Microwave level measuring apparatus

The invention relates to a operating with microwaves, on a container to be fastened level meter, with a housing and a rod-shaped antenna in the Container has and which consists of a dielectric, esp. From Polytetrafluoroethylene (PTFE) or polyphenylene sulfide (PPS), consists.

In the level measurement by means of a microwave Sending antenna to the surface of a product and to the surface receiving the reflected echo waves. It will an echo amplitudes as a function of the distance Performing echo function is formed, from which the determines probable useful echo and its maturity will. For the duration of the distance between the will Filling material surface and the antenna is determined.

It can be applied, all known processes, the allow relatively short distances by to measure reflected microwaves. The most famous Examples are the pulse radar and frequency modulation continuous wave radar (FMCW radar).

Pulse radar are periodically short microwave transmission pulses, hereinafter referred to as wave packets, sent that reflected by the filling material surface and received after a distance-dependent travel time again will. The received signal amplitude as a function of Time represents the echo function. Each value of this Echo function corresponds to the amplitude of a certain distance from the antenna the reflected echoes.

The FMCW method, a continuous microwave transmitted, which is periodically linearly frequency modulated, For example, according to a sawtooth. The frequency Therefore, the received echo signal has, compared with the Instantaneous frequency of the transmitted signal at time Receiving, has a difference in frequency of the Duration of the echo signal depends. The frequency difference between the transmission signal and reception signal obtained by mixing both signals and evaluating the Fourier spectrum of the can be mixed signal obtained corresponds, therefore the Distance of the reflecting surface of the antenna. Further match the amplitudes of the spectral lines of the by Fourier transform obtained frequency spectrum to Echo amplitudes. This Fourier spectrum therefore provides in this case, the echo function.

With microwaves working level meters are in very many industries, for example in chemistry or in the food industry, are used. Typically, the level to be measured in a container. These containers typically have an opening to which a connection piece provided or a flange for mounting of measuring instruments is.

Rod-shaped antennas, often called rod antenna or dielectric rod radiator referred, are always preferably used when the container opening a having small diameter. Rod antennas have an in Compared to small horns on outside diameter.

In DE-A 44 05 855 a operates with microwaves, on a container to be fastened level meter described, with a housing and with a rod-shaped mounted in the housing antenna into the container has and consists of a dielectric.

In DE-A 44 43 055 a operates with microwaves described level measuring device, in which on a rod-shaped antenna of dielectric apart spaced metallic elements are arranged. These are interconnected electrically conductive. These metallic elements are used on the one hand, the bundling microwaves, and on the other to a charging of the dielectric rod antenna prevent.

A disadvantage of the aforementioned operating with microwaves Liquid level is that a rod antenna Microwave radiation both substantially forward as Main lobe in the interior of the container transmits and due to their directivity perpendicular to Antenna axis in the form of side lobes radiates. Typically, the amplitude of a first side lobe only 20 dB to 25 dB below the amplitude of the main lobe. Bundling described in DE -A 44 43 055 by Although metallic elements here leads to a However, improvement is not suitable, a radial to prevent radiation.

The radially transmitted proportion of microwave radiation at least partially at the edge of the opening of the container and reflected at the Meßgerätbefestigung and provides thereby constitutes an interference that significantly the measurement can affect. This interference occurs already then, if the amount of very low Meßgerätbefestigung is, for example, is only a few centimeters. The part of Interference depends on the installation method of the meter and as is the case of fixed on tubular nozzle Measuring instruments particularly large.

This interference occurs, for example, by a portion of the transmitted microwave energy directly to the socket reflected and then received by the antenna is without entering into the container or by the in Stub multiple reflections occur. These reflections or multiple reflections lead to an artificial Extension of short to send microwave pulses. A level measurement according to the pulse radar method is however, only feasible if the duration of the Measurement signal greater than the duration of the transmission pulse and the Time in which the last-mentioned interference signals faded are. applies similarly to the FMCW method, the Frequency shift of the measurement signal must be greater than the Frequency shift of the interference signals.

The smaller the distance between Meßgerätbefestigung, eg Spigot, and the antenna is, the greater the Impact, for the propagation of the microwaves in Near field of the antenna.

In addition to the land used for level measurement microwave radiation, the transmitted from the antenna directly to the contents is where it is reflected directly to the antenna, also arrives Interference that a detour via mindestenst Reflection has come at the Meßgerätbefestigung, to Antenna. This leads to an erroneous determination of the Runtime and thus erroneous test results.

