SENSOR FOR DETECTING THE LEVEL OF A LIQUID IN A TANK AND FUEL CELL

SYSTEM WITH SUCH A SENSOR

The invention relates to a sensor for detecting the level of a liquid in a tank, having a metal housing with an opening, out of which there extends an element which is made from a dielectric material in which a detector element is embedded. The invention also relates to a fuel cell system having such a sensor.

A sensor of this type is shown by way of example in a sectional representation in Fig. 1. The sensor 1 comprises the metal housing 2 with an opening 3 at the front, out of which there extends a bar-type element 5, which comprises a detector element made from brass, which is surrounded by the dielectric material 6, e.g. a plastics sleeve, and is thus embedded therein. In addition, control and evaluation electronics 8 are arranged in the housing 2, these being in electrical contact with the housing 2 and with the detector element 7.

A sensor shown in Fig. 1 is also used in a fuel cell system, in order to be able to detect therein for example the level of water in a tank associated with the fuel cell system. The sensor 1 operates according to the capacitive principle.

The medium disposed in the tank of the fuel cell system, to which the sensor 1 is then also exposed, also contains constituents flushed out of the system, such as carbon fibers, aluminates and silicates. After a short time, these settle on the outside of the bar-type element 5 and there form a dense encrustation. This encrustation is identified by reference numerals 9 and 10.

Disadvantageous properties of these contaminants, which settle as the encrustation 9 and 10, are on the one hand that they may store water in them and on the other hand that they are electrically conductive. Thus the contaminants in the form of the encrustation 9 or 10 form a conductive bridge between the bar-type element 5 and the metal housing 2, in particular the front edge 4. This virtually forms an electrode around the tip and thus the bar-type element 5, such that the sensor 1 is virtually no longer able to recognize any water in the tank and thus does not detect the level or detects it incorrectly.

A further sensing device for detecting the level of media introduced into tanks is known from DE 100 21 059 B4, and is constructed in a similar manner to the sensor 1 shown in Fig. 1.

Furthermore, DE 38 12 687 A1 discloses a capacitive sensor for determining the level of a liquid in a tank. This sensor comprises two electrodes, which consist of titanium and are in each case surrounded with an outer layer of titanium dioxide.

It is the object of the present invention to provide a sensor with which level detection may be effected precisely, even in media comprising flushed-out constituents.

This object is achieved by a sensor comprising the features as claimed in claim 1 and a fuel cell system comprising the features as claimed in claim 8.

A sensor according to the invention for detecting the level of a liquid in a tank comprises a metal housing with an opening, out of which there extends a detector element which is surrounded by a dielectric material. The dielectric material is surrounded by a coating layer.

The dielectric material preferably comprises a ceramic layer and a coating layer disposed thereon. Both layers are electrical insulators and act as a dielectric. The coating layer has very good non-stick properties, such that it is difficult for dirt particles to become attached thereto. In this way, substantially improved functionality of the sensor may be achieved with regard to level detection. The development of contaminants into a layer on the element in the form of an electrode may thereby be prevented.

Preferably, the coating layer is a Teflon® (PTFE) coating or another equally suitable polymer coating, which contains in particular perfluorinated plastics.

In addition to its very good electrically insulating properties, the ceramic layer also acts as a coupling agent for coupling the coating layer to the detector element, which thereby adheres particularly well. The surface of the ceramic layer displays sufficient surface roughness for the coating layer to become well anchored in the indentations, which act as anchoring points. In addition, the ceramic layer has the advantage of elevated mechanical stability. Since the sensor with these layers is disposed in a moving liquid, which contains particles, the latter may act tribologically on the layer. In the case of tribological loading, the mechanically unstable polymer layer is only abraded down to the asperities of the rough ceramic layer, such that at these points the extremely stable ceramic layer comes to the surface, so providing better protection against further abrasion than the coating layer. It has been noted that the nonstick properties are also retained at these points, since residues of the coating layer remain in the indentations of the ceramic layer, which provide sufficient non-stick properties even at these damaged portions. As a result of this configuration, with a specific dielectric material the surface texture may be exploited to achieve improved properties with regard to the functional behavior of the sensor and in particular with regard to the attachment of contaminants contained in the liquid in the form of flushed-out constituents.

It has proven particularly preferable for the dielectric material in the ceramic layer to be an oxide ceramic. Precisely this material has a surface texture with which, in cooperation with the coating layer, the attachment of contaminants does not occur or occurs only to a very limited extent and the adhesion of contaminants may thus be markedly reduced. Furthermore, this configuration of such a specified dielectric material consisting of an oxide ceramic with a coating layer formed thereon is relatively easy to clear of contaminants, for example by simple rinsing.

Furthermore, in this way such a configuration of the dielectric material, preferably with an oxide ceramic, with a coating layer applied thereon, may be regarded as less hydrogen- permeable than the conventional configuration with a plastics sleeve.

In particular, the dielectric material contains an aluminum oxide ceramic or a titanium oxide ceramic in the ceramic layer. Both specific configurations of an oxide ceramic are

particularly suitable, in order to improve the functionality of the sensor and prevent the attachment of contaminants.

A fuel cell system according to the invention comprises a tank for accommodating water and a sensor according to the invention or an advantageous development thereof, which is arranged in the tank for level measurement. In particular, the fuel cell system is a mobile fuel cell system, which may be arranged in a motor vehicle. The fuel cell system comprises at least one fuel cell, preferably a fuel cell stack with a plurality of fuel cells, the fuel cell preferably taking the form of a PEM fuel cell.

An exemplary embodiment of the invention is explained in greater detail below with reference to schematic drawings, in which:

Fig. 1 is a schematic sectional representation of a sensor known from the prior art; and

Fig. 2 is a schematic sectional representation of an exemplary embodiment of a sensor according to the invention.

In the figures, elements which are the same or have the same function are provided with the same reference signs.

Fig. 2 is a sectional representation of a sensor 1 , which is designed to detect the level of a liquid in a tank. The tank and the sensor 1 are associated with a fuel cell system in a vehicle, the liquid being water and possibly containing contaminants in the form of for example carbon fibers and/or aluminates and/or silicates.

The sensor 1 comprises a housing 2 made of a metallic material, which comprises an opening 3 at the front (Fig. 1).

A bar-type element 5 extends forwards out of this opening and comprises a dielectric material 6, into which a detector element 7 is incorporated in the form of a brass tip and is surrounded by this dielectric material 6. The bar-type element 5 thus constitutes an electrode. Also arranged in the housing 2 are control and evaluation electronics 8.

The detector element 7 is surrounded by the dielectric material 6 in the form of an oxide ceramic, or embedded in this oxide ceramic outside the opening 3. The oxide ceramic may preferably be an aluminum oxide ceramic or a titanium oxide ceramic.

The dielectric material 6 is additionally covered by a coating layer 11. In the exemplary embodiment, the coating layer 11 is a PTFE (Teflon®) coating.

The coating layer 11 is formed on the dielectric material 6 in such a way that the surface texture of the oxide ceramic is displayed at least in places by the surface texture of the coating layer 11 applied thereto, without the oxide ceramic having been pressed through the coating layer 11 and thus exposed.

By configuring the sensor 1 in accordance with Fig. 2, formation of the layer or of the encrustation 9 and 10 may be at least substantially reduced.

List of reference signs

1 Sensor

2 Housing

3 Opening

4 Front edge

5 Bar-type element

6 Dielectric material

7 Detector element

8 Control and evaluation electronics

9, 10 Encrustation

11 Coating layer