Method for determining the moisture content of a material sample and a moisture content measuring cell

Abstract

Method for determining the moisture content (60) of a material sample (100), comprising the following steps: - Heating a material sample (100) within a closed chamber (2) so that water contained in the material sample (100) degasses into the closed chamber (2), - Determining the humidity in the closed chamber (2) using a coulometric sensor (3) arranged in the closed chamber (2), - Determining and outputting a value for the moisture content (60) of the material sample (100) based on the determined humidity.

Description

The invention relates to a method for determining a material humidity in a material sample and to a material humidity measuring cell.Various methods and devices for determining a material moisture are known from the prior art.In gravimetry, the material sample is weighed before and after the water has been heated and the water content of the sample is calculated therefrom. A disadvantage of gravimetry is that the sample, which has been baked out, comprises not only the evaporated water but also all other volatile substances (e.g. essential oils, CO 2) being detected. An oxidation or reduction of the material to be measured during the heating of the water can also lead to a change in weight and thus to a corruption of the water content measurement.In Karl-Fischer titration, the sample to be investigated is dissolved in an organic solvent, for example dehydrated methanol. By adding (titration) a measurement solution of iodine, sulfur dioxide, pyridine (or other organic bases) and a suitable solvent ("Karl-Fischer reagent"), the water contained in the sample is reacted until all water is consumed and no chemical reaction takes place any longer. This method is frequently used for liquid samples. Solid samples which are poorly soluble or somewhat coarser are heated and the heated water is transported into the solvent by an air stream. This method is the standard method for small amounts of water (<10 mg). An advantage of this method is that it is only selective with respect to water. Disadvantages are the consumption of relatively expensive chemicals and their disposal. Side reactions can also distort the measured value.Furthermore, a device which operates by means of a coulometric sensor is known. Such a system is known, for example, from WO 93 / 12418. In this case, a dry gas stream is passed over heated moist material. The dry gas stream takes the baked water and transports it to the coulometric sensor. The water is absorbed at the coulometric sensor coated with phosphoric acid, electrolysed at a DC voltage and the water content is deduced on the basis of the electrolysis current which results. The underlying chemical electrolysis equation is:Overall reaction:Partial reaction at the cathode:Partial reaction at the anode:The water content in the gas can be deduced from the resulting stream. The gas stream dried by electrolysis is then blown off into the atmosphere. The disadvantage of this measuring system is the use of a gas flow and a drying unit necessary in this respect.A coulometric sensor operated with alternating voltage is known from EP 2 264 445 A1. A coulometric sensor with a gel-like electrolyte is known from DE 10 2016 206 445 A1.From the technical article LARIKOVA, G. G. [et al.]: Ob ispol'zovanii kulonometriebeskogo izmeritelja vlaeannosti gazov "Bajkal-3" dlja opredelenija vlagi v tverdych ve sespecach=On the use of a coulometric measure of gas humidity "Baikal-3" for determining moisture contents of solids. In: New Annual Analysis Kimii, Vol. 35, 1980, No. 9, pp. 1777-1780. - ISSN 0044-4502 discloses a method for determining a material moisture in a material sample, comprising the steps:heating a material sample within an open chamber such that water contained in the material sample outgassing into the chamber,determining a humidity of the chamber by means of a coulometric sensor arranged on the chamber,determining and outputting a value for the material humidity in the material sample on the basis of the determined air humidity.From the technical article PYPER, J. W.: The Determination of Moisture in Solids. Further moisture sensors are known in: Analytica Chimica Acta, 170 (1985) 159-175 and CN 107 064 233 A.The object of the invention is to provide a method for determining a material humidity in a material sample and a material humidity measuring cell, in which the determination of the material humidity can be carried out without the use of an additional gas flow.The object is achieved according to the invention by a method having the features of claim 1 and a material humidity measuring cell having the features of claim 6. Advantageous embodiments of the invention are evident from the dependent claims.In particular, a method for determining a material humidity in a material sample is provided, comprising the following steps: heating a material sample within a closed chamber such that water contained in the material sample outgassing into the closed chamber, determining an air humidity in the closed chamber by means of a coulometric sensor arranged in the closed chamber, determining and outputting a value for the material humidity in the material sample on the basis of the determined air humidity.Furthermore, a material humidity measuring cell is proposed which is suitable for determining a material humidity in a material sample, comprising a closed chamber, a coulometric sensor within the closed chamber for detecting a humidity in the closed chamber, a sample holder within the closed chamber, and