DE102018113671B4 - Method for determining the moisture content of a moisture-absorbing material - Google Patents

Abstract

Method for determining the moisture content of a moisture-absorbing material (8) with a moisture sensor (1) comprising a heating element (3), which is coupled to a control unit (5) and an evaluation circuit (6), characterized in that the time control and Analysis of the temperature change of the heating element (3) using the finite propagation speed of the heat pulse in the material (8) as well as the different temperature effects due to the different heat capacities depending on the moisture content a distance to the heating element (3) in which the moisture content is measured is influenced by a To determine moisture in thin layers, contact with the material to be examined (8) being sufficiently good to detect the moisture.

Description

The invention relates to a method for determining the moisture content of a moisture-absorbing or moisture-containing material with a moisture sensor comprising a heating element, which is coupled to a control unit, a power supply and an evaluation circuit.

In many technical fields of application, knowledge of the moisture content of materials is important and is measured, whereby the measurement in relatively thin material layers often proves to be problematic.

In the literature, for example, in "Material moisture measurement: Fundamentals - Measurement methods - Applications - Standards, contact & studies, Volume 513, Klaus Kupfer, Expert-Verlag, 1997" A large number of measurement methods for different purposes are described, whereby methods of building and soil moisture measurement are often explained, which have the common feature that relatively large areas are measured and a local delimitation is often not possible. Special requirements, for example measurements close to the body, are only insufficiently met with the known methods.

The gravimetric method enables the water content of a material to be measured by comparing the dry weight of the material with its wet weight. For this purpose, a sample is taken from the material to be examined and, if necessary, dried, whereby the sampling turns out to be difficult in some cases, since it is not always possible to separate a sample from an overall system. A continuous measurement of the water content is almost impossible, with measurement results not indicating the actual water content if the sample cannot be weighed promptly, since part of the moisture absorbed in the material of the sample evaporates. Gravimetric methods are for example in the U.S. 4,316,384 A1 , U.S. 4,666,007 A1 , U.S. 4,750,143 A1 , U.S. 4,798,252 A1 and U.S. 4,889,201 A1 specified.

Furthermore, electrical methods, in particular electrical resistance measurement and capacitance measurement, are used to determine the water content in a material, for example from the US 5,402,075 A1 , US 6 647 782 B2 , US 6,222,376 B1 and the U.S. 5,861,758 A1 known. Here, electrodes are arranged in or on the material to be examined and the electrical measurement signal or electrical measurement values are recorded for evaluation. These methods are basically suitable for use in connection with relatively thin material layers, but the influence of the measured values by ionic impurities or parasitic, often time-varying capacitances, such as occur, for example, in measurements close to the body, proves to be disadvantageous.

In thermal measurement methods, the material moisture is determined on the basis of the thermal properties that change with the water content, such as thermal conductivity or heat capacity, whereby these measurement methods are less prone to interference in practice and their main application is in the construction sector.

The EP 1 844 323 B1 discloses a device for determining the moisture in a medium, in particular earth, on the basis of a thermal measurement. The device has at least one measuring sensor, which consists of a temperature sensor and a means for heating the temperature sensor. Furthermore, the device comprises evaluation electronics and a control unit, which are used to heat the heatable temperature sensor with a heating pulse and to determine the moisture content of a medium surrounding the measuring sensor. The sensor is surrounded by an intermediate layer made of an absorbent, mechanically deformable and thermally insulating material made of synthetic fibers and / or felt. A statement regarding the spatial measurement area, which is influenced by varying the shape of the heating pulse and the time of the measurement, cannot be inferred from this disclosure.

