WO1992010730A1

WO1992010730A1 - Method and sensor for measuring low air pressures

Info

Publication number: WO1992010730A1
Application number: PCT/FI1991/000289
Authority: WO
Inventors: Urpo PIETILÄ
Original assignee: PIETIKO Oy
Current assignee: PIETIKO Oy
Priority date: 1990-12-10
Filing date: 1991-09-23
Publication date: 1992-06-25
Anticipated expiration: 1993-06-10
Also published as: FI906057A0; AU8504391A; FI93993C; FI93993B; FI906057L

Abstract

The object of the invention is a method and sensor (10) for measuring air pressure by means of a membrane (14) in such a way that air pressure is allowed to affect both sides of the membrane, whereupon the pressure difference between the different sides of the membrane can be measured. According to the invention air pressure is measured so that the sensor membrane (14) is kept in position by means of the electrostatic pressure created by an electric field, whereupon the intensity of the electric field required indicates the effective pressure difference. The sensor (10) comprises a device which keeps the membrane (14) in position by means of the electrostatic pressure created by an electric field, and a device which indicates the effective pressure difference on the basis of the intensity of the electric field required.

Description

METHOD AND SENSOR FOR MEASURING LOW AIR PRESSURES

The object of the invention is a method for measuring air pressure, especially low air pressures, by means of a thin membrane placed in a sensor, to both sides of which membrane air is introduced, whereupon the pressure difference on the different sides of the membrane tends to move the membrane, the position of which is measured.

THE NEED FOR LOW-PRESSURE SENSORS

Sensitive air pressure sensors are needed in examining and adjusting heating and air-conditioning systems, because it is often necessary to measure extremely low air speeds and volume flows. The need to measure pressure differences arises particularly in connection with air conditioners, where the pressure sensor's performance must be good particularly in measuring low pressures. The sensor should also be such that it can be used in both fixed and portable air flow measuring devices.

Very sensitive sensors are, however, not available. This is because the known sensors used to measure low air pressures are usually based on a flexible membrane, from the deflection of which air pressure can be measured.

Determining the deflection of a thin membrane accurately is, however, difficult.

In known portable measuring devices the lower limit of the sensors' measuring pressure is in practice 1.2 Pa without expensive special arrangements. Lower pressures than this cannot be measured. This pressure corresponds to an air speed of 1 m/s. If, on the other hand, the measurements could be made at a 0.1 Pa pressure difference, air flow speeds as low as 0.3 m/s could be measured. Since known pressure sensors are based on measuring the deflection of the flexible membrane, the weight of the membrane also has some significance for the measurement result. The membrane may not be very thin either. Since it is known that a pressure of 1 Pa corresponds to the weight of a 0.1 mm water column, the weight of a 0.1 mm membrane corresponds to a pressure of several pascals.

It is technically difficult to control the flexibility properties of a thin membrane. This means that often each sensor must be calibrated individually and separately and that the sensor's zero point is unstable. Since an elastic sensor is in addition based on measuring movement, low air pressures must be able to move the membrane. This means that in a measuring situation, a. small amount of air must be introduced into a measuring chamber. As extremely low pressures are concerned, the sensor requires time to function. For this reason such membrane sensors are slow.

The aim of the present invention is to eliminate the above- mentioned problem and to achieve a new measuring method, which does not have the above disadvantages. It is characteristic of the method relating to the invention that air pressure is measured so that tfhe sensor's - electroconductive membrane is kept in position by means of the electrostatic pressure created by an electric field, the said pressure being equal to the air pressure affecting the membrane, which means that the intensity of the required electric field indicates the pressure difference between the different sides of the^'membrane.

By means of the method relating to the invention.it is possible to measure low pressures in a new way, which gives more precise measuring results on lower pressures than before.

It is also an aim of the invention to achieve a novel sensor for measuring air pressure, especially low air pressures, the said sensor comprising a thin membrane, to both sides of which air is introduced, whereupon the pressure difference between the different sides of the membrane tends to move the membrane, the position of which is measured.

