Device for the capacitive measurement of the quality and/or deterioration of a fluid, including a capacitive sensor that is mechanically uncoupled from the element in which it is encapsulated

The invention concerns a device for the capacitive measurement of the quality and/or deterioration of a fluid, wherein the device includes a sensor encapsulated in a perforated case, wherein the sensor is connected to the encapsulation so as to be mechanically uncoupled therefrom.

This is a National Phase Application in the United States of International Patent Application No. PCT/EP2008/054745 filed Apr. 18, 2008, which claims priority on Swiss Patent Application No. 00653/07, filed Apr. 20, 2007. The entire disclosures of the above patent applications are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention concerns a device for the capacitive measurement of the quality and/or deterioration of a fluid, in particular an oil. The invention particularly concerns a device of this type that has a capacitive sensor for measuring the quality and/or deterioration of cooking oil, which is arranged directly in the cooking apparatus, and wherein the capacitive sensor, which is encapsulated in a protective case fixed in a submerged area of the vat of the cooking apparatus, is mechanically uncoupled from the element in which it is encapsulated.

BACKGROUND OF THE INVENTION

It is well know that edible oils deteriorate during cooking, particularly when they are repeatedly heated to high temperatures. These oils are typically heated to temperatures of the order of 180° C. to fry food. A multitude of chemical reactions occur at these temperatures, such as polymerisation, thermo-oxidation, etc., which significantly alter the quality of the oil. The quantity of some products of these reactions must not exceed a threshold imposed by legislation, since the oil is deemed unfit for consumption beyond the threshold. It is thus important to be able to detect the threshold in a reliable manner, so that the oil is replaced as soon as it becomes necessary. For a long time, it was left to cooks to judge, after a visual and/or olfactory inspection, whether the oil was still fit for consumption. Of course, that method is entirely subjective and is consequently unreliable.

EP Patent No, 1 588 158, which corresponds to U.S. Patent Application Publication No. US 2006/0288877 A1, discloses a device for the capacitive measurement of the quality and/or deterioration of a cooking oil to overcome these drawbacks. The content of EP Patent No, 1 588 158 and corresponding U.S. Patent Application Publication No. US 2006/0288877 A1 is incorporated herein by reference. In this device, the capacitive sensor is directly arranged in the vat of the cooking apparatus, with the sensor encapsulated in a perforated protective case, secured in a submerged area of the vat.

Although the device disclosed in that Patent Application operates satisfactorily, performing a capacitance measurement inside a deep fat fryer remains a highly delicate operation. Indeed, a variation of a few picofarads between the new oil and used oil, greatly influenced by temperature, water and impurities, is not easy to detect. Added to this is the fact that the device has to operate in a very harsh environment in which the capacitive sensor, and, possibly, the temperature sensor associated therewith, are subjected to temperatures higher than 200° C., and to shocks when careless operators strike the sensor with the baskets holding food for frying.

Protection of the capacitive sensor, and possibly, the temperature sensor associated therewith, thus constitutes an extremely important problem that needs to be addressed to avoid deterioration in measurement accuracy and/or reducing the life time of the measuring device, since the measurements are directly dependent thereon.

SUMMARY OF THE INVENTION

It is thus an object of the invention to overcome this problem by providing a device for the capacitive measurement of the quality and/or deterioration of an oil that includes a sensor, encapsulated in a perforated case, wherein the sensor is connected to the case so that it is mechanically uncoupled therefrom.

The invention mainly concerns the separation between the assembled and precisely aligned, capacitive sensor, which is relatively fragile, and the case in which it is encapsulated. The function of the case is to protect the sensor as far as possible in the following situations:

In oil, in normal use: to protect against impact from the baskets, while still allowing the oil to flow in an optimum manner, and against shocks due to any other instrument.

The mechanical uncoupling of the sensor and its encapsulating case increases the level of reliability (fewer parts under stress), extends the lifetime of the sensor and simplifies assembly. Another advantage lies in the separation of the measuring and encapsulating functions, which thus means that:one part of the system (encapsulation) can be subcontracted yet control of the sensor assembly process is maintained.alterations can be made to one of the parts without affecting the other.most of the encapsulation parts can be simplified, significantly reducing constraints thereon and hence, obviously, costs:materials: thermal stability and mechanical stress at lower temperatureslower machining tolerances, since alignment does not have to be optimal.

Thus, in accordance with a first non-limiting illustrative embodiment of the present invention, a device for the capacitive measurement of the quality and/or deterioration of a fluid is provided, wherein the device includes a sensor encapsulated in a perforated case, wherein the sensor is connected to the encapsulating case so that it is mechanically uncoupled therefrom. In accordance with a second non-limiting illustrative embodiment of the present invention, the first non-limiting embodiment is modified so that the mechanical uncoupling between the sensor and its encapsulating case is achieved by means of two strip springs that also fulfil the function of electrical contact between the sensor electrodes and contact elements of the sensor that will connect the sensor to the exterior. In accordance with a third non-limiting illustrative embodiment of the present invention, the second non-limiting embodiment is further modified so that the contact elements take the form of elastic clamps. In accordance with a fourth non-limiting, illustrative embodiment of the present invention, the second and third non-limiting embodiments are further modified so that the strip springs ensuring the mechanical separation between the sensor and its encapsulating case are made of stainless steel and have a thickness of the order of 100 microns.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

Referring first of all toFIG. 1, an embodiment of a device for the capacitive measurement of the quality and/or deterioration of a fluid, particularly oil, is shown, designated by the general reference1.

