Abstract:
The invention relates to a device for measuring the quality and/or degradation of a fluid, especially an oil, comprising a sensor consisting of at least one pair of electrodes which are distanced from each other. Said sensor is immersed in the fluid which is to be measured. The electrodes and the fluid form a capacitive element whose capacity varies according to the dielectric constant of the fluid. The sensor can provide an electric output signal representing said dielectric constant. The inventive device also comprises processing means which receive the output signal and which can determine the degree of quality and/or degradation of the fluid on the basis of said output signal. The invention is characterized in that the electrodes extend substantially on the same plane and in that the fluid surrounds the two surfaces of the electrodes on both sides of said plane.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. application Ser. No. 10/542,636 filed Jul. 19, 2005, now U.S. Pat. No. 7,504,836 issued Mar. 17, 2009, which is a 371 of PCT/EP03/14955 filed Dec. 30, 2003. 
    
    
     BACKGROUND OF INVENTION 
     The present invention concerns a capacitive device for measuring the quality and/or degradation of a fluid, particularly an oil. The invention concerns in particular a device of this type for measuring the quality and/or degradation of a food oil, such as frying oil, directly in the cooking apparatus. 
     It is well known that food oils deteriorate during cooking, particularly when they are repeatedly brought to high temperatures. Typically, in order to fry food, these oils are brought to temperatures of the order of 180° C. At such temperatures a multitude of chemical reactions occur, such as polymerisations, thermo-oxidisations etc, which significantly alter the quality of the oil. The quantity of certain products of these reactions must not exceed a threshold imposed by law, since beyond such thresholds the oil is considered unfit for consumption. It is thus important to be able to detect the threshold in a reliable way in order to replace the oil as soon as it is necessary to do so. For a long time determination of the threshold was left to the cooks, who, after a visual or olfactory inspection, determined whether the oil was still fit for consumption. Of course, such a method is purely subjective and is consequently unreliable. 
     Various devices have been proposed in the prior art until now to try to overcome this problem in order to be able to measure objectively the quality and/or degradation of food oils. Since the degradation of food oils results in particular from the thermo-oxidisation thereof and this reaction produces polar compounds, devices have been envisaged in which the degree of degradation of the oil has been correlated with the dielectric constant of the oil, by measuring the capacitance of a capacitor in which the oil to be monitored forms the dielectric. 
     Such a device is disclosed for example in U.S. Pat. No. 5,818,731. This document discloses a device for measuring the quality of food oils to be mounted in a cooking apparatus such as a deep fryer. This device simultaneously monitors variations in the capacitance and optical transmission of the oil within the cooking or frying temperature range. The capacitive measuring unit comprises two sets of parallel plates interlaced with each other to define a measuring capacitor. When the sets of plates are immersed in oil, the latter forms the dielectric of the measuring capacitor for said unit and the variation in capacitance is measured by means of a direct current oscillator bridge circuit. However, this device has several drawbacks. A first drawback lies in the fact that the spaces been the plates are small and that, when the plates are plunged into the oil, the latter does not flow easily between the plates because of capillary phenomena. A regular change of the oil present between the plates is not, therefore guaranteed, which can lead to erroneous oil degradation measurement results. Moreover, solid particles present in the oil can also get caught between the plates, which has a negative effect on the measured signal. It should also be noted that, given the small space between the plates, the configuration of the capacitor in parallel plates does not allow easy access to these spaces, which makes it complex to carry out maintenance operations on the device. Another drawback lies in the fact that the capacitor with plates is bulky and occupies a significant amount of space on the cooking apparatus. Moreover, the single measuring sensor formed by the capacitor is subject to temperature variations, which can lead to erroneous capacitance measurements, such that the device has to provide means for compensating for such errors. The solution proposed in this document consists in using a temperature sensor that provides an indication to a suitable processing circuit to take account of temperature variations measured by software means integrating data relating to the oil to be monitored. Consequently, if the quality of the oil changes or if new oil is used, the software means have to be updated which makes the device inflexible to use. 
