Moisture sensor element, method for producing a moisture sensor element, moisture or dew point sensor and moisture-measuring method

Abstract

In order to permit a robust, energy-efficient and precise moisture sensor, the invention relates to a moisture sensor element (10) for a moisture sensor (12) for measuring a moisture content in a gas, comprising at least one vibrating element (14) and at least one material (16, 18) on the vibrating element (14), wherein the at least one material (16, 18) is designed in such a way that the mass thereof changes rapidly with moisture changing over a moisture value. The invention also relates to a moisture-measuring method for measuring a moisture in a gas, comprising: using a moisture sensor element (10), wherein the course of the measurement signal thereof has at least one non-linearity according to the moisture; and determining a reference value based on the at least one non-linearity.

Claims

1. A moisture sensor element for a moisture sensor for measuring a moisture content in a gas, comprising: at least one vibrating element; and at least one material on the vibrating element, wherein a mass of the at least one material changes in response to a moisture value, wherein the at least one material is a porous material, and deviation of pore sizes of pores from an average pore size of the porous material is chosen such that the mass changes in response to the moisture value.

2. The moisture sensor element according to claim 1, characterized in that a first material with a first mass that changes in response to a first moisture value and a second material with a second mass that changes in response to a second moisture value are provided.

3. The moisture sensor element according to claim 2, characterized in that the first material and the second material are disposed 3.1 on a same side or on opposite sides of the at least one vibrating element; 3.2 on a same region or on different regions of the at least one vibrating element; 3.3 side by side, one above the other or in an annular or surrounding arrangement; and/or 3.4 on different arms or on a same arm of a vibrating element designed as a tuning fork.

4. The moisture sensor element according to claim 1, characterized in that the at least one material is a defined-porous material.

5. The moisture sensor element according to claim 2, characterized in that the first and the second material differ 5.1 by their average pore diameter; and/or 5.2 by their wettability; and/or 5.3 by being provided in quantities which contain a substantially different pore volume; and/or 5.4 by their capability to absorb light or infrared or heat radiation.

6. The moisture sensor element according to claim 1, characterized by at least one of the following features: 7.1 that the at least one material is chosen from the group consisting of microporous, mesoporous and macroporous materials; 7.2 that at least a part of the at least one material is treated with a surface-derivatizing reagent to change wettability; 7.3 that at least a part of the at least one material is treated with an organosilane or organosiloxane as a surface-derivatizing reagent to change the wettability; 7.4 that at least a part of the at least one material is provided or treated with an additive to change a radiation absorption capacity towards light or infrared or heat radiation; or 7.5 that the at least one vibrating element is designed as a quartz crystal microbalance and/or a quartz crystal tuning fork.

7. A method for producing the moisture sensor element according to one of the preceding claims, comprising: 8.1 providing a vibrating element; and 8.2 coating the vibrating element with at least one material, a mass of which changes in response to a moisture value, wherein step 8.2 includes: 8.2.1 applying a precursor material to the vibrating element; and 8.2.2 creating pores in the precursor material to form the material, the mass of which changes in response to a moisture value.

8. A moisture sensor for determining a moisture content in a gas, comprising: a housing with at least one opening; the moisture sensor element according to claim 1 disposed inside the housing; and an electronic component for driving the vibrating element and providing a measurement signal.

9. The moisture sensor according to claim 8, characterized in that the housing 10.1 comprises a temperature control element for influencing temperature of the at least one material; and/or 10.2 comprises a heating element for influencing the temperature of the at least one material; and/or 10.3 a light, infrared or heat radiation source for irradiating the at least one material with light, infrared or heat rays; and/or 10.4 an LED component for irradiating the at least one material with light, infrared or heat rays; and/or 10.5 is metallic; and/or 10.6 has a planar surface vis-a-vis the at least one vibrating element; and/or 10.7 includes a thermal mass which is more than 1000 times higher than a thermal mass of the at least one material applied to the at least one vibrating element; and/or 10.8 is closed at its opening with a water vapor-permeable but water-repellent membrane.

10. A dew point sensor for determining a dew point in a gas, comprising the moisture sensor according to claim 8, wherein the electronic component is designed to provide a measurement signal that allows the dew point to be determined.

11. A moisture measuring method for measuring moisture in a gas, comprising: using the moisture sensor element according to claim 1, wherein a measurement signal response thereof has at least one non-linearity; and determining a reference value based on the at least one non-linearity.

