FI93993C - Method and detector for measuring low air pressure - Google Patents

Description

93993

METHOD AND SENSOR FOR MEASURING LOW AIR PRESSURE FÖRFARANDE OCH DETEKTOR FÖR MÄTNING AV LÄGA LUFTTRYCK

The invention relates to a method for measuring atmospheric pressure, in particular low atmospheric pressures, in a sensor by means of a thin measuring sensor groove arranged between two plates, which is arranged in the sensor so that a gap defined by the membrane and the plates is formed on both sides of the membrane. The air whose pressure is to be measured is introduced into the air gap on the other side of the membrane, whereby the pressure difference between the different sides of the membrane tends to move the membrane. On the air gap side, the film is subjected to an electrostatic field with an electrostatic pressure equal to the air pressure to be measured to keep the film in its original position, after which the position of the film is measured. From the measurement of the position of the membrane, feedback is provided to control the electrostatic pressure, and the strength of the required electrostatic field 15 is used to indicate the pressure difference between the different sides of the membrane.

The invention also relates to a sensor for measuring atmospheric pressure, in particular low atmospheric pressures, which sensor comprises a thin measuring sensor-20 membrane interposed between two plates, which is fitted to the sensor so that a gap delimited by the membrane and the plates is formed on both sides. air gap, is connected to the supply air of the air to be measured, whereby the pressure difference between the different sides of the membrane tends to move the membrane. The anaturator further includes means for generating an electrostatic field in the air gap, means for generating an electrostatic pressure corresponding to the measured air pressure in the air gap, means for measuring the membrane position, means for providing feedback on measuring the membrane position to the electrostatic field generating means. to indicate the prevailing pressure difference.

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NEED FOR LOW PRESSURE SENSOR

Sensitive air pressure sensors are needed to study and control heating and air conditioning systems, as it is often necessary to measure very low air velocities 5 and volume flows. The need to measure pressure differences occurs especially in air conditioners, in which case the pressure sensor must have good performance, especially when measuring low pressures. The sensor should also be able to be used in both fixed and portable airflow measuring devices.

10 However, very sensitive sensors are not available. This is because the known sensors used to measure low barometric pressures are usually based on a flexible membrane whose deflection can be used to measure barometric pressure.

However, it is difficult to accurately determine the deflection of a thin film.

With known portable measuring devices, the lower limit of the measuring pressure of the sensors is practically 1.2 Pa without expensive special arrangements. Pressures lower than this cannot be measured. This pressure corresponds to an air speed of l m / s. If, instead, 20 measurements could be made with a pressure difference of 0.1 Pa, then air flow velocities could be measured up to speeds as low as 0.3 m / s.

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Since the known pressure sensors are based on the measurement of the deflection of a flexible membrane, the weight of the membrane also has a bearing on the measurement result. The film cannot be very thin either. When it is known that a pressure of 1 Pa corresponds to the pressure of a 0.1 mm water column, then the weight of a 0.1 mm film corresponds to the pressure of several Pascals.

It is technically difficult to control the elastic properties of a thin film. This means that often each sensor has to be calibrated individually and. the zero point of the sensor is unstable. While the elastic • · * sensor is based on motion measurement, low air pressures must be able to move the membrane. This means that in a measuring situation, a small amount of air must be led into the measuring chamber. Because of the very low pressures involved, the sensor takes time to operate. From the joh-5 support such a membrane sensor is slow.

The object of the present invention is to eliminate the above-mentioned problem and to provide a new measuring method which does not have the above-mentioned drawbacks. The method according to the invention is characterized in that the air pressure measurement 10 uses a measuring sensor film formed of a thin hard plastic film having a high dielectric constant and the other side away from the air gap coated with an electrically conductive thin metal such as an aluminum layer. is formed by applying a small electric voltage to the metal layer of the film.

The electrically conductive membrane of the sensor is held in place by an electrostatic pressure generated by an electric field equal to the atmospheric pressure acting on the membrane, the strength of the required electric field indicating the pressure difference between the different sides of the membrane.

By means of the method according to the invention, it is possible to measure low pressures in a new way, whereby the measurement results 25 can be made more accurate from even lower pressures.

It is also an object of the invention to provide a new sensor for measuring atmospheric pressure, in particular low atmospheric pressures, which sensor comprises a thin membrane with air applied to both sides, the pressure difference between the different sides of the membrane tending to move the membrane.

