DE102008062301A1 - Inclined seat-standard backflow preventer for use in e.g. central water supply plant, has guide pin extending from valve seat in valve opening direction and entering into receiving guide or guide sleeve, which is component of valve cover - Google Patents

Abstract

The preventer has a valve element arranged in a housing (6) and mounted displaceably by a guide pin (2) that is in connection with the element. The pin is designed such that a sensor (10) i.e. giant magneto resistive measuring cell, arranged in proximity to the pin is subjected to interferences during change of position of the pin. The guide pin extends from a valve seat (5) in a valve opening direction and enters into a receiving guide or a guide sleeve (8), which is a component of a valve cover (4), where the guide or sleeve is made of magnetic neutral material.

Description

The The invention relates to a standard non-return valve with a housing, one arranged in the housing Valve element, which has a guide means is displaceably mounted, and a valve seat according to the preamble of claim 1

The The invention further relates to an attachment for a Backflow preventer and a system of insert part and Attachment for a backflow preventer.

Backflow preventer serve the self-prevention of backflow or sucking back of fluid media, for example from Drinking water immediately behind a consumption meter system for z. B. central water supply systems. Backflow preventer are also used in heating circuits to provide backflow to prevent heated water.

In spring-loaded backflow preventer or check valves, the actual closing element in one direction by a biasing force, the z. B. can be generated by a spring, closed. In the opposite direction, the closing element is released from the flowing medium. In domestic installations, check valves are used to prevent the backflow of water from the customer's system into the public network in the event of pressure fluctuations, which can lead to considerable hygienic problems. According to the rules in force DIN 1988, part 4 , as well as the regulation on the general condition for the supply of water, the function of a backflow preventer in home installations must be checked at least once a year. For such a check there are test or drain plugs available with commercially available non-return valves. For the check a shut-off of the drinking water pipe must be made before the backflow preventer. The test screw should then be removed. If a larger amount of water from the downstream domestic water system, then it is to conclude a defect of the valve.

Out From the above, it can be seen that this testing effort basically connected with an intervention in the supply is what happens with larger water supply systems regularly leads to problems and at commercial or industrial equipment hardly feasible is.

It There is therefore a need to provide a backflow preventer with a corresponding test sensor.

From the generic DE 201 05 191 U1 a backflow preventer is known, which is used in piping or fittings of piping used to shut off the flow through a pipe can. The known backflow preventer has a valve element movably mounted in a sleeve against a spring force. In the case where the non-return valve releases the flow through the pipeline, the valve element is opened against the spring force. In order to be able to easily determine the presence or absence of a flow through the non-return valve, the teaching is based on DE 201 05 191 U1 prior to assigning a magnet to the movable valve element. The magnet is then displaceable together with the valve element against the spring force of the spring element. On the outside of a connection element is a sensor, in particular a so-called reed sensor, which acts as a position sensor.

The solution after DE 201 05 191 U1 Due to the quasi-concentric position between the magnet and the sensor, the distances between these components which are as close as possible are eliminated, since otherwise no reliable switching function can be obtained. Due to the necessary small distance, however, the free cross-section is reduced when the valve is fully open, with the result of an in many cases unacceptable pressure drop.

From the DD 57 983 is a check valve with electrical flow indicator previously known. In this check valve, the conical seat is extended by a cylindrical hollow body, wherein the cylindrical hollow body has recesses. At the opposite end of the conical seat, a guide shaft is arranged, at the outer end of a permanent magnet is attached. The guide shaft is supported by the permanent magnet attached at its end in a sleeve closed on one side, which is fastened in the valve cover of the check valve. Outside the sealed sleeve, a sensor is mounted in the form of a magnetic switch. To ensure adequate guidance with the valve open, the mentioned extension of the conical seat by a cylindrical hollow body is required. Even if this hollow body has recesses, it is located in the flow path and therefore influences the flow conditions in the valve interior. The valve fitting of the previously known solution is still in several parts and requires an additional sleeve for receiving the moving inside the sleeve permanent magnet. However, the sleeve itself does not assume any guiding tasks during the movement of the valve element.

