DE19714351C2 - Method and device for detecting gas and liquid volumes with volume counters - Google Patents

Description

The invention relates to a method for detecting Gas and liquid volumes with volume counters, especially rotary lobe, turbine and Oval gear meter, in which the non-contact Moving one past the shaft of the measuring element fixed permanent magnets on fixed Pulse wire sensors due to their magnetic reversal Voltage pulses are generated with which the Measuring device determined volume with an electronic Pulse processing device detected and by one electronic counter is displayed.  

The invention further relates to a device for for recording gas and liquid volumes with Volume meters, especially rotary lobe, turbine and Oval gear meter, with a housing, in the measuring space one or several measuring element (s) are rotatably mounted, the shaft of one only carries a magnet that is transverse to the shaft axis Axial direction is spaced from the measuring element, with several pulse wire sensors that are perpendicular to the magnetic axis in the Area of action of the magnet from a wall part of the housing are held stationary, and with an electronic Pulse processing device, which is an electronic Counter with display device is assigned.

DE 42 11 704 A1 describes a measuring arrangement for non-contact detection of the speed one on one Shaft arranged component with several revolving Known magnets in their magnetic fields stationary pulse wire sensor is arranged. The magnets are radial and arranged in such a way that every pulse of two axially spaced opposite polar poles (N, S) is triggered.

Furthermore, DE 87 14 182 U1 is a digital one Speed sensor with magnet-equipped sensor wheel for stationary arranged pulse wire sensors known by a circumferential air gap from that with a Machine shaft coupled encoder wheel are separated. There are one or more pulse wire sensors in one stretched over part of the encoder wheel  Sensor cassette housed, the electrical and mechanically with a rigid circuit board to one Unit is connected. This unit is adjustable on attached to a flat front surface. Are on the encoder wheel on the circumference with setting and resetting magnets arranged on the fixed pulse wire sensors be passed without contact.

One is also known from EP 0484 716 A1 electromagnetic encoder for determining the speed and / or direction of rotation of a rotor with at least one permanent magnets attached to the rotor and with at least one arranged laterally next to the rotor bistable magnetic switching element with associated Sensor coil that jumps into a big one Barkhausen jump is reversible.

Triggering the Barkhausen jump in the bistable magnetic switching element and resetting the bistable magnetic switching element is made by the same permanent magnet. The permanent magnet is offset to the axis of the rotor so that its two magnetic poles in the circumferential direction of the rotor are arranged side by side.

For speed direction-dependent speed detection at least two bistable magnetic switching elements provided that are electrically connected in series. The direction of rotation is from the sign of the Voltage pulses recognized.  

With this known encoder it is possible besides the speed also capture the direction of rotation it however, only one signal channel is available, the no redundancy monitoring is permitted.

Furthermore, DE 29 06 795 A1 describes one Pulse generator, preferably containing a rotor trained runner, which with respect to a stator is movable, and means for generating a Magnetic field consisting of at least one Wiegand wire acts to move the runner through the Wiegang effect caused impulses in a coil produce. The magnets are arranged so that the gained impulses a positive or negative Have polarity.

In DE 37 29 949 A1 there is a rotary encoder Pulse wire sensors depending on the setting of one with Set magnet equipped encoder wheel and means for Resetting the respectively set sensors disclosed. Arrangement according to other known solutions All of these known solutions do not allow Redundancy check.

This leads to legal-for-trade accounting Problems because no redundancy check is possible. The use of this known measuring arrangement is therefore unsuitable for custody transfer.

Knowing this state of the art Invention based on the object, a method and a Device of the type mentioned are available place that allows, in addition to the speed also the direction of rotation and measured value transmission in independent signal channels with simple Construction, high accuracy in a large measuring range and with lower power consumption under extensive Exclusion of braking forces on the measuring elements to capture.

This task is accomplished with the procedure of the beginning mentioned type according to the invention solved in that with two pulse wires with the same polarity short successive, serving as counting impulses in the same direction voltage pulses are generated by Up and down counters counted and the displayed one Meter reading used for a redundancy check and that the counts have a common, of one to the same-polarized pulse wire sensors counter-polarized pulse wire sensor generated as Directional pulse serving voltage pulse assigned which is converted into a direction bit with the the counters recognize the counting direction.