The sensor according to the invention is characterized in that the measuring sensor film is formed of a thin hard plastic film with a high dielectric constant and the other side of the air gap is coated with an electrically conductive thin metal, such as an aluminum layer, and electrically connected to the film to conduct to the metal layer of the film, to generate an electrostatic pressure 5 in the air gap.

The membrane of the sensor according to the invention is thus at least partially electrically conductive and the sensor comprises a device which measures the movement of the membrane and adjusts the electrostatic field so that the membrane remains stationary by the electrostatic pressure caused by the electric field. Laiate reports the pressure difference across the membrane based on the required electric field strength.

The sensor according to the invention is a new type of pressure sensor with good performance when measuring low pressures. It 15 is particularly suitable for systems where it is necessary to measure very low pressures. In connection with the sensor, it is possible to use Pitot tubes or throttle flanges to measure substantially lower airflows than before. The structure of the sensor is simple and it is easy to calibrate. In addition, the sensor can be used in both fixed and portable airflow measuring devices.

The sensor according to the invention is based on the fact that the membrane of the sensor is substantially rigid. The film is held in place by an electric field. The barometric pressure value is obtained by measuring the 25-racket voltage at which the membrane is held in place.

Such a sensor is accurate because it is not dependent on the deflection of the film. The sensitivity of the sensor is only affected by the air ,,, dielectric constant and the size of the air gap.

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The measurement can be performed by placing a fixed reference capacitance parallel to the sensor 30. In this case, the voltage at which the capacitance of the sensor is made equal is measured.

OPERATING PRINCIPLE OF THE ELECTROSTATIC SENSOR

ELECTROSTATIC FORCES ON THE CAPACITOR

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In a plate capacitor whose plate area is A and the distance between the plates is d, the capacitance is given by the formula: C = e A / d 5 where e is the dielectric constant or = 8'8 PF / ra

An electrostatic pressure is created between the plates, the magnitude of which depends on the strength of the electrostatic field: P = e E2 = e (U / d) 2 10 where E is the electrostatic field, U is the voltage between the plates of the capacitor d is the distance between the plates.

Example:

Capacitor with A = 10 cm2 and d = 0.1 mm.

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

The electrostatic pressure between the plates is 1 Pa when the electrostatic field has E = (P / e) 1/2 = 337 kV / m = 337 V / mm, which corresponds to a voltage of 33.7 V between the plates of the capacitor shown above. If the air acts as an intermediate insulating layer, then the maximum possible field is 3 kV / mm and the maximum electrostatic pressure is 80 Pa.

ELECTROSTATIC BALANCE

25 Electrostatic mechanical forces and pressures can be exploited in a pressure sensor where air is conducted to the membrane. This is done by holding the membrane in its original position by setting the electrostatic pressure in the device 93993 6 to correspond to the pressure of the air to be measured.

To achieve this, the second plate of the sensor is formed by a membrane which is affected on the other side by the pressure to be measured.

5 If a voltage is now applied between the plates of the capacitor, the voltage can be adjusted so that the electrostatic pressure corresponds to the air pressure. This can be done automatically, for example, by measuring the capacitance of the condenser and arranging the feedback circuit so that the capacitance-10 si is kept constant at all air pressure values. Then the square of the capacitor voltage is directly proportional to the air pressure as follows: P = k * U2

The sensor works like a scale. Instead of adding 15 known weights to the other side of the balance to achieve equilibrium, the capacitor voltage is increased to keep the film in equilibrium.

The feedback system indicating the balance can be made in many different ways. It is important to obtain information on the position of the 20 films or part thereof. If the voltage is not high enough, * the film will move to its outer extreme position. If, on the other hand, the voltage is too high, then it moves to its inner extreme position. Instead of measuring the capacitance, mechanical coupling, optical devices or the like can be used. Also changes in capacitance and its effects. for other electrical measurements can be used to indicate the voltage at which the film transition occurs.

SUMMARY OF BASIC PRINCIPLES

An electrostatic pressure sensor can be .. 30 implemented in many different ways. However, in each implementation, the following guiding principles apply.

Il 7 93993 1) The electrostatic field generated creates a pressure on the membrane which is affected by the pressure to be measured. The pressure produced by the electrostatic field is opposite to the direction of action of the air pressure.

