KR20080100773A - Capacive liquid level detector for direct methanol fuel cell systems - Google Patents

Abstract

A capacive liquid level detector for a direct methanol fuel cell system is provided to measure the liquid level with accuracy, to improve the measured capacitance and to use inexpensive detection circuit. A capacive liquid level detector for a direct methanol fuel cell system is to detect the liquid level of the store(1). The liquid has the conductivity. The capacitive liquid level detector comprises a capacitor. The capacitor has the fixed size and includes the first plate(7) insulated with the liquid and the second plate(14) formed by the liquid itself, in which the size can be varied according to the liquid level inside the store. The thin film dielectric layer(8) is formed on the surface of the first plate and is positioned between the first plate and the second plate. The first plate formed with the thin film dielectric layer on the surface is arranged to cover the opened part of the exterior wall of the store and is adhered to the store with the sealing uint(4) to seal in order to prevent the liquid leak.

Description

Capacitive liquid level detector for direct methanol fuel cell systems

The present invention relates to a direct methanol fuel cell system, and more particularly to a capacitive liquid level detector for a direct methanol fuel cell system.

A fuel cell is a galvanic cell that converts chemical reaction energy of a continuously injected fuel and oxidant into electrical energy. In general, a fuel cell consists of two electrodes separated by a membrane or electrolyte. The anode is surrounded by a flow of fuel, such as hydrogen, methane or methanol, which is oxidized here. The cathode is surrounded by a stream of oxidant such as oxygen or hydrogen peroxide, which is reduced here. Depending on the type of fuel cell, the materials used to implement one component are chosen differently.

Small direct methanol fuel cell systems are currently the focus of development for many electronics companies. Smaller direct methanol fuel cell systems are expected to replace or modify the power source of mobile electronics with longer operating times and faster charging. Direct methanol fuel cells are low temperature fuel cells that operate in the low temperature range of about 60 ° C to 120 ° C. This type of fuel cell uses a polymer membrane as the electrolyte. Methanol (CH ₃ OH), which has not been previously reformed, is fed directly to the anode with water to be oxidized at the anode. Carbon dioxide (CO ₂ ) is formed at the anode as waste gas. Atmospheric oxygen, supplied to the cathode as an oxidant, reacts with hydrogen ions (H ⁺ ) and electrons to form water. An advantage of direct methanol fuel cells is the use of liquids, which are easy to store and very cheap energy sources, for example, which can be distributed to plastic cartridges. Moreover, massively expanded infrastructures for methanol already exist in many fields, such as those used as antifreeze additives in the windshield washer solutions of vehicles. Depending on the design, this type of fuel cell can provide a power range from several mW to several 100 kW. In particular, direct methanol fuel cells are suitable for portable use as an alternative or supplement to conventional storage batteries in electrical devices. Typical fields of use for direct methanol fuel cells are in communication and power supply of notebooks.

The oxidation of methanol with the catalyst of the anode proceeds step by step, with various reaction routes having various intermediate products being discussed. In order to maintain the high efficiency of the fuel cell, it is necessary to quickly remove the reactants in the peripheral region of the anode. As a result of the temperature and the underlying chemical reaction, a liquid / gas mixture of carbon dioxide (CO ₂ ), water, water vapor and non-reacted methanol is formed. To maintain the fuel cell's self-efficiency for as long as possible, water and methanol must be removed from this liquid / gas mixture. Furthermore, carbon dioxide must be removed from this mixture equilibrium, which is done by a carbon dioxide separator. The removal of carbon dioxide from this liquid / gas mixture is for refeeding the liquid fuel mixture to the anode after controlling the methanol concentration. Separation of gases is effective by means of carbon dioxide separator.

Similarly, a liquid / gas mixture consisting of non-consumed air, water and water vapor is formed in the cathode. In order to achieve the long lasting magnetic efficiency of the fuel cell, as much water as possible must be separated from the mixture, and this water separated mixture must be fed back to the cathode cycle. To achieve this purpose, a heat exchanger is arranged downstream of the cathode outlet of the fuel cell for the purpose of cooling the mixture and concentrating the water vapor.

