Measuring the quality and/or degradation of food oil, cooking appliance

The present invention relates to the use of a capacitive measuring device of the quality and / or degradation of a fluid for measuring the quality and / or degradation of a food oil. The invention particularly relates to such a device for measuring the quality and / or degradation of cooking oil, such as a frying oil, directly into the cooking appliance.

It is well known that edible oils degrade during cooking, particularly when worn repeatedly at high temperatures. Typically, for frying foods such oils are carried out at temperatures of about 180 ° C. At these temperatures to a multitude of chemical reactions, such as polymerization, thermo-oxidation, etc., which significantly alter the quality of the oil. The quantity of certain products of these reactions should not exceed a threshold set by the legislation, because beyond that threshold oil is considered unfit for consumption. It is therefore important to detect it reliably threshold to replace the oil when necessary. For a long time it was left to the chefs who following a visual inspection and / or olfactory determined if the oil was still fit for consumption. Of course such a method is purely subjective and is therefore not reliable.

Various devices have been proposed in the prior art so far to try to address this problem in order to objectively measure power quality and / or degradation of edible oils. As the breakdown of dietary oils results in particular from their thermal oxidation and that this reaction product of the polar compounds, it was envisaged devices wherein correlating the degree of degradation of the oils to the dielectric constant of the oil, measuring the capacitance of a capacitor in which the oil to form the dielectric monitor.

Such a device is for example described in US Patent No. 5 818 731. This document discloses a device for measuring the quality of cooking oils intended to be mounted in a cooking appliance such as a fryer. This device simultaneously monitors changes in capacitance and optical transmission of the oil in the cooking temperature range or frying. The capacitance measuring unit comprises two sets of parallel plates interleaved fun in each other to define a measuring capacitor. When the sets of plates are immersed in the oil, the latter form the dielectric of the measuring capacitor of said unit and the change in capacitance is measured by a bridge circuit DC oscillator. However, this device has various disadvantages. A first drawback lies in the fact that the spaces between the plates are small and, when the plates are immersed in the oil, the latter does not easily flow between the plates due to capillary phenomena. Regular replacement of oil present between the plates is not ensured, which can lead to erroneous measurement of the degradation of the oil results. In addition, solid particles present in the oil can also be found trapped between the plates, which adversely affects the measured signal. Also note that the configuration of the parallel plate capacitor can not, given the small space between the plates, easy access to these areas which makes the complicated maintenance of the device. Another drawback lies in the fact that the plate capacitor is bulky and occupies an important place in the cooking apparatus. Furthermore, the single sensor formed by the capacitor is subject to temperature variations which can lead to erroneous measurements of capacity, so that the device must provide means to compensate for these errors. The solution proposed in this document consists in using a temperature sensor which provides an indication to a processing circuit able to take into account the temperature changes measured by software means integrating data relating to the oil to be monitored. Therefore, if the quality of the oil changes or new oil is used, it is necessary to update the software in ways which makes it very flexible device usage.

US-A-4,728,882 discloses a device for measuring the quality of a fluid comprising a sensor measuring the capacity of the fluid, the electrodes of said sensor being in a same plane (see Fig.3, 6), means signal processing and presentation of earnings (see Fig. 9, col. 9, 1.49-53 and claim 21) and a reference sensor (42), see Fig. 7 and 9. This device is suitable for liquids containing hydrocarbons or other fluids (see col. 8, 1.18-28). However, the fluid does not bathe both sides of the electrodes "on either side of said plane." 7 relates to a measuring electrode and a reference electrode fixed back to back on a support 52.

GB-A-2136130 describes a device suitable for measuring the quality of a fluid. This device includes electrodes in a comb-located in the same plane, arranged so that the fluid washes the two faces of each electrode (see Fig. 1, 2, 4, 5). It also includes a pair of reference electrodes, which can swim in another fluid (see p.3, column 2, 1.85-97 and p.7, column 1, I.1-5). The electrodes are coated with a reactive film 26 or 226 and placed on a support 22 or 222. In the embodiment of Fig. 5, this support can let the fluid flow within the electrode (see p. 7, column 1, 1.23-30). The dielectric film 24 or 224 is a common feature in the prior art. According to p.7 passage, column 1, 1.58-59, it is still optional. In this device, the fluid may come circulate "in the vicinity of the teeth of electrodes".

