CA2331230A1

CA2331230A1 - Arrangement for filling level measurement

Info

Publication number: CA2331230A1
Application number: CA002331230A
Authority: CA
Inventors: Clive Van Biljon; Craig Mcintyre; Alexander Hardell; Peter Gerst
Original assignee: Endress and Hauser SE and Co KG
Current assignee: Endress and Hauser SE and Co KG
Priority date: 2000-02-17
Filing date: 2001-01-12
Publication date: 2001-08-17
Also published as: HUP0100673A3; JP2001255196A; EP1126251A2; HUP0100673A2; EP1126251A3; HU0100673D0

Abstract

An arrangement for filling level measurement is provided in which reliable filling level measurement is possible during filling or emptying of a container, which arrangement comprises: a container (1), in which a filling material (3) is located, of which the filling level (5) is to be measured, a bypass (7), which is connected to the container (1) via an upper transverse pipe (9) arranged above a highest filling level (13) to be measured, and a lower transverse pipe (11) arranged below a lowest filling level (15) to be measured, a filling level meter (21) which is arranged on the bypass (7) above the upper transverse pipe (9) and operating with microwaves, and a microwave conductor (25, 35), which during operation guides microwaves from the filling level meter (21) into the bypass (7) into a region below the uppermost transverse pipe (9) and which during operation guides to the filling level meter (21) microwaves reflected at a filling material surface.

Description

November 28, 2000 Arrangement for filling level measurement FIELD OF THE INVENTION
The invention relates to an arrangement for filling level measurement in which a filling level meter operating with microwaves is arranged on a bypass.
BACKGROUND OF THE INVENTION
In filling level measurement, microwaves are sent by means of an antenna to the surface of a filling material, and the echo waves reflected at the surface are received. An echo function representing the echo amplitudes as a function of the distance is formed and used to determine the probable useful echo and the propagation time thereof. The propagation time is used to determine the spacing between the filling material surface and the antenna.
Dielectric rod antennas and horn antennas are regularly used in industrial instrumentation for sending and/or receiving. Use is typically made of a housing exhibiting a housing section which has the geometry of a short-circuited waveguide. Inserted into the latter is an exciter element, via which microwaves are sent and/or received through the housing section. In a horn antenna, a funnel-shaped section which widens in the direction facing the container and forms the horn adjoins the housing. In the case of the rod antenna, provision is made of a rod which is made from a dielectric and points into the container.
Usually, the interior of the housing is virtually completely filled up with an insert made from a dielectric. In the case of the horn antenna, the insert November 28, 2000 has a conical end pointing into the container. In rod antennas, the rod-shaped antenna adjoins the insert.
In order to determine the filling 1_evel, it is possible to use all the known methods which permit comparably short distances to be measured by means of reflected microwaves. The best known examples are pulse radar and frequency modulation continuous wave radar (FMCW
radar) .
In pulse radar, short microwave main pulses, denoted below as wave packets, are sent periodically and are reflected from the filling material surface and received again after a propagation time dependent on spacing. The received signal amplitude as a function of time represents the echo function. Each value of this echo function corresponds to the amplitude of an echo reflected at a specific spacing from the antenna.
In the FMCW method, a continuous microwave is sent which is periodically frequency modulated in linear fashion, for example in accordance with a saw-tooth function. Consequently, with reference to the instantaneous frequency which the transmitted signal has relative to the instant of reception, the frequency of the received echo signal has a frequency difference which depends on the propagation time of the echo signal. The frequency difference between the transmitted signal and received signal, which can be obtained by mixing both signals and evaluating the Fourier spectrum of the mixed signal, thus corresponds to the spacing of the reflected surface from the antenna. Furthermore, the amplitudes of the spectral lines of the frequency spectrum obtained by Fourier transformation correspond to the echo amplitudes. This Fourier spectrum therefore represents the echo function in this case.