It is characteristic of the sensor relating to the invention that the sensor membrane is at least partially electroconductive and that the sensor comprises a device which measures the movement of the membrane and adjusts the electrostatic field to be such that the membrane remains in position by means of the electrostatic pressure created by the electric field, and a device which indicates the pressure difference affecting the different sides of the membrane on the basis of the intensity of the electric field required.

The sensor relating to the invention is a new type of pressure sensor with good performance in measuring low pressures. It is particularly well suited for equipment in which the measuring of extremely low pressures is necessary. It is possible to use Pitot tubes or orifice plates in connection with the sensor to measure substantially weaker flows of air than before. The sensor is of simple construction and it is easy to calibrate. The sensor can in addition be used in both fixed and portable air flow measuring devices.

The sensor relating to the invention is based on the fact that the sensor membrane is mainly inflexible. The membrane is kept in position by means of an electric field. The air pressure value is obtained by measuring the voltage which makes it possible to keep the membrane in place. This type of sensor is accurate, as it is not dependent on the deflection of the membrane. The sensitivity of the sensor is only affected by the dielectric constant of the air and the size of the air gap. The measurement can be carried out so that a fixed reference capacitor is placed next to the sensor. Thus the voltage is measured with which the capacitance of the sensor can be made equal to that of the reference capacitor.

THE FUNCTIONING PRINCIPLE OF AN ELECTROSTATIC SENSOR

THE ELECTROSTATIC FORCES AFFECTING THE CAPACITOR

In a plate capacitor, the area of the plates of which is A and the distance between the plates d, capacitance is calculated with the formula:

C = e A/d,

in which e is the dielectric constant e_air = 8.8 pF/ .

Between the plates an electrostatic pressure is created, the magnitude of which depends on the intensity of the electrostatic field:

P = e E² = e (U/d)²,

in which E is the electrostatic field

U is the voltage between the capacitor plates, d is the distance between the plates.

Example:

A capacitor, where A = 10 cm² and d = 0.1 mm.

Capacitance C = 88 pF, if the insulation between the plates is air.

The 1 Pa electrostatic pressure between the plates is obtained by the electrostatic field E = (P/e)^% = 337 kV/m = 337 V/mm,

which corresponds to a 33.7 V voltage between the plates of the above-mentioned capacitor. If air acts as the insulating layer between them, then the most intensive field possible is 3 kV/mm and the highest electrostatic pressure 80 Pa. Higher pressures can be obtained by using other insulation materials.

ELECTROSTATIC EQUILIBRIUM

Electrostatically produced mechanical forces and pressures can be utilized in a pressure sensor in which air is introduced to the membrane. This is done by keeping the membrane in its original position by setting the electrostatic pressure in the equipment to correspond to the pressure of the air to be measured.

In order to achieve this, one of the sensor plates is formed by a membrane, which is affected on one side by the pressure to be measured.

If a voltage is now supplied between the capacitor plates, the voltage can be regulated so that the electrostatic pressure corresponds to the pressure of the air. This can be carried out automatically by measuring, for example, the capacitor capacitance and by arranging a feedback circuit so that the capacitance is kept constant at all air pressure values. Thus the square of the capacitor voltage is directly proportional to air pressure as follows:

P = k * U²

The sensor operates like scales. Instead of adding known weights to one side of the scales to achieve an equilibrium, the capacitor voltage is increased to keep the membrane in an equilibrium position.

The feedback system indicating the equilibrium can be realized in many ways. The important factor is that information on the position of the membrane or its part is obtained. If the voltage is not sufficiently high, the membrane moves to its extreme outermost position. If, on the other hand, the voltage is too high, it moves to its extreme innermost position. Instead of measuring capacitance, mechanical connections, optical devices or other similar arrangements can be used. Changes in capacitance and its effects on other electrical measurements can also be used to indicate the voltage at which the shifting of the membrane takes place.