From the above it is therefore the task the invention to provide a further developed backflow preventer with a housing and a valve element arranged in the housing, which leads to no significant pressure drop in its use and which has a simple construction, which ultimately also retrofitting existing facilities with appropriate test sensors should be possible.

The Solution of the object of the invention is carried out by a standard backflow preventer according to the feature combination according to claim 1 with an attachment consisting of a cap with means for Attach to the housing of the non-return valve according to claim 15 and a backflow preventer system according to claim 18, wherein the dependent claims at least expedient embodiments and further developments.

Of the in particular oblique seat standard non-return valve has a housing, wherein in the housing a valve element is arranged. The valve element is via a guide means movably moveable. Furthermore, in the housing a Valve seat formed and it is in one embodiment a magnetic field generating means and a magnetic field sensor available.

The Guide means is in communication with the valve member standing or connectable guide pin, wherein the guide pin is formed such that at a Change in position of the guide pin in the area of z. B. magnetic field sensor present magnetic field changes subject. It is assumed that the guide pin performs a movement similar to that of the valve element corresponds or depends on the movement of the valve element.

Of the Guide mandrel is therefore at least partially as a magnetic field influencing body is formed or comprises such a magnetic field influencing body.

The Valve element is in a variant of a preferred solution realized as a valve plate or poppet, wherein the guide pin extending from the valve seat in the valve opening direction. The guide pin occurs in its movement in a guide receptacle or a guide sleeve, which component the valve cover is as a valve closure element. With that you can otherwise necessary, located in the valve interior and the flow behavior influencing management funds accounted for.

Farther the guide pin indicates at least on or in its, from Valve element distal end of the mentioned magnetic field influencing body on.

Of the Magnetic field sensor, the z. B. formed as a magnetoresistive measuring cell can be, is on the outside in the area of the guide or the guide sleeve, the latter means essentially outside the flow path for the respective fluid medium.

In Another embodiment, the magnetic field sensor in radial and / or axial direction, starting from the guide receptacle or the guide sleeve located.

The Guide receptacle or guide sleeve is according to the invention therefore part of a non-magnetic or magnetic-field-neutral terminating element, in particular an end cap or lids, which are screwed to the housing of the Return valve is connectable. To receive a sufficient guide path, the guide sleeve extends in the outer direction of the end cap. Here is the guide sleeve preferably an integral part of the end cap and after outside liquid-tight. The outward protruding Part of the guide sleeve is used according to the invention Attaching an attachment which the at least one sensor receives.

The Closing element therefore has means for securing a cap-like Mount for the magnetic field sensor on.

In this holder, the magnetic field sensor can be introduced or be integrated.

The Bracket can now not only the magnetic field sensor, but also a magnetic field generating means such that with the Dipping the guide pin and the magnetic field located there affecting body in the area of the present Magnetic field undergoes this change, z. B. in shape field enhancement or field weakening. Of the Magnetic field influencing body can in this case as Magnetic field concentrating metallic, especially steel core executed be.

Of the Guide pin can affect next to a passive magnetic field Body also have an active magnetic material or consist of such a material. For example, can in the guide pin a permanent magnet can be arranged.

Of the Magnetic field influencing body or the permanent magnet can span a given area in the longitudinal direction extend on or in the guide pin. This results following further design possibilities.

First is the magnetic field sensor starting from the direction of the Guide mandrel radially arranged, with over the Length expansion of the magnetic field influencing body or permanent magnets the signal threshold and its displacement depending on the course is selectable. Alternatively, you can in radial arrangement of the magnetic field sensor influencing the magnetic field Body or the permanent magnet over its longitudinal extent in the guide pin a structure or structure with continuous or discontinuously changing magnetic or Possess magnetic field changing properties. If those Properties known about the longitudinal extent are and as a result of a calibration process related to the Position shift to the sensor were determined is a determination the displacement itself possible.