A further disadvantage of the aforesaid level measuring devices is that as condensate or moisture to may result in that a direct electrical connection forming between the antenna and the container mounting. Such a short circuit causes like shorting a coaxial line, a reflection of the entire Microwave energy to the transmitter and prevents you from sending of microwaves.

Even if it does not come to such a short-circuit, condensate and / or moisture acting very disadvantageous , since they and a direct influence on the propagation have the reflection behavior of microwaves. Thus, for example Interference that only after the installation of the device small degree occurred, by condensation and / or humidity increase so much that a level measurement no longer is possible. This is particularly disadvantageous because the Equipment operator assumes a reliable to have functioning instrument.

It is therefore an object of the invention to provide a with specify microwaves working level meter in the antenna in the Meßgerätbefestigung no Microwave sends and where they do not in the region Meßgerätbefestigung receives reflected microwaves.

It is a further object of the invention, a measuring device indicate in which the transmission power by condensate or Moisture is unimpaired.

End, the invention according to a first Variant solution in a working with microwave, on to be attached to a container, fill level with<ul><li>a housing and</li><li>a rod-shaped fixed in the housing antenna,<sl><li>- Which points into the container and</li><li>- Consisting of a dielectric, in particular from. Polytetrafluoroethylene (PTFE) or polyphenylene sulfide (PPS), is,</li></sl></li><li>wherein a portion of the adjacent to the housing Antenna is enclosed by a metallic sleeve.</li></ul>

According to a second variant of the solution, the invention in an operating with microwaves, on a container to be fixed, level measuring device with<ul><li>a housing,</li><li>an integrally formed on the housing sleeve and</li><li>a rod-shaped fixed in the sleeve antenna,<sl><li>- Which points into the container and</li><li>- Consisting of a dielectric, in particular of polytetrafluoroethylene. (PTFE) or polyphenylene sulfide (PPS), consists.</li></sl></li></ul>

are In a further development of the first variant of the solution Microwaves through a coaxial line in a housing introduced exciter element supplied.

According to one embodiment of the invention, one of the Solution variants, the antenna has one of the metallic sleeve enclosed solid cylinder and a it formed, directed into the container interior Transmitting bar on.

According to another embodiment one of the Alternative solutions, the sleeve is a tube whose in the Tank interior is slanted directional end.

According to a further embodiment of the invention is the Sleeve of a protective sleeve, esp. From Polytetrafuorethylen (PTFE) surrounded.

According to a further embodiment of the first Solution variant, the housing portion, in which the Antenna is mounted, a side short-circuited Circular waveguide and the sleeve is an extension of shorted circular waveguide and is located on the same electrical potential as the portion of the The housing in which the antenna is mounted.

According to a further embodiment of the first Possible solution is the inner diameter of the sleeve is greater than the inner diameter of the portion of the housing, in the attached antenna.

According to a further embodiment of the invention is the The protective sleeve in a region adjoining the housing region a flange, esp. of polytetrafluoroethylene (PTFE), formed.

According to a further embodiment of the first Possible solution is the transmission rod with the full cylinder on a threaded screw, the full cylinder with in the Casing fixed portion of the antenna via a thread screwed and plugged the antenna into the case or screwed.

are In a further development of the second variant of the solution Microwaves guided through a through sleeve Coaxial one in an antennae-facing Portion of the sleeve arranged exciter element, esp. one arranged on a base body transmitting wire, supplied.

Furthermore, the invention resides in a method of Preparation of a level in which a in the Sleeve to be arranged portion of the antenna in cooled State is inserted into the sleeve, and in a Process for the preparation of a level, at the protective sleeve in the heated state on the sleeve is pushed.

An advantage of the invention is that by the Sleeve from the region of the Meßgerätbefestigung Antenna incident microwave radiation completely is hidden. The to be transmitted microwave radiation passed in the interior of the sleeve into the container. In the area the Meßgerätbefestigung is therefore no transmitted microwave radiation and also receiving no.

An advantage of the invention is that the Transmission power of the fill level by moisture or condensate is not affected. An electrically conductive connection between the sleeve and the Meßgerätbefestigung has no effect.