DE-A-19649366 discloses a sensor for measuring the quality of a fluid (for example a gasoline-alcohol mixture, column 1, 1.10-11). This device includes electrodes in a comb-located in the same plane, arranged so that the fluid washes the two faces of each electrode (see abstract, claims, column 2, 1.32-36, column 4,1.12-16 and 1.55-58). This sensor can also be used for measuring the quality / degradation of engine oil (see column 6, 1.48-50).

US-B1-6 469 521 describes a portable measuring device for measuring the quality of a food oil or engine. It includes interdigitated electrodes in the same plane and placed on a support (see Fig. 1A). The fluid to be measured can not cross the plane of the electrodes.

WO / 00 62057 discloses a cooking apparatus comprising a set of electrodes for measuring the quality of an edible oil. These electrodes are not located in the same plane (see Fig. 4).

The present invention has the main purpose of overcoming the drawbacks of the aforementioned prior art by using an improved device for measuring the quality and / or degradation of a fluid by a capacitive measurement, having a simple construction, compact and inexpensive .

The present invention also aims at the use of such a device wherein the capacitive measuring sensor has a structure allowing movement facilitated by the fluid to be measured in the vicinity of the electrodes while maintaining a high sensitivity of measuring change in the capacity.

The present invention also aims at the use of such a device in which the likelihood of particles in the fluid are trapped between the electrodes of the measuring capacitor is reduced.

The present invention also aims at the use of such a device, the maintenance, and in particular the measuring sensor cleaning is facilitated.

The present invention also aims at the use of such a device allowing to overcome the temperature dependence of the measurement of the capacity while maintaining high flexibility, that is to say a device that does not require an update systematic software updates means the processing circuit when one wishes to use the device with a different kind of fluid.

To this end, the invention relates to the use of a quality measurement device and / or degradation of a fluid, especially an oil, comprising a sensor having at least one pair of electrodes spaced from each other, intended to be immersed in the fluid to be measured, the electrodes and the fluid forming a measuring capacitive element whose capacitance varies depending on the dielectric constant of the fluid. The sensor is capable of providing an electrical output signal representative of said dielectric constant. The device further comprises processing means receiving said output signal and capable of determining the degree of quality and / or dégrada6on fluid based on said output signal. This device is characterized in that said electrodes extend substantially in the same plane and in that the fluid bathing the two faces of the electrodes, on either side of said plane.

Thanks to these characteristics, the fluid to be measured can flow easily and quickly on either side of the electrodes of the capacitive measuring element. The fluid present in the vicinity of the electrodes can be thus constantly renewed, which improves the reliability and accuracy of the measurements provided by the device to the extent that they are representative of the evolution of the quality of the fluid as a whole. This structure also serves to greatly reduce the risk that particles get stuck in the gap of the capacitive element. Another advantage of the device of the invention lies in the fact that access to the gap is easy, which facilitates maintenance operations on the sensor. Also note that as the fluid bathes the faces of the measuring electrodes on both sides of the median plane, it is possible to obtain a high measurement sensitivity.

According to a preferred embodiment of the invention, the sensor further includes a reference capacitive element, comprising at least one pair of reference electrodes spaced apart from each other, intended to be immersed in a reference fluid , the electrodes and the reference fluid forming a reference capacitive element whose capacitance varies depending on the dielectric constant of the reference fluid, said reference capacitive element being capable of providing a reference signal representative of said dielectric of reference to constant processing means and the processing means being arranged to compare the output signal with the reference signal.

The reference capacitive element can thus continuously measuring the dielectric properties of a reference fluid "new" or in other terms undegraded and provide a dielectric constant value of reference of the fluid which can be compared to the dielectric constant value provided by the capacitive measuring element. The use of two sensors also makes it possible to overcome variations of the dielectric constant due to variations in temperature.

when the measuring device is associated with a cooking appliance comprising a vessel containing the cooking edible oil, the measuring capacitive element can dip in the oil in which the foods are fried while the reference capacitive element immersed in a reference food oil having the same characteristics as cooking edible oil, but contained in an insulated enclosure thereof.