Pulse wire sensors switch on, for example Approximation of the magnetic north pole depending on the polarity of the Pulse wire sensors. The sensor principle is based on the Wiegand effect, which is that when passing  a pulse wire on the poles of a permanent magnet a sudden magnetic reversal takes place that increases a short voltage pulse. The south pole of the Magnet prepares the pulse wire for ignition, the north pole triggers the ignition. On the electrical Output of the pulse wire sensor is at the moment Ignition of this voltage pulse as a signal Available.

The signals from the three pulse wire sensors are through Transistors amplified. The two like-minded Count pulses are sent to the inputs via flip-flops a microcontroller and one each Standard up / down counter counted. With the impulse of reverse-polarized pulse wire sensor becomes a direction bit won, with which the up and down counter the Recognize the counting direction before the impulses of the two processed the same polarized pulse wire sensors become. By comparing the meter reading which matched both counters and the redundancy of the Counter checked. This is conveniently done with suitable software.

According to a preferred embodiment of the The inventive method is a two-channel Pulse delivery carried out. The same-minded Count pulse of one of the two same polarity Pulse wire sensors are each equipped with a hardware  Counter counted. With the impulse of the opposite polarity Pulse wire sensor and the pulse of the other polarized pulse wire sensor is this hardware Counter assigned the counting direction.

It is of course also possible to get the impulses of the two polarized pulse wire sensors on two separate Hardware counters to put and compare the two Meter readings to perform a redundancy check.

In the case of the method described above, the Object achieved by a device that two same-polarized pulse wire sensors for one two-channel pulse delivery and another however oppositely polarized pulse wire sensor for the Direction detection are provided, their longitudinal axes arranged parallel to each other and close together laid bundles are summarized, and that the distance of the same polarized pulse wire sensors from the axis of rotation (Shaft axis) of the magnet versus the distance of the reverse-polarized pulse wire sensor from the axis of rotation either is larger or smaller.

In a preferred embodiment of the device according to the invention are the electrical outputs of the same polarity Pulse wire sensors each via separate Amplification arrangements with the S input of a first and  second clocked, static RS flip-flops and the electrical output of the reverse polarized pulse wire sensor each connected to the R input of both RS flip-flops, a D flip-flop with its D input on the Q output of the first RS flip-flop and with its CP input is connected to the S input of the second RS flip-flop, and that the Q output of the first RS flip-flop is set to a pre / Return counter, the Q output of the D flip-flop on one Input pin and the Q output of the second RS flip-flop another up / down counter of a microcontroller for the Counting, display and redundancy check is performed.

In a further preferred embodiment of this The device according to the invention is electrical Output of an equally polarized pulse wire sensor with the S input of an RS flip-flop, the electrical output of the other same-polarized pulse wire sensor with the CP input of a D flip-flop and the electrical Output of the reverse polarized pulse wire sensor each connected via separate reinforcement arrangement, the D flip-flop with its D input on the Q- Output of the RS flip-flop is set. The Q output of the RS flip-flop is connected to an input of a forward / backward Counter of a hardware module and the Q output the D flip-flop is with a front and back input of the hardware module for assigning the counting direction in connection.  

The task continues with a procedure solved that by two same polarity Pulse wire sensors in quick succession, as Counter pulses serving in the same direction for processed a two-channel pulse output in this way be that the times T (n) for one revolution of the Magnets and the time differences ΔT (n) between the generated voltage pulses with two control and latchable counters determined and then by Counting in several cascaded Software counters with a predetermined counting frequency, which is faster than the measuring frequency, T (n) and ΔT (n) are found in another cycle n '≠ n and that by comparing the counts of a measuring counter with a comparison counter according to the condition ΔT (n) <T (n) / 2 and ΔT (n) <T8n) / 2 the direction of rotation is determined, using a software counter Redundancy check is made.

The microcontroller has two of them separate controllable and latchable counters.

With these counters and the downstream ones Software counters change times from round to round T (n) and ΔT (n) newly determined and by comparing the Counter readings determine the direction of rotation.  

Count frequencies that are at least 100 times higher than the frequency to be detected are perfect sufficient.

The task is also carried out with a device for Implementation of the aforementioned procedure solved according to the invention in that two to one Bundles arranged in the same polarity Pulse wire sensors for two-channel pulse delivery are provided, the longitudinal axes of which are parallel to one another and arranged close to one another, approximately the same Have distance to the axis of rotation of the magnet, and that the electrical outputs of the two Pulse wire sensors directly with interruptible ones Connections of a microcontroller are connected, of at least two controllable and latchable counters exists, each of which has separate downstream Software counter and a common counter clock assigned are.