5 2) The electrostatic field is adjusted using feedback so that the electrostatic pressure is equal to the pressure to be measured. Alternatively, the electrostatic field is varied and monitored when the electrostatic pressure and barometric pressure are equal.

10 3) The air pressure is obtained from the electrical values at which the electrostatic field is generated.

FACTORS AFFECTING FILM SELECTION

The diaphragm used to measure the pressure has its own weight, which can affect the measurement. It is difficult to separate this 15 weight from the pressure if it is required that the sensor must operate in all positions. The measurement result from the sensor varies depending on whether the pressure to be measured pushes the membrane up, down or to the side. In order to minimize the directional dependence, the weight of the film should be as small as 20 possible.

The film should have a high dielectric constant and its overvoltage voltage should be high in order to be able to measure higher pressures. The maximum pressure that can be measured is equal to the maximum electrostatic pressure that can be generated, ie: «

Pmax - «* IW2 - er * 8.» * (Mother * / <* »/)) 2

The zero point stability of the sensor depends only on the weight of the sensor and can be easily compensated unless • the sensor is required to operate in all positions.

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If the electrostatic pressure is obtained from the voltage passed 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 voltage: 5 P = e * (U / d) 2

Thus, variations in dielectric constant and film insulating material thickness are critical to stability. The insulation must be of a material with a stable dielectric constant.

If a precision instrument is to be provided, temperature compensation must also be used. The thickness of the insulation is important and should be the same for all sensors. Otherwise, the sensors must be calibrated individually.

FACTORS AFFECTING THE Membrane

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

If the sensor is to be made to measure both positive and negative pressures, then this can be achieved, for example, by constructing the sensor identical on both sides.

20 In other words, the parts below and above the film * - are similar. In this case, the film itself is double-acting, so that the conductive area is in the middle. The electronic equipment uses this dual capacitor so that the other side acts as a fixed capacitor in the circuit diagram.

25 The feedback amplifier has two outputs, one for the upper electrode and one for the lower electrode. In the case of overpressure, the upper electrode is attached to the membrane and the electrostatic pressure acts on the lower copper plate. In the case of a vacuum, the lower electrode 30 is connected to the membrane and the electrostatic pressure acts on the upper copper plate.

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CALIBRATION

There is a good chance of calibrating the sensor with a reasonable accuracy in the range of 0.1 Pa. In this case, however, good calibration equipment is required. One way to calibrate the sensor 5 is to close the second inlet of the sensor with a valve that closes without affecting the volume. Now that the sensor is moved up and down, the sensor forms an electronic altimeter. 1 Pa corresponds to a height difference of approximately 10 cm. Greater accuracy can be achieved by calculating 10 atmospheric pressures and temperatures. Using the existing pressure as a standard, a height of, for example, 1 m, corresponding to about 10 Pa, can be measured and calibrated.

However, the arrangement may require great care, 15 as temperature gradients and variations in external pressure affect the measurement.

ALTERNATIVE MEASURING CIRCUIT

Instead of a feedback system, an alternative method may be used, which may be more appropriate when using micro-20 processor hardware. In this case, an electric. the static voltage can be varied using a square wave. When the electrostatic pressure is equal to the atmospheric pressure, the applied voltage can be obtained from the voltage peak that occurs when the capacitance of the sensor varies greatly when the film detaches from the copper plate.

DESCRIPTION OF THE EXAMPLES

The invention will now be described by way of example with reference to the accompanying drawings, in which

Figure 1 schematically shows a cross-section of an embodiment of the sensor.

Figure 2 shows the sensor of Figure 1 seen from another direction.

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Fig. 3 corresponds to Fig. 1 and shows a cross section of a second embodiment of the sensor.

Figure 4 shows axonometrically the parts of the third embodiment of the sensor in the assembly order.

Figure 5 shows a measuring device according to the invention, comprising a sensor placed on a circuit board and related components.

Fig. 6 corresponds to Fig. 5 and shows a second embodiment.

Figure 7 shows a block diagram of the control equipment associated with the sensor.

Figure 8 shows a sensor connection diagram.