In a level according to the first Solution variant assumes the sleeve functions as a Outer conductor and shields the space perception. The metallic Sleeve acts as a mirror for microwaves. Typically are grounded container. Both sleeve and the Meßgerätbefestigung are preferably on a Ground.

That stated above for a level measuring device according to the second solution variant also. Here, however, only for a portion of the sleeve, which container facing to the Side of the excitation element is. In a on the container-side remote from the exciter element disposed portion of the sleeve is carried out, the electrical Shielding by the fact extending coaxial line is mechanically protected by the sleeve.

The invention and further advantages will now be based on the Figures of the drawing in which two embodiments a level meter operating with microwaves according the first solution variant and an embodiment a level meter operating with microwaves according the second solution variant are shown, in detail explained; like parts in the figures with the same Reference numerals.<dl tsize="13"><dt>Fig. 1 shows</dt><dd>a longitudinal section through a with Microwave level measuring labor according to the first variant of the solution;</dd><dt>FIG. 2</dt><dd>a longitudinal section through a with Microwave level measuring work, according to the first variant of the solution in which the Sleeve one end on one side sloping Tube; and</dd><dt>FIG. 3</dt><dd>a longitudinal section through a microwave working level meter according to the second variant of the solution.</dd></dl>

In Fig. 1 is a working with microwave Level measuring device 1 according to the first variant of the solution shown schematically. As a serving Meßgerätbefestigung Piece 2, on which the level measuring device 1 is fixed. The nozzle 2 is tubular and in a circular Opening 31 of a container 3 is mounted, eg welded. In the container 3 is a medium.

The level transmitter 1 serves the filling level of this Medium to determine in the container. It has a housing 11 that on one side the form of a pot or of a end closed tube possesses. At this housing 11, leaving the pot or tube opening a Flange 12 is formed. The nozzle 2 has its container-side remote from a corresponding Flange 21. Flange 12 and flange 21 are connected to one another by means of screws that Pot or tube opening of the opening 31 of the container 3 faces and the axes of symmetry of the housing 11 and Piece 2 are identical.

The microwaves are not represented by a Microwave generator and a coaxial line 4 a laterally introduced into the housing 11 Exciter element 5 is supplied.

The microwave generator is, for example, a pulse radar device, an FMCW device or a continuously oscillating A microwave oscillator.

The housing 11 consists of an electrically conductive Material, such as aluminum, stainless steel or with a conductive material coated plastic, and has a circular cross-section. It is thus its Geometry to a shorted circular waveguide, in which the to be transmitted microwave mode is formed. The Of radiation of the microwaves by an arrow 6 indicated.

In the housing 11 is a rod-like antenna 7 is made of a Dielectric attached. The antenna 7 fills the interior of the housing 11 to a for receiving the Exciter element 5 provided recess completely and extends through the opening 31 and into the Container. 3

The antenna is rod-shaped and comprises three sections, a first fixed portion 70 in the housing 11, an adjacent solid cylinder 71 and a thereto end to its container-facing side molded, directed into the container interior Transmitting rod 72. All three sections consist of a Dielectric, preferably made of polytetrafluoroethylene (PTFE) or polyphenylene sulfide (PPS). Both are materials which are easy to handle and high chemical exhibit resistance.

At the 70 end portion is an external thread 701 provided with a corresponding opening on the an internally threaded cylinder 71 Full is screwed. The full cylinder 71 instructs at its the transmission rod 72 adjacent the end of an opening with a Female thread 711. is into this internal thread 711 correspondingly shaped extension 721 of the transmission rod 72 screwed. Of course, other types of Connecting the antenna sections as well as the chosen the use of a one-piece antenna possible.

The section 70 may be in the housing 11 also through a Thread'll secured. In the embodiment but opted for another attachment, which in a the following paragraph is explained.

The portion of the housing 11, in which the antenna 7 is attached, and the antenna 7 may be other have cross-sectional geometries, which should be sent to the Fashion and or the transmission frequencies are compatible. The Geometry of a shorted circular waveguide and a corresponding circular cross-section are antenna advantageous in case of such a geometry fashions are capable of propagation, the radiation with a a pronounced forward lobe arise.

An in-the neck 2 portion of the antenna 7 is enclosed by a metallic sleeve eighth The sleeve 8 consists for example of stainless steel and is located directly on the antenna 7 to. In the embodiment of Fig. 1, the sleeve 8 is just like the antenna tubular and encloses the Full cylinder 71 closely. The close fit of the neck 8 of the Antenna is required to allow for microwave only a single interface to which an impedance mismatch occurs, namely between the sleeve 8 and antenna 7, is present. Other interfaces, eg caused by trapped Bubbles, degrade the quality of the sleeve 8 as Extension of the waveguide.