Prefers is the sensor, z. B. magnetic field sensor integrated in an attachment, which has a recess, the outer Contour of the guide sleeve of the end cap corresponds. If the outer contour z. B. a key hex has, the recess in the attachment is complementary executed. It is easily possible with this variant, aufzustecken the attachment and z. B. by a stud on the guide sleeve and thus on the end cap to fix.

The retrofit and replaceable according to the invention Insert for a backflow preventer exists from a valve element with valve cone, valve cone or valve disc and with a guide mandrel connected or connectable thereto, wherein the guide pin is formed such that at a Position change, z. B. shift, the same in the Range of a sensor, z. B. magnetic field sensor is subject to magnetic field changes.

Of the also retrofittable attachment for the backflow preventer is attached to the valve cover of the non-return valve, wherein on or in the essay at least one sensor, for. B. a magnetic field sensor is arranged. Ausgestaltend can also be on or in the essay Magnetic field generating agent may be present, for. B. in the form of a Permanent magnets. Preferably then are permanent magnet and Magnetic field sensor substantially opposite, leaving a central space remains for approaching or immersion magnetic field changing means designed is.

It scans the existing sensors, eg. B. an inductive or magnetic Sensor through the guide sleeve of the valve cover Through the movement of the influencing body, the again part of the guide pin is. The valve cover must Therefore, have no through holes, so that leakage problems be avoided. The valve cover with guide sleeve Can be as a one-piece part of the same material as the valve housing are made. It also exists the ability to use the one-piece valve cover Guide sleeve made of a non-metal, z. B. electric non-conductive plastic.

Of the Sensor can be screwed into an appropriate hole or Recess in the attachment to be fixed.

The Invention will be described below with reference to embodiments and explained in more detail with the aid of figures become.

in this connection demonstrate:

1 ; 2 a first embodiment of a non-return valve with oblique seat valve closed ( 1 ) as well as open ( 2 ) and sensor holder in sectional view;

3 the sensor attachment 1 and 2 in exploded view;

4 an oblique seat valve with a sensor attachment and cast-in coil arrangement for an inductive sensor;

5 a further embodiment of a Schrägsitzventils with a sensor attachment and coaxial arrangement of the sensor;

6 a representation similar to that after 5 but in a variant embodiment with a sensor attachment receiving a radially disposed sensor;

7 a Schrägsitzventil with a sensor attachment, which has an axially parallel bore based on the extension of the guide pin to receive a sensor, wherein the sensor holder in addition to the space for receiving the actual magnetic field sensor and space for a magnetic field generating means, for. B. has a permanent magnet;

8th a representation similar to that after 6 However, with an embodiment in which the radially arranged sensor substantially opposite in the sensor attachment or in the sensor holder, a receptacle for a magnetic field generating means is provided;

9 a detailed representation of the sensor attachment or the sensor holder after 8th in exploded view;

10a to c is a functional diagram of a Arrangement according to 6 together with valve with guide pin in two operating positions, the 10c represents the relevant waveform;

11a to c is a functional diagram of the arrangement with sensor holder and sensor after 6 in addition to showing various positions of the guide pin with a relatively long over the longitudinal extent of the mandrel extending magnetic insert, wherein 11c represents a fundamental signal curve, as it results from the arrangement according to the 11a and 11b results;

12a to c is a functional diagram with the representation of a guide pin with magnets in two displacement positions and a sensor which is located in the longitudinal axis direction relative to the extension of the guide mandrel this substantially opposite, wherein 12c represents a signal waveform, as in the application of the arrangement according to the 12a and 12b results;

13a to c is a schematic representation of a guide pin with magnetic field influencing body, which enters a sleeve space, wherein the sensor holder with respect to the longitudinal axis of the guide pin radially opposite a magnet and the actual sensor or a measuring cell, wherein 13c shows a typical principal waveform, as it results from an arrangement according to the 13a and 13b results;