SUMMARY OF THE MAIN PRINCIPLES

A pressure sensor functioning on the electrostatic principle can be realized in various ways. The same main principles are, however, valid in each realization.

1) The electrostatic field achieved creates pressure in the membrane, which is affected by the pressure to be measured. The direction of the pressure created by the electrostatic field is opposite to the effective direction of air pressure.

2) The electrostatic field is adjusted by applying feedback so that the electrostatic pressure is as high as the pressure to be measured. Alternatively, the electrostatic field is varied and it is observed at which point the electrostatic pressure and air pressure are.equally high.

3) The air pressure is obtained from the electric values by which the electrostatic field is created. FACTORS AFFECTING THE CHOICE OF MEMBRANE

The membrane used in measuring pressure has its own weight, which may affect the measurement. It is difficult to distinguish this weight from the pressure if the sensor is required to function in all positions. The measurement result obtained from the sensor varies depending on whether the pressure to be measured pushes the membrane up, down or to the side. In order for the dependency on direction to be minimized, the weight of the membrane should be as low as possible.

The membrane should have a high dielectric constant and its breakdown voltage should be high in order to enable measurement of higher pressures. The maximum pressure that can be measured is as high as the highest electrostatic pressure that can be achieved, that is:

*•_» = β * E_mx ² = e_r * 8.8 * (E_maχ/(kV/mm))²

The stability of the sensor's zero point depends only on the weight of the sensor, and this can be easily compensated unless the sensor is required to function in all positions.

If the electrostatic pressure is obtained from the voltage supplied through the capacitor, as in the example, then the factors affecting the stability of the reading can be seen from the following formula, which gives the pressure as a function of the voltage:

P = e * (U/d)²

Variations in the dielectric constant and the thickness of the membrane insulation material are thus critical from the point of view of stability. The insulation should be of a material that has a stable dielectric constant. If a precision instrument is to be achieved, temperature compensation should also be applied. The thickness of the insulation is significant and it should be the same in all sensors. Otherwise the sensors have to be calibrated individually.

FACTORS AFFECTING THE MEMBR^E

The basic sensor measures only the overpressure in the cavity between the membrane and the copper laminate.

If the sensor is to measure both positive and negative pressures, this can be achieved by constructing the sensor so that it is identical on both sides. In other words, the parts above and below the membrane are similar. In this case the membrane itself is also double-acting, so that the conductive area is in the middle. Electronic apparatus uses this double-capacitor in such a way that one side acts as a fixed capacitor in the circuit diagram. The feedback amplifier has two outputs, one of which is for the upper electrode and the other for the lower electrode. In the case of overpressure, the upper electrode is connected to the membrane and the electrostatic pressure affects the lower copper laminate. In the case of underpressure, the lower electrode is connected to the membrane and the electrostatic pressure affects the upper copper laminate.

CALIBRATION

There are good possibilities to calibrate the sensor with reasonable accuracy within the area of 0.1 Pa. Good calibration equipment is, however, required. One way to calibrate the sensor is to close one of the sensor's inlet openings with a valve which closes the opening without affecting the volume. When the sensor is now moved up and down, the sensor forms an electronic altimeter..1 Pa corresponds to a difference in height of about 10 cm. Greater accuracy can be achieved through calculation from atmospheric pressure and temperature. By using the ambient pressure as a standard, a height of, for example, 1 m can be measured, corresponding to some 10 Pa, and through it the calibration can be carried out.

This arrangement may, however, require extreme care as the temperature gradients and variations in external pressure affect the measurement.

ALTERNATIVE MEASURING CIRCUIT

Instead of a feedback system, an alternative method can be used which may be more suitable when using microprocessor equipment. In this case the electrostatic voltage is allowed to vary by using a square wave. When the electrostatic pressure is as high as the air pressure, the voltage to be used can be obtained from the voltage peak, which is indicated when the sensor's capacitance varies significantly as the membrane detaches from the copper laminate.