at least one heating element for heating a material sample arranged on the sample holder.In the following, the term "within the closed chamber" with respect to a sensor means that at least the part of the sensor sensitive for a measurement provided by means of the sensor is arranged within the closed chamber. Other parts of the sensor, such as housing parts, additional electronics, etc., can also be arranged outside the closed chamber.Since the material humidity measuring cell according to the invention no longer requires an additional gas flow, the material humidity measuring cell can be designed to be significantly smaller, less expensive and easier to transport; additional pumps, feeds and discharges, etc. for the additional gas flow and a drying unit are omitted.During heating, the material sample is heated, for example, to a temperature of 105° C., so that the water present in the material sample completely outgassing into the closed chamber. As a result, all of the water bound in the material sample is available for electrolysis at the coulometric sensor.The air humidity in the closed chamber is determined from an electrolysis current detected at the coulometric sensor by means of Faraday's laws. The procedure is sufficiently known for coulometric sensors. Assuming that the water reacted in electrolysis originates from the material sample, the determined amount of water in the air in the closed chamber is equal to the amount of water originally bound in the material sample (material humidity of the material sample).In a preferred embodiment, it is provided that the coulometric sensor is operated by means of alternating voltage. It has been found that a DC voltage operation is less suitable for the coulometric sensor, since the time for the electrolysis increases significantly with the same amount of water. Deposits of phosphorus pentoxide (P 2 O 5) on the anode are probably responsible here. This P 2 O 5 is no longer converted back into phosphoric acid and is thus no longer available for electrolysis of water after deposition. The P 2 O 5- deposits increase over time. As a result, electrolyte is constantly removed from the coulometric sensor and the conversion per time decreases significantly. By operating with alternating voltage, on the other hand, the coulometric sensor remains stable for a longer time, so that maintenance effort is thereby reduced. The material humidity measuring cell can thereby be operated in a longer stable and more cost-effective manner.A coulometric sensor operated with alternating voltage is described in EP 2 264 445 A1.Accordingly, in the case of the material moisture measuring cell, it is provided that the coulometric sensor is a coulometric sensor operated with alternating voltage.It can also be provided that the coulometric sensor uses a gel as electrolyte. As a result, the electrolyte can be locally bound, as a result of which it can be prevented that the phosphoric acid drains off, in particular at high humidity, and can likewise no longer be used completely for the electrolysis. In addition, this prevents contamination of the measuring cell. Such a coulometric sensor is described in DE 10 2016 206 445 A1.The volume of the closed chamber is preferably less than 100 cm 3, particularly preferably less than 50 cm 3, very particularly preferably less than 20 cm 3. These volumes allow a very compact and small-sized design of the material humidity measuring cell.In a further embodiment, it is provided that a pressure in the closed chamber is detected simultaneously by means of a pressure sensor and a further value for the material moisture in the material sample is determined and output from the detected pressure.Accordingly, in this embodiment, the material humidity measuring cell comprises a pressure sensor which is arranged and designed in such a way as to detect a pressure in the closed chamber.By using a closed chamber, it is possible to detect a rise in pressure in the closed chamber which is caused by the evaporation and electrolysis of the water. The amount of water converted can likewise be determined from the pressure increase. It can be seen from the above equation for the overall reaction in the electrolysis of water that 1 mol of H 2 O gives 1 mol of H 2 and 0.5 mol of O 2 i.e. after the electrolysis 0.5 mol of additional gas is present in the closed chamber, which leads to a rise in pressure. A further advantage is that the water is present in liquid form before the heating and is evaporated by the heating. This also leads to a significant increase in pressure in the measuring cell. The following example calculations show the potential of the pressure measurement. 1 mol of H 2 O is about 18 cm 3 of liquid water. If this water is evaporated, a gas volume of about 24,500 cm 3; is obtained by electrolysis, the volume increases further to about 36,700 cm 3( at 25° C.). The volume increase is thus approximately 2040 times. When converted to the material humidity measuring cell, with a typical water quantity of 5 mg water and 16 cm 3 volume content of the closed chamber, these 5 mg H 2 O cause a pressure increase of about 0.6 bar. Even small amounts of water can therefore be detected without problems by means of the pressure sensor.The material quantity or the material moisture can then be determined by means of the following equation, which was derived from the equation of state for an ideal gas: in this case, n is 0 the starting material quantity in the closed chamber before heating the material sample, n is the substance quantity after outgassing and electrolysis, V is the volume of the