The method with the triple sensor has compared to that Procedure with the double sensor has the advantage that it too is suitable for rotational frequencies <0.1 Hz and the Microcontroler does not time with the Direction of rotation detection is loaded, causing the Battery life of the microcontroller noticeably increased becomes.  

It is also advantageous if the pulse wire sensors in a pressure-tight sleeve are arranged, the in turn axially adjustable depending on the magnet received by an immersion sleeve on the housing cover of the Gas meter is attached.

By inserting the sleeve in the direction of the shaft Detection point for the sensor signals sensitive and can be optimally adjusted.

The pulse wire sensors are in a preferred one Embodiment in an electrically non-conductive Embedded plastic, causing the location of the Pulse wire sensors are fixed to each other.

In a further advantageous embodiment of the The device according to the invention is a shielding element on the shaft of the measuring element in the effective range of the Magnetic field arranged. This shielding element serves as a magnetic shield against the measuring element.

With the double or triple sensor according to the invention can be installed in a simple manner and Way a two-channel signal acquisition and evaluation realize that also meet the demands in custody transfer after redundancy monitoring enough.  

This gives the advantage of high flexibility and variability of the method according to the invention and the device according to the invention when detecting Detect gas and liquid volumes.

Further advantages and details emerge from the following description of two preferred Embodiments with reference to the attached drawings.

The individual shows:

Fig. 1 is a schematic representation of the dual channel pulse detection with a double sensor,

Fig. 2 pulse images with rotation detection in the forward and reverse rotation,

Fig. 3 is a schematic diagram of the signal evaluation for a triple sensor with two separate counting outputs and static direction of rotation detection with microcontroller and redundancy check,

Fig. 4 is a schematic diagram of the signal evaluation for a triple sensor of FIG. 3 without a redundancy check,

Fig. 5 shows the pulse waveform with rotation direction detection by assigning a direction bits of Fig. 3 and 4,

Fig. 6 is a side view of the device according to the invention with a triple sensor arrangement and

Fig. 7 is a section along the line BB of Fig. 6.

Embodiment 1

Figs. 1 and 6 show the arrangement of two pulse wire sensors with associated evaluation circuit and details. A bar magnet 4 is fastened to the shaft end 30 transversely to the shaft axis 1 of a rotary piston 2 of the gas meter 3 and, together with the rotary piston 2, carries out a rotary movement about the shaft axis 1 . In the effective range of the magnetic field of the bar magnet 4 , two pulse wire sensors 5 and 6 are fixed in place in a sleeve 7 with an immersion sleeve 8 on a housing cover 9 of the gas meter 3 . The two sensors 5 and 6 are parallel to each other with their longitudinal axes A and touch. As soon as the bar magnet 4 approaches the pulse wire sensors 5 and 6 during its rotation with its magnetic north pole N, short voltage pulses amounting to approximately 3 V and a pulse duration of approximately 10 microseconds are generated in succession by magnetic reversal. The voltage pulses or sensor signals S1 and S2 of the two sensors 5 and 6 that are generated in each case are amplified in commercially available amplification arrangements 10 and 11 which are separate from one another and are led directly to interruptible connections 12 and 13 of a microcontroller 14 . The microcontroller 14 has two controllable and latchable counters, a measuring counter 15 and a comparison counter 16 . With these counters and downstream software counters 24 , the times T (n) from cycle to cycle and the time differences between the sensor signals S1 and S2 as ΔT (n) are determined again and again as a function of the cycle by counting in a faster counting frequency than the measuring frequency. The counting frequency should be chosen significantly higher than the times to be recorded. In order to detect a signal of 30 Hz, for example, a counting frequency with a factor of 100 times the 30 Hz signal is sufficient.

By comparing the meter readings to the conditions using appropriate software

ΔT (n) <T (n) / 2 and

4T (n) <T (n) / 2

it is determined (see FIG. 2) whether the rotary piston is clockwise or counterclockwise.

With failure monitoring of sensor signals S1 and S2, redundancy monitoring is carried out by comparing counters 15 and 16 .

Embodiment 2

The arrangement of the bar magnet and pulse wire sensors corresponds to Example 1 as shown in FIG. 3. In addition to the two pulse wire sensors 5 and 6 , a third pulse wire sensor 17 is combined in the sleeve 7 to form a triple sensor.

The longitudinal axes A of the pulse wire sensors are aligned parallel to one another. The pulse wire sensors 5 , 6 and 17 touch each other and form a bundle.