DESCRIPTION OF THE EXAMPLES

The invention is described in the following with examples, with reference to the attached drawings, in which

Figure 1 shows diagrammatically the cross-section of an embodiment of the invention. Figure 2 shows the sensor of figure 1 as seen from a different direction.

Figure 3 corresponds to figure 1 and shows the cross- section of a second embodiment of the sensor. Figure 4 shows axonometrically the parts of a third embodiment of the sensor in order of assembly. Figure 5 shows the measuring device relating to the invention, which comprises a sensor mounted on a circuit board and the components relating to the sensor. Figure 6 corresponds to figure 5, showing a different embodiment.

Figure 7 shows a block diagram of the sensor's control equipment. Figure 8 shows the sensor's circuit diagram. Figure 9 corresponds to figure 1 and shows the cross- section. of the third embodiment of the invention.

Figure 10 shows a section of figure 9 along line X-X.

Figure 1 shows diagrammatically the cross-section of an embodiment of the sensor. The mechanical part of the sensor 10 comprises a top plate 11, which is made, for example, of epoxy laminate. In the figure, the top plate 11 is shown to be solid, but if a pressure difference is being measured, a hole must be made in the top plate 11, into which hole a tube is connected leading to the negative pressure side.

Below the top plate 11 is an electroconductive ring 12 which is, for example, of copper laminate. The ring 12 acts as an electric conductor in the capacitor function and thus an electric conductor leading to the control equipment is also soldered onto the ring, which conductor is not shown in the figure.

Under the top plate 11 and inside the ring 12 is formed the sensor's negative pressure side cavity 13. On the opposite side of the cavity 13 is the actual measuring sensor membrane 14, which is a 0.01 mm thick membrane of thermosetting plastic in this example. The upper surface 15 of the membrane 14 is coated with a thin aluminium layer.

Under the membrane 14, on its edges, is a thin insulator ring 16 or a layer of glue, by means of which the membrane 14 is connected to the copper laminate 17 under the membrane. The insulator ring 16 or corresponding layer of glue must be very thin so that only a small air gap 18 remains between the membrane 14 and the copper laminate 17. Onto the copper laminate 17 an electric conductor is also • soldered leading to the control equipment, which conductor is not shown in the figure.

Under the copper laminate 17 is a support plate 19 which is made, for example, of epoxy laminate plate. The copper laminate 17 may be loose or fixed to the support plate 19. Plates 17 and 19 can also be formed using a copper-plated circuit board plate, so that both of the required layers 17 and 19 are thus together. Both plates 17 and 19 are provided with holes 20, through which the air pressure can affect the lower surface of the membrane 14. Under the support plate 19, on the sensor 10, there is in addition an insulator ring 21 and a bottom plate 22. Inside the insulator ring 21, between the support plate 19 and the bottom plate 22 there remains an air space 23.

The sensor 10 in figure 1 functions so that, from the space to be measured, air is introduced inside the sensor through the supply tube 24 on the positive pressure side. The air pressure measured from the air space 23 passes through the holes 20 into the air gap 18. The air pressure thus pushes the membrane 14 upwards and the electrostatic pressure created in the sensor on the other hand presses the membrane 14 downwards. When the membrane 14 is in a state of equilibrium, it rests partially on the copper laminate 17 below it. In other words the sensor of figure 1. measures overpressure in the air gap 18 between the membrane 14 and copper laminate 17.

Figure 2 shows the sensor 10 of figure 1 as seen from the direction of the bottom plate 22. The figure shows the air supply tube 24, through which the air pressure to be measured is supplied inside the sensor 10. The inner edge of the insulator ring 21 above the bottom plate 22 is shown by a broken line. Figure 3 shows the cross-section of the second embodiment of the sensor 10. This sensor is constructed so that it has two layers by making it identical on both sides, that is, the parts below and above the membrane are identical. The aim of this construction is to achieve a sensor with which both positive and negative pressures can be measured.

