EP3679361A1

EP3679361A1 - Quality sensor

Info

Publication number: EP3679361A1
Application number: EP18769303.1A
Authority: EP
Inventors: Pierre-André GUITARD; Fagnier ARTHUR; Mamadou Saliou BALDE
Original assignee: Plastic Omnium Advanced Innovation and Research SA
Current assignee: Plastic Omnium Advanced Innovation and Research SA
Priority date: 2017-09-04
Filing date: 2018-09-04
Publication date: 2020-07-15
Also published as: WO2019043266A1; FR3070762B1; FR3070762A1

Abstract

The subject matter is a sensor (100) for sensing the quality of a medium, comprising two interdigitated electrodes (12, 14), a member (28) that is capable of determining a temperature value of a medium at the electrodes (12, 14), a flexible support (26) covered by a metal oxide layer (32) and a heating member that is comprised between the metal oxide layer and the flexible support (26). The sensor (100; 200) is capable of being attached to a vehicle tank.

Description

The invention relates to vehicle tanks.

It relates more particularly to the quality sensors of the media contained in the vehicle tanks, as part of the selective catalytic reduction (SCR) of pollutant emissions of vehicles.

This technique is used to reduce the nitrogen oxides emitted by the thermal engines of vehicles. It involves injecting a reducing agent, usually a precursor of ammonia, into the nitrogen oxides emitted during combustion in order to generate a redox reaction, transforming the nitrogen oxides into dinitrogen and in water. This process, resulting in reducing the pollutant particles emitted into the air, is in particular made necessary by the establishment of standards that require the pollution of the particles emitted into the air.

Thus, a heat engine vehicle having to meet these standards has a reservoir for containing such a reducing agent. It may for example be a medium having urea, such as the solution marketed under the name AdBlue. However, for oxidation-reduction to be effective, the reducing medium must have certain characteristics in terms of concentration and purity in particular. It must especially be liquid. However, it is known that a reducing agent such as AdBlue freezes, at ambient pressure, around -11 ° C. It is a temperature sometimes reached during the life cycle of the vehicle. However, the injection of solid AdBlue is undesirable.

To prevent the AdBlue from being in the solid state at the time of injection, the AdBlue heating is switched on before injection, at an arbitrary temperature and for an arbitrary duration. This temperature and this duration are not adapted in real time to the environmental conditions, and are therefore not optimal. AdBlue may remain strong due to too short or not powerful heating, while at other times heating an already liquid AdBlue is not necessary.

In addition, AdBlue quality sensors are used in the tanks. However, they are likely to be altered by such changes of state of the AdBlue, in particular because of the change in volume of the medium that can exert pressure on certain components of the sensor.

An object of the invention is therefore to improve the control of the contents of the tank.

According to the invention, a quality sensor of a medium, comprising:

- two interdigital electrodes;

an organ capable of determining a temperature value of a medium at the level of the electrodes;

a flexible support covered with a layer of metal oxide; and a heating element comprised between the metal oxide layer and the flexible support,

the sensor being adapted to be attached to a vehicle tank.

In case of freezing of the medium, the sensor is not altered because the electrodes are interdigitated, that is to say flat and intersecting, so that the change in volume of the medium when it changes state does not vary. the distance between the electrodes. Whether the medium is liquid, solid or gaseous, the measurement allowed by the electrodes will therefore remain reliable. In addition, the volume taken by the sensor is minimal due to the intercrossing of the electrodes and their flatness.

Since the electrodes are able to circulate an electric current, it is possible to determine a capacitance value at the terminals of the electrodes, the medium acting as a dielectric between the electrodes. In addition, the temperature being determined at the electrodes, it is relevant to couple the measured value to that of the capacitance, the two capacitance and temperature values characterizing the medium in the same place. By these two values, one is able to identify characteristics of the medium, in particular its liquid, solid or gaseous state, but also its concentration and its level of impurities, or even its composition. Depending on the results, it is then possible to adapt the heating strategy of the environment, in particular AdBlue, or even to adapt its composition. AdBlue is then put in the best possible conditions for its injection for the oxidation-reduction of nitrogen oxides. Finally, the low number and simplicity of its components, this sensor is also inexpensive, easy to manufacture and easy to fix.