Downstream of the heat exchanger is an air separator arranged to separate the air flow from the liquid water in order to replenish the anode cycle with water. Therefore, various separators are mainly used in water management and to remove carbon dioxide. Conventional separators separate gas or vapor components from a phase mixture of liquid and gas or vapor components and discharge them to the environment.

Above all, a liquid storage tank is required for stable system operation. The liquid level inside such tanks should be known to the system to control the amount of water recycled.

Continuous level measurement is very beneficial for system control. This is because such level measurement makes it possible to supplement the control proportional-integral-derivative (PID) control algorithm with the control software which enables a faster and better controlled response to system changes.

Measuring liquid levels in small recepatacles is a difficult problem. Common problems are the stability and accuracy of the measured signal. Most established measurement principles can only be used in rather large tanks.

Conventional capacitive measurement principles use liquid as the dielectric between capacitor plates. Some inventions include a series of plates that precipitate in a liquid, while in others the plates are arranged outside of the reservoirs.

One of the liquid level sensors is disclosed in US Pat. No. 5522545. As the liquid rises and falls in the container, the dielectric effect of the liquid changes the effective capacitance of the sensing capacitor sensed by the electronic circuitry coupled to the liquid level sensor. One plate of sensing capacitor becomes one probe disposed inside the reservoir, while the grounded conductive portion of the reservoir becomes the second plate of the sensing capacitor.

A non-intrusive fluid level sensor is disclosed in US Patent No. 5017909 that includes a single point capacitive sensor mounted to an outer surface of the reservoir. A nonconductive container or a nonconductive window of a conductive container is used to position a sensor having an insulating plate that is not in direct contact with the liquid. The sensor assembly is disposed on the outer wall of the reservoir.

US Patent No. 5152545 and US Patent No. 5017909 both use fluid as the dielectric of the capacitor. However, the capacitance of the plate capacitor is inversely proportional to the distance between the plates, so using a fluid as a dielectric or placing the plate outside of the plate results in a large plate-to-plate distance. Therefore, these sensors exhibit disadvantages when the capacitance and capacitance variation measured are very small. Small capacitances result from small plate areas and relatively large distances between the plates. This prior art requires relatively expensive measuring electronics and the measured signal is relatively inaccurate and also affected by external electrical devices.

Sensors applied to or integrated into a container wall are disclosed in US Pat. No. 6,397,566. Container walls made of plastic or glass fibers constitute a dielectric and the conductive fluid itself constitutes a second plate. That is, the capacitor consists of the contents of the sensor plate and the container. However, in this prior art, the distance between the plates is still severe and the capacitance, since the walls of plastic or glass fibers form a dielectric and the walls must have a thickness sufficient to contain the liquid therein, i.e. at least several millimeters. And sensitivity to capacitance changes is reduced. Furthermore, no metal parts are allowed to exist between the sensor plate and the fluid, because if such metal parts are present, the capacitance of the system is determined by these metal parts. The accuracy and linearity of the measurements depend on the thickness and uniformity of the dielectric constant of the wall material over the entire area of the sensor plate. The latter is difficult to achieve with conventional plastic production methods.

SUMMARY OF THE INVENTION An object of the present invention is to provide a capacitive liquid level detector capable of measuring liquid level with improved accuracy, which is to solve the above-mentioned problems of the prior art.

Another object of the present invention is to provide a capacitive liquid level detector having an improved capacitance measured.

It is yet another object of the present invention to provide a capacitive liquid level detector that can use cheaper detection circuits.

The present invention provides a capacitive liquid level detector for detecting a liquid level in a reservoir, wherein the liquid is conductive, the capacitive liquid level detector has a capacitor, the capacitor has a fixed size and is insulated from the liquid. And a second plate formed by the liquid itself, the size of which may vary depending on the liquid level inside the reservoir, and a thin film dielectric layer is formed on the surface of the first plate and the first plate and the first plate. Located between two plates, the first plate having a thin film dielectric layer formed thereon is arranged such that it covers an open portion of the outer wall of the reservoir, and the first plate having a thin film dielectric layer formed thereon Capacitive liquid level attached to the reservoir by the sealing means to seal the reservoir to prevent liquid leakage. Start the bell detector.