12. The moisture measuring method according to claim 11, characterized by: using the moisture sensor element, wherein the measurement signal response thereof has a first non-linearity and a second non-linearity according to moisture; and determining the reference value based on a comparison of the first non-linearity and the second non-linearity.

13. The moisture measuring method according to claim 11, characterized by at least one of the following steps: 15.1 using two or more changes of vibration behavior of the at least one vibrating element for reference value determination and/or calibration; 15.2 heating the at least one vibrating element and detecting one or more changes in frequency of the vibrating element during the heating; or 15.3 repeatedly heating the at least one vibrating element and detecting changes in frequency of the at least one vibrating element during a cooling or cold phase between two heating cycles, to detect frequency decrease as a number of heating/cooling phases decreases, wherein the moisture value or a dew point is determined from the frequency decrease.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) One embodiment of the invention will be described in more detail with reference to the attached drawings wherein it is shown by:

(2) FIG. 1 a perspective view of a first embodiment of a sensor element for a moisture sensor for measuring a moisture content in a gas, the sensor element comprising at least one vibrating element and at least a first material and a second material on the vibrating element;

(3) FIG. 2 a perspective view of a second embodiment of the sensor element;

(4) FIG. 3a a top view of a third embodiment of the sensor element;

(5) FIG. 3b a lateral view of the sensor element of FIG. 3a;

(6) FIG. 4a a top view of a fourth embodiment of the senor element;

(7) FIG. 4b a lateral view of the sensor element of FIG. 4a;

(8) FIG. 5a a top view of a fifth embodiment of the sensor element;

(9) FIG. 5b a lateral view of the sensor element of FIG. 5a;

(10) FIG. 6a a top view of a sixth embodiment of the sensor element;

(11) FIG. 6b a lateral view of the sensor element of FIG. 6a;

(12) FIG. 7 a diagram showing a frequency signal of a vibrating element according to one of the embodiments of FIGS. 1 to 6b together with a voltage for driving a heating element for heating the vibrating element, plotted over time;

(13) FIG. 8 a perspective view of one embodiment of a moisture sensor with a housing and an opening of the housing, in the interior of which one of the sensor elements of FIGS. 1 to 6b is accommodated;

(14) FIG. 9a a perspective view of another embodiment of the moisture sensor in which the opening is closed by a gas-permeable but water-repelling membrane;

(15) FIG. 9b a section through an area of the opening of the sensor of FIG. 8a; and

(16) FIG. 10 a section through a housing of another embodiment of the moisture sensor; and

(17) FIG. 11 a linear characteristic curve of a moisture sensor according to prior art compared to a characteristic curve with at least one non-linearity of embodiment of the moisture sensor.

DETAILED DESCRIPTION

(18) FIGS. 1 to 6b show various embodiments of a sensor element 10 for a moisture sensor 12. The sensor element 12 has at least one vibrating element 14 that can be made to vibrate. On the at least one vibrating element 14, a first material 16 and a second material 18 are provided. In a preferred embodiment, at least one of the materials 16, 18 is designed in such a way that a mass changes rapidly in response to moisture changing over a range of a moisture value.

(19) FIGS. 8 to 10 show various embodiments of a moisture sensor 12 having a housing 500 with an opening 510. At least one vibrating element, preferably at least one vibrating element 14 of the type shown in FIGS. 1 to 6b, is accommodated in the housing 500. Further, the moisture sensor 12 comprises an electronic component not shown in the drawings and a temperature control element 20 for temperature control of the vibrating element 14. The opening 510 can be closed with a gas-permeable membrane. The temperature control element 20 is preferably designed as radiating element 560 for irradiating the vibrating element 14 with heat or infrared radiation and/or light.

(20) In one embodiment of the moisture sensor 12, the at least one vibrating element 14 is coated in different regions with materials with different wettability. This can also be done in such a way that one vibrating element or a region thereof is coated with a hydrophilic material and another vibrating element or a region thereof is provided with a hydrophobic coating.

(21) In the embodiment of the sensor element 10 according to FIG. 1, a quartz crystal vibrating element 100 with two carriers 101 and 102 is provided as vibrating element 14, said carriers also serving as electrodes. A metal electrode 110 is also visible in this view. The quartz vibrating element 100 contains as first and second material 16, 18 two defined-porous materials 151 and 152 which are mounted on the quartz vibrating element 100 next to each other with (or without) a distance to each other. A concentric arrangement (not shown), in which the first material 16 surrounds the second material 18 e.g. annularly completely or even only partially, is also conceivable.