closed chamber, R is the universal gas constant, p is 0 and T is 0 the pressure or the temperature before heating the material sample, p and T is the pressure or the temperature after heating the material sample and complete electrolysis of the water. The quantity of difference material n-n 0 then yields the quantity of material or the moisture of the material in the material sample.By determining the pressure, a further measurement is available in parallel with the determination of the material humidity by means of the coulometric sensor, which provides a further value for the material humidity.In a further embodiment, it is provided that a water activity is detected by means of a capacitive sensor before the air humidity in the closed chamber is determined and a value for the water activity is additionally determined and output. Accordingly, in this embodiment, the material humidity measuring cell comprises a capacitive sensor within the closed chamber, wherein the capacitive sensor is configured to sense a water activity within the closed chamber. The use of the closed chamber enables the determination of the water activity, i.e. the relative humidity of the material sample in the equilibrium state, before the material sample is heated and thereby dried out. The capacitive sensor can be, for example, a capacitive polymer humidity sensor. It is thus possible to determine both the water activity and the water content of the material sample.Ideally, the material sample and the closed chamber are tempered to a constant temperature during the determination of the water activity. For this purpose, the heating element on the sample holder can be used, for example. This can be designed, for example, as a Peltier element. The closed chamber can likewise be tempered to the constant temperature by means of a further heating element, for example likewise a Peltier element. The temperature is between 25° C. and 30° C. in this case.In one embodiment, it is provided that a quality estimate and / or a plausibility check of the values is carried out from the value for the material moisture determined by means of the coulometric sensor and the further value for the material moisture determined by means of the pressure sensor by comparison with one another. Ideally, the values for the material moisture in the material sample determined by means of the coulometric sensor and the pressure sensor should coincide with one another.If the value determined by means of the pressure sensor does not correspond to the value determined by means of the coulometric sensor, this can have one of the following causes, for example:still other constituents are exhausted from the sample,the closed chamber is leak-tight,Strong recombination takes place at the coulometric sensor or one of the sensors is defective.For example, when gases are heated, components other than water are exhausted, the pressure sensor determined value is greater than the coulometric sensor determined value, since the coulometric sensor selectively electrolyzes only water, but the pressure increases through all exhausting components. If the value determined by means of the pressure sensor is greater, this is an indication that still further constituents have been outgassed from the material sample.Conversely, if the value determined by means of the pressure sensor is less than the value determined by means of the coulometric sensor, this can be an indication, for example, that the closed chamber is leaking, since a part of the gas escapes.Such indications derived from the comparison of the two values are advantageous for the fault analysis or fault avoidance, for example in production processes, since the measured values can be evaluated and checked for plausibility immediately with regard to their quality.In a further embodiment, the material humidity measuring cell provides that the closed chamber has a first sub-region and a second sub-region, wherein the first sub-region and the second sub-region can be separated from one another by separating means, and wherein the capacitive sensor is arranged in the first sub-region and the coulometric sensor and, if present, the pressure sensor is arranged in the second sub-region. This makes it possible to first examine the material sample exclusively in the first subregion by means of the capacitive sensor without a gas flow to the second subregion of the closed chamber being possible. After the determination of the water activity by means of the capacitive sensor in the first sub-region, for example, the separating agent is removed, so that an exchange with the second sub-region is possible. In this state, the material moisture can then be determined according to the described embodiments of the method by means of the coulometric sensor arranged in the second sub-region and, if present, the pressure sensor.The separating means can be formed, for example, by a sample pot lid. The sample holder with the material sample is located in a sample pot. The sample pot cover seals the sample pot in an airtight manner. The sample pot and the sample pot cover are then screwed, for example, to a valve rod arranged on the material humidity measuring cell, by means of which valve rod the material sample can be brought into different positions within the closed chamber. In a first position, there is no communication between the sample pot and the closed chamber. However, by moving the valve rod, the sample pot lid can be brought into a second position. In this second position, there is a connection between the first partial region of the closed chamber and the sample