It will be noted that the following description will concern an application of a device1for measuring the quality and/or deterioration of an edible oil or similar, used for frying food in a cooking apparatus that has a vat2in which the oil can be heated, typically up to around 200° C.

Measuring device1has an encapsulated sensor4including a pair of electrodes6,8, which are spaced apart from each other and will be submerged in a fluid F (FIG. 2), for example the oil of a deep fat fryer, whose quality and/or deterioration one wishes to measure, in order to determine whether it is still fit for use. With oil F, electrodes6,8form a capacitive measuring element EFM, whose capacitance varies as a function of the dielectric constant of the oil. When the oil deteriorates, the quantity of polar components present therein increases and causes an increase in the dielectric constant thereof. Thus, by measuring the evolution of the capacitance of capacitive measuring element EFM, one can determine the degree of quality and/or deterioration of the oil. Sensor4, and more specifically its capacitive element EFM, is thus capable of providing an electrical output signal representative of the dielectric constant of the oil across a broad temperature range, in particular between 20° C. and 200° C. An electronic processing circuit10, arranged outside vat2, processes the electrical signal. Sensor4is connected to the electronic processing circuit by electric contacts4a. The sensor is, for example, secured in a removable manner underneath heating body12, via a securing and connecting support14integral therewith. Typically, sensor4can be plugged into support14via its electric contacts4, which may, for example, take the form of elastic clamps. The securing and connecting support14is connected to the electronic circuit by means of cables16which are protected, for example in tubes.

Each electrode6,8of the pair takes the form of a comb with a plurality of teeth6a,8a, which are approximately parallel to each other and extend from a base6b,8b. Electrodes6,8are arranged in relation to each other such that the teeth6aof one electrode6fit between the teeth8aof the other electrode8. The teeth of electrodes6and8are thus arranged in approximately the same plane.

It will be noted in this regard that electrodes6and8are, for example, formed from the same flat plate cut in a suitable manner, with the plate being sufficiently rigid for the electrodes to keep their shape when they are handled. In the example described, the electrodes are made from a plate of steel used for food (low carbon austenitic 18-10 stainless steel) with a thickness of between 0.1 and 3 mm. Other types of steel used for food may also be used, for example Z7CN18-09, Z3CND18-12-02, Z6CNDT17-12 and Z7CNU16-04. The plate is cut using a laser beam, which can make air gaps between the teeth of the electrodes of between 10 nm and 1 mm. It is clear that, the smaller the air gap, the greater the sensitivity of the capacitive element. According to a variant, one could also envisage making electrodes formed of a substrate coated with a conductive material, for example a substrate coated with a layer of gold, platinum or suchlike.

Electrodes6and8are arranged in a perforated encapsulating case. This case is formed of flat, perforated, metal plates18,29between which electrodes6and8extend, with two pairs of spacers22a,22band24a,24bmade of insulating material inserted at the ends, between which electrodes6,8, which form the impedimetric sensor, are sandwiched. Electrodes6,8are secured to plates18,20via spacers22a,22bat one end and are guided freely between spacers24a,24bat their other end.

The perforations in plates18,20of the encapsulating case are arranged opposite the measuring area of electrodes6and8, i.e. opposite air gaps defined by the spaces between teeth6aof electrode6and teeth8aof electrode8. Owing to this configuration, the fluid to be measured, in this case oil, bathes the other two faces of electrodes6and8on either side of the plane of the electrodes such that it can flow in proximity to teeth6aand8aof electrodes6and8.

This electrode encapsulation structure optimises the flow of oil around the two faces of the flat electrodes and, in particular creates two channels C1, C2, respectively defined between a first surface of electrodes6,8and the perforated plate18and a second surface of electrodes6,8, opposite the first surface, and perforated plate20.

Electrodes6and8are secured to the spacers by elastic means, namely two strip springs26,28, which also fulfil the function of electric contact between the electrodes and contact elements4aof sensor2.

Mechanical uncoupling of the sensor from its encapsulating case is achieved via this elastic securing method. The strips, cut, via electro-erosion, into a stainless steel sheet that is 100 microns thick, position the sensor elastically in the encapsulating case. The sensor is guided in a perpendicular direction to the plane of plates18and20by securing elements30a,30b, which are housed in bores in spacers24a,24b. A small amount of play is left so that the sensor is “free” in the perpendicular direction to the plane of plates18and20in its position, simply resting on the insulating parts.

The spacers are preferably made of a material that is resistant to temperatures of between 20° C. and 200° C. and has a low thermal expansion coefficient, such as a ceramic material. However, they can be made of any other insulating material compatible with the application envisaged for the measuring device. By way of example, for a food-related application that has to be stable within the aforementioned temperature range, the spacers could also be made of a fluoride polymer such as Teflon.