November 28, 2000 Filling level meters operating with microwaves are used in very many branches of industry, for example in chemistry or in food manufacturing. Typically, the filling level in a container is to be measured. Waves can form on the surface during filling and emptying of the container. It is therefore usual to provide a so-called bypass in a multiplicity of applications. A
bypass is a vertical pipe arranged directly next to the container and sealed at a lower end. The pipe is connected to the interior of_ a container by a lower transverse pipe at the height of the lowest filling level to be measured, and by an upper transverse pipe at the height of the highest filling level to be measured, or thereabove. The filling level in the interior of the bypass therefore follows the filling level in the container. However, compared with the surface of the filling material in the container, in the bypass the surface of the filling material is at rest and smooth. Thus, a very much more accurate filling level measurement can be performed in the bypass than in the container. For this purpose, a filling level meter is mounted at the top on the bypass.
The bypass acts like a waveguide for the microwaves, and the upper transverse pipe, and each further one not covered by filling material, in each case represents a source of interference signals which considerably impair the measurement.
Interference signals are produced, for example, by a proportion of the transmitted microwave energy being directly reflected at or in a transverse pipe and subsequently being received by the antenna. In addition to the microwave radiation used for filling level measurement, which is sent from the antenna directly to the filling material and reflected there directly to the antenna, interfering radiation which has covered November 28, 2000 the detour via at least one reflection at a transverse pipe also reaches the antenna. This can lead to erroneous determination of the propagation time, and thus to erroneous measurement results.
Different microwave modes propagate at different propagation rates. Consequently, a single microwave mode is preferably used for filling level measurement.
In the case of a round waveguide, this is, for example, the transverse electric 11 mode or the transverse electric Ol mode. As dictated by geometry, other boundary conditions for the propagation of microwaves apply in the region of a transverse pipe than in the remaining regions of the bypass. Because of the changed boundary conditions, the case can occur that at least a portion of the transmitted microwave radiation experiences a mode-conversion iri the region of a transverse pipe. This portion can become noticeable, for example, in the form of increased interfering radiation, and the useful signal power is reduced by this portion.
This effect is particularly marked when a transverse pipe is located in the near field of the antenna. The desired microwave mode is not yet fully formed in the near field, and is therefore particularly sensitive to a change in the boundary conditions.
SiJN~PaRY OF THE INVENTION
It is an object of the invention to specify an arrangement for filling level measurement in which reliable filling level measurement is possible during filling or emptying of the container.
For this purpose, the invention consists of an arrangement for filling level measurement which comprises:

November 28, 2000 - a container, -- in which a filling material is located, --- of which the filling level is to be measured, - a bypass, -- which is connected to the container via an upper transverse pipe arranged above a highest filling level to be measured, and a lower transverse pipe arranged below a lowest filling level to be measured, - a filling level meter which is arranged on the bypass above the upper transverse pipe and operating with microwaves, and - a microwave conductor, -- which during operation guides microwaves from the filling level meter into the bypass into a region below the uppermost transverse pipe and -- which during operation guides to the filling level meter microwaves reflected at a filling material surface.

In accordance with a development of the invention, the microwave conductor is a tubular waveguide.
In accordance with another development, the filling level meter has a horn antenna which forms the microwave conductor.
In accordance with a refinement of the last-named development, a tubular extension element is provided between the filling level meter and the horn antenna.
In accordance with a refinement, the microwave conductor has ventilation openings.
An advantage of the invention consists in that the microwave conductor covers the upper transverse pipe in each case. As a result, there are defined boundary November 28, 2000 conditions in the near zone of the antenna, and the excited mode or modes can form undisturbed under these conditions.
Reflection of microwave radiation at or in the upper transverse pipe is ruled out. Consequently, virtually all of the microwave power emitted by the antenna is available as useful signal for filling level measurement.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention and further advantages are now explained in more detail with the aid of the figures of the drawing, in which four exemplary embodiments are represented; identical parts are provided in the figures with identical reference symbols.
Fig. 1 shows an arrangement for filling level measurement with a waveguide inserted into the bypass, the filling level meter having a rod antenna;
Fig. 2 shows an arrangement for filling level measurement with a waveguide inserted into the bypass, the filling level meter having a horn antenna;
Fig. 3 shows an arrangement for filling level measurement in which the filling level meter has a horn antenna which extends into the bypass; and Fig. 4 shows the arrangement of Fig. 3, a tubular extension being provided between the horn antenna and the filling level meter.