pot. By a further movement of the valve rod, the sample pot lid is moved into a third position, in which there is a connection both to the first subregion and to the second subregion. In this way, it is possible selectively to first enable a gas flow or gas exchange exclusively to the capacitive sensor arranged in the first subregion (second position). Subsequently, a gas flow or gas exchange to the coulometric sensor arranged in the second sub-region and, if present, pressure sensor can additionally be enabled (third position).Furthermore, it can be provided that the material moisture measuring cell comprises a lock means which is designed in such a way that a material sample can be introduced into the closed chamber. The lock means can be formed, for example, from the sample pot described above, the sample pot cover and the valve rod screwed thereto.The method in the various embodiments can be carried out, for example, by means of a controller configured accordingly for this purpose. The controller controls, for example, the heating element and reads out the electrolysis current of the coulometric sensor over time. The controller then determines the air humidity from the electrolysis current by calculation and derives the value for the material humidity in the material sample from this. The value determined in this way for the material moisture is then output by the controller.If further sensors are present, for example the pressure sensor and / or the capacitive sensor, these can likewise be controlled or read out by the controller and the corresponding values for the material moisture or the water activity determined and output from the detected measured values.The comparison between the values for the material moisture determined by means of the coulometric sensor and the pressure sensor can also be carried out accordingly in a control system.The described method and the material moisture measuring cell can be used, for example, in laboratories of the food industry, in the pharmaceutical sector, in medicine, in connection with chemical products and in biotechnology for determining the material moisture in material samples.The invention is explained in more detail below with reference to preferred exemplary embodiments with reference to the figures. The following are shown here: FIG. 1 shows a schematic illustration of an embodiment of the material humidity measuring cell; FIG. 2 shows a schematic illustration of a further embodiment of the material humidity measuring cell; FIG. 3 shows a schematic illustration of a further embodiment of the material humidity measuring cell; FIG. 4 ashows a schematic representation of the embodiment shown in FIG. 3, wherein the valve rod and the sample pot lid are in a first position; FIG. 4 bshows a schematic illustration of the embodiment illustrated in FIG. 3, wherein the valve rod and the sample pot lid are in a second position; FIG. 4 cshows a schematic illustration of the embodiment illustrated in FIG. 3, wherein the valve rod and the sample pot lid are in a third position; FIG. 5 shows an exemplary profile of a relative humidity detected by means of a capacitive sensor over time; FIG. 6 shows a differential pressure in the closed chamber and the electric charge converted during electrolysis at the coulometric sensor over time of the measurement; FIG. 7 shows a pressure change in the closed chamber derived from the values shown in FIG. 6 and a current derived from the electric charge converted during electrolysis at the coulometric sensor over the course of time of the measurement; FIG. 8 shows values for the water content (material moisture) in the material sample, derived from the measured values of the coulometric sensor and of the pressure sensor, over time.FIG. 1 shows an embodiment of the material humidity measuring cell 1. the material humidity measuring cell 1 comprises a closed chamber 2, a coulometric sensor 3 within the closed chamber 2 for detecting a humidity in the closed chamber 2, a sample holder 4 within the closed chamber 2, and a heating element 5 for heating a material sample 100 arranged on the sample holder 4.The material humidity measuring cell 1 is controlled by a controller 6. The controller 6 controls, for example, a heating power at the heating element 5 and detects an electrolysis current at the coulometric sensor 3.In order to determine a material moisture content of the material sample 100, the material sample 100 is heated to 105° C. by means of the heating element 5. For this purpose, the controller 6 controls a heating power of the heating element 5. temperature sensors for monitoring and regulating the temperature can also be provided (not shown). By the heating, the water bound in the material sample 100 is released, and the water content of the air in the closed chamber 2 increases. The water molecules are electrolysed at the coulometric sensor 3, which results in a corresponding electrolysis current, which is detected by the controller 6. The controller determines the air humidity in the closed chamber 2 from the electrolysis current. Assuming that all water in the material sample 100 has been gasified and electrolysed at the coulometric sensor 3, a material amount of water in the air in the closed chamber 2 corresponds to the material amount of water originally bound in the material sample 100. The determined value for the amount of water in the material sample 100, i.e. the value for the material humidity of the material sample 100, is then output by the controller 6.In order to ensure a state of the coulometric sensor 3 that is stable over a long time, the