By forming the thin film dielectric layer on the first plate as described above, the distance between the capacitor plates can be remarkably reduced to several micrometers corresponding to the thickness of the thin film dielectric layer formed on the first plate. Capacitance and capacitance change rise linearly as the distance between capacitor plates decreases. For example, the capacitance reached by 10 ³ times larger than that of a conventional measuring device can make the measuring signal much more stable, accurate and free from external electric fields.

Preferably, in the present invention, a thin film passivation layer is provided between the thin film dielectric layer and the second plate to prevent the liquid from penetrating into the thin film dielectric layer or penetrating the thin film dielectric layer. It can be provided.

Both the thin film dielectric layer and the passivation layer allow the first plate to be insulated from the liquid.

Preferably, in the present invention, the thin film passivation layer may be hydrophobic. This prevents the measurement error from occurring that the liquid level decreases due to the adhesion of the liquid to the surface of the thin film passivation layer.

Preferably, in the present invention, the surface of the liquid in contact with the thin film dielectric layer may form the second plate.

Preferably, in the present invention, the sealing means may cover the edges of the thin film dielectric layer formed on the first plate and attach the thin film dielectric layer to the reservoir such that the first plate does not contact the liquid. . The sealing means may also cover the edges of the thin film passivation layer formed in the thin film dielectric layer and attach the thin film passivation layer to the reservoir so that the first plate does not contact the liquid.

Preferably, in the present invention, the opened portion of the outer wall of the reservoir may be a hole formed in the side wall of the reservoir. The top of the hole may correspond to the maximum liquid level of the reservoir and the bottom of the hole may correspond to the minimum liquid level of the reservoir. The sealing means may attach the first plate having a thin film dielectric layer formed on a surface to the top edge of the hole and the bottom edge of the hole, which are open portions of the reservoir.

More preferably, in the present invention, the opened portion of the outer wall of the reservoir may be the entire sidewall of the reservoir. The sealing means may attach the first plate having the thin film dielectric layer formed on a surface to an edge of an open portion of the outer wall of the reservoir.

Preferably, in the present invention, the sealing means may be a sealant or an adhesive.

The dielectric constant of the thin film dielectric layer is preferably a constant over the entire surface of the first plate.

In the present invention, a first contact may be provided to the first plate and a second contact may be provided to the second plate. The second contact may be provided by a contact electrode having a shape that is in permanent contact with the liquid. The second contact may be provided at the bottom of the reservoir. The second contact may be implemented by a wire arranged on the inner bottom of the reservoir. Alternatively, the second contact can be realized by a metal plate provided on the inner bottom of the reservoir. The second contact may also be implemented by a wire frame provided at the inner edges of the reservoir and the wire frame may be arranged to contact the liquid in any direction of the reservoir. The second contact can also be implemented with a metal plate inserted into a bottom or one side window of the reservoir. The second contact may be implemented with multiple metal plates arranged to contact the liquid in any direction of the reservoir. The second contact may be provided at the inlet of the reservoir.

In the present invention, the first plate is preferably a polished stainless steel sheet.

In the present invention, the thin dielectric layer is preferably a layer of sputtered TiO ₂ to achieve a high capacitance, by a large dielectric constant of the TiO _2.

In the present invention, the thin film passivation layer consists of a sputtered SiO ₂ layer, a Si ₃ N ₄ layer, a SiOxNy layer, and a spincoated lacquer layer using a chemical resistant photolithography lacquer. It may be any one layer selected from the group. This ensures long term chemical stability and inhibits the penetration of the liquid into the thin film dielectric layer.

In the present invention, it is preferable that the distance between the first plate and the second plate is smaller than 70 μm, and the distance between the first plate and the second plate is even more preferably 1 μm to 10 μm.

In the present invention, the thickness of the thin film dielectric layer is preferably smaller than 50 mu m, and even more preferably, the thickness of the thin film dielectric layer is 0.5 mu m to 5 mu m.

In the present invention, the thickness of the thin film passivation layer is preferably smaller than 20 μm, and even more preferably, the thickness of the thin film passivation layer is 0.5 μm to 5 μm.

In the present invention, the thin film dielectric layer preferably has a dielectric constant of at least 20, and even more preferably the thin film dielectric layer has a dielectric constant of at least 80.