From the EP 0 981 737 A1 a sensor for determining the moisture content is known, which is preferably manufactured as a microsystem in silicon technology and comprises a membrane with a first temperature sensor, which can be heated either permanently or by means of a short pulse, and a second temperature sensor, thermally insulated from the membrane, for determining the ambient temperature. An absorbent material, for example wood, paper, cardboard or a porous ceramic, is arranged on the membrane. The heat generated when the membrane is heated is dissipated to the environment through the absorbent material. The difference in temperature between the two temperature sensors represents a reference value for the material moisture. Heating by means of a heating pulse minimizes the energy requirement, but it is not possible to limit the measuring range.

The US 3,550,439 A1 describes a so-called clothing hygrometer with which the moisture content of textiles can be determined, using a structure similar to a capacitor and a hygroscopic material. Of the The moisture content is determined without a heat pulse by means of an electrical capacitance measurement.

The invention is based on the object of creating a method and a device of the type mentioned at the outset which enable a statement to be made regarding a moisture content in a moisture-absorbing or moisture-containing material, the spatial measuring range being variable and the moisture content being thin Material layer or in the immediate vicinity of the moisture sensor can be measured and wherein the spatial measurement area is not through a physical barrier, i. H. is not influenced by the influence of a surrounding material.

According to the invention, the object is achieved by the features of the independent claims.

The subclaims represent advantageous configurations.

In a method of the type mentioned above, the time control and analysis of the temperature change of the heating element using the finite speed of propagation of the heat pulse in the material and the different temperature effects due to the different heat capacities depending on the moisture content, the distance to the heating element in which the moisture content is measured , influenced.

The heating element, a resistance heating element, is subjected to a defined electrical current for heating for a short defined period of time, the heat being absorbed and dissipated by the material surrounding the moisture sensor. The method enables the determination of the moisture content in a material that can also have a relatively thin cross-section, for example a textile of a piece of clothing or a shoe or the like, by means of a thermal method, using the finite propagation speed of a heating pulse and the associated limitation of the measurement volume becomes. The temperature of the heating element as a function of time and the moisture content describes a temperature curve that is determined to determine the moisture content.

The temperature change generated by the defined heating pulse or the temperature profile over time can thus be used as the measured value for the ambient humidity. Since the heat spreads into the environment at a finite speed, it is possible to define the spatial area of influence by the duration of the heating pulse or the times of the temperature measurements. The influenced measurement volume of the material to be examined or monitored is effectively limited by precise control of the time sequence of the temperature measurement. It is also possible, through a clever choice of the temporal measurement window, to measure areas that are indirectly adjacent to the sensor, i.e. the measurement can be focused on certain discrete layers that are at a defined distance from the sensor. It is thus possible to measure the water content in thin layers, especially of textile material that is used in the manufacture of clothing, without the additional environment such as physical barriers influencing the measurement result. In particular, the method is suitable for determining the moisture in thin layers, the surroundings of which are not defined in detail. Textile layers that are in more or less well-defined contact with the human body and / or other textile layers and / or with the ambient air are to be mentioned as an example.

Due to the high thermal conductivity and the high thermal capacity of water compared to air, the thermal energy dissipated from the heating element by the environment is heavily dependent on the water content in its environment.

The heat flow itself can vary during the heat pulse, but the course should be the same for the individual measurements. It is possible to analyze the entire temperature profile during the heating pulse or at defined times. It is important that the heat propagation front remains within the desired measurement volume at the time of measurement or that contact with the environment outside the desired measurement volume cannot yet affect the measurement on the sensor.

The heating element of the humidity sensor is protected from the environment by a thin, electrically non-conductive layer.

The heating element itself can be used as the temperature sensor in which its temperature-dependent electrical resistance is monitored and evaluated, or a temperature sensor attached directly to the heating element is provided, which is of course also coupled to the evaluation circuit and the control unit. Of course, in addition to memory elements, interfaces and CPU, the control unit can include further electronic components, in particular also coupled to input and / or output units, or be designed for wireless data transmission. The energy supply can be stationary via a power pack or in the form of a battery for mobile applications or applications close to the body, for example. By choosing the length of the heat pulse, the intensity or the frequency of a measurement the energy consumption can be adapted to the respective situation.