14a and b is a schematic diagram similar to that of FIGS 13a and 13b but here the guide pin is almost entirely made of a magnetic field influencing material;

15a to c is a schematic representation of an arrangement similar to that of the 13a and 13b respectively. 14a and 14b However, wherein here in the guide pin a magnetic field influencing body is present, which varies in a longitudinal axis direction, z. B. has pointed cone-like structure, wherein 15c represents a fundamental signal curve, as it emerges from an arrangement according to the figures 15a and 15b in the corresponding positions;

16 a further embodiment of the backflow preventer in a basic sectional view without a substantially outwardly emerging guide sleeve, wherein a flattened cap is provided and

17 a variant of the non-return valve with a sensor attachment comprising sensor holder and sensor in a coaxial design, wherein the essay also forms a means for guiding the mandrel.

The Figures show the exemplary embodiments of the invention Backflow preventer with an angle seat valve.

According to the representations z. B. after the 1 . 2 and 16 extends to the valve disk 1 facing away from a guide pin 2 ,

This guide thorn 2 simultaneously forms a receptacle for a spring 3 , This spring 3 rests against the valve disk 1 on the one hand and in relation to a final element 4 on the other hand. Under the action of the spring 3 results in a bias of the valve disk 1 so that the backflow prevention function of the device is ensured. It should be noted that the valve plate 1 with closed valve (see 1 ) in sealing contact with the valve seat 5 is applied.

The backflow preventer shown is part of a tubular housing 6 of which there is a housing section 7 extends.

At the housing section 7 is the final element 4 fixable, z. B. via a corresponding thread pairing. In this way, easy access to the valve plate 1 as well as the guide pin 2 , z. B. for the purpose of replacement and easy assembly.

Starting from the non-magnetic material, such as brass, plastic or the like existing end element 4 extends an outwardly projecting guide sleeve 8th as preferably integrated part of the valve cover as a final element 4 ,

With sufficient flow conditions S (see arrow illustration), the force of the spring 3 overcome and it moves the valve disk 1 as shown by 2 to the top left, leaving the corresponding fluid medium through the tube 6 can flow. As a result of the movement of the valve disk 1 the guide thorn emerges 2 in the guide sleeve 8th one.

To the guide sleeve 8th is a sensor holder 9 in the form of a z. B. pluggable attachment disposed.

The sensor holder may be formed as an extension portion of the attachment part and a bore for receiving the sensor element 10 include. The bore may have an internal thread with an external thread of the sensor element 10 is complementary. But there is also the possibility of the sensor element 10 in the sensor holder 9 to fix by a circular wedge connection. Here are on the cylinder surface of the sensor element 10 several circular wedges in the circumferential direction arranged. The hole or recess in the sensor holder 9 has a corresponding number of corresponding wedges. It can then be the sensor element 10 inserted into the correspondingly shaped bore and radially clamped by twisting and thus fixed.

The sensor holder 9 takes a sensor element 10 , z. B. a GMR measuring cell.

Am from the valve plate 1 Far end has the guide pin 2 an insert with a magnetic field generating means or with a magnetic field influencing body 11 ,

When the valve plate 1 from the valve seat 5 solves, the magnetic field generating means passes 11 in the magnet-selective region of the sensor 10 and a switching signal is triggered. If the switching signal is present, it can be seen that the backflow preventer is in the free flow state. A non-existent signal, on the other hand, is proof of proper closing of the valve.

The Threshold of the triggered switching event is freely adjustable and allows monitoring of the entire opening or closing the suitably equipped Backflow preventer.

The 4 to 9 show possible variants of the arrangement of the sensor 10 in the sensor holder 9 , where the 4 from an attachment with cast coil arrangement 17 an inductive sensor emanates.

According to the 5 extends the sensor 10 based on the orientation of the mandrel 2 axially, ie in the direction of the greatest sensitivity of the sensor.