In the sensor 10 shown in figure 3, a double-capacitor is formed so that support plates 19 coated with copper laminates 17 are mounted on both sides of the membrane 14. Copper laminates 17 are arranged so that they are situated on the side of the membrane 14 and in the immediate vicinity of the membrane 14 as seen from the support plates 19, so that between the copper laminate 17 and the membrane 14 remains only the thin air gap 18 determined by the thin insulator ring 16 or a corresponding layer of glue.

In the sensor shown in figure" 3, membrane 14 is also made double-acting so that its electroconductive area is in the centre. The air pressures to be measured are introduced into the sensor 10 on opposite sides of the membrane 14 through the tubes 24 and 26 connected to the bottom plate 22 and top plate 11.

The electronic equipment uses this double-capacitor in such a way that one half of it acts as a fixed capacitor in the circuit diagram. The feedback amplifier 48 has two outputs, one of which is for the upper electrode and the other for the lower electrode. In the case of overpressure, the upper electrode is connected to the membrane 14 and the electrostatic pressure affects the lower copper laminate. In the case of underpressure the lower electrode is connected to the membrane 14 and the electrostatic pressure affects the upper copper laminate.

Figure 4 shows axonometrically the parts of the third embodiment of the sensor in order of assembly. The main parts of the sensor 10 are a top plate 11 provided with an air tube 26, a bottom plate 22 provided with another air tube 24 and the membrane 14 between them. I order to supply the air pressure evenly on both surfaces of the membrane 14, both the top plate and bottom plate are provided with radial grooves 27.

The purpose of sensor 10 is to form a capacitor, in which the electrostatic pressure keeps the membrane 14 in place despite the tendency of the air pressure to cause the membrane to deviate from its position of equilibrium. Since this type of construction can be formed in several ways, the diagram in figure 4 only shows the order of assembly in principle. The construction of the sensor may thus vary considerably, and the parts shown in the figure may be very different.

The essential factor is that membrane 14 acts as one of the plates of the capacitor. In the exemplary construction shown in figure 4, the membrane 14 may be coated with an aluminium layer 15, to which the electric current is supplied through the copper ring 12 on top of the membrane 14. Thus inside ring 12, between top plate 11 and membrane 14 remains the negative pressure side cavity.

The other capacitor plate is preferably formed so that circuit board 19 coated with a copper surface 17 is used as the bottom plate 22 of the sensor 10. The electrical conductor required by the capacitor is easily connected to the copper surface 17, and thus the entire sensor can be integrated with the circuit board. The intermediate layer 16 can in this case be a thin insulator layer, to create a narrow air gap between the membrane 14 and the copper surface 17.

Figure 5 shows the measuring device relating to the invention, which comprises a sensor 10 mounted on a circuit board 19 and the components relating to it. The sensor's air tubes 26 and 24 are situated on different sides of the circuit board. In a preferred embodiment the positive pressure side air tube 24 is below and the negative pressure side air tube 26 above the circuit board 19. On the circuit board are also situated the other components required in the sensor's control circuit and the current feed connector.

Figure 6 shows a measuring device corresponding to figure 5, with which underpressures can, however, be measured in addition to overpressures. This has been achieved by mounting on the circuit board two sensor elements 10 and 30, the air tubes of which are connected to each other so that sensors 10 and 30 function in joint operation. The connection is executed so that from below both sensors 10 and 30, that is, tubes 31 and 3^'2 are conducted from the respective positive pressure tubes 24 to the respective negative pressure tubes 26 of each sensor.

Figure 7 shows a block diagram 40 of the control equipment relating to the sensor, wherein an oscillator 41 produces a 20-50 kHz sine wave. The frequency of the oscillator 41 depends on sensor type and can be, for example, 25 kHz. From oscillator 41 the signal is supplied to double low- pass filters 42 and 43, of which -filters one filter 42 uses the fixed reference capacitor 44, and the other filter 43 uses the sensor 10 as the low-pass element. The fixed filter can be adjusted to suit different types of sensors.