coulometric sensor 3 is preferably operated by means of an AC voltage.FIG. 2 shows a further embodiment of the material humidity measuring cell 1, which, in addition to the embodiment shown in FIG. 1, also comprises a pressure sensor 7 which is arranged in the closed chamber 2. The pressure sensor 7 is likewise actuated and read out by the controller 6, for example. The controller 6 detects a pressure in the closed chamber 3 by means of the pressure sensor 7 simultaneously with the method steps described above. From the detected pressure, the controller 6 determines a further value for the material moisture in the material sample 100 and outputs this further value.The controller 6 can furthermore additionally be configured to perform a quality estimation and / or a plausibility check of the values from the value for the material moisture determined by means of the coulometric sensor 3 and the further value for the material moisture determined by means of the pressure sensor 7 by comparison with one another. This quality estimate, which can be, for example, an absolute or a relative difference value between the two values, is likewise output by the controller 6. This difference value can serve, for example, as an indication that the sensors are not operating correctly or that, in addition to water, further constituents have been outgassed from the material sample 100 into the closed chamber 2.FIG. 3 shows a schematic illustration of a further embodiment of the material humidity measuring cell 1. identical reference numerals here denote identical features having identical functions as in the embodiments shown in FIGS. 1 and 2. In addition to the coulometric sensor 3 and the pressure sensor 4, the material moisture measuring cell 1 additionally comprises a capacitive sensor 8 for detecting and determining a water activity (relative humidity of the material sample in equilibrium). Furthermore, the material humidity measuring cell 1 comprises a sample pot 9 which surrounds the sample holder 4 and a sample pot cover 10 which can cover the sample pot 9 in an airtight manner. The sample pot 9 and the sample pot cover 10 arranged thereon are screwed to a valve rod 11 for introducing the material sample into the closed chamber 2. After screwing, the material sample is located inside the closed chamber 2 and can enter into a gas exchange with the sample pot lid 10 by opening it, in order to be able to carry out the measurement by means of the coulometric sensor 3, the pressure sensor 4 and / or the capacitive sensor 8.FIGS. 4 a, 4 band 4 c show a schematic illustration of the embodiment illustrated in FIG. 3, wherein the valve rod and the sample pot lid 10 are in a first position 21 (FIG. 4 a), a second position 22 (FIG. 4 b) and a third position 23 (FIG. 4 c).The material sample to be examined is introduced into the sample pot 9 and sealed off from the surrounding atmosphere by the sample pot lid 10. The material sample can consist, for example, of 100 mg of porous spheres of Al 2 O 3- powder. For example, about 5 mg of water is adsorbed on the spheres, i.e. physically bound to the surface of the spheres. The sample holder 4 is screwed into the sample pot 9. A Peltier element is located on the sample holder 4 as a heating element 5, which can temperature control the sample pot 9.For insertion into the closed chamber 2 of the material moisture measurement chamber 1, the sample pot lid 10 is screwed into the valve rod 11 (first position 21, FIG. 4 a ). In this first position 21, the water vapor cannot yet escape from the sample pot 9, since the latter is hermetically sealed by the sample pot cover 10.Thereafter, the valve rod 11 is pushed into the second position 22. This creates a small cavity, which enables the path to the capacitive sensor 8 within a first partial region 12 of the closed chamber 2. The capacitive sensor 8 detects the water activity (relative humidity of the material sample in equilibrium). A stable temperature is essential for this measurement. Therefore, the material humidity measuring cell 2 is precisely tempered (not shown), for example, with a Peltier element. The material sample itself is tempered by means of the heating element 5 arranged on the sample holder 4 (second position 22, FIG. 4 b ). This heating element 5 can likewise be a Peltier element, for example. The selected temperature is between 25°C and 30°C and is kept constant throughout the measurement.FIG. 5 shows an exemplary profile of a relative humidity 30 (in %) detected by means of the capacitive sensor 8 over time 31 (in hours). The points in time 32, 33 at which the sample is conveyed from the first position 21 into the second position 22 or from the second position 22 into the third position 23 can be clearly seen. The relevant value for the relative humidity 30 is that at which an equilibrium state is reached in the closed chamber in the second position 22 (i.e. approximately shortly before the time 33, i.e. at a value of approximately 19% relative humidity).After measuring the water activity (FIG. 5 ), the sample holder is slid into the third position 23 (FIG. 4 c ) and the material sample is heated up to 105° C. in order to expel the water. In the third position 23 (FIG. 4 c), the path is free to a second partial region 13 of the closed chamber 2 and thus to the coulometric sensor 3 and to the pressure sensor 7, so that a gas exchange can take place with these.FIG. 6 shows the differential pressure 40 (left y-axis, in kPa) in the closed chamber and the electric charge 41 (right y-axis, in As) converted during the electrolysis at the coulometric sensor 