The pulse wire sensors 5 and 6 are polarized in the same direction, ie electrically connected in series. Compared to the pulse wire sensors 5 and 6 , the third pulse wire sensor 17 is polarized in opposite directions, that is to say negatively. The pulse wire sensors 5 and 6 thus switch at the north pole, while the pulse wire sensor 17 at the south pole. The pulse wire sensor 17 has a somewhat larger distance E than the distance D of the pulse wire sensors 5 and 6 from the shaft axis 1 or axis of rotation of the magnet (see FIG. 7).

It is arranged so that it rests slightly above the two pulse wire sensors 5 and 6 on them.

The electrical outputs of the pulse wire sensors 5 , 6 and 17 are routed to separate, conventional transistor amplification arrangements 10 , 11 and 18 , in which the sensor pulses S1, S2 and S3 are amplified.

The amplified sensor pulses S1 and S2 are fed to the S inputs of separate RS flip-flops 19 and 20, respectively. The amplified sensor signal S3 is applied to the R input of the RS flip-flop 19 and 20 . Both RS flip-flops 19 and 20 are linked via a D flip-flop 21 , by connecting the D input of the D flip-flop 21 to the Q output of the RS flip-flop 20 and the amplified sensor signal S2 in the CP input of the D flip-flop 21 is performed.

The Q output of the RS flip-flop 19 is connected to the pulse input 22 of a counter 23 and the Q output of the RS flip-flop 20 is connected to the counter input 34 of a counter 35 of the microcontroller 14 .

The Q output of the D flip-flop 21 is present at the direction pin 36 of the microcontroller 14 , as a result of which a direction bit "1" or "0" is assigned to the sensor signals S1 and S2 (see FIGS. 3 and 5), and counterclockwise rotation of the rotary lobes can be distinguished.

The redundancy check is carried out by comparing the counters 23 and 35 .

Alternatively, as shown in FIG. 4, the microcontroller 14 can also be omitted. The Q output of the D flip-flop 21 is then present at a forward / reverse input 25 and the Q output of the RS flip-flop 20 with the signal pulse S1 is present at the counter input 32 of the counter 26 of the hardware counting module 33 .

Fig. 6 shows the installation of the triple sensor according to the invention in a rotary piston gas meter 3. In the housing cover 9 there is an opening 27 in which an immersion sleeve 8 is arranged, which is parallel to the shaft axis 1 of the rotary piston 2 in the interior of the housing 28 of the Gas meter 3 extends. In the immersion sleeve 8 , the sleeve 7 is inserted with the pulse wire sensors 5 , 6 and 17 embedded therein in non-conductive plastic so as to be adjustable in depth and angular position. The sleeve 7 is inserted to such a depth that optimal sensor signals S1 to S3 are achieved. The bar magnet 4 causing these signals is held by a magnet carrier 29 made of magnetizable material. The magnet carrier 29 is attached to the shaft end 30 , so that the bar magnet 4 rotates accordingly when the rotary piston 2 rotates. A shielding element 31 , for example an oil splash plate, is fastened with the magnetic carrier 29 and is aligned with the magnetic axis C of the bar magnet 4 . It shields the components lying on the bearing side of the shaft 1 against magnetic interference, so that the braking forces which could act on the rotary pistons 2 due to the magnetic field are kept small.

In the changing magnetic field, the pulse wire sensors 5 and 6 generate sensor signals S1 and S2 at a frequency that is proportional to the speed. The sensor signal S3 detects the direction of rotation in accordance with the pressure gradient acting on the rotary pistons 2 .

Reference list

1

Shaft axis

2nd

Rotary lobe

3rd

Gas meter

4th

Permanent magnet, bar magnet

5

,

6

same-direction pulse wire sensors

7

Sleeve

8th

Immersion sleeve

9

Housing cover

10th

,

11

Reinforcement arrangement

12th

,

13

connections

14

Microcontroler

15

Measuring counter

16

Target counter

17th

opposing pulse wire sensor

18th

Reinforcement arrangement

19th

,

20th

RS flip-flop

21

D flip-flop

22

Counter input to the counter

23

23

Counter of the microcontroller

24th

Software counter

25th

Front / back entrance

26

Counter in the hardware counter module

33

27

Opening in the housing cover

9

28

casing

29

Magnetic carrier

30th

Rotary piston shaft end

31

Shielding element

32

Input of the hardware counter module

33

33

Hardware counting module

34

Counter input of the counter

35

35

Counter in the microcontroller

14

36

Input pin in the microcontroler

14

A Longitudinal axis of the pulse wire sensors
S1, S2, S3 sensor signals
CMagnetic axis
DA distance between the pulse wire sensor and the magnetic axis
E Self magnification pulse wire sensor magnetic axis