     SUMMARY OF THE INVENTION 
     It is a main object of the present invention to overcome the drawbacks of the aforementioned prior art by providing an improved device for measuring the quality and/or degradation of a fluid by a capacitive measurement, having a simple, compact and inexpensive structure. 
     It is also an object of the present invention to provide a device of this type wherein the capacitive measuring sensor has a structure facilitating the flow of the fluid to be measured in proximity to its electrodes while maintaining a high level of capacitance variation measuring sensitivity. 
     It is also an object of the present invention to provide a device of this type in which the probability of particles present in the fluid becoming caught between the electrodes of the measuring capacitor is reduced. 
     It is another object of the present invention to provide a device of this type, wherein maintenance and particularly the cleaning of the measuring sensor, is facilitated. 
     It is another object of the present invention to provide a device of this type wherein the capacitance measurement is no longer temperature dependent, while maintaining great flexibility of use, i.e. a device whose processing circuit software means do not require systematic updating when one wishes to use the device with a fluid of a different nature. 
     The invention therefore concerns a device for measuring the quality and/or degradation of a fluid, particularly an oil, comprising a sensor including at least one pair of electrodes spaced apart from each other, to be immersed in the fluid to be measured, the electrodes and the fluid forming a capacitive measuring element whose capacitance varies as a function of the dielectric constant of the fluid. The sensor is capable of providing an electrical output signal representative of said dielectric constant. The device further includes processing means receiving said output signal and capable of determining the degree of quality and/or degradation of the fluid on the basis of said output signal. This device is characterized in that said electrodes extend in substantially the same plane and in that the fluid bathes both surfaces of the electrodes, on either side of the said plane. 
     Owing to these features, 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 proximity to the electrodes can thus be constantly renewed, which improves the reliability and precision of the measurements provided by the device insofar as the latter are representative of the evolution of the quality of the fluid as a whole. This structure further greatly reduces the risk of particles becoming caught in the air gap of the capacitive element. Another advantage of the device according to the invention lies in the fact that access to the air gap is easy, which facilitates the sensor maintenance operations. It should also be noted that since the fluid bathes the surfaces of the measuring electrodes on either side of their median plane, it is possible to obtain a high level of measuring 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, to be immersed in a reference fluid, the reference electrodes and fluid forming a reference capacitive element whose capacitance varies as a function of the dielectric constant of the reference fluid of the reference fluid, said reference capacitive element being capable of providing a reference signal representative of said reference dielectric constant to the processing means and the processing means being arranged to compare the output signal with the reference signal. 
     The reference capacitive element can thus permanently measure the dielectric properties of a “new” or in other words non-degraded reference fluid, and supply a reference dielectric constant value for the fluid, which can be compared to the dielectric constant value provided by the capacitive measuring element. The use of two sensors also removes dielectric constant variations due to temperature variations. 
     When the measuring device is associated with a cooking apparatus comprising a vat containing the cooking oil, the capacitive measuring element can be immersed in the oil in which the food is fried while the reference capacitive element is immersed in another reference food oil having the same features as the cooking oil, but contained in a enclosed space insulated therefrom. 
     Preferably, the enclosed space containing the reference food oil is in thermal contact with the cooking oil. The reference food oil can of course be renewed periodically, for example once a day or, if necessary, continuously in order to provide a well defined reference dielectric constant value for a non-degraded oil. This renewal can be performed automatically or manually. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the present invention will appear in the following description of preferred embodiments of measuring devices according to the invention, given by way of non-limiting examples with reference to the annexed drawings, in which. 