In the present invention, the reservoir is preferably a liquid storage tank or mixer of the direct methanol fuel cell system.

Further alternative embodiments of the invention may relate to dependent claims which may be supplemented individually or in combination.

The present invention provides a capacitive liquid level detector for detecting a liquid level in a reservoir, by forming a thin film dielectric layer on a first plate, whereby the distance between capacitor plates corresponds to the thickness of the thin film dielectric layer formed on the first plate. A significant reduction in micrometers can be obtained. As the distance between capacitor plates decreases, the capacitance and capacitance change rise linearly, making the measurement signal much more stable, accurate, and unaffected by external electric fields.

The invention is explained in more detail by the following examples in conjunction with the accompanying drawings.

1 shows the basic structure of a sensor according to the basic measuring principle used in the present invention.

The basic idea of the present invention is to deposit the well defined isolated thin film dielectric layer 8 on the first plate 7 corresponding to the sensor electrode using a well defined layer deposition method. The high capacitance of the capacitor is achieved by the thin thickness of the thin film dielectric layer 8, preferably between 0.5 micrometers and 5 micrometers. The use of the thin film dielectric layer 8 makes it possible to use materials with high dielectric constants, for example TiO ₂ , which in turn improves capacitance and measurement accuracy.

The capacitance of the plate capacitor is

C = ε ₀ ε _r (A / d),

here,

ε ₀ = vacuum dielectric constant

ε _r = dielectric constant of the dielectric

A = area

d = distance of capacitor plates

to be.

A thin film passivation layer 9 may be deposited in the thin film dielectric layer 8 to prevent erosion of the dielectric layer material.

In the present invention, the distance between the capacitor plates is reduced by using the liquid itself as one "liquid plate" and using the thin film layer structure as the dielectric.

The electrolyte capacitor is one invariant plate 7 corresponding to the first plate 7 having a constant area which is insulated from the liquid by the thin film dielectric layer 8, preferably by the thin film passivation layer 9 as measuring device. And a liquid plate 14 corresponding to a second plate 14 made of the liquid 6 itself, having a variable area varying with the liquid level 10. The contact electrode 5 is in contact with the liquid 6. The liquid 6 needs to be conductive. Thus, the capacitance of the electrolyte capacitor depends on the area of the liquid plate 14 and thus the liquid level 10. An electric field is formed in the area of the liquid plate 14 below the liquid level 10 when voltage is applied to the invariant plate 7 and the liquid plate 14. In order to measure the capacitance, the invariant plate 7 and the liquid plate 11 must be in contact with the power source. The invariant plate 7 may be in direct contact with the power source. The liquid plate 14 is contacted with a power source using a contact electrode 5 shaped to contact the liquid 6.

The distance between the constant plate 7 and the liquid plate 14 constituting the electrolyte capacitor as described above is the thickness of the thin film dielectric layer 8 formed on the constant plate 7 or the thin film dielectric layer 8 and the thin film passivation layer 9. By reducing to a few micrometers, which corresponds to the thickness of, the capacitance and capacitance change are increased in proportion to the reduced distance between the plates. For example, in the present invention compared to conventional measuring principles, such as shown in U.S. Patent No. 6,943,566 arc may reach higher capacitance by 10 ^three times, the measured signal is much more reliable and accurate by the external electric field It is not affected. Capacitance 100 times higher than the conventional measurement principle as above is achieved from a very small dielectric thickness of 10 μm for a tank wall thickness of 1 mm. Furthermore, a 10 times higher capacitance is achieved from the TiO ₂ dielectric constant of 86 compared to plastic dielectric constants such as ABS 4.3, PVC 3.4, where the effective dielectric constant of the layer structure is smaller than that of pure TiO ₂ . Thus low cost capacitance measurement circuits and cheap multimeters can be used. The thin film dielectric layer 8 and the thin film passivation layer 9 formed on the constant plate 7 perform the following functions. First, the thin film dielectric layer 8 and the thin film passivation layer 9 achieve high capacitance by implementing a high dielectric constant. Second, the thin film passivation layer 9 penetrates the liquid 6 into the thin film dielectric layer 8. Or to penetrate the thin film dielectric layer 8.