(22) FIG. 2 shows a further embodiment of the sensor element 10 with the quartz vibrating element 100 as vibrating element 14, wherein again two defined-porous materials 161 and 162 are provided as first and second materials 16, 18, which here, however, are partially mounted one above the other on the quartz vibrating element 100.

(23) FIG. 3 shows a further embodiment of the sensor element 10, wherein a quartz tuning fork 200 in top view (FIG. 3a) and in side view (FIG. 3b) with holder 202 (left) and vibrating region 204 is provided as vibrating element 14. Both arms 210, 220 of the tuning fork 200 are coated with corresponding electrodes that are connected to the holder 202 by suitable soldering joints 211 and 221. Two defined-porous materials 301 and 302 are applied as first and second material 16, 18 to the same arm 220 of the quartz tuning fork 200.

(24) FIG. 4 shows a further embodiment of the sensor element 10 having the quartz tuning fork 200 with two defined-porous materials 303 and 304 which are applied to both arms 210 and 220.

(25) FIG. 5 shows a further embodiment of the sensor element 10 having the quartz tuning fork 200 with two defined-porous materials 305 and 306 which are applied to the same arm 220 in an overlapping manner.

(26) FIG. 6 shows a further embodiment of the sensor element 10 having the quartz tuning fork 200 with two defined-porous materials 307 and 308 which are applied to the same arm. A first material 307 is applied to the front and second material 308 is applied to the rear.

(27) FIG. 8 shows one embodiment of the moisture sensor 12 having the housing 500 with the opening 510 through which the environment can interact with the coated vibrating element 14, in particular with the quartz 100, 200 (not shown in FIG. 8, in the housing).

(28) FIG. 9 shows the same housing 500 with a membrane 550 fitted across the opening 510 (concealed, not visible). In addition to the side view (FIG. 9a, left), a cross-section through the opening 510 with diaphragm 550 fitted over it is shown (FIG. 9b, right).

(29) FIG. 10 shows a further embodiment of the moisture sensor 12 with the housing 500 with the quartz tuning fork 200 and one of the above-mentioned coatings (for the sake of clarity not shown in detail), the opening 510 and a light or infrared emitting element 560.

(30) It is obvious that other variants can be used. Depending on the design, more than two materials 16, 18 can be used or these materials 16, 18 can be applied to the same side or to the same vibrating arm 210, 220 or to different vibrating arms 210, 220 and opposite sides of the vibrating element 14.

(31) A variety of systems are suitable as vibrating elements 14, which are known to the expert and have been described above. Preferred vibrating elements 14 are based on vibrating quartz or quartz tuning forks and contain at least one piezoelectric material and at least two electrodes. Suitable vibrating elements 14 are therefore also quartz oscillating crystals, such as those used in quartz crystal microbalances.

(32) Defined-porous materials 151-162, 301-308 exist in a wide variety of materials and are characterized by the fact that the materials have pores with a narrowly defined diameter. According to the International Union of Pure and Applied Chemistry, IUPAB, a distinction is made between microporous (pore diameter less than 2 nm), mesoporous (pore diameter between 2 and 50 nm) and macroporous (pore diameter greater than 50 nm) materials.

(33) The material class of zeolites and metal-organic framework (MOF) materials belongs mainly to the microporous materials.

(34) The material class of the MCM-41 (Mobil Composition of Matter No. 41) and SBA-15 materials includes mesoporous materials, wherein the pore size can be adjusted by special selection of the synthesis conditions.

(35) In general, porous glasses with pores from 2 to 1000 nm can be produced by a variety of methods, the extraction of alkali borosilicate glasses after a phase separation probably being the best-known method. Such glasses are commercially available under the name Vycor glasses.

(36) The measurement of pore diameters is sufficiently well known to the expert and can be performed either directly by suitable microscopic methods such as electron microscopy or indirectly by determining the specific surface area of a material by an absorption method and calculating the pore diameter. A frequently used method is the absorption of nitrogen (N2) at −196° C., the so-called Brunauer-Emmet-Teller (BET) method. This can be done using devices such as a TriStar or Gemini, as commercially distributed by the company Mikromeritics. This allows the specific surface area of the sample to be determined and the mean pore diameter and pore size distribution to be calculated. The procedure is sufficiently well known to the expert. For further details on the porous materials, reference is also made to the above-mentioned doctoral thesis.