Preferably, the enclosure containing the food oil in thermal contact with the reference food cooking oil. The reference food oil can naturally be renewed periodically, for example once a day or, if necessary, continuously to provide a dielectric constant value reference to a well-defined non-degraded oil. This renewal can be performed automatically or manually.

Other features and advantages of the present invention appear from the following description of preferred embodiments of measurement devices presented by way of nonlimiting examples with reference to the accompanying drawings, in which:<ul><li>Figure 1 is a schematic view of a first embodiment of the measuring device;</li><li>Figure 2 is a schematic sectional view of a vessel of a conventional cooking apparatus on which is fixed the measuring device shown in Figure 1, the latter being partially shown in section along line 11-11 of Figure 1 and the processing means having been omitted;</li><li>3 shows a graph, depending on the temperature, the variation of the capacitance of a capacitive element of the measuring device when the capacitive member is immersed in a new oil and in used oil;</li><li>Figure 4 is a schematic sectional view and perspective view of a vessel of a conventional cooking apparatus on which is fixed a measuring device according to a second embodiment;</li><li>5 shows a variant of the second embodiment of the measuring device</li><li>6 shows an embodiment variant illustrated in Figure 5; and</li><li>7 shows an embodiment of a support structure of the capacitive elements of the device;</li></ul>

By first referring to Figure 1, there is shown a first embodiment of a capacitive measurement device of the quality and / or degradation of a particular oil fluid, designated by the general reference 1.

Note that the following description will be made in an application of the device 1 for measuring the quality and / or degradation of a food oil or the like, used for frying foods in cooking apparatus comprising a tank in which the oil may be heated typically to about 200 ° C.

The measuring device 1 comprises a sensor 2 comprising a pair of electrodes 4, 6 spaced from each other and adapted to be immersed in a fluid F (Figure 2), for example oil of a fryer, whose quality is to be measured and / or damage and determine if it is still serviceable. The electrodes 4, 6 form with oil F a capacitive measuring element EFM whose capacitance varies depending on the dielectric constant of the oil. When the oil is degraded, the amount of polar compounds present therein increases and leads to an increase in its dielectric constant. Thus, by measuring the change in capacitance of the capacitive measuring element EFM, one can determine the degree of quality and / or degradation of the oil. The sensor 2, more precisely its capacitive element EFM, is therefore capable of providing an electrical output signal representative of the dielectric constant of the oil over a wide range of temperatures, particularly between 20 ° C and 200 ° C.

Each electrode 4, 6 of the pair has the shape of a comb having a plurality of teeth 4a, 6a substantially parallel to each other and extending from a base 4b, 6b. The electrodes 4 and 6 are arranged with respect to each other so that the teeth 4a of an electrode 4 are interleaved between the teeth 6a of the other electrode 6. The teeth of the electrodes 4 and 6 are thus arranged substantially in the same plane.

Note here that the electrodes 4 and 6 are for example formed from a single cut flat plate appropriately, the plate being rigid enough so that the electrodes retain their shape when handled. In the example described, the electrodes are made from a plate of food steel (austenitic stainless steel type 18-10 low carbon) having a thickness between 0.1 and 3 mm. Other types of food steels may also be used e.g. Z7CN18-09, the Z3CND18-12-02, Z6CNDT17-12 and Z7CNU16-04. The plate is cut by means of a laser beam, to perform between the teeth of the electrodes of the air gaps between 10 nm and 1 mm. It is understood that the more the gap E, the lower the sensitivity of the capacitive element is great. According to a variant embodiment, it is also possible to realized electrodes formed of a conductive material coated with a substrate, for example a substrate coated with a gold layer of platinum or the like.

