temperature will have no affect on the dielectric constant of the space defined between the capacitor plate and the charge retaining region.

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which:

FIG. 1 is a plan view of the aneroid capsule of the present invention; and

FIG. 2 is a section view taken along line II--II of FIG. 1.

Referring now to the drawings wherein like reference numerals designate identical on corresponding parts throughout the several views, the aneroid capsule or cell of the present invention, generally designated 10 is of generally conventional construction and includes a first, upper membrane 11 and a second lower membrane 12 which are sealingly engaged to each other along their perpherally extending side edge regions 13. Thus, a sealed interior space 20 is defined between first and second membranes 11, 12, which interior space is partially evacuated in a conventional manner.

The first, upper membrane 11 is formed having circular corrugations 21 extending over the substantial entire surface thereof. The second, lower membrane 12 also is provided with corrugations 22 which circumferentially extend around the outer regions thereof. Thus, each of the membranes 11, 12 have effective spring constants which govern the extent of deflection of the membranes as a result in changes in ambient pressure.

The second, lower membrane 12 is formed having a central planar region 18 which comprises a charge retaining portion. Thus, planar portion 18 is preferably of a circular configuration and is circumferentially surrounded by the corrugations 22.

A capacitor plate 17 is disposed within the evacuated sealed interior space 20 in opposed relationship to the charge retaining planar portion 18 of second membrane 12. Capacitor plate 17 preferably comprises a circular disc shaped member which is substantially similar in outer configuration to the planar charge retaining portion 18 of membrane 12. Capacitor plate 17 is supported within the sealed interior space 20 by connection, such as by resistance butt-welding, as its center portion 19 to the end of an electrically conductive pin 16. Pin 16 is fixed to the first membrane 11 by a sleeve 14 fixed to an opening centrally formed in membrane 11, sleeve 14 housing a cylindrically shaped member 15 formed of electrically insulative material.

The distance, designated d(p) between capacitor plate 17 and the charge retaining region 18 of the second, lower membrane 12 is substantially linearly dependent on the ambient pressure acting on aneroid capsule 10 in a conventional manner. Thus, the higher the ambient pressure, the smaller is the distance d(p). The sensitivity of the capsule, i.e., the extent of deflection resulting from a change in pressure, is determined by appropriately selecting the radius of the charge retaining region 18 and capacitor plate 17. Capacitor plate 17 is located at an appropriate distance from the charge retaining region 18 to achieve the desired sensitivity. It is preferable to locate the capacitor plate 17 and the opposed charge retaining region 18 at the substantial midsections of membranes 11, 12 since, at such location, maximum deflection of the membranes for a given change in pressure will occur.

Means are provided in the form of electrical leads 23 by which the capacitance C can be measured. Thus, the leads are connected to the ends of pin 16 and to second membrane 12, respectively.

Thus, as the first and second membranes 11, 12 deflect pursuant to pressure fluctuations, the distance d(p) will vary thereby changing the capacitance. This change in capacitance thus reflects changes in pressure.

Since the movable or "hot" capacitor plate 17 is disposed within the sealed, evacuated space between the membranes, the stability and mechanical integrity of the structure is improved. The space between the capacitor plate 17 and charge retaining region 18, as mentioned above, is partially evacuated and, therefore, is not subject to changes in its dielectric constant due to ambient moisture, pressure or temperature.

Obviously, numerous modifications and variations of the present invention are possible in the light of the above teachings. Accordingly, it is understood that within the scope of the claims, the invention may be practiced otherwise than as specifically recited herein.