(37) To distinguish between porous materials and defined-porous materials, the so-called pore size distribution is used: Here the pore volume fraction (y-axis; units: cm3/g per nm or angstrom) is plotted against the pore diameter (x-axis; units: nm or angstrom). A material is defined-porous if the following two conditions apply:

(38) The width of the peak in the above illustration at half height of the peak (full width at half height; units: nm or angstrom) is less than one quarter of the mean pore diameter (units: nm or angstrom) as determined by the BET method.

(39) The pore volume (units: cm3/g) within the width of the peak (integral of the pore volume fractions from the lower end of the width of the peak at half height to the upper end of the width of the peak at half height) is at least one fifth (20%) of the total pore volume (units: cm3/g) of the material.

(40) To determine the total pore volume of the material, pores having a diameter from zero to ten times the average pore diameter are considered. For this purpose, the integral of the pore volume fractions from zero to 10 times the mean pore diameter is determined in the above representation of the pore size distribution.

(41) The first material 16 and the second material 18 in the defined-porous materials 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 can differ in pore diameter. In this case, the differentiation is purely by the pore diameter while the material composition is the same. As an alternative or additionally, two defined-porous materials 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 can be defined by the selection of the substances of which they are composed or by the coating of the inner side of the pores. In these cases, the pore diameter of different materials 16, 18; 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 can be identical, provided that these materials 16, 18; 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 do not differ in their wettability. Such different wettabilities are known to the expert and are referred to as hydrophilic or hydrophobic materials 16, 18. The wettability is often quantitatively determined by the so-called contact angle. For this purpose, devices such as the DAS 100 from Kruss or the Theta device from Biolin Scientific are used.

(42) Two or more different materials 16, 18, in particular two or more defined-porous materials 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 can be applied to the vibrating element 14 in numerous arrangements and variable quantities. The materials 16, 18; 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 can be applied in one or more process steps, wherein also additional process steps may be involved depending on the specific design.

(43) Suitable methods for applying especially the defined-porous materials 16, 18; 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 include spin-coating of suitable dispersions or dispersions of precursor materials. The layers are solidified preferably by heating or are given their actual form of use in an additional processing step. Application methods which are also suitable are coating by gas phase or plasma coating, whereby typically a precursor layer is applied first and later the actual form of use of the defined-porous material 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 is obtained by an additional processing step. As an option, auxiliary materials known to the specialist and containing binders or adhesives can be used to improve adhesion.

(44) Particularly suitable methods employ the deposition of one or more drops of a dispersion of the materials 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 or dispersions of the precursor materials on the vibrating element 14, the individual drops being applied at the place or at different places on the vibrating element 14.

(45) In a simple form of execution, which is shown in particular in FIG. 1 and FIG. 3, two or more materials 151, 152; 301, 302 are applied side by side to the vibrating element 14, and the regions with the coatings may touch each other or even overlap or be clearly separated by non-coated regions. Such an arrangement is shown, for example, in FIG. 1 where the materials 151 and 152 are applied side by side to the vibrating element 14.

(46) In another simple form of execution, two or more materials 161, 162; 305, 306 are applied one on top of the other to the vibrating element 14. In this case, also additional processes can be used between the individual steps, such as heating the vibrating element or using an extraction solution for converting a precursor material into a defined-porous material. Such an arrangement is shown in FIG. 2 or FIG. 5. The two materials 161, 162; 305, 306 are arranged one on top of the other.

(47) When using overlapping layers or superimposed layers of materials 161, 162; 305, 306, it is advantageous to deposit the layer with the smaller pore diameter first on the vibrating body of the vibrating element 14. Concerning the arrangement shown, for example, in FIG. 2, the preferred selection of the materials 161 and 162 is such that material 161 placed closer to the vibrating element 14 has a smaller pore diameter than the material 162 placed above it.

(48) When using different materials with similar pore diameters, it is advantageous to deposit the more hydrophilic material on the vibrating body of the vibrating element 14 beforehand.

(49) The arrangement of the individual coated regions of the vibrating body of the vibrating element 14 can be symmetrical or asymmetrical with regard to the shape of the vibrating body.

(50) The quantity and size of the individual coated regions can vary greatly.

(51) The height, form and extent of the coated regions can vary greatly.

(52) In a preferred embodiment, the quantities of defined-porous materials 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 used are different.

(53) In a preferred embodiment, the quantities of materials used are chosen in such a way that the pore volume that can be filled by water clearly differs from each other.

(54) In a particularly preferred embodiment, the pore volume of the material with the smallest pore diameter is clearly smaller than the pore volume of the material with the larger pore diameter.