By "flexible" for the support, we want to mean "easily deformable" and reversibly. In particular, the flexible nature of the support corresponds to a bending stiffness (defined as being equal to (E ^* h ^3/12 (1 -v ²⁾⁾ where E is the Young's modulus of the flexible portion measured according to ASTM D790-03, h is its thickness, and v is the fish coefficient of its constituent material) less than or equal to 1000 Nm, or even 100 Nm or even even 10 Nm and very particularly preferably, less than or equal to 1 Nm The materials corresponding to such flexible supports include plastics, metals, semi-metallic materials, or glass. Thus, thanks to this flexibility of the support, it is possible to fix the sensor in many places that do not necessarily have a perfectly flat surface.

The metal oxide covering the support makes it possible to facilitate the fixing of molecules of the medium at the level of the electrodes and thus to make more accurate the measurement of the capacity of the medium. In other words, the metal oxide layer increases the sensitivity of the sensor. The oxide is preferably non-crystalline, but rather polymorphic. The flexible nature of the oxide layer is obtained during the formation of the oxide layer by means of an electrochemical anodization process according to a method known per se. The The growth of the oxide layer and its flexibility are thus controlled as a function of the density of the electric current generated during the anodizing process. By density is meant density of current, expressed in particular in Ampere per square meter.

Preferably, the metal oxide is chosen from the following oxides:

aluminum oxide AI2O3;

TiO ₂ titanium oxide;

magnesium oxide MgO;

silica oxide SiO ₂ ; or any metal oxide whose dielectric properties vary specifically with respect to a chemical compound such as H ⁺ , OH ^" , NH ⁺ ₃ , CO, or with respect to a chemical compound comprising a methyl group or a ketone group.

Regarding the heating member, it allows, at defined time intervals, to desorb the support, that is to say, to remove the molecules attached to the support and resulting reactions between the metal oxide and the medium. This desorption is thus performed by heating the support at a temperature and for a predetermined duration. Thanks to this regular desorption, the life of the sensor is increased.

Preferably, the metal oxide layer has a porosity value of between 10 ⁹ and ^{11 11} cm ² .

Thus, this range of values is particularly effective as regards the attraction of the molecules of the medium and their attachment to the electrodes.

Advantageously, the electrodes interdigitatively cover a total area of less than 2 cm ² .

Thus, this surface corresponds to the sensitive part of the sensor. This sensitive part is therefore of small size, which makes it possible to place the sensor in places that are usually not very accessible.

Preferably, the sensor comprises a member adapted to determine a capacitance value at respective terminals of the electrodes.

Thus, this member, by measuring the characteristics of the current flowing between the electrodes via the medium, makes it possible to determine a capacitance value across the electrodes. Subsequently, this capacitance value makes it possible to find other data, such as a conductivity value and a relative permittivity value of the medium. This body can be embedded on the sensor, or connected to the latter.

According to the invention there is also provided a vehicle fluid reservoir comprising a quality sensor previously described.

Advantageously, the sensor is attached to a wall of the tank, preferably at a suction point of a pump.

Thus, the sensor is fixed closer to the place where the medium will be collected to be injected into the exhaust gas. In doing so, the data recovered at this location are the most relevant possible.

According to the invention, a vehicle comprising a reservoir according to the invention is also provided.

Preferably, the vehicle comprises a sensor without a member capable of determining a capacitance value, the vehicle further comprising a device able to determine a capacitance value.

Thus, it is the vehicle that includes this body, which can be housed anywhere in the vehicle, either in the tank at a different location than the sensor, or outside the tank. This may include an electronic control unit performing various tasks within the vehicle, including obtaining measurements of the current flowing in the sensor electrodes.

Also provided according to the invention a method of manufacturing a sensor described above, comprising a step of printing the electrodes by screen printing.

Also provided according to the invention a method of manufacturing a sensor described above, comprising a roller-like step for manufacturing the electrodes.

Also provided according to the invention a method of manufacturing a sensor described above, comprising a step of welding the support by ultrasound.

The invention also provides a method for determining a quality of a medium, comprising the implementation of the following steps:

an alternating current is circulated in two interdigital electrodes in contact with the medium;

a temperature value of the medium is determined;

an impedance data is determined;

a value of a capacitance across the electrodes is determined from the impedance data;

a permittivity value and a conductivity value are deduced from the capacitance value and a frequency value of the current; and

a characteristic value of the medium is deduced from the conductivity value, the permittivity value and the temperature value.

Thus, from data obtained by means of tests carried out upstream, and represented for example in the form of graphs illustrating characteristic values as a function of the conductivity, the permittivity and the temperature, this characteristic value of the medium can be determined. .