In the embodiment according to 7 is the sensor 10 in the sensor holder 9 radially with respect to the longitudinal extent of the immersed in the space shown end of the guide pin 2 arranged, wherein the sensor body, the z. B. is designed as a cylindrical screw, quasi from above into a corresponding bore of the sensor holder 9 can be introduced.

According to the 6 the sensor body itself becomes a radial recess in the sensor holder 9 brought in.

In the representations after the 5 and 6 it is assumed that the influencing body 11 z. B. is formed as a permanent magnetic part and immediately upon a corresponding relative movement of the guide pin 2 a magnetic field change in the corresponding measuring cell of the sensor 10 is effected.

In the presentation after 7 to 9 a variant is shown in which the sensor 10 not or not just the magnetic field of the magnetic body 11 detected, but a magnetic field of a measuring cell 12 of the sensor 10 opposite magnets 13 , where again the sensor body is axially ( 7 ) or radial ( 8th ) on or in the sensor holder 9 can be fixed.

The magnetic field influencing body 11 is designed so that when introducing the same between the sensor cell 12 and magnets 13 either the magnetic field located there is weakened or even amplified. Such a magnetic field influencing body 11 can z. B. made of steel. Alternatively, the entire guide pin 2 from such, z. B. steel material.

The 10a and b show a guide pin 2 with a magnet as influencing body 11 in a first position P1 and a second position P2, wherein the in 10c represented signal voltage US can be tapped.

In position P1 ( 10a ) is the valve plate 1 and thus also the guide pin 2 in its quasi-rest position, ie the valve is closed. The signal voltage of the rest position can be set in a control or display device to a zero point value, if necessary, for. B. if it is ensured that the valve actually has the closed state.

When opening the valve, z. B. by water extraction, lifts the valve disk 1 from the valve seat, bringing the distance between the magnet 11 and the measuring cell 12 decreases, so that an increase of the signal voltage is caused.

Increases the signal voltage over a freely selectable signal threshold on, a switching signal, a display or further signal processing can take place.

Does the magnet move? 11 in the area substantially completely in front of the sensor cell 2 , the magnetic field does not change or only slightly, ie the signal voltage US remains substantially constant. If then, when the valve is opened again, the magnet 11 again from the sensitive area of the cell 12 emerges, the signal drops. In the region of the now falling signal, a further threshold value can be interrogated or monitored, which represents, for example, an end position of the guide pin or of the valve.

The 11a and b show an embodiment form, where the length of the magnet 11 extending over a larger area in the longitudinal direction within the guide pin 2 extends. The length of the magnet 11 is z. B. such that even with a complete opening of the valve, the magnet 11 still within the detection range of the measuring cell 12 of the sensor 10 located.

The waveform as it moves during a movement of the guide pin 2 starting from position 1, 11a after position 2, 11b is in the 11c shown.

The arrangement after 11a and 11b is advantageous if, for example, there is a risk that in the embodiment, as in the 10a , 10b is shown, the falling edge of the signal voltage falls below the local signal threshold (zero point).

In the arrangement according to the 12a and 12b is the guide pin 2 with magnets 11 essentially axially opposite the measuring cell 12 of the sensor 10 , In this embodiment, a movement is carried out from position 1 to position 2, that is, the magnet approaches 11 in the guide pin 2 the measuring cell 12 on, the signal US takes a course, as in the 12c is shown. When the signal threshold is exceeded z. B. be given an opening of the valve and displayed accordingly.

The 13a and b represent an embodiment which, from the basic technical function, corresponds to that already explained. Here dives a magnetic field influencing body 11 who is in the thorn 2 is located in a room in which there is a field coming from the magnet 13 is generated, which is located in the sensor holder 9 located.

The magnet 13 essentially opposite is then the sensor 10 with measuring cell 12 located. In the example shown is of an influencing body 11 assumed that has a magnetic field weakening effect.