(55) In a preferred embodiment and when using materials having a similar pore diameter, the pore volume of the more hydrophilic material is clearly smaller than the pore volume of the less hydrophilic material.

(56) In this case, the material is preferably produced in two steps in which a precursor material is applied to the vibrating body of the vibrating element 14 first (step 1) and then a part of the precursor material is converted into a material with different wettability in a further process step.

(57) Particularly preferred materials in this case are silicate-based micro- or mesoporous materials and macroporous glasses, a part of which is converted into a material with different wettability by a surface modification step.

(58) Suitable reagents for changing the wettability of the pores are sufficiently known and include the class of organosilanes, organoboranes, surface active substances, amphiphilic polymers and numerous other materials.

(59) Particularly suitable groups of materials for changing the wettability are trimethoxy silanes commercially available from the company Gelest, for example.

(60) A suitable combination of mesoporous materials is based on defined-porous silicate materials like MCM41 and derivatized organotrimethoxy silanes.

(61) Suitable organosilanes are alkyl and aryl trimethoxysilanes for lowering the wettability of the defined-porous materials to water.

(62) Suitable organosilanes with amine, ammonium, carboxy, sulfonate, phosphonate, polyglycol, polyglycerol or polyacrylate functionality are suitable for improving the wettability of defined-porous materials.

(63) The use of ionic organosilanes or organosilanes that allow salt formation can be used in preferred variants to change the wettability.

(64) The use of ionic organosilanes is advantageous for sensors with fast response to changes in moisture or dew point.

(65) When three and more materials are used, it is advantageous if the proportions of the material quantities of the individual materials are different.

(66) When three and more materials are used, it is advantageous if the quantity of pore volume of the material with the smallest pore diameter is the smallest.

(67) When using three and more materials and when using materials with similar pore diameter, it is advantageous if the quantity of the pore volume of the most hydrophilic material is smallest.

(68) Optional heating elements are known to the expert and include resistive heating elements based on platinum.

(69) Optionally, the vibrating element can be periodically heated by an infrared or light emitting element 560. This is shown, for example, in FIG. 7, with a voltage to be applied to the light-emitting element in the lower part and a typical frequency signal of the vibrating element 14 in the upper part.

(70) The use of an infrared or light emitting element 560 is advantageous in certain designs.

(71) The length of the heating phases, the power and the frequency of the heating intervals depend on the application field of the sensor and can be optionally adjusted via the electronics.

(72) The length of the irradiation, the power and the frequency of the irradiation depend on the application field of the sensor and can optionally be adjusted via the electronics.

(73) The use of a light emitting element 560 in a partially transparent housing 500 or in a housing 500 with an opening 510 can also be used to provide a user with information about the operation or operating status of the sensor 12.

(74) The parameters of the irradiation or heating of the vibrating element 14 are adapted in a preferred version to the previously determined measured values of the sensor 12.

(75) The duration and intensity of the irradiation of vibrating element 14 is preferably higher under rapidly changing moisture conditions than under comparatively constant moisture conditions.

(76) When detecting an increase in relative moisture in a gas mixture, the duration and intensity of the irradiation of the vibrating element 14 is shorter than when the relative moisture in a gas mixture decreases.

(77) The selection of the defined-porous materials 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308, the number of the materials 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 used and the selection of a heating element or an infrared or light emitting element 560 depends on the application field of the sensor 12.

(78) Sensors 12 of the type described here with at least one material which due to its pore distribution or other properties ensures at least one non-linearity in the response behavior are particularly suitable for reliably finding the zero point or retrieving a reference point. Such sensors are especially suitable for long-term monitoring, as they are less or not susceptible to drift of the sensor signal.

(79) Heating or irradiation of the vibrating element 14 leads to one or more successive changes in the vibration behavior of the vibrating element 14.

(80) Changes in the vibration behavior of vibrating element 14 are read out by the electronics. The procedures, calculation and evaluation methods as well as circuits used for this purpose are known to the expert e.g. from prior art cited above. In suitable vibrating elements 14, the changed vibration behavior can be easily converted into a mass change of the vibrating element 14.

(81) As a result of heating or irradiation, the electronics deliver one or more successive mass changes of the vibrating element 14, the mass change not occurring continuously but rather abruptly.

(82) In a preferred embodiment, the vibration behavior of the vibrating element 14 is evaluated in such a way that a distinction is made between areas in which no sudden change in mass occurs (continuous areas) and areas in which a sudden change in mass occurs (erratic areas).