         FIG. 1  is a schematic view of a first embodiment of the measuring device according to the invention; 
         FIG. 2  is a schematic cross-section of a vat of a conventional cooking apparatus to which the measuring device shown in  FIG. 1  is fixed, the latter being partially shown in cross-section along the line II-II of  FIG. 1  and the processing means having been omitted; 
         FIG. 3  shows a graph representing, as a function of temperature, the capacitance variation of a capacitive element of the measuring device according to the invention when the capacitive element is immersed in new oil and in used oil; 
         FIG. 4  is a schematic perspective cross-section of a vat of a conventional cooking apparatus to which a measuring device according to a second embodiment of the invention is fixed; 
         FIG. 5  shows a variant of the second embodiment of the measuring device according to the invention; 
         FIG. 6  shows a variant of the embodiment illustrated in  FIG. 5 ; and 
         FIG. 7  shows an example embodiment of a support structure for the capacitive elements of the device of the invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference first of all to  FIG. 1 , there is shown a first embodiment of the capacitive device for measuring the quality and/or degradation of a fluid, particularly an oil, designated by the general reference 1. 
     It will be noted that the following description will be made in an application of device  1  to the measurement of the quality and/or degradation of a food oil or similar, used for frying food in cooking apparatuses comprising a vat in which the oil can be heated typically to around 200° C. 
     Measuring device  1  comprises a sensor  2  including a pair of electrodes  4 ,  6  spaced apart from each other and to be immersed in a fluid F ( FIG. 2 ), for example the oil of a deep fryer, whose quality and/or degradation one wishes to measure to determine whether it is still fit for use. Electrodes  4 ,  6  form with oil F a capacitive measuring element EFM whose capacitance varies as a function of the dielectric constant of the oil. When the oil is degraded, the quantity of polar compounds present therein increases and causes an increase in its dielectric constant. Thus, by measuring the evolution of the capacitance of the capacitive measuring element EFM, the degree of quality and/or degradation of the oil can be determined. Sensor  2  and more specifically its capacitive element EFM, is thus capable of providing an electrical output signal representative of the dielectric constant of the oil over a wide temperature range, in particular between 20° C. and 200° C. 
     Each electrode  4 ,  6  of the pair has the shape of a comb having a plurality of teeth  4   a ,  6   a  substantially parallel to each other and extending from a base  4   b ,  6   b . Electrodes  4  and  6  are arranged in relation to each other such that teeth  4   a  of one electrode  4  are interdigited with the teeth  6   a  of the other electrode  6 . The teeth of electrodes  4  and are thus arranged in substantially the same plane. 
     It will be noted in this regard that electrodes  4  and  6  are for example formed from the same flat plate cut in a suitable manner, 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 and food grade steel (18-10 carbon based austenitic steel) having a thickness comprised between 0.1 and 3 mm. Other types of food grade steel can also be used, for example Z7CN18-09, Z3CND18-12-02, Z6CNDT17-12 and Z7CNU16-04. The plate is cut by means of a laser beam, which enables air gaps to be made between the teeth of the electrodes comprised between 10 nm and 1 mm. It is clear that the smaller the air gap E, the greater the sensitivity of the capacitive element. According to an alternative embodiment, electrodes formed of a substrate coated with a conductive material, for example a substrate coated with a layer of gold, platinum or similar, could be envisaged. 
     In the embodiment shown, electrodes  4  and  6  are secured to an insulating substrate  8 , which, in conjunction with alignment means  10 , holds the electrodes in a fixed position in relation to each other. More specifically, electrodes  4  and  6  are each fixed to substrate  8  via a fixing lug extending from their base  4   b ,  6   b , via any appropriate means, for example screws or suchlike. Alignment means  10  comprise for example positioning pins driven into substrate  8  and cooperating with holes for this purpose in electrodes  4  and  6 . 
     Substrate  8  has the shape of a frame having a central aperture  12  arranged facing the region for measuring electrodes  4  and  6 , i.e. facing air gaps defined by the spaces between teeth  4   a  of electrode  4  and teeth  6   a  of electrode  6 . Owing to this configuration, the fluid to be measured, in this case oil, bathes both faces of electrodes  4  and  6  on either side of the plane of the electrodes, such that it can circulate in proximity to teeth  4   a  and  6   a  of electrodes  4  and  6 . 