Advantageously, in addition, the frequency being a first frequency and the value of the capacity being a first value of the capacity,

- an alternating current is circulated with a second frequency in both electrodes;

a second value of the capacitance across the electrodes is determined; and

the permittivity value and the conductivity value of at least one of the capacitance values and at least one of the frequency values are deduced.

Thus, the same measurements are made according to two frequencies. Here again, according to data obtained by means of preliminary tests, the characteristic value of the medium is determined more precisely. At least a part of the composition of the medium can also be determined.

Preferably, in addition, the metal oxide layer is heated to desorb at least one element.

Advantageously, the characteristic value of the medium is relative to a phase state, a concentration or a level of impurities in the medium.

Thus, it is determined in particular whether the medium is liquid, solid or gaseous.

Preferably, the medium comprises a precursor of ammonia such as urea, a fuel, or water.

Advantageously, the method is implemented by means of a sensor described above.

According to the invention there is also provided a method for regulating a temperature of a medium of a vehicle tank, in which, as a function of at least one value obtained by means of a process described above, a temperature value of a medium heating member and a heating time of the medium.

Finally, according to the invention, a computer program is provided, comprising code instructions able to control the implementation of the steps of a method described above when it is executed on a computer.

We will now present an embodiment of the invention by way of non-limiting example and with reference to the drawings in which:

- Figures 1 and 2 illustrate a sensor according to a first embodiment of the invention;

- Figure 3 illustrates a portion of the sensor of Figure 1 schematically; - Figure 4 is a schematic sectional view of the sensor of Figure 3;

FIG. 5 is a circuit diagram of a first mode of implementation of a method according to the invention;

FIG. 6 illustrates a method of determining data according to this first embodiment of the invention;

FIG. 7 illustrates a diagram of a sensor according to the first embodiment implementing the method of FIG. 6;

FIGS. 8 and 9 are charts of a second mode of implementation of the invention;

FIG. 10 is an abacus of a third mode of implementation of the invention; and

- Figures 11 and 12 respectively illustrate from above and from a sensor side according to a second embodiment of the invention.

The sensor 100 of Figures 1 and 2 has a generally rectangular shape. It could of course present another form, for example circular. It is 7 millimeters wide, 22.8 millimeters long and 3 millimeters thick in this example. The sensing portion 11 of the sensor has a crossing of fingers of two electrodes 12 and 14. On this portion 11, the electrodes are so-called "interdigitated". More precisely, as can be seen schematically in FIGS. 3, 7 and 11 and 12, the electrodes 12 and 14 have, in all the embodiments, rectilinear fingers 16 and 18 orthogonal to respective straight branches 15 and 17 of each electrode 12 and 14. The fingers of the two electrodes are parallel to each other. The fingers of one of the electrodes extend at a distance and facing the fingers of the other electrode between which they are interposed. This interlacing, corresponding to the sensitive portion 11, covers a maximum area of 2 cm ² . This sensitive part is therefore of small dimensions. The branches 15 and 17 of the electrodes 14 and 12 each have a terminal 22, 24 to which any dipole can be connected, for example to carry out a measurement. In this case, as will be described below in the quality determination process, the voltage at the terminals 22 and 24 of these electrodes 14 and 12 is measured, in particular in order to deduce therefrom a capacitance value.

The electrodes of the sensor 100 are fixed on a support, or substrate 26. It is made of a plastic material and is particularly flexible. In this way, the sensor 100 can be placed on surfaces that are not completely flat, for example on the wall of a vehicle tank. In addition to these elements, a temperature sensor 28 is fixed on the support 26 and illustrated in FIGS. 1 and 3. This sensor is in this case a thermocouple and its operation will not be described in detail, since it can be any temperature sensor able to be fixed closer to the electrodes. It is preferable that the temperature measurement be as close as possible to the characteristic values of the medium 40 determined by the current flowing in the electrodes, as will be described below.

The sensor 100 is connected to an electrical circuit according to the diagram illustrated in FIG. 5. The two electrodes 12 and 14 are represented by a determined impedance capacitor Zst, the capacitor being connected in series with a conventional capacitor of known impedance. Zc1, in parallel with two known impedances Z1 and Z2 and a generator Vps of a known voltage. This assembly is called Pont de Sauty. In In this case, only the unknown impedance capacitor Zst, formed by the two electrodes 12 and 14, is fixed on the support 26. The other elements of this circuit form an electronic card to which the sensor 100 is connected and which is the species located outside the tank. Alternatively, it may be in the reservoir, for example below the support 26 or in the latter. It can therefore be either separate or integrated into the sensor.