(83) The differentiation into these two areas allows a simplified data analysis of the sensor signal by using continuous areas to define a baseline correction (linear drift). After deducting the baseline correction, the start and end points of an erratic area are more strongly defined and thus the measurement signal is more accurate.

(84) The relative size of two or more erratic areas depends on the materials used 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 on the vibrating element 14.

(85) In a preferred embodiment, information on the pore volume ratios of the materials used 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 is used in the evaluation of the raw data of the electronics to improve the signal quality.

(86) In a particularly preferred embodiment, a number of materials in different quantities with significantly different pore volumes are used. The series of erractic aeras from the raw data can be evaluated in an improved way, since the relative mass changes (height of the jumps) are known and always occur in the same order.

(87) For clarification or illustration, it should be mentioned here that when the relative moisture in an initially dry gas volume increases, a sensor 12 of the type described here containing a pore volume 1 of a first defined-porous material 1 (examples: 151, 161, 301, 303, 305, 307) and a pore volume 2 of a second defined-porous material 2 (examples: 152, 162, 302, 304, 306, 308) and under the conditions
(pore volume 2)=3×(pore volume 1)  (i)
(pore diameter of material 1)<(Pore diameter of material 2)  (ii)
will first exhibit a small jump and then later (at a higher moisture) a further, larger jump in the mass (measured as a rapid change in frequency). The calculated change in mass from the respective rapid changes in frequency is given for such a sensor 12 in good approximation by:
(mass change 2)=k*3*(mass change 1).

(88) The quality of this correlation can be expressed by the factor “k”. For identical materials with different pore diameters the factor “k” is approximately 1. For materials with different wettability, the deviations of the value of k=1 can also be larger and can range from some % to some 10%. In practice, however, this is not a particular disadvantage, as this factor can be included in the calculation method. Of practical use is the fact that this value is constant for the sensor 12, which means that the value is subject to little or no significant drift.

(89) Since the relative magnitude of the two rapid changes is thus known in advance, the raw data from a sensor 12 can be reliably evaluated, even for data with high noise. Corresponding algorithms are known and often used in telecommunications and computer science, for example.

(90) Accordingly, the use of numerous materials leads to series of erratic areas each of which can be detected due to its characteristic size. As a result, a sensor 12 can be heated or irradiated even for a short time in a preferred embodiment. This means that only some of the erratic areas are made visible, and irradiation or heating is more efficient than if all areas were activated.

(91) The use of such series of materials 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 allows the production of sensors 12 with a large range of dew point temperatures or moisture contents.

(92) The use of few materials allows easy production of sensors 12 for monitoring a set point or alarm or limit value.

(93) The energy requirement of the sensor 12 depends on the demands on the sensor, the moisture range, the measuring frequency and other factors.

(94) Preferred sensors 12 for monitoring a huge gas volume with very slow changes of the moisture content are heated or irradiated periodically and can thus retrieve the zero point or reference point and are thus better protected against slow drift of the sensor signal. Such sensors have a very low energy consumption and are especially suitable for places that are difficult to access or for sensors 12 that are powered by solar cells or battery, or sensors 12 that are completely enclosed in another device or room. This is especially advantageous if an environment needs to be especially clean or inaccessible due to toxic components or fire or explosion hazards. The latter areas are of particular interest, as the use of electrical cables is preferably avoided under so-called Ex-conditions.

(95) Sensors 12 for monitoring a limit value are typically used to detect an increase in moisture in an environment and are referred to as monitoring sensors. In a preferred embodiment, the sensor 12 is vibrated only periodically and a decision is made based on its vibration behavior, and on the basis of the vibration behavior it is decided whether no increase in moisture has taken place or whether there is an increase in moisture. In the first-mentioned case, the result (no increase in moisture) can be confirmed by a heating/irradiating step and by the absence of an erratic area. In the case of an increase in moisture, this can be confirmed by a heating/irradiating step and by the detection of an erratic area. The time course of the oscillation behavior after termination of the heating/irradiation step can also be used to determine the magnitude of the moisture increase.

(96) The current consumption of preferred monitoring sensors is low, and the sensors can also provide an indication of the extent to which the limit value has been exceeded in the event of a response (detention of exceeding the limit value). Preferred monitoring sensors therefore have a preferred arrangement of the opening 510 in the housing 500 and of the thermal or irradiation element 560.

(97) Preferred sensors 12 for monitoring rapidly changing moisture conditions preferably comprise a series of materials and are relatively often heated or irradiated.