In order to achieve a dense, defect free thin film layer, the invariant plate 7 corresponding to the substrate preferably has a polished surface with very low roughness and is a high energy coating such as magnetron sputtering. It is appropriate to use a process.

The minimum thickness of the thin film dielectric layer 8 is determined by the doposition method, ie the uniformity of how the pinholes are formed. Preferably, the maximum thickness of the thin film dielectric layer 8 is 50 μm. At this time, the sensor having the TiO ₂ layer as the thin film dielectric layer 8 has a capacitance with respect to an area of 1 cm ² equal to 1 nF. The thickness of the thin film dielectric layer 8 is preferably in the range of 0.5 μm to 5 μm, in which case the sensor derives a capacitance of 14 nF to 140 nF for an area of 1 cm ² .

The thickness of the thin film passivation layer 9 is preferably smaller than 20 μm and even more preferably in the range of 0.5 μm to 5 μm.

The distance between the first plate 7, which is the constant plate 7, and the second plate 14, which is the liquid plate 14, is therefore preferably less than 70 μm, even more preferably in the range of 1 μm to 10 μm. Do. To set up the measurement completely in order to measure the capacitance in a simple way, it is necessary to have an AC power supply 17 supplying the voltage V at the frequency f, which is shown in FIG. Two terminals of the AC power source 17 are connected to the first plate 7 and the contact electrode 5, respectively. An AC current meter 16 is mounted in the circuit.

For positive sine waves of the AC power source 17 having a positive voltage at the contact electrode 5 and a negative voltage at the first plate 7, the positive charge is the interface between the first plate 7 and the thin film dielectric layer 8. Negative charge is induced at the interface between the second plate 14 and the thin film passivation layer 9. The resulting dipole electric field 11 is shown by the arrow in FIG.

When the AC voltage is reversed, current flows from the first plate 7 to the second plate 14 and the dipole field 11 is reversed. The flowing current is measured by AC current meter 16. The measured current I is proportional to the AC voltage V, the frequency f and the capacitance C of the sensor.

I = V * 2Πf * C

Knowing the geometrical conditions of the sensor, one can calculate the wetted area of the capacitor, ie the charge level.

Alternatively, any other method of calculating capacitance can be applied.

The reservoir 1 shown in FIG. 2 is a direct methanol fuel cell mixing tank having an air outlet 2 and a fluid inlet 3. The reservoir 1 has an open structure and consists of only three side walls, and the wall facing the wall having the air outlet 2 and the fluid inlet 3 is omitted. A layer structure having a first plate 7, a thin film dielectric layer 8 and a thin film passivation layer 9 is bonded to this open structure and to the open edges of the top and bottom walls. A sealant or adhesive 4 prevents leakage of the liquid 6 from the reservoir 1 and prevents the first plate 7 from contacting the liquid 6. Contact electrodes 5 are formed in the bottom wall and pass through the bottom wall to form contact with the surrounding environment. Preferably, the thin film dielectric layer 8 completely covers one side of the first plate 7 facing the liquid 6, and if a thin film passivation layer 9 is formed, it completely covers the thin film dielectric layer 8. It is preferable to cover. However, both the thin film dielectric layer 8 and the thin film passivation layer 9 leave the edges of the first plate 7 open. The sealing means 4 then need to be arranged to prevent the first plate 7 from contacting the liquid 6.

In order to reduce the size of the sensor, only part of one side of the reservoir 1 can be covered by the first plate 7 as shown in FIG. 3. To this end, the window 18 on one side of the storage 1 is in an open state. The upper and lower edges of the window 18 are preferably approximately equal to the permitted minimum and maximum liquid levels inside the reservoir 1, respectively. That is, the measurement range of the sensor is defined by the window 18.

The first plate 7 covered with the thin film dielectric layer 8 and the thin film passivation layer 9 is attached to the outer surface of the reservoir 1 using a sealant or adhesive 4. The advantage of this arrangement is that the sealant or adhesive 4 can be in contact with the first plate 7, which allows good mechanical attachment of the first plate 7 corresponding to the sensor electrode to be established. In addition, covering the first plate 7 with the thin film dielectric layer 8 and the thin film passivation layer 9 prevents the contact (short circuit) of the first plate 7 and the liquid 6 and the thin film dielectric layer 8 ) And the material of the first plate 7 can be prevented from being eroded by the liquid 6.