The sensor 100 is intended to be placed in a reservoir containing a medium 40, otherwise called solution, which may be liquid, solid or gaseous, or having a mixture of some of these states, as illustrated in FIG. it is located in a tank of a solution for the selective catalytic reduction of the exhaust gases. The medium 40 contained in this reservoir here comprises a precursor of ammonia, such as urea. In this case, it is a eutectic solution of urea such as AdBlue, the composition of which is known to those skilled in the art. The AdBlue therefore acts as a dielectric between the electrodes 12 and 14 of the sensor 100. The sensor is attached to one of the inner walls of the reservoir, as close as possible to the position where a pump sucks the AdBlue. Indeed, it is the most relevant place to take measurements on the environment.

The vehicle comprising a reservoir having such a sensor, also comprises a member adapted to determine a capacitance value at the terminals 22 and 24 of the electrodes. This member may be an electronic control unit already present in the vehicle for other uses, and which therefore has the particular function of determining a capacitance value at the terminals 22 and 24, or more simply to measure a characteristic a current, such as a voltage, an intensity or a frequency. This member or another member is capable of extracting from a measured value data characteristic of the medium 40, as will be seen below. It is therefore connected to the electronic card of the sensor.

Alternatively, such a capacitance determination member may be integrated with the sensor 100 within the tank.

We will now describe a method for determining a quality of AdBlue 40 contained in the tank of the vehicle.

Via the Sauty bridge circuit of FIG. 5, the generator circulates an alternating voltage current Vps in the circuit. By measuring the output voltage Vmes at the terminals 14 and 12 of the electrodes, it is possible to obtain an impedance data item Zst by means of the first formula of FIG. 6. By "impedance data", is meant in particular the real part of the impedance, its imaginary part, the set of two or any data directly related to the impedance. The following data are made possible by the following formulas of FIG. 6 which the person skilled in the art knows and knows how to handle: the capacitance Cst is determined from the impedance Zst, the capacitance Ce between each Finger 16 and 18 of the electrodes is determined from the capacitance Cst, knowing that N is the number of fingers and L the length of the electrodes. The objective is then to obtain εκ, which is a relative permittivity value of the medium 40, again using the formulas of FIG. 6. For this, the dimensions a and b of the sensor 100 illustrated in FIG. the constants ε ₀ and ε _Γ , respectively denoting the permittivity of the vacuum and the relative permittivity of the support 26.

After having obtained εκ by means of the formulas of FIG. 8, the real part £ k 'is extracted and the imaginary part εκ "of εκ. Εκ' represents the contribution of the electric permittivity of the medium 40, while εκ" represents the contribution of the electrical conductivity of the medium 40. However, each of these parts does not behave in the same way as a function of the frequency of the current flowing in the electrodes and therefore through the AdBlue 40 which acts as a dielectric , as shown in the graph in FIG. 8. These two data make it possible to characterize the medium 40.

In the graph of measure 9, it is shown that the measured capacitance C varies as a function of the Ccon concentration in AdBlue 40, and as a function of the temperature t of AdBlue 40.

Thus, by combining the teachings of these two graphs, it is observed that the permittivity of the medium 40 and its conductivity vary each according to:

the frequency of the current flowing through the electrodes 12 and 14 in the medium 40;

the temperature of the medium 40; and

- the concentration of the medium 40.

However, since it is possible to control the frequency and determine the temperature by means of the temperature sensor 28, it becomes possible to determine the urea concentration of the AdBlue 40.

Thus, during tests, the correlations between concentration, frequency, temperature and permittivity and conductivity values are saved in a database. This database can be located in the electrical control unit of the vehicle, or directly within the sensor 100.

Then, conductivity and conductivity measurements are made as previously described, which makes it possible, at known frequency and temperature and thanks to the database, to find the concentration of AdBlue.

In conclusion, the sensor 100 thus makes it possible to determine the urea concentration value of the AdBlue contained in a tank of the vehicle.

In a second mode of implementation, the frequency of the current is varied.

Thus, a permittivity and conductivity measurement is made with a current flowing with a first frequency, and then these measurements are again performed with a second frequency. Since the permittivity and conductivity change with frequency, as shown in FIG. 8, performing these measurements with several frequencies makes it possible to increase the accuracy of the determination of the concentration.