(98) In a preferred embodiment of the sensor 12, the housing 500 and the arrangement of the vibrating element 14 and the heating or irradiating element 560 are chosen in such a way that a part of the housing 500 serves as a condensation surface during the heating or irradiation step. This part of the housing 500 is preferably arranged close to at least one of the defined-porous materials 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308, preferably in an arrangement of parallel surfaces, one surface being defined by the vibrating element 14 and the second surface being defined by the corresponding part of the housing 500.

(99) In this embodiment of the sensor 12, the housing 500 is not only a protection for the vibrating element 14, but also serves as a thermally inert element and temporary cooling unit which can cool water vapor from the defined-porous material 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 and temporarily fixes it in the form of condensate.

(100) After completion of the heating or irradiation phase, the defined-porous material 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 cools down and the water inside the housing 500 will partly again deposit in the defined-porous material 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308. Another part of the water can escape to the environment through the opening 510 in the housing. Therefore, the vibration behavior of the sensor 12 can be detected several times before and after a heating or irradiation phase and thus delivers the mass of water in a defined-porous material 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308.

(101) With rapid succession of heating or irradiation phases, the amount of bound water decreases after each heating or irradiation phase. The decrease in mass as a function of the number, type and duration and distance of the heating or irradiation phases can be used as an additional measured parameter to determine a dew point, dew point temperature or moisture. Such behavior is shown in FIG. 7. The vibrating element 14 has a higher frequency (f0) after the start of irradiation. After the end of the irradiation phase, a cooling phase begins. Some of the moisture in the housing 500 is slowly deposited on the vibrating element 14 (frequency drops). Towards the end of the cooling phase the frequency increase becomes slow and almost linear. The difference Δf1 compared to the frequency of the sensor 12 before the start of the first irradiation phase is characteristic of the geometry of the housing 500, the type of materials and the arrangement and size of the opening 510.

(102) A next heating phase again leads to a very rapid increase in frequency. A cooling phase starts again after the end of the irradiation. The frequency decreases again, but less. The difference Δf2 with respect to the frequency of the sensor before the start of the first irradiation phase, Δf1, and optionally other such frequency differences Δf1 when carrying out further (i−2) irradiation phases are characteristic for a certain moisture, geometry and the housing 500 as well as the length and type of irradiation.

(103) When using the sensors 12 and a light or infrared irradiation element 560, it can be advantageous to provide the defined-porous materials 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 all or some of them with an additive which changes the absorption of the radiation.

(104) Preferred additives to the defined-porous materials 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 are light or infrared absorbing materials.

(105) Preferred additives are mineral or organic pigments or dyes. Suitable materials for absorbing light or infrared radiation are well known and include materials that are commercially available as inks.

(106) The use of suitable additives can serve to reduce the duration and intensity of the heating or irradiation phases. This is of interest because it directly reduces energy consumption.

(107) In a specific design, several materials 16, 18, 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 are arranged on a vibrating element 14, the materials 16, 18, 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 differing in the pore diameter and the content of additives.

(108) In this specific design, pairs of materials 16, 18, 151, 152; 161, 162; 301, 302; 303, 304; 305, 306; 307, 308 where a defined-porous material is used once without and once with the additive.

(109) The use of arrangements with materials 161, 162; 305, 306 on top of each other, as shown in FIG. 2 and FIG. 5, can be advantageous for this special embodiment.

(110) In such special embodiments it can be advantageous to deposit a material 162, 306 with a smaller pore diameter on a material 161, 306 with a larger pore diameter and to place the material with smaller pore diameter in such a way that it comes into contact with the material with larger pore diameter.

(111) In another embodiment of the sensor 12 using a light emitting element 560, the additive is designed to remove the deposit of contaminants on the sensor 12. Suitable materials are known and include titanium dioxide in its anatase crystal form and light emitting elements 560 with light in the blue or ultraviolet range of the spectrum. Such sensors 12 are suitable for use in gas mixtures containing gases or volatile substances that cause deposits on the sensors 12. Examples of such environments that lead to deposits on the sensors 12 are known to the expert and include working environments in the paint and varnish industry, manufacturing technology or other industrial environments where volatile organic compounds occur. Another such environment is found in agriculture or in the processing of food and beverages, where volatile components of organic origin can lead to deposits. Such deposits can then consist of condensed terpene derivatives.

(112) In another embodiment of the sensor 12, further defined-porous materials (e.g. more than two) are placed inside the housing 500. These additional materials support the function of sensor 12 and may improve the accuracy or reliability of sensor 12 in certain environments.