     It will be noted that capacitive element EFM is surrounded by a metal frame CM. This metal frame forms a protective screen as regards external electrical disturbance and thus reduces the influence of such disturbance during measurements. This frame is typically formed by a metal grid. 
     Substrate  8  is preferably made of a material resistant to temperatures comprised between 20° C. and 200° C. and having a low thermal expansion coefficient, such as a ceramic material. However, it can be made in any other insulating material compatible with the envisaged application of the measuring device. By way of example for a food application that has to be stable within the aforementioned temperature range, substrate  8  could also be made of a fluorinated polymer, such as Teflon. 
     In order to give an idea, the Applicant carried out conclusive tests with electrodes  4 ,  6  made of stainless steel having a thickness of the order of 0.8 mm. Electrode  4  comprised 9 teeth and electrode  6  comprised 8 teeth defining  18  air gaps of 100 μm each, the teeth having a width of the order of 1 mm. The substrate was made of ceramic material and had a thickness of the order of 0.6 mm for an external surface of 5×5 cm. 
     The capacitance of the capacitive measuring element EFM defined by electrodes  4  and  6  in combination with the oil is measured by processing means  14  comprising for example an analogue capacitance/voltage converter  16  well known to those skilled in the art and associated with a microcontroller  18 . By way of example, the capacitance/voltage converter circuit bearing the reference XE2004 marketed by the Xemics company could be used. 
     Electrodes  4  and  6  are connected to the input of circuit  16 , which delivers at output an analogue voltage signal S V  representative of the capacitance of the capacitive measuring element. Signal S V  is supplied to the input of microcontroller  18  that converts signal S V  into a digital signal S N . One could typically use a microcontroller bearing the reference 68HC11 and marketed by the Motorola company. The digital signal at the output of the microcontroller is then supplied to display means  20 , made for example in the form of a liquid crystal display or by an LED display. This latter display a digital value representative for example of the dielectric constant of the oil. According to an alternative embodiment, this digital value can be processed in an appropriate manner to indicate the polar compound rate measured in the oil. 
     Referring to  FIG. 2 , one can see how the capacitive measuring element EFM of measuring device  1  is arranged in a vat  22  of a conventional cooking apparatus  24  containing the food oil to be examined. Processing means  14  and display means  20  are omitted from this Figure. These means will be for example arranged in a case associated with the cooking apparatus but separate from vat  22 . The cooking apparatus is of course associated with heating means, which are not shown. In this embodiment, the capacitive measuring apparatus is suspended, by hooking means  26  secured to substrate  8 , from the top edge of the lateral wall of vat  22  and extending substantially parallel to said wall. 
     In  FIG. 3 , two curves C 1  and C 2  are shown, respectively illustrating the variation in capacitance C of the capacitive measuring element of device  1  as a function of temperature T for the same oil respectively new and used. “Used oil” means oil that has undergone several cooking cycles. Curve C 1  shows the capacitance variation of the capacitive measuring element when electrodes  3  and  4  are immersed in new oil, whereas curve C 2  shows the capacitance variation of the capacitive measuring element when electrodes  3  and  4  are immersed in used oil. It will be noted that these two curves generally evolve in the same way as a function of temperature and particularly for a given temperature, the difference between the capacitance measured with a new oil and the used oil is substantially constant. Consequently, the capacitance measurement of the capacitive element EFM easily enables a good oil to be distinguished from used oil within a broad temperature range. 
     With reference now to  FIG. 4 , a second embodiment of a measuring device according to the invention can be seen, in which those elements that are identical to those already described are designated by the same reference numerals. This device will also be described within an application to measurement of the quality and/or degradation of a food oil F contained in vat  22  of a cooking apparatus  24 . 