As indicated by the waveform US through positions 1 and 2 13c can be seen, is initially at a fairly high signal voltage, when inserting or dipping the guide pin 2 body influencing the magnetic field 11 among those in the 13c shown signal threshold drops, so that a corresponding switching event can be triggered.

In the embodiment of the 14a and 14b , the construction according to the representation 13a . 13b corresponds, is the entire guide pin 2 from a magnetic field influencing material. Here, for example, steel, but also a ferritic material is used.

The solution after the 15a and 15b again assumes that in the sensor holder 9 Essentially opposite a magnet 13 and the sensor body 10 with measuring cell 12 located. The magnetic field affecting body 11 has z. B. over its longitudinal extent different cross-sections, z. As a cone or cone shape, or properties. By introducing the body 11 in the space between the magnet 13 and the measuring cell 12 the magnetic field is changed depending on the penetration depth, in particular weakened, so that a path-dependent signal voltage US as shown in FIG 15c compared to the distance traveled from position 1 to position 2.

In the constructive solution of the non-return valve 16 is the basic structure of tubular housing 6 , Guide pin 2 , Feather 3 , Valve disc 1 and valve seat 5 similar to that, as based on the 1 explained. In the part 7 Here is a cap-like closing element 4 However, this has no outwardly emerging guide sleeve 8th but has a flatter shape with a z. B. hexagonal process 15 to get a key to assemble or disassemble the closure element 4 to be able to start. The guide shot 16 for the guide pin 2 is here on the inside of the final element 4 formed.

The sensor holder 9 has an inside recess, the contour of the extension 15 essentially corresponds. The thus designed sensor holder 9 with sensor incorporated therein 10 and measuring cell 12 can z. B. made of an elastomeric material and on the key surface of the extension 15 slipped over and fixed there by form fit or adhesion.

In addition to the examples shown, an embodiment is possible in which the sensor is not fastened via a separate holder, but an integral part of the non-return valve or of the closing element 4 is.

The guide pin 2 can also in principle from a hydrophobic material, eg. As polyoxymethylene (POM) exist.

As well is a coating z. B. a steel core with polyoxymethylene conceivable.

valve disc 1 and guide thorn 2 can be made in one piece, but also consist of two parts that can be connected to each other, each optimized on the material side for the sealing properties of the plate and the wear and magnetic feldspe become the spine's characteristics. The magnetic field influencing body can be in a recess at a free open end of the guide mandrel, also interchangeable, introduced. The sensor holder is preferably made of a plastic or brass material, ie a material that is not magnetizable itself and does not significantly affect the magnetic field, which is to be detected.

The Sensor holder can with the end element or the end cap screwed or with a laterally introduced transverse screw be fixed by clamping on the cap.

The magnet 13 according to the embodiments according to the 13a , b; 14a , Federation 15a , B can simultaneously as a fixing screw for fixing the sensor holder 9 be used on the guide sleeve or the closing element. Additionally or alternatively, there is the possibility of the sensor 10 as Einschraubsensor form and selbigen as a transverse screw for attaching the sensor holder 9 to use.

17 discloses an embodiment with a sensor mount 9 that the sensor 10 Coaxially receives. Here is the sensor mount 9 even with a guide 16 for the guide pin 2 Mistake. The sensor holder 9 has a recessed male thread, with an internal thread, located in the housing section 7 is complementary. This allows the sensor holder 9 in the housing section 7 be attached by screwing sealing.