As a contact electrode in contact with the liquid 6, a wire frame 13 can be arranged inside the reservoir 1. This wire frame 13 is arranged in such a way that at least a portion of the wire frame 13 remains in contact with the liquid 6 even when the reservoir 13 is inclined toward the viewer or away from the viewer. have. The sensor therefore operates in different directions.

In order to be able to operate the sensor also in different directions, as shown in Fig. 4, a contact electrode made of multiple metal plates 5a, 5b, 5c may be employed. In FIG. 4 a first plate covered with a thin film dielectric layer and a thin film passivation layer is attached to the back side of the reservoir 1. Only the thin film passivation layer 9 is shown in FIG. 4. When multiple metal plates 5a, 5b, 5c are arranged on the left, bottom and right wall of the reservoir 1, the latter can be inclined as indicated by the arrow. By employing such contact electrodes made of such multiple metal plates 5a, 5b, 5c, the sensor can measure the level in all directions of +/- 90 degrees or less.

The reservoir 1 has an open structure on the one hand and a first plate 7 and a thin film dielectric layer 8, and optionally a thin film passivation layer 9, which are attached to the reservoir 1 by an adhesive on the other hand. It has a configured layer structure, which enables the manufacture of the detector in two different steps, thereby improving the flexibility of the manufacture.

The material of the thin film passivation layer 9 is preferably slightly hydrophobic in order to reduce the adhesion of the liquid 6 to the thin film passivation layer 9 during the process of raising and lowering the liquid level resulting in correction of the liquid level measurement. .

The first plate 7 may be designed as an entirety of one side of the reservoir 1 or a portion inserted into a window on one side of the reservoir.

The reservoir 1 can be a tank or mixing tank of a direct methanol fuel cell. The tank may consist of a polished stainless steel sheet.

The contact electrode 5 comprises a wire or metal plate arranged at the inner bottom of the reservoir 1, a wire frame 13 arranged at the inner edges of the reservoir 1 in contact with the liquid in any direction, of the reservoir 1. It can be designed as a metal plate inserted into the bottom or window 18 on one side or multiple metal plates 5a, 5b, 5c in contact with the liquid in any direction.

A thin film dielectric layer 8 may be sputtered TiO ₂ layer in order to obtain the capacitance and by the dielectric constant of the TiO _2.

The thin film passivation layer 9 is a sputtered SiO ₂ , Si ₃ N ₄ or SiO x Ny layer or chemically resistant to ensure long term chemical stability and to inhibit the infiltration of the liquid 6 into the thin film dielectric layer 8. It may be a spincoated lacquer layer using a photolithographic lacquer.

The present invention relates, in particular, to a capacitive liquid level detector for direct methanol fuel cell systems, which can be used in direct methanol fuel cell systems.

1 shows the basic structure of a sensor according to the basic measuring principle used in the present invention.

Figure 2 relates to one embodiment of the present invention, having a full sidewall of the reservoir as a capacitor.

3 illustrates another embodiment of the present invention having a wire frame as a contact electrode with a window in a reservoir.

Figure 4 shows yet another embodiment of the present invention having a contact electrode formed of multiple metal plates.

Brief description of symbols for the main parts of the drawings

1: reservoir 2: air inlet and outlet

3: fluid inlet and outlet 4: sealing means (sealant or adhesive)

5: contact electrode 6: fluid

7: first plate (constant plate) 8: thin film dielectric layer

9: thin film passivation layer 10: liquid level

11: dipole field below liquid level 12: area above liquid level

13: wire frame 14: second plate (liquid plate)

Claims (34)