In the embodiment shown, the electrodes 4 and 6 are secured to an insulating substrate 8 which maintains, in conjunction with alignment means 10, the electrodes in a fixed position relative to each other. More specifically, the electrodes 4 and 6 are each fixed to the substrate 8 by means of a fastening tab extending from their base 4b, 6b by any suitable means, for example by screws or the like. The alignment means 10 comprise for example positioning pins driven into the substrate 8 and cooperating with holes for this purpose in the electrodes 4 and 6.

The substrate 8 has the shape of a frame having a central opening 12 arranged opposite the measurement region of the electrodes 4 and 6, that is to say facing the air gaps defined by the spaces between the teeth 4a of electrode 4 and the teeth 6a of the electrode 6. with this configuration, the fluid to be measured, in this case oil, bathe both sides of the electrodes 4 and 6 on either side of the plane of these electrodes so that it can come to circulate in the vicinity of the teeth 4a and 6a of the electrodes 4 and 6.

Note that the capacitive element EFM is surrounded by a metal frame CM. This metal frame forms a protective shield vis-à-vis external electrical interference and allows to reduce the influence of these disturbances during measurements. This frame is typically formed by a metal fence.

The substrate 8 is preferably made of a material resistant to temperatures between 20 ° C and 200 ° C and having a low coefficient of thermal expansion, such as a ceramic material. However it may be made of any other insulating material compatible with the application of the proposed measuring device. For example for a food application to be stable in the above temperature range, the substrate 8 may also be made of a fluoropolymer such as Teflon.

To fix ideas, the applicant conducted tests concluding with electrodes 4, 6 stainless steel with a thickness of 0.8mm. The electrodes 4 comprised teeth 9 and the electrode 6 with 8 teeth 18 defining gaps of 100 .mu.m each, the teeth having a width of about 1 mm. The substrate being made of ceramic and had a thickness of about 0.6 mm to an outer surface of 5x5 cm.

The capacitance of the capacitive measuring element EFM defined by the electrodes 4 and 6 in combination with the oil is measured by processing means 14 comprising for example an analog circuit capacity / voltage converter 16 is well known to the skilled person associated with a microcontroller 18. for example, the converter circuit capacity / voltage bearing the reference XE2004 marketed by the company Xemics can be used.

The electrodes 4 and 6 are connected in input to the circuit 16 which outputs an analog signal voltage S_V representative of the capacitance of the capacitive measuring element. The signal S_V is input to the microcontroller 18 which converts the signal Sv into a digital signal S_NOT. typically a microcontroller can be used with the reference 68HC11 and marketed by Motorola. The digital signal output from the microcontroller is then supplied to display means 20, produced for example in the form of a liquid crystal cell or by a LED display. The latter shows a representative numerical value for example of the dielectric constant of the oil. According to one embodiment, this numerical value can be dealt with appropriately to indicate the level of polar compounds as measured in the oil.

Referring to Figure 2, we see how the capacitive measuring element EFM of the measuring device 1 is disposed in a tank 22 of a conventional cooking apparatus 24 containing the food oil to be examined. The processing means 14 and display 20 are omitted in this figure. These will be for example arranged in a housing associated with the cooking appliance but away from the tank 22. The cooking apparatus is of course associated with heating means, not shown. In this embodiment, the capacitive measuring element is suspended by hooking means 26 fixed to the substrate 8, from the upper rim of the side wall of the vessel 22 and extending substantially parallel to this wall.

In Figure 3, two curves C1 is shown, C 2 respectively illustrating the variation of the capacitance C of the capacitive measuring element of the device 1 according to the temperature T for the same respectively new and used oil. As used here means an oil oil having undergone several cooking cycles. The curve C1 corresponds to the variation of the capacitance of the capacitive measuring element in the case where the electrodes 3 and 4 are immersed in a new oil, while the curve C2 corresponds to the capacitance variation of the capacitive element of measurement in the case where the electrodes 3 and 4 are immersed in a waste oil. It is found that these two curves generally follow the same trend as a function of temperature and in particular that for a given temperature, the difference between the measured capacitance with the fresh oil and the used oil is substantially constant. Accordingly, the measurement of the EFM capacitive element capacity easily allows to discriminate a good oil to a used oil in a wide temperature range.