In a third implementation mode, the state, or phase state, (solid, liquid or gaseous) of the AdBlue 40 is determined. As shown in FIG. 10 obtained during cold room tests, the voltage Vmes output measured at the terminals of the electrodes 12 and 14 can directly determine the state of the AdBlue. Curve 1 is the temperature setpoint of a cold room, curve 2 is the temperature of the AdBlue measured by means of the temperature sensor 28, and curve 3 is the output voltage Vmes measured across the electrodes 12 and 14, as illustrated in Figure 7. In this cold room, AdBlue was installed at -11 ° C, in three different states: liquid, solid, or a mixture between liquid and solid. -11 ° C is indeed the melting temperature of AdBlue at ambient pressure. However, it is observed that the measured voltage can be correlated to the AdBlue state: at less than 200 mV, the AdBlue is liquid. Between 200 and 800 mV, it has both a solid and liquid state, and above 800 mV it is solid. Thus, while the temperature and the concentration are identical, the sensor 100 makes it possible to determine the state of the AdBlue.

This data is particularly useful for AdBlue, which has to be injected into exhaust gases. Indeed, it is possible with this data to adapt the heating strategy of the AdBlue, so as to pass it to the liquid state before injection, or conversely so as to avoid unnecessary heating if the AdBlue is already in the liquid state before injection. The heating strategy of AdBlue for injection is therefore optimized.

Of course, these methods can be implemented to characterize any medium, not just the AdBlue. In particular, any fuel or water can be characterized by these processes, whether by their concentration or their state.

By such a method, based on correlations measured in the laboratory, it is possible to determine other characteristic measures of the medium. For water, it is possible to estimate a level of minerality or purity. It is necessary for this purpose to carry out measurements evoked in the laboratory and to establish correlations between all or some of them and the minerality or the purity of the water, via a calibration and calibration phase.

In general, all the characteristic measurements of the medium that can be correlated in the laboratory with measurements of voltage, frequency, temperature, capacitance, permittivity or conductivity can therefore be stored and serve to characterize a given medium by means of the sensor. and the method according to the invention. In a second embodiment 200 illustrated in FIGS. 11 and 12, there are the elements of the sensor 100, in particular the same electrical diagram, with the difference that the electrodes 12 and 14 have branches 15 and 17 connected differently with respect to the sensor. 100, and the number of these fingers is greater. Such parameters (number of fingers, space between the fingers, length of the electrodes) can of course vary from one embodiment to another of the invention, but they are taken into account in the determination of the capacity, such as mentioned above.

In addition, this sensor 200 has an aluminum oxide layer 32 Al 2 O 3 above the support 26, this layer being installed just below the interdigitated portion of the electrodes 12 and 14. This could be another metal oxide. , for example titanium oxide ΤΊΟ2, magnesium oxide MgO, silica oxide S102, or any metal oxide whose dielectric properties vary specifically with respect to a chemical compound such as H ⁺ , OH ^" , NH ⁺ ₃ , CO or with respect to a chemical compound comprising a methyl group or a ketone group It is furthermore advantageous for the porosity of the oxide chosen to be between 1 × 10 ⁹ and 1 × 10 ¹¹ cm ² . The oxide is formed by an anodizing process known to those skilled in the art, and in this process the growth of the oxide layer and its flexibility are controlled as a function of the density of the generated electric current.

This sensor 200 has, between the oxide layer 32 and the support 26, a driver 34. This driver can be of any type and will not be described in more detail.

A fourth mode of implementation of the method will now be described by means of the sensor 200, again within a tank comprising AdBlue 40. In this sensor 200, the aluminum oxide layer 26 makes it possible to facilitate the fixing urea molecules at the electrodes 12 and 14 and thus making the measurement of capacity more relevant. In other words, the metal oxide layer increases the sensitivity of the sensor. This way of increasing the attraction of certain molecules by means of a metal oxide is well known to those skilled in the art and will not be described in more detail.

In this context, the fourth embodiment comprises an additional step with respect to the previously described methods: the driver 34 is activated on command or in a predetermined manner so as to heat the metal oxide layer 32, to desorb it. This heating may for example be 5 minutes and take place every 10 months. In fact, the chemical reactions between the oxide and the AdBlue cause the fixing of molecules within the oxide which from time to time require desorption so that they are separated from the oxide. The driver 34 is therefore not comparable to the heating system of the AdBlue and it is designed only for this task. Alternatively, it is possible to imagine a global heating system at a time AdBlue and metal oxide.

Thus, this sensor 200 makes it possible to increase the accuracy of the measured values which are characteristic of the medium but may require desorption by heating from time to time, for example for 5 to 10 seconds once a year.