A capacitive liquid level detector for detecting a liquid level in a reservoir, wherein the liquid is conductive, the capacitive liquid level detector comprises a capacitor, the capacitor having a fixed size and insulated from the liquid And a second plate formed by the liquid itself and whose size can vary depending on the liquid level inside the reservoir, A thin film dielectric layer is formed on the surface of the first plate and is located between the first plate and the second plate, The first plate having a thin film dielectric layer formed on the surface thereof is arranged so as to cover the open portion of the outer wall of the reservoir, A capacitive liquid level detector, wherein a first plate having a thin film dielectric layer formed on the surface is attached to the reservoir by sealing means to seal the reservoir to prevent liquid leakage. The method of claim 1, A capacitive liquid level detector further comprising a thin film passivation layer provided between said thin film dielectric layer and said second plate to prevent said liquid from penetrating said thin film dielectric layer or penetrating said thin film dielectric layer. . The method of claim 2, And the thin film passivation layer is hydrophobic. The method of claim 1, A capacitive liquid level detector, wherein the surface of the liquid in contact with the thin film dielectric layer forms the second plate. The method of claim 1, The sealing means covers the edges of the thin film dielectric layer formed on the first plate and attaches the thin film dielectric layer to the reservoir such that the first plate is not in contact with the liquid. The method of claim 2, Said sealing means covering the edges of a thin film passivation layer formed in said thin film dielectric layer and attaching said thin film passivation layer to said reservoir such that said first plate is not in contact with said liquid. The method of claim 1, An open portion of the outer wall of the reservoir is a hole formed in the sidewall of the reservoir. The method of claim 7, wherein A capacitive liquid level detector corresponding to an upper end of the hole corresponding to a maximum liquid level of the reservoir and a lower end of the hole corresponding to a minimum liquid level of the reservoir. The method of claim 7, wherein Said sealing means attaching said first plate having a thin film dielectric layer formed thereon to a top edge of said hole, said open portion of said reservoir, and a bottom edge of said hole. The method of claim 1, An open portion of the outer wall of the reservoir is the entirety of the sidewall of the reservoir. The method of claim 10, The sealing means attaching the first plate having the thin film dielectric layer formed on a surface to an edge of an open portion of the outer wall of the reservoir. The method of claim 1, Wherein said sealing means is a sealant or an adhesive. The method of claim 1, The dielectric constant of the thin film dielectric layer is a constant over the entire surface of the first plate. The method of claim 1, A capacitive liquid level detector provided with a first contact on the first plate and a second contact on the second plate. The method of claim 14, Wherein the second contact is provided by a contact electrode having a shape that is in permanent contact with the liquid. The method of claim 15, The second contact is provided at the bottom of the reservoir. The method of claim 16, Wherein the second contact is implemented by a wire arranged at the inner bottom of the reservoir. The method of claim 16, Wherein the second contact is implemented by a metal plate provided at an inner bottom of the reservoir. The method of claim 15, Wherein the second contact is implemented by a wire frame provided at the inner edges of the reservoir and the wire frame is arranged to contact the liquid in any direction of the reservoir. The method of claim 15, And the second contact is implemented with a metal plate inserted into a bottom or one window of the reservoir. The method of claim 15, Wherein the second contact is implemented with multiple metal plates arranged to contact the liquid in any direction of the reservoir. The method of claim 15, The second contact is provided at the inlet of the reservoir. The method of claim 1, Wherein said first plate is a polished stainless steel sheet. The method of claim 1, Wherein said thin film dielectric layer is a sputtered TiO ₂ layer. The method of claim 2, The thin film passivation layer is any one selected from the group consisting of a sputtered SiO ₂ layer, a Si ₃ N ₄ layer, a SiOxNy layer and a spincoated lacquer layer using a chemical resistant photolithography lacquer. Capacitive liquid level detector. The method of claim 1, And a distance between the first plate and the second plate is less than 70 μm. The method of claim 1, And a distance between the first plate and the second plate is between 1 μm and 10 μm. The method of claim 1, A capacitive liquid level detector having a thickness of the thin film dielectric layer less than 50 μm. The method of claim 1, Wherein the thin film dielectric layer has a thickness of 0.5 μm to 5 μm. The method of claim 2, A capacitive liquid level detector having a thickness of the thin film passivation layer of less than 20 μm. The method of claim 2, Wherein the thin film passivation layer has a thickness of 0.5 μm to 5 μm. The method of claim 1, And the thin film dielectric layer has a dielectric constant of at least 20. The method of claim 1, And the thin film dielectric layer has a dielectric constant of at least 80. The method according to any one of claims 1 to 33, The capacitive liquid level detector of claim 1, wherein the reservoir is a liquid storage tank of a direct methanol fuel cell system.

Images