     In this embodiment, sensor  2  comprises, in addition to capacitive measuring element EFM which is immersed in the oil to be measured, a reference capacitive element EFT which is immersed in a reference oil F ref , arranged in an enclosed space to be separated from the oil to be measured. The reference oil is oil having the same features as the new oil to be measured. The structure of the reference capacitive element EFR is preferably identical to that of capacitive measuring element EFM although this is not indispensable. Reference capacitive element EFR is thus formed of electrodes  4   ref  and  6   ref  in combination with reference oil F ref . Reference capacitive element EFR is thus capable of providing a reference signal representative of the dielectric constant of the reference oil, said signal being able to be compared with the measuring output signal from the capacitive measuring element by processing means  26 . The connection of the capacitive measuring element and the reference capacitive element to processing means  26  is schematically shown in  FIG. 4 . In this example, processing means  26  typically comprise a capacitance/voltage converter  27  with three inputs and one analogue voltage output connected to a microcontroller  28 , which is in turn connected to 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 whereas 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. By way of example, the capacitance/voltage converter  27  bearing the reference XE2004 marketed by the Xemics company can be used and microcontroller  28  is of the same type as that described in conjunction with the first embodiment. 
     In the embodiment illustrated, the reference capacitive element EFR and the measuring capacitive element EFM are arranged in the vat  22  of cooking apparatus  24 . Reference capacitive element EFR is arranged in an enclosed space  32  immersed in the oil to be measured, enclosed space  32  being sealed in a watertight manner such that the reference oil contained therein does not mix with the oil to be measured contained in vat  24 . Measuring capacitive element EFM is arranged in an open-worked enclosed space  34 , for example whose walls are formed of a grid, such that it is immersed in the oil to be measured. The use of such walls forms a filter that protects the electrodes of measuring capacitive element EFM and thus prevents solid particles suspended in the oil coming into contact with the electrodes, which could interfere with the measurement. Of course, according to another variant, these walls could be omitted. 
     It will be noted that the walls of enclosed space  32  and the walls of open-worked enclosed space  34  respectively form a protective metal frame or screen against external electrical interference and thereby enable the effect of such interference to be reduced during measurements. 
     For reasons of convenience, enclosed spaces  32  and  34  are secured to each other and form a single structure  36  that is fixed to vat  22  of cooking apparatus  24 . Preferably and as illustrated in  FIG. 4 , the capacitive elements are fixed in their respective enclosed space by insulating supports cooperating with their substrate. 
     It will be noted that enclosed space  32  containing the reference oil comprises a filling channel  38 , the orifice of which can be sealed in a watertight manner by a cap or a lid (not shown). According to a variant that is not shown, enclosed space  32  can further comprise drainage means arranged in its bottom part. 
     It will also be noted that structure  36  is advantageously arranged in proximity to an inner vertical wall  22   a  of the vat, which leaves sufficient space free for cooking food while the measurements are carried out. 
     Structure  36  further comprises on one of its lateral walls, hooking means  40   a ,  40   b  for cooperating with complementary hooking means  42   a ,  42   b  secured to wall  22   a . In the example illustrated, hooking means  40  and complementary hooking means  42  respectively comprise two hooks  40   a ,  40   b ,  42   a  and  42   b  cooperating in pairs. Structure  36  can also be suspended in a removable manner inside vat  22 . It will be noted that this removable assembly of structure  36  facilitates the filling and drainage of enclosed space  32  as well as maintenance operations for capacitive elements EFM and EFR. Moreover, since structure  36  is of simple construction and in particular, does not comprise any moving mechanical parts, it is extremely reliable. 