Figure 1 schematically shows a cross-section of an embodiment of a sensor. The mechanical part 15 of the sensor 10 includes a cover 11 made of, for example, epoxy laminate. The lid 11 is shown closed in the figure, but if the pressure difference is measured, then a hole must be formed in the lid 11 to which a pipe leading to the low pressure side is connected.

Below the cover 11 there is an electrically conductive ring 12, which is 20 copper plates, for example. The ring 12 acts as an electrical conductor in capacitor operation, so the ring is also soldered with an electrical wire going to the control equipment, which is not shown in the figure.

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The cover 11 under ring 12 and the sensor 25 is formed in the low-pressure side of the cavity opposite the side 13 of the cavity 13 is the actual measuring sensor membrane 14, which in this embodiment is 0.01 mm thick hard plastic wells.

.. The upper surface 15 of the film 14 is coated with a thin layer of aluminum.

30 Below the film 14, at its edges there is a thin insulating ring 16 or adhesive layer by means of which the film 14 is connected to the copper plate 17 below the film. The air layer of the insulating ring 16 or ♦ must be very thin so that only a small air gap 11 18. A copper conductor, not shown, is also soldered to the copper plate 17 to the control equipment.

Below the copper plate 17 there is a support plate 19, which is, for example, an epoxy laminate plate. The copper plate 17 can be detached or attached to a support plate 19. The plates 17 and 19 can also be formed by using a copper-coated circuit board, which then has both necessary layers 17 and 19 together. Holes 20 are formed in both plates 17 and 19, through which air pressure 10 can act on the lower surface of the film 14. Below the support plate 19, the sensor 10 still has an insulating ring 21 and a base 22. Inside the insulating ring 21, an air gap 23 remains between the support plate 19 and the base 22.

sensor 10 of Figure 1 operates so that the dimensions from the space 15 air is introduced into the sensor in the bottom 22 connected to je-pressure side through the supply pipe 24. The air pressure to be measured from the air gap 23 enters the air gap 18 under the membrane 14 through the holes 20. In this case, the air pressure lifts the membrane 14 upwards and the electrostatic pressure 20 applied to the sensor again pushes the membrane 14 downwards. When the membrane 14 is in equilibrium, it partially rests on the copper plate 17 below it. That is, the sensor of Fig. 1 measures the overpressure in the air gap 18 between the membrane 14 and the copper plate 17.

Fig. 2 shows the sensor 10 of Fig. 1 seen from the direction of the base 22. The figure shows an air supply pipe 24 provided with a hole 25, through which the air pressure to be measured is introduced into the sensor 10. The inner edge of the insulating ring 21 above the base 22 is shown in the figure by a broken line.

Figure 3 shows a cross-section of a second embodiment of the sensor 10. This sensor is built in two layers by making it identical on both sides, i.e. the parts below and above the membrane are similar. The purpose of the structure is to provide a sensor, 93993 12 t, with which both positive and negative pressures can be measured.

A dual capacitor is formed in the sensor 10 of Fig. 3 by placing support plates 19 coated with copper plates 17 on both sides of the film 14. Copper plates 17 are positioned so as to be on the film side 14 and in the vicinity of the film 14 as seen from the support plates 19 Only the thin air gap 18 defined by the thin insulating ring 16 or the corresponding adhesive layer 10 remains between the 14.

In the sensor of Figure 3, the membrane 14 is also constructed as double-acting so that its electrically conductive area is in the middle.

The air pressures to be measured are conducted to the sensor 10 on opposite sides of the membrane 14 through pipes 24 and 26 connected to the base 22 and the cover 11.

The electronic equipment uses this dual capacitor so that the other side acts as a fixed capacitor in the circuit diagram. The feedback amplifier 48 has two outputs, one for the upper electrode and one for the lower electrode. In the case of an overpressure, the upper electrode is connected to the membrane 14 and the electrostatic pressure acts on the lower copper plate. In the case of • negative pressure, the lower electrode is connected to the membrane 14 and the electrostatic pressure acts on the upper copper plate.

Fig. 4 is an axonometric view of the parts of the third embodiment of the sensor in the assembly order. The main parts of the sensor 10 are a cover 11 with an air tube 26, a base 22 with a second air tube 24 and a membrane 14 therebetween. In order to apply air pressure evenly to both surfaces of the membrane 14 30, both the cover 11 and the base 22 are provided with radial grooves 27.