The measurement volume detected directly by the moisture sensor can consist of the material to be examined for its moisture content. Alternatively, it is possible to embed the moisture sensor in a reference material that is in close moisture exchange with the material to be examined. The medium to be examined for its moisture content can thus be, for example, a textile itself and act directly on the moisture sensor or indirectly via the moisture-absorbing reference material firmly connected to the moisture sensor, which is in a moisture exchange with the environment. If the water absorption in the reference material is almost hysteresis-free and fast enough compared to the change in the water content of the environment, then the moisture content in the reference material can be assumed to be an equipotential value, which in soil physics is often referred to as "suction stress" for the ambient moisture.

The humidity sensor suitable for heating and temperature measurement can be implemented in different ways and integrated accordingly into the measurement environment. When setting up the moisture sensor, care should be taken to ensure that the heat capacity of its structure is low and that the contact with the material to be examined, which can also be referred to as a measuring body, is sufficiently good to detect the moisture.

It is possible to have a structure of the moisture sensor on a conventional circuit carrier z. B. build a circuit board. Alternatively, the sensor structure on a flexible circuit carrier z. B. a film or a flexible and stretchable circuit carrier z. B. a stretchable film can be applied so that the moisture sensor nestle closely to the material to be monitored or examined or can be built into the material z. B. in a textile.

The moisture sensor preferably comprises a thin-film heating element made of gold, which is applied to a thin, film-like polyimide substrate. The heat pulse propagates through the polyimide and then reacts to the humidity in the immediate vicinity of the sensor.

As a further option, the structure can also be designed as a thread, fiber, wire or as a conductive yarn and can accordingly be integrated into e.g. textile fabric, i.e. directly into clothing or footwear. As a result, textiles can be built with largely integrated sensor structures that do not interfere with the normal moisture transport of the textile or the item of clothing, whereby significant advantages can be achieved in the construction of intelligent clothing.

The thermal moisture measurement method is robust against a variety of interfering influences such as ionic contamination, parasitic capacitances or electrical interference fields, which is why it can be used in a harsh environment, in particular realistically and under particular stress from a wearer of the clothing and environmental influences. The thermal moisture measurement method is also a non-destructive method, so that the moisture sensor can be used directly on the material without a sample having to be separated from the material. The short heat pulse or heat pulse makes it possible to limit the measuring range to a small area. It is therefore possible to measure thin layers without having to accept a major influence on the wider environment.

In contrast to many other measurement methods such as B. the capacitive moisture measurement, the thermal moisture measurement method is also very sensitive in areas with very high material moisture. If the moisture sensor is incorporated into a textile in the form of a thread-like structure, various measurement tasks in the areas of sport, health, ambient assisted living (AAL) can be solved relatively easily without affecting normal moisture transport. It is also possible to build sensors based on perforated foils so that normal moisture transport is not hindered.

Due to the robust operation of the measuring principle, the moisture sensor can be used in systems worn close to the body, for example in portable electronic devices (wearables) or directly in textiles or clothing, in particular functional or protective clothing, in order to quantify the humidity climate or to actively control ventilation systems. The very short heating pulses make energy-saving small systems possible. The ventilation systems themselves can be of an active or passive nature, e.g. flaps or fans.

What is essential is the ability to control the distance from the heating element at which the moisture content is measured. This is done via the time control of the heating pulse and using the finite speed of propagation of the heat pulse in the material as well as the different temperature effects due to the different heat capacities depending on the moisture content.

In the simplest case, you can limit the spatial measurement area, so that the measurement is based on a thin layer can be limited around the humidity sensor. It is also possible to analyze layers defined over the transit time of the heat pulse at a certain distance from the heating element.

The sensor can be constructed in different ways. Practical embodiments are e.g. thin conductor tracks made of gold on a very thin polyimide film or as a semiconductor component or thin conductive threads. It is essential that the heat capacity can be kept very small through a miniaturized design, e.g. in thin-film technology or microtechnology, in order to improve the sensor sensitivity.