Referring now to Figure 4, there is shown a second embodiment of a measuring device, wherein the elements identical to those already described are designated by les- same reference numerals. This device will also be described in the context of an application to measuring the quality and / or degradation of a food oil F contained in the tank 22 of a cooking apparatus 24.

In this embodiment, the sensor 2 includes, in addition to the capacitive measuring element EFM infused in oil to be measured, a reference capacitive element EFR bathed in a reference oil F_ref. disposed in an enclosure to be separated from the oil to be measured. Benchmark oil is an oil with the same characteristics as To- measure new oil. The structure of the FE reference capacitive element is preferably identical to that of the capacitive measuring element EFM although this is not essential. The FE reference capacitive element is formed of the electrodes 4_ref and 6_ref. in combination with F reference oil_ref. The FE reference capacitive element is therefore capable of providing a reference signal representative of the dielectric constant of the reference oil, which signal can be compared with the measurement output signal from the capacitive measuring element EFM by processing means 26. the connection of the capacitive measuring element and the capacitive element reference to the processing means 26 is shown schematically in Figure 4. in this example, the processing means 26 typically comprise a capacitance converter / voltage 27 with three inputs and an analog voltage output connected to a microcontroller 28 which is in turn connected to a display means 30. a first electrode 6, 4_ref. of each capacitive element EFM and EFR is connected to a first common input of the converter while the second electrodes 4, 6_ref. of each capacitive element EFM and EFR are respectively connected to second and third inputs of the capacitance / voltage converter. For example, the capacitance / voltage converter circuit 27 bearing the reference XE2004 marketed by the company Xemics can be used and the microcontroller 28 is the same type as that described with the first embodiment.

In the embodiment illustrated, the FE reference capacitive element and the EFM capacitive measuring element are arranged in the tank 22 of the cooking apparatus 24. The FE reference capacitive element is disposed in a closed chamber 32 bathed in oil to be measured, the chamber 32 being sealed so that the reference oil in the enclosure does not mix with the oil to be measured in the tank 24. the capacitive element measuring FSM is in turn arranged in a perforated enclosure 34, for example the walls of which are formed with a grating, so that it is immersed in the oil to be measured. The use of such walls form a filter that protects the electrodes of the capacitive measuring element EFM and in particular prevents solid particles suspended in the oil come into contact with them which could disturb the measurement . Of course, according to an alternative embodiment, these walls could be omitted.

Note that the walls of the enclosure 32 and the walls of the perforated enclosure 34 respectively form a frame or screen of metal vis-a-vis protect against external electrical disturbances and allows to reduce the influence of such disturbances during measurements.

For reasons of convenience, the speakers 32 and 34 are integral with one another and form a unitary structure 36 that is fixed to the tank 22 of the cooking apparatus 24. Preferably and as illustrated in the 4, the capacitive elements are fixed in their respective enclosure by insulating supports cooperating with their substrate.

It is noted that the enclosure 32 containing the reference oil comprises a filling channel 38, the orifice may be optionally sealed by a plug or a cover (not shown). According to a variant not shown, the enclosure 32 may further drain means provided in its lower part.

Note also that the structure 36 is advantageously disposed adjacent an inner vertical wall 22a of the tank, thereby releasing a sufficient space to cook food while performing measurements.

The structure 36 further comprises on one of its side walls of the attachment means 40a, 40b for cooperating with complementary fastening means 42a, 42b secured to the wall 22a. In the example illustrated, the fastening means 40 and the complementary fastening means 42 comprise two hooks 40a respectively 40b, 42a, 42b and cooperating in pairs. The structure 36 can thus be removably suspended within the tank 22. This removable assembly It is noted that the structure 36 facilitates the enclosure of the filling and emptying operations 32 as well as maintenance operations capacitive elements EFM and EFR. Furthermore, as the structure 36 is of simple construction and in particular has no moving mechanical parts, it has a very high reliability.