Claims (17)

1. A method for detecting gas and liquid volumes with volume counters, in particular rotary lobe, turbine and oval wheel counters, in which, when a permanent magnet attached to the shaft of the measuring element moves past contactlessly, fixed pulse wire sensors are generated by magnetic reversal by means of their remagnetization, with which voltage pulses are generated by the measuring element Certain volumes are recorded with an electronic pulse processing device and displayed by an electronic counter, characterized in that with two polarity-polarized pulse wire sensors ( 5 ; 6 ), shortly successive voltage pulses (S1; S2) serving as counting pulses are generated by up and down counters ( 35 ; 23 ) are counted and the displayed counter reading is used for a redundancy check, and that the counting pulses have a common pulse wire sensor ( 17 ) generated by a counter-polarized to the same polarized pulse wire sensors ( 17 ) as a directional pulse di Ending voltage pulse (S3) is assigned, which is converted into a direction bit with which the counters recognize the count direction. 2. The method according to claim 1, characterized in that a two-channel pulse delivery to the pulse processing establishment is carried out. 3. The method according to claim 1 and 2, characterized in that the pulses (S1; S2) of the pulse wire sensors ( 5 ; 6 ) each in a standard up / down counter ( 35 ; 23 ) of a microcontroller ( 14 ) and counted with the pulse (S3) of the third pulse wire sensor, a direction bit is obtained with which the up and down counters recognize the counting direction before the pulses of the two polarity sensors with the same polarity are further processed, the counter redundancy being checked by a counter comparison between the counters. 4. The method according to claim 3, characterized in that the Pulses (S1; S2) counted with a software counter become.   5. The method according to claim 1 and 2, characterized in that the voltage pulse (S1) or (S2) one of the two same polarized pulse wire sensors ( 5 or 6 ) each counted with a hardware counter ( 26 ), with the pulse of the reverse polarized pulse wire sensor ( 17 ) and the other same-polarized pulse wire sensor ( 6 or 5 ) the hardware counter ( 26 ) is assigned the counting direction. 6. A method for detecting gas and liquid volumes with volume counters, in particular rotary lobe, turbine and oval wheel counters, in which, when a permanent magnet attached to the shaft of the measuring element moves past contactlessly, fixed pulse wire sensors are generated by magnetic reversal by means of their remagnetization, with which voltage pulses are generated by the measuring element Certain volumes are detected in an electronic pulse processing device and displayed by an electronic counter, characterized in that two consecutive voltage pulses (S1; S2) serving as counting impulses (S1; S2) are processed for two-channel pulse processing in such a way by two pulse wires ( 5 ; 6 ) with the same polarity that they are processed the times T (n) for one revolution of the magnet and the time difference .DELTA.T (n) between the generated voltage pulses are determined with two controllable and latchable counters and then by counting in several series-connected ones Software counters with a predetermined counting frequency that is higher than the measuring frequency, T (n) and ΔT (n) are determined in another cycle (n '≠ n) and that by comparing the counter readings of a measuring counter ( 15 ) with a comparison counter ( 16 ) after the condition ΔT (n) <T (n) / 2 and Δ (n) <T (n) / 2 the direction of rotation is determined, the displayed counter readings of the software counters ( 24 ) being used for a redundancy check. 7. The method according to claim 6, characterized, that the counting frequency is at least 100 times higher than that of detecting frequency is. 8. Apparatus for carrying out the method according to claim 1 to 5, with a housing in the measuring chamber of which one or more measuring element (s) are rotatably mounted, the shaft of which carries a single magnet which is arranged transversely to the shaft axis in the axial direction at a distance from the measuring element, with a plurality of pulse wire sensors, which are held in a fixed position perpendicular to the magnet axis in the area of action of the magnet by a wall part of the housing, and with an electronic pulse processing device to which an electronic counter with a display device is assigned, characterized in that two polarity-reversed pulse wire sensors ( 5 ; 6 ) for one Two-channel pulse processing and another, however, opposite-polarized pulse wire sensor ( 17 ) are provided for the direction detection, the longitudinal axes (A) of which are arranged parallel to one another and which are combined to form a tightly packed bundle, and that the distance (D) of the same-polarized pulse wire sensor s ( 5 ; 6 ) of the axis of rotation (shaft axis) of the magnet is either larger or smaller than the distance of the counter-polarized pulse wire sensor ( 17 ) from the axis of rotation. 