From the low-pass filters 42 and 43 the signal is supplied to the double rectifier, which comprises rectifiers 45 and 46 for both low-pass filters 42 and 43. The signal is supplied further to comparator 47, which indicates the difference between the outputs of both filters. In the diagram the high-voltage power supply 49 supplies a 200 V voltage to the feedback amplifier, which on the other hand supplies the voltage required for measuring to sensor 10.

The electronics part measures the capacitance of the sensor 10 by using the low-pass filters 42 and 43, rectifiers 45 and 46 and the comparator 47. If the capacitance of the sensor 10 is lower than the comparative capacitance, the feedback amplifier 48 increases the voltage of the sensor 10, thus intensifying the electrostatic field and increasing the pressure between the plates. This force decreases the distance between the sensor membrane 14 and the copper laminate 17, which in turn increases capacitance. In this way the capacitance of the sensor 10 is kept the same as the comparative capacitance, independent of air pressure. To maintain this operation, the feedback electronics equipment supplies voltage and, as a result of it, electrostatic pressure between the plates. The pressure is proportional to the pressure of air.

Figure 8 shows, as an example of figure 7, a circuit diagram 50 of sensor 10, corresponding to the block diagram of figure 7, in which are shown the corresponding components, such as an oscillator, a comparator, etc.

Figure 9 shows the third embodiment of the sensor 10, which comprises a second membrane 60, in addition to membrane 14.

The structure of the sensor is symmetrical in such a way that essentially similar structural elements are mounted on both sides of the membranes 14 and 60 in the sensor.

Topmost are the top plates 61 and 62, which are metal, for example copper or brass. Under them are rings 63 and 64 made of reinforced plastic, which are coated with a copper layers 65 and 66 on three sides.

The thin membranes 14 and 60 are made of plastic, such as Mylar plastic, and they are coated with a thin aluminium layer, for example through vaporization. Membranes 14 and 60 are glued to the copper surfaces 65 and 66 of the plastic rings 63 and 64 so that the aluminium surface of each membrane is against the copper surface of the corresponding ring. Thus, through top plate 61 an electric current can be conducted to the aluminium surface of membrane 14, through the copper layer 65 of ring 63. Through top plate 62 an electric current is correspondingly- conducted to the aluminium surface of membrane 60.

Figure 9 shows the assembled sensor 10, in which membranes 14 and 60 are glued to rings 63 and 64. Rings 63 and 64 are also glued to each other by the glue layer 67. Membranes 14 and 60 are tightly against each other on their plastic sides, which means that there is no electrical contact between the membranes in this embodiment. Between membrane 14 and top plate 61 is formed a small slot 68. There is a corresponding slot 69 also between the second membrane 60 and the second top plate 62. The purpose of the slots 68 and 69 is to allow membranes 14 and 60 to move slightly and to act at the same time as a limiters preventing excessive movements which might damage the membranes.

In rings 63 and 64 grooves are made at the same point on the membrane side so that membranes 14 and 60 are kept slightly apart at the groove to form channel 70. Air pressure is allowed to pass through this channel 70 from tube 71 between membranes 14 and 60. This detail is shown more accurately in figure 10. Correspondingly, on the opposite sides of rings 63 and 64, channels 72 are formed, through which the opposite sides of membranes 14 and 60 are connected to the second air tube 73.

Figure 10 shows a section of figure 9. The figure depicts the top plates 61 and 62, the rings 63 and 64 inside them and the membranes 14 and 60 glued to the rings. At the grooves formed on the same points in rings 63 and 64 an air channel 70 is formed, from which the air pressure can pass between the membranes 14 and 60.

To a person skilled in the art it is obvious that the various embodiments of the invention may vary within the limits of the claims presented below.

