Level measuring arrangement

[0002] This application claims the priority of application No. 198 10 601.7 filed in Germany on Mar. 12, 1998, the rights of priority of which are claimed and the subject matter of which is incorporated herein by reference.

[0003] The present invention relates to a level measuring arrangement for measuring the liquid level in a tank by using a microwave transmitting/receiving devices and a waveguide that extends into the tank for guiding a coupled-in microwave.

[0004] In addition to mechanical measuring methods using floaters, arrangements for measuring the liquid level in tanks are known and are used primarily in large tank plants and tank plants with floating lids. These arrangements use microwave radiation to determine the distance to the liquid, and thus the liquid level, from the echoes reflected by the liquid surface. The measuring accuracy is in the millimeter range to centimeter range.

[0005] An arrangement of the above type is shown in U.S. Pat. No. 5,136,299, in which a microwave transmitter excites a wave in a first hollow conductor segment. A specific polarization direction of the electric field vector is forcibly imposed on this wave by means of a polarizing plate. The polarized wave travels from the first hollow conductor segment via a transition piece into a measuring tube having a considerably larger diameter. The measuring tube has a plurality of openings along a casing line to allow an interchange with the surrounding liquid, so that it is ensured that the filling level inside the measuring tube is equal to the surrounding filling level. The tube also functions as a settling tube (stilling well) to balance rapid fluctuations of the liquid surface.

[0006] The measuring tube functions as a hollow conductor for the coupled-in microwave. For specific types of oscillation, the large tube cross section can provide propagation conditions with particularly low losses. However, it also allows for the propagation of various modes with potentially different propagation speeds. Depending on the excited type of oscillation, the openings serving as interruptions in the tube wall can, moreover, influence the propagation conditions, so that the signal analysis in the microwave device cannot be scaled linearly to the filling level despite the calibration markings that are provided. In particular, the propagation conditions in the measuring operation and the calibration operation can differ with the known arrangement.

[0007] It is therefore the object of the invention to specify an arrangement of the aforementioned type which, while requiring little expenditure, makes it possible to obtain an exact and in particular scalable level measuring with high precision.

[0008] With the above objects in view, the present invention resides in the provision of measuring arrangement of the above type in which the wave guide has clear and constant wave propagation characteristics for a wave with the operating frequency.

[0009] As a result of the unambiguous and constant waveguide propagation characteristics, as referred to the operating frequency, the echo delay time of the microwave signal can be imaged unambiguously and linearly on the filling level. The arrangement can be set up inexpensively since commercially available radar level sensors without mode adapter can be used.

[0010] FIG. 1 is a schematic illustration of the level measuring arrangement according to the invention;

[0011] FIG. 2aand bare cross-sectional views of waveguides according to preferred embodiments;

[0012] FIG. 3 represents a section of a waveguide schematically illustrating flow openings; and

[0013] FIG. 4 is an illustration of another level measuring arrangement according to the invention.

[0014] As shown in FIG. 1, an arrangement for measuring levels of a liquid in a tank 2 includes a microwave transmitting/receiving device 4 and a waveguide 6 that extends into liquid 8 contained by tank 2. Waveguide 6 guides a coupled-in microwave from transmitting/receiving device 4 into tank 2 and liquid 8.

[0015] The waveguide 6 according to the present invention can be designed as a hollow conductor, which is preferably dimensioned such that for an excited fundamental mode, especially a microwave signal with H₁₁wave or mode, the operation occurs in a round hollow conductor 6′ (see FIG. 2a), or for a H₁₀wave or mode in a rectangular hollow conductor 6″ (see FIG. 2b) in the unambiguous range.

[0016] The defined wave propagation characteristics for a hollow conductor are given for operating frequencies f_op, which exceed a critical propagation frequency f_c, wherein for a round hollow conductor, the critical wave length α_cfor propagation is given with the inside diameter d_cof the round hollow conductor:

[0017] d_c=α_c=c/f_c; f_c<f_op

[0018] and for a rectangular hollow conductor, the critical wavelength α_cfor propagation is given with twice the internal width a of the hollow conductor:

[0019] 2a=α_c=c/f_c; f_c<f_op.

[0020] Similarly to the known arrangement, the arrangement according to the invention can have flow openings 10 for the liquid and/or calibration markings 12 along the path of the hollow conductor as shown in FIGS. 3 and 1. In order to retain the defined propagation characteristics, the openings arranged in particular along a casing line of the hollow conductor are positioned in a plane through the center axis of the hollow conductor, which plane is rotated far enough, relative to the polarization plane for the excited wave, so that these openings influence the propagation characteristics as little as possible.

[0021] One embodiment with calibration markings 12 along a path of the hollow conductor 6 provides that these are preferably arranged in such a way that they do not significantly interfere with the wave radiated in for the level measuring. For a calibration operation, a wave having a polarization plane that is rotated relative to the level measuring operation is radiated in, in a manner known per se, which leads to unambiguous echoes as a result of the calibration markings 12. The polarization planes between level measuring and calibration operation, in particular, are perpendicular to each other. The openings for an embodiment with calibration markings and flow openings are preferably positioned such that the propagation conditions for the level measuring operation and the calibration operation are influenced equally and as little as possible. In particular, the openings can be positioned in a plane extending through the longitudinal axis of the hollow conductor. This plane is rotated by the same angle relative to the polarization planes for the level measuring on the one hand and the calibration operation on the other hand, e.g. an angle of 45°.

[0022] If the waveguide is designed as a leakage waveguide or a dielectric waveguide, output components of the conducted wave are radiated purposely, and the echoes from these radiated components are also used for the analysis.

[0023] The waveguide can be surrounded by a larger pipe 14, so that in particular with existing plants that have a pipe or settling tube 14 with large diameter, an arrangement according to the present invention can subsequently be retrofitted with a waveguide 6 that is inserted into the settling tube 14.

[0024] As shown in FIG. 4, pipe or settling tube 14 has a plurality of flow openings 16 incorporated into the course of the pipe. Waveguide 6 can be a hollow conductor with a cross-section as shown in FIG. 2a. Pipe 14 has a diameter at least that of waveguide 6.

[0025] The invention is capable of being modified and all variations and equivalents are deemed to be encompassed by the appended claims.