• · The purpose of the sensor 10 is to form a capacitor in which the electrostatic pressure holds the membrane 14 in place despite the fact that the atmospheric pressure tends to deviate the membrane from its equilibrium position. Since such a structure can be formed in several different ways, the assembly diagram shown in Fig. 4 is in principle. Thus, the structure of the sensor 5 can vary greatly and the parts shown in the figure can be very different.

Essentially, the membrane 14 acts as the second plate of the capacitor. In the exemplary structure of Figure 4, the film 14 may be coated with an aluminum layer 15 into which an electric current 10 is conducted through a copper ring 12 on the film 14. In the tire 12 between the cover 11 and the film 14 remaining in the low-pressure side of the cavity.

The second plate of the capacitor is preferably provided in such a way that a circuit board 19 coated with a copper surface 15 17 is used as the base 22 of the sensor 10. It is easy to connect the electrical conductor needed by the capacitor to the copper surface 17 and thus the entire sensor can be connected to the circuit board. The intermediate layer 16 may in this case be a thin insulating layer so that a narrow air gap is left between the film 14 and the copper surface 17.

Figure 5 shows a measuring device according to the invention, comprising a sensor 10 placed on a circuit board 19 and associated components. The sensor air tubes 26 and 24 are located on different sides of the circuit board. Most preferably, the earring-pressure side air tube 24 is below the low-pressure-side air tube 25 above the 19 to 26 of the circuit board. Other components required in the sensor control circuit and the power supply connector are also placed on the circuit board.

Fig. 6 shows a measuring device corresponding to Fig. 5, with which, however, in addition to overpressures, it is also possible to measure underpressures. This is achieved by mounting on the circuit board 19 two sensor elements 10 and 30, the air tubes of which are connected to each other so that the sensors 10 and 30 cooperate. The connection is made in such a way that pipes 31 and 32 are led from below the two sensors 10 and 30, i.e. from the high-pressure pipe 24, to the low-pressure pipe 26 of the second sensor 93993 14.

Figure 7 shows a block diagram 40 of the control equipment associated with the sensor, in which the oscillator 41 provides a stable 20-50 5 kHz sine wave. The frequency of the oscillator 41 depends on the type of sensor and may be, for example, 25 kHz. The signal from the oscillator 41 is applied to a dual low pass filter 42 and 43, of which one filter 42 uses a fixed reference capacitor 44 and the other filter 43 uses a sensor 10 10 as a low pass element. The fixed filter can be adjusted for different types of sensors.

From the low-pass filters 42 and 43, the signal is applied to a dual rectifier, which includes rectifiers 45 and 46 for both low-pass filters 42 and 43. The signal is further routed to a comparator 47, which indicates the difference in the outputs of both filters. In the diagram, the high voltage power supply 49 supplies a voltage of 200 V to the feedback amplifier 48, which in turn supplies the voltage required for the measurement to the sensor 10.

The electronics measure the capacitance of the sensor 10 using low pass filters 42 and 43, rectifiers 45 and 46 and. reference element 47. If the capacitance of the sensor 10 is smaller than the reference capacitance, then the feedback amplifier 48 increases the voltage of the sensor 10, thereby increasing the electrostatic field and pressure between the plates. This force reduces the distance between the sensor film 14 and the copper plate 17, which in turn increases the capacitance. In this way, the capacitance of the sensor 10 is kept the same as the reference capacitance regardless of the air pressure. To maintain this operation, the feedback electronic equipment supplies voltage and, as a result, electrostatic pressure between the plates.

The pressure is proportional to the atmospheric pressure.

· Fig. 8 shows by way of example the block diagram of Fig. 7 corresponds only to the circuit diagram 50 of the sensor 10, which shows

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93993 15 corresponding components such as oscillator, reference element, etc.

It will be apparent to those skilled in the art that various embodiments of the invention may vary within the scope of the 5 claims below.