In this embodiment, the electrical connections between the two capacitive elements and the processing means 26 arranged outside of the vessel is via the hooks 40a, 40b, 42a, and 42b. To this end, the hooks 40a, 40b comprise contact pads 44a, 44b respectively connected to the capacitive elements EFM and EFR. Hooks 42a, and 42b comprise complementary contact pads 46a, 46b connected to the processing means 26 and designed to come into contact with the contact pads 44a, 44b when the structure 26 is suspended in the vessel. The electrical connection between the contact pads 44a, 44b and the capacitive elements is formed by sealingly son through the wall of the enclosure 32. Similarly, the electrical connection between the tracks 46a, 46b and the processing means is son performed by passing in sealed manner the wall of the vessel 22. of course, according to an alternative embodiment, the various connections between the capacitive elements and the processing circuit can be independent of the suspension means in the vessel 22, to be directly connected in a separate housing that may or may not include the display means 30 and forming the structure 36 independent portable measurement unit of any cooking appliance.

In Figure 5 is shown a variant of the second embodiment of the measuring device, wherein the device is associated with a reference oil renewal system. Renewal system comprises a tank 48 containing fresh oil and disposed outside the tank 22. The tank 48 is connected to the enclosure 32 by a pipe inlet on which are placed a pump P and a solenoid valve EV1. The chamber 32 is connected by an outlet to a pipe provided with a solenoid valve EV2 and opening into the tank 22. Oil Used reference can be reinjected into the well 22 and reused tank.

Note that the pump P and the two EV1 and EV2 are preferably placed outside the vessel 22 to protect against the effects of temperature.

Note again that the pump may be omitted in alternative embodiments in which the reservoir 48 is placed high enough so that the oil measure does not go back into the enclosure containing the reference oil when the valves are open.

This renewal system may advantageously be associated with means for controlling automatically and programmed the pump and solenoid valves. These means may typically take the form of a microprocessor. Such a system facilitates the operation of the measuring device.

In Figure 6 is shown a variant of the embodiment shown in Figure 5 in which the tank 48 is directly connected to the inlet chamber 32 via a pipe 52. The chamber 32 is connected through an orifice output to a pipe passing through a wall of the tank 22 and opening into a recovery tank 50 disposed outside the tank 22. the tank 48 is preferably formed of a sealed pocket connected to the pipe 52 by a flow device-controlled drip type drop 54a and the outlet of the enclosure 32 is also associated with a flow control device 54b of the same type. Of course the flow rates of the two flow control devices are identical and can advantageously be adjusted so that the volume of the enclosure 32 is renewed daily. Another advantage of this variant is that the reference oil is maintained at all times in the absence of oxygen.

The advantage of this variant resides in the fact that it does not include moving mechanical parts, thereby improving its reliability and easy maintenance.

With the measuring device according to the second embodiment, the capacitive reference element which is immersed in a fresh oil at substantially the same temperature as the oil to be measured in which is arranged the measuring capacitive element allows, by performing a Wheatstone bridge of the two capacitive elements, to distinguish the variations of the dielectric constant of these elements, from the degradation of the oil, to variations of temperature fluctuations. Such a bridge circuit is described for example in the article entitled "Application of technology in capacitance sensor design" by Willem Chr. Heerens published in J. Phys. E: Scientific Instruments 19: 897 906 (1986), which is incorporated in this application by reference.

It will be understood that various modifications and / or improvements evident to those skilled in the art can be made to the embodiment described in the present description without departing from the scope of the present invention defined by the appended claims. In particular may be considered to have a capacitive elements any orientation in tank containing the fluid to be measured.

We can also envisage making the unitary structure 36 according to the configuration shown in Figure 7 in which the speakers 32 and 34 are arranged more compactly, the walls of the speakers being omitted to reveal the capacitive elements EFM and EFR.

One could also envisage in the second embodiment of arranging a common electrode to the two capacitive elements in order to limit the number of connections.

It goes without saying that the measuring device according to the invention which has just been described is not limited to application to the measurement of food oils and can be used for measuring the quality and / or degradation any fluid which changes its dielectric constant is representative of the quality and / or degradation.