Claims

1. A method for measuring air pressure, particularly low' air pressures, by means of a thin membrane (14, 60) mounted on a sensor (10), to both sides of which membrane is introduced air, whereupon the pressure difference on the different sides of the membrane will tend to move the membrane, the position of which is measured, characterized in that the air pressure is measured so that the sensor's (10) electroconductive membrane (14, 60) is kept in position by means of the electrostatic pressure created by an electric field, the said pressure being equal to the air pressure affecting the membrane, whereupon the intensity of the required electric field indicates the pressure difference between the different sides of the membrane.

2. A method as claimed in claim 1, characterized in that from the measurement of the position of the membrane (14,

60) feedback is supplied for the control of the electric field kept in position by electrostatic pressure so that the electric field is at any given moment intensive enough to only just keep the membrane in position.

3. A method as claimed in claim 1 or 2, characterized in that such a minimum voltage is supplied to the membrane (14) by means of which the membrane can only just be kept in contact with the other surface (17, 60) connected to the sensor (10).

4. A method as claimed in claim 1, 2 or 3, characterized in that the membrane's (14) remaining in position by means of static pressure is ascertained by means of the change in capacitance between the membrane and the other surface (17, 60) connected to the sensor (10).

5. A method as claimed in claim 1, 2 or 3, characterized in that the membrane's (14) remaining in position by means of static pressure is ascertained by means of the change in electric current between the membrane and the other surface (17, 60) connected to the sensor (10).

6. A method as claimed in claim 1, 2 or 3, characterized in that the membrane's (14, 60) remaining in position by means of static pressure is ascertained optically.

7. A method as claimed in any of the claims 1-6, characterized in that the point is measured when the membrane (14) becomes detached from the other surface (17, 60) connected to the sensor (10), and at the same time the voltage causing the membrane to move is measured.

8. A method as claimed in any of the claims 1-7, characterized in that, using an electronic control circuit (50), the voltage between the membrane (14) and the other surface (17, 60) is set to such a level that the electrostatic pressure created by it neutralizes the air pressure between the membrane andi the surface, whereupon the distance between the membrane and the surface remains constant regardless of variations in pressure difference.

9. A sensor (10) for measuring air pressure, particularly low air pressures, which sensor comprises a thin membrane

(14, 60) on both sides of which has been introduced air, whereupon the pressure difference on the different sides of the membrane tends to move the membrane, the position of which is measured, characterized in that the sensor (10) membrane (14, 60) is at least partially electroconductive and that the sensor comprises a device (50) which measures the movement of the membrane (14, 60) and sets the electrostatic field to such a level that the membrane remains in position by means of the electrostatic pressure created by the electric field, and a device which indicates the pressure difference affecting the different sides of the membrane, on the basis of the intensity of the electric field required. 10. A sensor (10) as claimed in claim 9, characterized in that the sensor's (10) electroconductive membrane (14) forms a capacitor together with the other electroconductive surface (17, 60) connected to the sensor, whereupon the membrane's remaining in position can be ascertained by measuring the capacitance between the membrane and the other surface.

11. A sensor (10) as claimed in claim 9 or 10, characterized in that the sensor (10) comprises a device which indicates the point at which the membrane (14) becomes detached from the other surface (17, 60) connected to the sensor.

12. A sensor (10) as claimed in claim 9, 10 or 11, characterized in that the membrane (14, 60) is a metal- coated plastic membrane, such as an aluminium-coated, vaporized plastic membrane or a membrane made wholly of metal, for example, brass or stainless steel.

13. A sensor (10) as claimed in any of the claims 9-12, characterized in that two sensors (10 and 40) are interconnected in such a way that their air tubes (31 and 32) are connected to each other and that the sensors are connected to form a part of the electronic circuit board (19).