1

valve disc

2

guide pin

3

feather

4

Termination element / valve cover

5

valve seat

6

tubular casing

7

housing section

8th

guide sleeve

9

sensor mount

10

sensor

11

magnetic body

12

cell

13

magnet

15

Hexagonal extension / key face

16

guide seat

17

cast Kitchen sink

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - DE 20105191 U1 [0007, 0007, 0008]
• - DD 57983 [0009]

Cited non-patent literature

• - DIN 1988, Part 4 [0004]

Claims (18)

Standard non-return valve having a housing, a valve cover, a valve element arranged in the housing, which is displaceably movably mounted via a guide means, and a valve seat, wherein the guide means comprises a guide pin ( 2 ), wherein the guide pin ( 2 ) is designed in such a way that, when the position is changed, a sensor arranged or can be arranged in the vicinity ( 10 ; 12 ) Is subject to influences, characterized in that the guide pin ( 2 ) from the valve seat ( 5 ) extending in the valve opening direction and the guide pin ( 2 ) in a guide receptacle or guide sleeve ( 16 ; 8th ), which part of the valve cover ( 4 ). Backflow preventer according to claim 1, characterized in that the guide pin ( 2 ) at least in sections as a magnetic field influencing body ( 11 ) is formed or at least one magnetic field influencing body ( 11 ). Backflow preventer according to claim 2, characterized in that the guide pin ( 2 ) at least at or in its end remote from the valve element, the magnetic field influencing body ( 11 ) having. Backflow preventer according to claim 2, characterized in that a magnetic field sensor ( 10 ; 12 ) in the region of the guide receptacle or guide sleeve ( 8th ; 16 ) is arranged. Backflow preventer according to claim 4, characterized in that the magnetic field sensor ( 10 ; 12 ) in the radial and / or axial direction starting from the guide receptacle or the guide sleeve ( 8th ; 16 ) is arranged. Backflow preventer according to claim 1, characterized in that the guide receptacle or the guide sleeve ( 8th ; 16 ) as part of the valve cover ( 4 ) consists of a magnetic field neutral material. Backflow preventer according to claim 6, characterized in that the valve cover ( 4 ) Means for attaching a cap-like mount for the magnetic field sensor ( 10 ) having. Backflow preventer according to claim 7, characterized in that the magnetic field sensor ( 10 ) in the cap-like holder ( 9 ) is integrated. Backflow preventer according to claim 7 or 8, characterized in that the cap-like holder ( 9 ) the magnetic field generating means ( 11 ) and the magnetic field sensor ( 10 ; 12 ) has such that with the immersion of the guide mandrel ( 2 ) is subject to these changes in the region of the present magnetic field. Backflow preventer according to one of claims 1 to 9, characterized in that the guide pin ( 2 ) has permanent magnetic properties or at least one permanent magnet is located in the guide pin. Backflow preventer according to one of claims 3 to 10, characterized in that the magnetic field influencing body or the permanent magnet over a predetermined range in the longitudinal direction on or in the guide pin ( 2 ). Backflow preventer according to claim 11, characterized in that the magnetic field sensor ( 10 ; 12 ) starting from the direction of displacement of the guide pin ( 2 ) is arranged radially, wherein over the longitudinal extent of the magnetic field influencing body or the permanent magnet ( 11 ) the signal threshold and its Verschiebewegabhängiger course is selectable. Backflow preventer according to claim 11, characterized in that the magnetic field sensor ( 10 ; 12 ) starting from the direction of displacement of the guide pin ( 2 ) is arranged radially, wherein the magnetic field influencing body or the permanent magnet ( 11 ) over its longitudinal extent in the guide pin ( 2 ) has a structure or structure with continuously or discontinuously changing magnetic or magnetic field varying properties. Backflow preventer according to one of claims 7 to 13, characterized in that the cap-like holder is designed as an attachment part, which has a recess which corresponds to the outer contour of the valve cover ( 4 ) corresponds. Attachment for a backflow preventer after one of the preceding claims, consisting of a Cap with means for attachment to the valve cover of the non-return valve, wherein at least one sensor is arranged on or in the cap. Attachment according to claim 15, characterized in that in that on or in the cap a magnetic field generating means, in particular at least one permanent magnet is located. Attachment according to claim 16, characterized in that the material of the cap is non-magnetic or has such properties that does not or only insignificantly affect the magnetic field influence. Backflow preventer system, consisting of a Backflow preventer according to one of the claims 1 to 14 and an attachment according to the features of claim 15.