This close fit of the sleeve 8 is achieved by the in the sleeve 8 to be disposed portion of the antenna con 7 of here cooled solid cylinder 71 in the sleeve 8 before introduction becomes. The shrunken by cooling solid cylinder 71 is inserted into the sleeve 8 and expands when the subsequent heating again. Since Polytetrafuorethylen (PTFE) is a flowable material to a limited extent, adapts the solid cylinder 71 of the internal geometry of the Sleeve 8 at. Polyphenylene sulfide (PPS) is a material which has a thermal expansion coefficient that the of steel is very similar. Components made of polyphenylene sulfide (PPS) can be produced with low manufacturing tolerances be and behave about the temperature in substantially the same as the metallic sleeve eighth

The length of the solid cylinder 71 and the sleeve 8 in the axial Direction is preferably equal to the height of the nozzle. 2 By the action of sleeve 8 as a mirror for microwaves thus all of the microwave radiation in the range the connecting piece would impinge on the antenna 7, hidden. Noise from this area will not be receive.

The sleeve 8 is at the same electrical potential, as the portion of the housing 11, in which the antenna 7 is attached. In the embodiment of the figure 1 this is done by the sleeve 8 with a ring-shaped end face 81 directly on the housing 11 bears. The sleeve 8 is thus an extension of the formed by the housing 11, the short-circuited Circular waveguide.

occurs at the transmitting rod-facing end of the sleeve 8 Impedance jump on. To achieve a continuous Transition of impedance at a place of just cut pipe occurring impedance jump is at the fill level measuring device according to the illustrated in Fig. 2 first solution variant a sleeve 8 'is used, the The form of a rod at the send-facing end chamfered, ie an angle to the perpendicular to the Tube longitudinal cut, having pipe. Of the Plot of the section is shown in FIG. 2 by a broken Line shown.

A conditional by eg moisture or approach Short circuit has no effect on the transmission characteristics the level meter. If it is assumed that the Container 3 and the housing 11 are grounded, as would a electrically conductive connection between the nozzle 2 and the antenna 7 in the absence of the sleeve 8 a short circuit between the inner conductor of the coaxial line 4 and the Outer conductor of the coaxial line 4 correspond, for the The inner conductor is connected via the exciter element 5 with the antenna 7 connected and the outer conductor of the coaxial line 4 is about the exciter element 5 and the housing 11 with the nozzle 2 connected.

Experiments have shown that it is advantageous to use a To use sleeve 8 with an internal diameter which is larger than the inner diameter of the portion of the housing 11 is, in which the antenna 7 is fixed. power loss and dispersion can thereby be significantly reduced. Especially pronounced are these advantages in conjunction with the pulse radar method. Short pulses have frequency spectra with a large bandwidth.

In the embodiments, the sleeve 8 is of a Protective sleeve 9 surrounding the cylindrical closely at a outer surface of the sleeve 8 abuts. To this end, it is in the embodiments shown in the figures tubular and instructs the send-rod end facing a radially extending into the pipe interior shoulder 91 on. The shoulder 91 has a central through hole with an internal thread. The projection 721 of the transmission rod 72 is at its attachment to the full cylinder 71 this hole threaded through.

The protective sleeve 9 preferably also consists of a highly chemically resistant material, esp. from Polytetrafluoroethylene (PTFE). The use of the same Material for all elements ensures that all Elements withstand the same harsh conditions. The Protective sleeve 9 has no effect on the transmission and Reception performance of the antenna 7. It has the advantage, that for the sleeve 8 simple inexpensive materials are used, as they both sleeve 8 against moisture protects and prevents them with the possibly chemically aggressive or abrasive media comes into contact.

In the preparation of an inventive Level meter of the protective sleeve is heated. 9 Thereby extends it and is slightly deformed. In this heated state, the protective sleeve 9 via the Sleeve 8 is pushed. During subsequent cooling, shrinks the protective sleeve 9 on the sleeve. 8

On the sleeve 9 is at its housing-facing end an annular disc-shaped flange 92 integrally formed of the parallel extending to the flange 12 of the housing 11 and between the Flange 12 and the flange 21 is clamped. Since the Protective sleeve 9 is fixed to the antenna 7, by the clamping of the flange 92, the antenna 7 in the Housing 11 is fixed.