14. A sensor (10) as claimed in any of the claims 9-13, characterized in that the other surface connected to the sensor (10) is a second membrane (60) corresponding to the first membrane (14). AMENDEDCLAIMS

-,-ϋreceived by the International Bureau on 7 May 1992- (07.05.92); original claims 1 — 14 replaced by amended claims 1 — 11 (3 pages)]

X - A. m*__^a-t_.t_ocl _E-o__r easu-_-Lng a.S- ~: pressure , E^>__L__r-fc-L _txαl__LxrXy low •a._i___r

_r fc>y means o__: _a -t_._n._Ln membirane ( 14 , 60 ) mDun- e on a sensor (10), to bot sides of which membrane is introduced air, whereupon the pressure difference on the different sides of the membrane will tend to move the membrane, the position of which is measured, characterized in that the air pressure is measured so that the sensor's (10) least partially electroconductive membrane (14, 60) is kept in position by means of the electrostatic pressure created by an electric field, the said pressure being equal to the air pressure affecting the'membrane, whereupon the intensity of the required electric field indicates the pressure difference between the different sides of the membrane, and in that from the measurement of the position of the membrane (14, 60) feedback is supplied for the control of the electric field kept in position by electrostatic pressure so that the electric field is at any given moment intensive enough to only just keep the membrane in position, and in that such a minimum voltage is supplied to the membrane (14) by means of which the membrane's (14) remaining in position by means of static pressure is ascertained by means of the change in capacitance between the membrane and the other surface (17, 60) connected to the sensor (10).

2. A method as claimed in claim 1, characterized in that the membrane's (14) remaining in position by means of static pressure is ascertained by means of the change in electric current between the membrane and the other surface (17, 60) connected to the sensor (10).

3. A method as claimed in claim 1 or 2, characterized in that the membrane's (14, 60) remaining in position by means of static pressure is ascertained optically.

4. A method as claimed in claim 1, 2 or 3, characterized in that the point is measured when the membrane (14) becomes detached from the other surface (17, 60) connected to the sensor (10), and at the same time the voltage causing the membrane to move is measured.

5. A method as claimed in any of the claims 1-4, characterized in that using an electronic control circuit (50), the voltage between the membrane (14) and the other surface (17, 60) is set to such a level that the electrostatic pressure created by it neutralizes the air pressure between the membrane and the surface, whereupon the distance between the membrane and the surface remains constant regardless of variations in pressure difference.

6. A sensor (10) for measuring air pressure, particularly low air pressures, which sensor comprises a thin membrane

(14, 60) on both sides of which has been introduced air, whereupon the pressure difference on the different sides of the membrane tends to move the membrane, the position of which is measured, characterized in that the sensor (10) membrane (14, 60) is at least partially electroconductive and that the sensor comprises a device (50) which measures the movement of the membrane (14, 60) and sets the electrostatic field to such a level that the membrane remains in position by means of the electrostatic pressure created by the electric field, and a device which indicates the pressure difference affecting the different sides of the membrane, on the basis of the intensity of the electric field required.

7. A sensor (10) as claimed in claim 6, characterized in that the sensor's (10) electroconductive membrane (14) forms a plate capacitor together with the other electroconductive surface (17, 60) connected to the sensor, whereupon the membrane's remaining in position can be ascertained by measuring the capacitance between the membrane and the other surface.

8. A sensor (10) as claimed in claim 6 or 7, characterized in that the sensor (10) comprises a device which indicates the point at which the membrane (14) becomes detached from the other surface (17, 60) connected to the sensor.

9. A sensor (10) as claimed in claim 6, 7 or 8, characterized in that the membrane (14, 60) is a metal- coated plastic membrane, such as an aluminium-coated, vaporized plastic membrane or a membrane made wholly of metal, for example, brass or stainless steel.

10. A sensor (10) as claimed in any of the claims 6-9, characterized in that two sensors (10 and 40) are interconnected in such a way that their air tubes (31 and 32) are connected to each other and that the sensors are connected to form a part of the electronic circuit board (19).

11. A sensor (10) as claimed in any of the claims 6-10, characterized in that the other surface connected to the sensor (10) is a second membrane (60) corresponding to the first membrane (14).