Claims (13)

A method for measuring air pressure, in particular low air pressure, by means of a thin measuring detector membrane (14) arranged in a detector (10) between two disks (11, 17) arranged in the detector, so that on both sides of the membranes (14), a gap (13,18) confined by the diaphragms and the discs (11,17) is formed, whereby - air whose pressure is to be measured is conducted into the air gap (18) on one side of the diaphragms, the pressure difference between the two the sides of the membranes strive to displace the membranes, - on the air gap side (18) if the membranes are formed by an electrostatic field, an electrostatic pressure equal to the measured air pressure, in order to keep the membranes in its original position, - the position of the membranes is measured. - from the diaphragm position measurement an feedback is arranged to control the electrostatic pressure, and - the strength of the necessary electrostatic field is used to indicate the pressure difference between the two sides a v the diaphragm, characterized in that - in the measurement, a measuring detector membrane (14) is used: made of thin thermosetting plastic membrane which has a high dielectric constant and whose facing away from the air gap is coated with a thin electrically conductive metal layer, t. ex. with an aluminum layer, and - to obtain an electrostatic pressure, an electrostatic field is obtained by applying a weak electrical voltage through the metal layer (15) of the diaphragms (14). 2. A method according to claim 1, characterized in that from the position measurement of the diaphragms (14), a feedback is applied to the regulation of the electric field, which, by means of electrostatic pressure, places the diaphragms in place, so that the electric field obtains a strength by means of which the diaphragms are just 93993. just maintains its position. Method according to claim 1 or 2, characterized in that a minimum voltage is applied to the diaphragms (14), by which the diaphragm is precisely and precisely attached to a second surface (17) connected to the detector (10). 4. A method according to claim 1, 2 or 3, characterized in that, from a change in capacitance between the membranes and a second surface (17) coupled to the detector (10), it is read if the diaphragm (14), using static pressure, retains its position. 5. A method according to claim 1, 2 or 3, characterized in that, from a change in the electric current between the diaphragms and a second surface (17) coupled to the detector (10), it is read if the diaphragm (14), using static pressure, retains its position. 6. A method according to claim 1, 2 or 3, characterized in that it is optically read if the diaphragm (14) has retained its position using static pressure. Process according to any one of claims 1-6, characterized in; 20, which measures the time at which the membrane (14) is released from a second surface (17) coupled to the detector (10) and simultaneously measures the voltage which causes the membranes to move. Method according to any one of claims 1-7, characterized in that the voltage between the diaphragms (14) and a second surface (17) is controlled by means of an electronic control circuit (50), so that the electrostatic pressure which the voltage causes the air pressure between the membranes and the surface, the distance between the membranes and the surface being kept constant despite variations in the pressure difference. in 93993 9. Detector (10) for measuring air pressure, in particular measurement of low air pressure, which detector has - a thin measuring detector membrane (14) arranged between two disks (11, 17), which diaphragm is arranged in the detector so as to sides of the diaphragms (14) form a gap (13, 18) bounded by the diaphragms and the discs (11, 17), of which one gap, the so-called. the air gap (18), connected to an inlet pipe (24) for the air to be measured, the pressure difference rising between the two sides of the membrane striving to displace the membranes. - means for providing an electrostatic field in the air gap (18), for providing an electrostatic pressure corresponding to the air pressure to be measured, - means for measuring the position of the diaphragm (14), 15. means for providing an feedback connection from the position measurement of the membrane to the means for providing the electrostatic field, for controlling the electrostatic pressure, - means for measuring the strength of the electrostatic field, for indicating the pressure difference occurring between the two sides of the membrane, characterized therefrom , - the measuring detector membranes (14) are formed by a thin thermosetting membrane having a high dielectric constant and one of which; The side facing away from the air gap is coated with an electrically conductive thin metal layer, such as with an aluminum layer, and - connected to the diaphragm means for providing electrical voltage in the metal layer of the diaphragm, to provide an electrostatic pressure in the air gap (18). 10. A detector (10) according to claim 9, characterized in that the electrical conductive membrane (14) of the detector (10) together with the second conductive surface (17) connected to the detector forms a capacitor, whereby the membranes can be ascertained. maintains its position by measuring the capacitance between the membranes and the second surface. The detector (10) according to claims 9 or 10, characterized in that the detector (10) includes a means which indicates when the diaphragm (14) is released from the second surface (17) connected to the detector. 12. A detector (10) according to claims 9, 10 or 11, 5, characterized in that the diaphragm (14) is a metal-coated plastic membrane, such as e.g. an aluminum-coated plastic membrane extension. Detector (10) according to any one of claims 9-12, characterized in that two detectors (10 and 40) are connected together, that their trachea (31 and 32) are interconnected, and that the detectors are arranged as part of a detector. electrical circuit board (19).