3, in each case over time 42 (in min). FIG. 7 shows the pressure change 50 (left y-axis, in kPa / s) in the closed chamber derived from FIG. 6 and the current 51 (right y-axis, in A) derived from the electric charge converted during the electrolysis at the coulometric sensor 3, in each case over time 52 (in min). FIG. 8 shows the values for the material moisture 60 (in mg), i.e. the water content, in the material sample over the course of time 63 (in min), which values are derived from the measurement values of the coulometric sensor 3 (curve 61) and of the pressure sensor 7 (curve 62).The course of the curves in FIGS. 6, 7 to 8 can be explained as follows. Since the water content in the air in the closed chamber increases due to the annealing, an electrolysis current also increases greatly at the coulometric sensor 3. After a while, electrolysis reaches maximum activity. As the duration increases, more and more of the heated water is converted at the coulometric sensor 3. When all water has been electrolysed, the electrolysis current (cf. FIG. 7 ) falls off again and reaches its initial value. During the annealing and electrolysis, the pressure in the closed chamber likewise rises further and further (cf. FIG. 6 ). First, it is very strong because the water is evaporated. This is followed by a flattening gradient, since less and less water is electrolysed at the coulometric sensor 3 over time.It can be clearly seen in FIG. 8 that the values determined by means of the coulometric sensor 3 and the pressure sensor 7 correspond well. The value for the water content or the material moisture of the material sample determined by means of the pressure sensor 7 can thus serve as a comparison value for the values determined by means of the coulometric sensor and quality estimation and plausibility checking of the values can be carried out in this way.List of reference characters1 Material humidity measuring cell 2 closed chamber 3 coulometric sensor 4 sample holder 5 heating element 6 controller 7 pressure sensor 8 capacitive sensor 9 sample pot 10 sample pot cover 11 valve rod 12 first sub-region 13 second sub-region 21 first position 22 second position 23 third position 30 relative humidity (water activity) 31 time 32 time 33 time 40 differential pressure 41 electric charge 42 time 50 pressure change 51 current 52 time 60 material humidity 61 material humidity (coulometric sensor) 62 material humidity (pressure sensor) 63 time 100 material sample

Claims

Method for determining a material humidity (60) in a material sample (100), comprising the following steps: - heating a material sample (100) within a closed chamber (2) such that water contained in the material sample (100) outgassing into the closed chamber (2), - determining a humidity in the closed chamber (2) by means of a coulometric sensor (3) arranged in the closed chamber (2), - determining and outputting a value for the material humidity (60) in the material sample (100) on the basis of the determined humidity.Method according to claim 1, characterised in that the coulometric sensor (3) is operated by means of alternating voltage.Method according to either of Claims 1 and 2, characterized in that a pressure in the closed chamber (2) is detected simultaneously by means of a pressure sensor (7) and a further value for the material moisture (60) in the material sample (100) is determined and output from the detected pressure.Method according to one of Claims 1 to 3, characterized in that, before the determination of the air humidity in the closed chamber (2), a water activity (30) is detected by means of a capacitive sensor (8) and, in addition, a value for the water activity (30) is determined and output.Method according to one of the preceding claims, characterized in that a quality estimate and / or a plausibility check of the values is carried out from the value for the material moisture (60) determined by means of the coulometric sensor (3) and the further value for the material moisture (60) determined by means of the pressure sensor (7).A material humidity measuring cell (1) suitable for determining a material humidity (60) in a material sample (100), comprising: - a closed chamber (2), - a coulometric sensor (3) within the closed chamber for detecting a humidity in the closed chamber (2), - a sample holder (4) within the closed chamber (2), and - at least one heating element (5) for heating a material sample (100) arranged on the sample holder (4).Material humidity measuring cell (1) according to claim 6, characterised in that the coulometric sensor (3) is a coulometric sensor (3) operated with alternating voltage.Material humidity measuring cell (1) according to claim 6 or 7, characterised bya pressure sensor (7) which is arranged and designed in such a way as to detect a pressure in the closed chamber (2).Material humidity measuring cell (1) according to one of claims 6 to 8, characterised bya capacitive sensor (8) within the closed chamber (2), wherein the capacitive sensor (8) is configured to detect a water activity (30) within the closed chamber (2).Material humidity measuring cell (1) according to claim 9, characterised in that the closed chamber (2) has a first sub-region (12) and a second sub-region (13), wherein the first sub-region (12) and the second sub-region (13) can be separated from one another by separating means, and wherein the capacitive sensor (8) is arranged in the first sub-region (12) and the coulometric sensor (3) and, if present, the pressure sensor (7) is arranged in the second sub-region (13).

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