(113) In a preferred embodiment, the opening 510 in the housing 500 of the sensor 12 is equipped with a membrane 550 that protects the sensor 12 against contamination or the penetration of liquids. The gas outside the housing 500 can only interact with the sensor 12 through the membrane 550. The housing 500 is otherwise tightly sealed so that no dirt or water can enter the housing 500.

(114) In a particularly preferred embodiment, the membrane 550, which protects the vibrating element 14, is well permeable to water vapor and at the same time a good barrier for liquid water. Suitable membranes for such requirements are Goretex membranes, Sympatex or other membranes that are also used in the production of sports and outdoor functional clothing. The suitability of the membrane 550 can be measured by the water vapor permeability and the so-called water column that describes the resistance against the penetration of liquid water. Suitable measurement methods for water vapor permeability are described in the ASTM E96 Standard, Procedure B (water method). Suitable methods for measuring the water column are described in the International Standard ISO 811-1981 (E). Membranes 550 with a water vapor permeability of more than 400 grams per square meter and day at 23° C. and a water column of more than 5 meters are particularly suitable.

(115) In another special embodiment, the above membrane 550 consists of a polymer film which is characterized by good water vapor permeability. Suitable films can be produced from polyurethane or copolymers with hydrophobic and hydrophilic components. Such diaphragms 550 are suitable for using sensors in areas containing fuel or other solvents or oil, especially areas where oily droplets may occur, especially near machines and engines or in electronic systems.

(116) The arrangement of the heating element or the infrared or light emitting elements 560 in sensor 12 is of special importance for specific applications. In particular, it is of interest to place the heating or irradiation element 560 close to the vibrating element 14 without disturbing the latter in its function as vibrating element 14. Suitable arrangements are therefore opposite or diagonally in direct visual contact with the vibrating element 14.

(117) In special arrangements it is of interest to mount the heating or irradiation element 560 in such a way that the opening of the housing or the membrane 550 mounted on it is also heated or irradiated.

(118) When using light-emitting elements 560, it may be advantageous to position the element 560 so that some of the light can escape to the outside, thus becoming part of a signal to a user of the sensor 12. The light-emitting element 560 in this embodiment therefore performs two tasks: irradiating the vibrating element 14 and sending a signal to the user of the sensor 12. Such signals are of interest for the maintenance, calibration and monitoring of the condition of such sensors 12 as they can easily take over the function of other data reading devices and peripheral equipment.

(119) FIG. 11 shows a comparison of an ideal characteristic curve 600 of a moisture sensor, as it has been strived for in the entire state of the art, with a characteristic curve 602 of a version of the moisture sensor 12 with a sensor element 10 according to one of the FIGS. 1 to 6b. The detected frequency f is shown schematically on the y-axis and the relative humidity % F on the x-axis. For a gas with a certain absolute moisture, the temperature T can also be indicated on the x-axis. The characteristic curve 602 according to the embodiments of the moisture sensors has a first non-linearity 602 and a second non-linearity 604. The non-linearities 602, 604 can be used to generate reference values.

(120) It is known, for example, that the first material 16 has an abrupt increase in mass at a certain relative moisture value, for example due to pore condensation, while the second material 18 has a comparable increase in mass at another certain relative moisture value. If, for example, the two non-linearities are found by continuously increasing or decreasing the temperature, it is possible to obtain two unambiguous reference values and, via the relationship of the non-linearities to each other or their distance from each other, a further value for monitoring the function of the sensor.

LIST OF REFERENCE SIGNS

(121) 10 sensor element 12 moisture sensor 14 vibrating element 16 first material 18 second material 20 temperature control element 100 quartz crystal vibrating element 101 first carrier 102 second carrier 110 metal electrode 151 first defined-porous material (separate arrangement) 152 second defined-porous material (separate arrangement) 161 first defined-porous material (below) 162 second defined-porous material (above) 200 quartz crystal tuning fork 202 holder 204 vibrating region 210 first arm 211 soldering joint 220 second arm 221 soldering joint 301 first defined-porous material 302 second defined-porous material 303 first defined-porous material 304 second defined-porous material 305 first defined-porous material 306 second defined-porous material 307 first defined-porous material 308 second defined-porous material 500 housing 510 opening 550 membrane 560 light or infrared emitting element 600 linear characteristic curve 602 characteristic curve of embodiment of moisture sensor 604 first non-linearity 606 second non-linearity