Preferably, the flange 92 consists of a for sealing suitable material, esp. of polytetrafluoroethylene (PTFE), so that the flange 92 to seal the Container opening forms.

Fig. 3 shows a microwave level measuring device working according to the second variant of the solution. It is also to a nozzle 2 mounted on a vessel. 3 Just like the prescribed level meters has it a Housing 11 to which a flange 12 is integrally formed, the on a arranged on the stub 2 counterflange 21 is attached. Between flange 12 and flange 21 an annular disk-shaped seal 121, for example from Polytetrafluoroethylene (PTFE), disposed.

On the housing 11 a is in the direction of the container 3 looking sleeve 8a formed. In the in Fig. 3 shown Embodiment, the sleeve 8a, a tube made of a Metal, such as a steel or a stainless steel, to the the housing 11 is welded. The housing 11 has a continuous oriented in the direction of the container 3 Opening 13 on whose inner diameter in the Embodiment equal to the inner diameter of the sleeve 8a. The inner diameter can also be different.

In the sleeve 8a is a rod-like antenna 7 is attached. Just as in the embodiments described above , the antenna 7 a solid cylinder 71a and a mind molded directed into the container interior transmission rod 72 and consists of a dielectric, esp. From Polytetrafluoroethylene (PTFE) or polyphenylene sulfide (PPS). The full cylinder 71a is of a container-facing enclosed portion of the sleeve 8a and in this formed by means of a to the full cylinder 71a The external thread 74 is screwed.

In an antennae-facing portion of the sleeve 8a an exciter element 5 arranged. This will Microwaves a through the opening 13 of the housing 11 and the Sleeve supplied 8a guided through coaxial line. 4

The driving member 5 is a body 51 on a base arranged transmission wire equipped with a inner conductor is coaxial 4. For this purpose a Implementation provided through the base 51st Of the Main body 51 is disc-shaped and consists of a Metal. One in antennae direction toward the front of the Body forms 51 arranged portion of the sleeve 8a together with the base body 51 a shorted Circular waveguide, in which the microwaves transmit through the wire 5 are coupled. is a front of the main body 51 Cylinder 75 of a dielectric, esp. From Polytetrafluoroethylene (PTFE) or polyphenylene sulfide (PPS) disposed of to a slot-shaped recess comprises receiving the transmitting wire.

The length of the sleeve 8a in the axial direction is preferably equal to the height of Meßgerätbefestigung, so here equal the height of the nozzle 2, or longer. The antenna 7 can then in the region of Meßgerätbefestigung no microwaves send and receive no microwaves. Also, the Transmission power by, for example between 2 and nozzle sleeve 8a befindliches condensate or moisture impaired.

8a of the mutually facing container that portion of the sleeve on the located side of the exciter element 5 has the Function of an external conductor and shields the space perception. The metallic sleeve 8a acts for microwave like a Mirror. Typically, containers are grounded. Both sleeve 8a and the Meßgerätbefestigung, here the neck 2, are preferably present at a ground potential.

In the remote container on the side of Exciter element 5 arranged portion of the sleeve 8a carried out the electrical shielding by the fact extending coaxial 4. This is through the sleeve 8a mechanically protected.

In accordance with operating with microwaves level meters the second solution variant, as in those according to the first variant of the solution, the possibility of an in the container interior directional end of the sleeve 8a beveled form. Likewise, the sleeve 8a of a protective sleeve, esp. of polytetrafluoroethylene (PTFE), be surrounded. For this example would be the geometry of the container-facing end of the solid cylinder 71 and the full cylinder-facing end of the transmission rod 72 to analog of the solid cylinder 71 and the transmission rod 72 of Fig. 1 form. The protective sleeve 9 shown in FIG. 1 would be also in the heated state on the sleeve 8a slide. Alternatively, a protective sleeve on the sleeve 8a postponed and end, eg with the transmission rod 72 of Antenna 7 to be welded. Furthermore, the The protective sleeve in an area adjacent to the housing 11 range a flange, esp. of polytetrafluoroethylene (PTFE), be formed. If such an external flange between the flange 12 and the flange 21 having clamped outer annular surface, the seal 121 omitted.