     In this embodiment, the electrical connections between the two capacitive elements and processing means  26  arranged outside the vat occur via hooks  40   a ,  40   b ,  42   a  and  42   b . Thus hooks  40   a ,  40   b  comprises complementary contact pads  44   a ,  44   b  respectively connected to capacitive elements EFM and EFR. Hooks  42   a  and  42   b  comprise complementary contact pads  46   a ,  46   b  connected to processing means  26  and intended to come into contact with contact pads  44   a ,  44   b  when structure  26  is suspended in the vat. The electrical connection between contact pads  44   a ,  44   b  and the capacitive elements is achieved by wires passing, in a watertight manner, through the wall of enclosed space  32 . Likewise, the electrical connection between pads  46   a ,  46   b  and the processing means is achieved by wires passing, in a watertight manner, through the wall of vat  22 . Of course, according to a variant, the various connections between the capacitive elements and the processing circuit can be independent of the suspension means in vat  22 , directly connected to a separate case that may or may not comprise display means  30  and forming, with structure  36 , a portable measuring unit independent of any cooking apparatus. 
       FIG. 5  shows a variant of the second embodiment of the measuring device according to the invention, wherein the device is associated with a system for renewing the reference oil. The renewal system comprises a container  48  containing new oil and arranged outside vat  22 . Container  48  is connected to an inlet orifice of enclosed space  32  by a pipe on which are placed a pump P and a solenoid valve EV 1 . Enclosed space  32  is connected, via an outlet orifice, to a pipe provided with a solenoid valve EV 2  and opening into vat  22 . The used reference oil can thus be reinjected into vat  22  and reused. 
     It will be noted that pump P and the two solenoid valves EV 1  and EV 2  are preferably placed outside vat  22  to protect them against the effects of temperature. 
     It will also be noted that the pump could be omitted in variants wherein container  48  is placed high enough that the oil to be measured does not rise up into the enclosed space containing the reference oil when the solenoid valves are open. 
     This renewal system can advantageously be associated with means for automatically programmed means for controlling the pump and the solenoid valves. These means can typically take the form of a microprocessor. This type of system facilitates use of the measuring device. 
       FIG. 6  shows a variant of the embodiment illustrated in  FIG. 5  wherein container  48  is directly connected to an inlet orifice of enclosed space  32  by a pipe  52 . Enclosed space  32  is connected by an outlet orifice to a pipe passing through one wall of vat  22  and opening out into a recovery tank  50  arranged outside vat  22 . Container  48  is preferably formed of a watertight pouch connected to pipe  52  by a drip type flow control device  54   a  and the outlet orifice of enclosed space  32  is also associated with a flow control device  54   b  of the same type. Of course, the flow rate of the two flow control devices are identical and can advantageously be adjusted so that the volume of enclosed space  32  is renewed daily. Another advantage of this embodiment is that the reference oil is permanently sheltered from oxygen. 
     The advantage of this variant lies in the fact that it does not comprise any moving mechanical parts, which improves its reliability and facilitates maintenance. 
     With the measuring device according to the second embodiment, the reference capacitive element, which is immersed in the new oil at substantially the same temperature as the oil to be measured, in which the measuring capacitive element is arranged, enables, by a Wheatstone bridge arrangement of the two capacitive elements, variations in the dielectric constant of these elements, caused by degradation of the oil, to be distinguished from variants caused by temperature fluctuations. This type of bridge arrangement is described, for example, in the article entitled, “Application of capacitance techniques in 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 annexed claims. In particular, one could envisage arranging the capacitive elements in any orientation in the vat containing the fluid to be measured. 
     One could also envisage making single structure  36  in accordance with the configuration illustrated in  FIG. 7  wherein the enclosed spaces  32  and  34  are arranged in a more compact manner, the walls of the enclosed spaces having been omitted in order to show capacitive elements EFM and EFR. 
     One could also envisage, in the second embodiment, having an electrode common to both capacitive elements in order to limit the number of connections. 
     It goes without saying that the measuring device according to the invention that has just been described is not limited to an application for measuring food oils and could be used for measuring the quality and/or degradation of any fluid wherein the evolution of the dielectric constant is representative of its quality and/or degradation.