The quality determination method, irrespective of the characteristic value of the desired medium, the medium heating strategy, and the desorption step, can be realized automatically, by means of an electronic control unit, or integrated in the sensor. , which is distinct from the sensor. This can be an electronic control unit of the vehicle, which performs various tasks. Correlations resulting from tests which then make it possible to characterize the medium can be integrated in a database forming part of this electronic control unit, or forming part of the sensor itself.

To manufacture a sensor 100 or 200, various techniques are usable. In particular, the electrodes 12 and 14 may be printed by screen printing. Alternatively, they can be manufactured using a so-called "roll-to-roll process" technique ("reel-to-reel processing"). For their fixation on the support, it is possible to weld them ultrasonically.

Of course, we can bring to the invention many changes without departing from the scope thereof.

Claims

1. A quality sensor (100; 200) of a medium (40), comprising:

two interdigital electrodes (12, 14);

a member (28) capable of determining a temperature value of a medium (40) at the level of the electrodes (12, 14);

- a flexible support (26) covered with a layer (32) of metal oxide; and

a heating element (34) between the layer (32) of metal oxide and the flexible support (26),

the sensor (100; 200) being adapted to be attached to a vehicle tank.

2. Sensor (100; 200) according to the preceding claim, wherein the metal oxide is chosen from the following oxides:

aluminum oxide AI2O3;

TiO ₂ titanium oxide;

magnesium oxide MgO; or

SiO ₂ silica oxide.

3. The sensor (100; 200) according to any preceding claim, wherein the layer (32) of metal oxide has a porosity value between 1 .10 and 1 .10 ⁹ ¹¹ cm ^"2.

4. Sensor (100; 200) according to any one of the preceding claims, wherein the electrodes (12, 14) interdigitatively cover a total surface (11) less than 2 cm ² .

5. Sensor (100; 200) according to any one of the preceding claims, comprising a member adapted to determine a capacitance value at respective terminals (22, 24) of the electrodes (12, 14).

A vehicle fluid reservoir, comprising a quality sensor (100; 200) according to any one of the preceding claims.

7. Tank according to the preceding claim, wherein the sensor (100; 200) is attached to a wall of the tank, preferably at a suction point of a pump.

8. Vehicle comprising a tank according to the preceding claim.

9. Vehicle according to the preceding claim comprising a sensor (100; 200) according to claims 1 to 5 and a member adapted to determine a capacitance value at respective terminals of the electrodes (12, 14) of the sensor (100; 200.

10. A method of manufacturing a sensor (100; 200) according to any one of claims 1 to 5, comprising a step of printing the electrodes (12, 14) by screen printing.

11. A method of manufacturing a sensor (100; 200) according to any one of claims 1 to 5, comprising a roll-to-roll step for manufacturing the electrodes (12, 14).

12. A method of manufacturing a sensor (100; 200) according to any one of claims 1 to 5, comprising a step of welding the support (26) by ultrasound.

13. A method for determining a quality of a medium (40), comprising the implementation of the following steps:

an alternating current is circulated in two interdigital electrodes (12, 14) in contact with the medium (40);

a temperature value of the medium (40) is determined;

an impedance data is determined;

a value of a capacitance across the electrodes (12, 14) is determined from the impedance data;

14. Method according to the preceding claim, wherein in addition, the frequency being a first frequency and the value of the capacity being a first value of the capacity,

circulating an alternating current having a second frequency in the two electrodes (12, 14);

a second value of the capacitance at the terminals (22, 24) of the electrodes (12, 14) is determined; and

The method of any one of claims 13 to 14, wherein further the metal oxide layer (34) is heated to desorb at least one element.

16. A method according to any one of claims 13 to 15, wherein the characteristic value of the medium (40) is relative to a phase state, a concentration or a level of purity of the medium.

The method of any one of claims 13 to 16, wherein the medium (40) comprises a fuel, water or an ammonia precursor such as urea.

18. A method according to any one of claims 13 to 17, implemented by means of a sensor (100; 200) according to any one of claims 1 to 5.

19. A method of regulating a temperature of a medium (40) of a vehicle tank, wherein, as a function of at least one value obtained by means of a method according to any one of claims 13 to 18, is adapted a temperature value of a medium heating member and a medium heating time.

20. Computer program, comprising code instructions adapted to control the implementation of the steps of a method according to any one of claims 13 to 19 when executed on a computer.