November 28, 2000 DETAILED DESCRIPTION OF THE RAWINGS
An arrangement for filling level measurement is represented in Fig. 1. The arrangement comprises a container 1 in which a filling material is located. A
filling level 5 of this filling material 3 is to be measured. A bypass 7 is arranged next to the container 1. The bypass 7 is a vertically arranged pipe which extends above the overall filling height of the container 1.
The container 1 is connected to the bypass 7 via an upper and a lower transverse pipe 9, 11. The upper transverse pipe 9 is arranged above a highest filling level 13 to be measured, and the lower transverse pipe 11 is arranged below a lowest filling level 15 to be measured. If the container 1, is filled, for example through an opening 17 in the container lid, the filling level 5 rises in the container 1; if filling material is removed, for example through an outlet valve 19 at the bottom of the container, the filling level 5 drops in the container 1. The filling material surface is perturbed during these changes in filling level. This is represented in Fig. 1 by a wavy line which symbolizes the current filling level 5. Each change in filling level is transmitted to the bypass 7 through the lower transverse pipe 11, and pressure compensation between the container 1 and bypass 7 is performed via the upper transverse pipe 9. In a corresponding fashion, a falling level 5a in the bypass 7 exactly follows the filling level 5 in the container 1. Since the change in filling level in the bypass 7 takes place indirectly via the lower transverse pipe 11, the filling material surface in the bypass 7 is very flat by comparison with the filling material surface in the container 1. This is represented in Fig. 1 by a straight line which symbolizes the filling level 5a. Of November 28, 2000 course, further transverse pipes can be arranged between the upper and the lower transverse pipes 9, 11.
The bypass 7 preferably ends distinctly below the lowest filling level to be measured so that an echo originating from a low filling level can be distinguished clearly from an echo originating from the bottom of the bypass 7.
A filling level meter 21 operating with microwaves is arranged on the bypass 7 above the upper transverse pipe 9. The filling level meter 21 has a rod-shaped antenna 23 made from a dielectric, for example from polytetrafluoroethylene and serves the purpose of determining the filling level 5a in the bypass 7.
A microwave conductor is provided which during operation guides microwaves from the filling level meter 21 into the bypass 7 into a region below the uppermost transverse pipe 9, and which during operation guides microwaves reflected at the filling material surface back to the filling level meter 21.
In the exemplary embodiment represented in Fig. l, the microwave conductor is a tubular waveguide 25 which is inserted from above into the bypass 7. It has a flange 27 with the aid of which it is fastened to a mating flange 29 arranged on an upper end of the bypass 7. The filling level meter 21 has an external thread 31 with the aid of which it is screwed into an upper end, projecting from the bypass 7, of the waveguide 25.
In the exemplary embodiment represented in Fig. l, the microwave conductor ends shortly below the upper transverse pipe 9. In order to avoid a jump in impedance necessarily occurring at a lower end of the waveguide 25, the microwave conductor can also extend - g - EH0372-CA
November 28, 2000 as far as below the lowest filling level 15 to be measured. This is indicated in Fig. 1 by a dashed line.
Figures 2 to 4 show further exemplary embodiments of an arrangement for filling level measurement. Since the principle of the design is identical in all the exemplary embodiments, merely the differences with respect to the exemplary embodiment represented in Fig. 1 are explained in more detail below.
In the exemplary embodiment represented in Fig. 2, the filling level meter 21 has a horn antenna 33. The microwave conductor is a tubular waveguide 25 here, as well. The latter has at its upper end a flange 37 on I5 which the filling level meter 21 is fastened in such a way that the horn antenna 33 is located inside the waveguide 25.
In the exemplary embodiments of Fig. 1 and Fig. 2, a commercially available stilling well can be used as tubular waveguide 25. Stilling wells are tubular elements which are used in filling level instrumentation in order to measure the filling level in containers with a perturbed and/or uneven filling material surface. In this case, the wave tube is inserted directly into the container from above through an opening, and the filling level meter is placed onto the wave tube above. In the case of the use according to the invention of a wave tube, the latter is not inserted, as usual, into the container, but into the bypass.
The filling level meter 21 likewise has a horn antenna in the exemplary embodiment represented in Fig. 3.
35 The filling level meter 21 is mounted directly on the bypass 7 by means of a flange connection 37, and the horn antenna 35 proj ects into the bypass 7 . It extends as far as into a region below the upper transverse pipe November 28, 2000 9, and thereby forms a microwave conductor which during operation guides microwaves from the filling level meter 21 into the bypass 7 into a region below the uppermost transverse pipe 9, and which during operation guides microwaves reflected at the filling material surface back to the filling level meter 21.
Should the upper transverse tube 9 be arranged very far below the upper end of the bypass 7, it is possible, as represented in Fig. 4, for a tubular extension element 38 to be provided between the filling level meter 21 and the horn antenna 35.
In all the exemplary embodiments represented, there are provided in the respective microwave conductors ventilation openings 39 through which in the event of a change in the filling level 5a in the bypass 7 air or gas can escape from the microwave conductor or penetrate into the latter. The ventilation openings perform pressure compensation. It is thereby ensured that the same filling level prevails inside and outside the microwave conductor. Ventilation openings 39 are, for example, necessary when filling material can rise into the microwave conductor because of the spatial conditions of the measuring arrangement, and/or to ensure the aeration and ventilation via the upper transverse pipe 9 when the ventilation path is blocked by the microwave conductor.

Claims

1. An arrangement for filling level measurement which comprises:
- a container (1), -- in which a filling material (3) is located, ---of which the filling level (5) is to be measured, - a bypass (7), -- which is connected to the container (1) via an upper transverse pipe (9) arranged above a highest filling level (13) to be measured, and a lower transverse pipe (11) arranged below a lowest filling level (15) to be measured, a filling level meter (21) which is arranged on the bypass (7) above the upper transverse pipe (9) and operating with microwaves, and - a microwave conductor (25, 35), -- which during operation guides microwaves from the filling level meter (21) into the bypass (7) into a region below the uppermost transverse pipe (9) and -- which during operation guides to the filling level meter (21) microwaves reflected at a filling material surface.

2. The arrangement for filling level measurement as claimed in claim 1, in which the microwave conductor is a tubular waveguide (25).

3. The arrangement for filling level measurement as claimed in claim 1, in which the filling level meter has a horn antenna (35) and, in which the horn antenna (35) forms the microwave conductor.

4. The arrangement for filling level measurement as claimed in claim 3, in which a tubular extension element (38) is provided between the filling level meter (21) and the horn antenna (35).

5. The arrangement for filling level measurement as claimed in one of the preceding claims, in which the microwave conductor (25, 35) has ventilation openings (39).