9. The device according to claim 8, characterized in that the electrical outputs of the same-polarized pulse wire sensors ( 5 ; 6 ) each via separate amplification arrangements ( 10 ; 11 ) with the S input of a first and second clocked static RS flip-flops ( 19th ; 20 ) and the electrical output of the reverse-polarized pulse wire sensor ( 17 ) are each connected to the R input of both RS flip-flops ( 19 ; 20 ), with a D flip-flop ( 21 ) having its D input the Q output of the first RS flip-flop ( 19 ) and its CP input is connected to the S input of the second RS flip-flop ( 20 ), and that the Q output of the first RS flip-flop Flop's ( 19 ) to an up and down counter ( 23 ), the Q output of the D flip-flop ( 21 ) to an input pin ( 36 ) and the Q output of the second RS flip-flop ( 20 ) to one further up and down counter ( 35 ) of a microcontroller ( 14 ) for counting, display and redundancy check is performed. 10. The device according to claim 8, characterized in that the electrical output of the pulse wire sensor ( 5 ) with the S input of an RS flip-flop ( 20 ), the electrical output of the pulse wire sensor ( 6 ) with the CP input of a D- The flip-flop ( 21 ) and the electrical output of the reverse-polarized pulse wire sensor ( 17 ) are connected to the R input of the flip-flop ( 20 ) via separate amplification arrangements ( 10 ; 11 ; 18 ), the D-flip-flop ( 21 ) is connected with its D input to the Q output of the RS flip-flop ( 20 ), and that the Q output of the RS flip-flop ( 20 ) is connected to an input ( 32 ) with a pre / Back counter ( 26 ) of a hardware module ( 33 ) and the Q output of the flip-flop ( 21 ) are connected to a front and back input ( 25 ) of the hardware counter module ( 33 ) for counting and display. 11. Apparatus for carrying out the method according to claim 6 and 7, with a housing in the measuring chamber of which one or more measuring element (s) are rotatably mounted, the shaft of which carries a single magnet which is arranged transversely to the shaft axis in the axial direction at a distance from the measuring element, with a plurality of pulse wire sensors, which is held stationary by a wall part of the housing perpendicular to the magnetic axis in the effective range of the permanent magnet and with an electronic pulse processing device to which an electronic counter and a display device are assigned, characterized in that two polarity-reversed pulse wire sensors ( 5 ; 6 ) for one Two-channel pulse delivery is provided, the longitudinal axes of which are arranged parallel and close to one another and have approximately the same distance (D) to the axis of rotation (shaft axis) of the magnet ( 4 ), and that the electrical outputs of the two pulse wire sensors ( 5 ; 6 ) are provided via amplification arrangements ( 10 ; 11 ) with interruptible connections ( 12 ; 13 ) are connected to a microcontroller ( 14 ) which consists of at least two controllable and latchable counters ( 15 ; 16 ), each of which is assigned a separate downstream software counter ( 24 ) and a common counter clock ( 32 ). 12. The apparatus according to claim 8 to 11, characterized in that the pulse wire sensors ( 5 ; 6 ; 17 ) are arranged in a pressure-tight sleeve ( 7 ), which in turn is axially dependent on the magnet ( 4 ) adjustable by an immersion sleeve ( 8 ) on Housing cover ( 9 ) of the meter is attached. 13. The apparatus according to claim 12, characterized in that the pulse wire sensors ( 5 ; 6 ; 17 ) are embedded in an electrically non-conductive plastic. 14. The apparatus according to claim 8 to 12, characterized in that the magnet ( 4 ) is a bar magnet. 15. The apparatus according to claim 8 to 14, characterized in that a shielding element ( 31 ) is provided, which is arranged parallel to the magnet ( 4 ) on the magnet carrier ( 29 ) in the effective range of the magnetic field and extends to the level of the immersion sleeve ( 8 ) . 16. The apparatus according to claim 15, characterized in that the shielding element ( 31 ) consists of magnetizable material. 17. The apparatus according to claim 8 to 12, characterized in that the sleeve ( 7 ) and the immersion sleeve ( 8 ) consists of non-magnetizable material.