It goes without saying that the features mentioned above and still to be explained below can be used not only in the respectively specified combination, but also in other combinations. The scope of the invention is defined only by the claims.

The invention is explained in more detail below using an exemplary embodiment with reference to the associated drawing.

• 1 eine schematische Darstellung einer Anordnung zur Durchführung des Verfahrens nach der Erfindung,
• 2 eine schematische Darstellung einer Wärmeverteilung,
• 3 Diagramme zur Darstellung der Zusammenhänge zwischen der Heizleistung und der Temperatur über die Zeit in trockenem und feuchtem Material,
• 4 ein Diagramm zur Darstellung der Eindringtiefe der Wärme in das Messmaterial und die Umgebung über der Zeit,
• 5 ein Diagramm einer Feuchtigkeitsmessung und
• 6 ein Diagramm einer Feuchtemessung, in dem die Widerstands- sowie die berechnete Temperaturänderung des Sensors über der Zeit aufgetragen sind, wobei unterschiedliche Feuchtezustände gezeigt sind (W=Wet, feucht; D=Dry, trocken).

It shows:

• 1 a schematic representation of an arrangement for performing the method according to the invention,
• 2 a schematic representation of a heat distribution,
• 3 Diagrams to show the relationships between heating power and temperature over time in dry and moist material,
• 4th a diagram showing the depth of heat penetration into the measuring material and the environment over time,
• 5 a diagram of a moisture measurement and
• 6th a diagram of a humidity measurement in which the resistance and the calculated temperature change of the sensor are plotted over time, with different humidity states being shown (W = wet, humid; D = dry, dry).

The humidity sensor 1 includes a structure 2 on which a heating element 3 is arranged, which has electrical connections 4th with an electronic control unit 5 that have an evaluation circuit 6th and of course, in addition to memory elements, also includes a CPU and an energy source 7th for example in the form of a battery or an accumulator is coupled to the heating element 3 to apply a defined heating current for a predetermined period of time and thus to generate a heating pulse or heat pulse and the temperature increase to determine the moisture content of the material 8th the humidity sensor 1 is assigned to measure.

The heating element 3 is designed as a thin-film heating element that sits on top of the thin polyimide structure 2 comprises vapor-deposited gold and is encapsulated with a further polyimide layer. Other materials and manufacturing processes are possible.

2 it can be seen that the heat distribution indicated by the arrows starting from the moisture sensor 1 into the material to be examined or monitored 8th which can be, for example, a textile of an item of clothing or also a diaper, and within the measurement time not beyond that into the environment 9 .

To 3 is illustrated how the abrupt course of a heat flow Q over time t according to the course of the line 11 to a core temperature T over time t affects being the curve 12th on dry material 8th and the curve 13th on damp / wet material 8th is determined. Due to the high thermal effusiveness of water, heat is transferred through the material 8th the higher the water content, the better of the moisture sensor 1 discharged. Other courses of heat flow, such as a very short pulse, are also possible.

A representation of the depth of penetration x the heat or the heat flow of the heating pulse into the material 8th depending on the time t is 4th to be taken, the depth of penetration also increasing with increasing time. The penetration depth is a root function of time and the thermal diffusivity of the environment and can be described using the "mean square displacement" of statistical physics. Even after switching off the heating current, the heat continues to spread. It is therefore also possible, for example, to use only a short heating pulse and, after this, the cooling behavior of the moisture sensor 1 consider.

5 represents how the temperature changes T of the humidity sensor 1 with a sudden heat flow Q behaves. Depending on the time t is the temperature profile through the polyimide structure 2 of the humidity sensor 1 , then through the immediate sensor environment, namely the material 8th and its moisture content and, in the case of longer measurements, through which on the material 8th subsequent environment 9 influenced. By considering a certain time interval, the influence of the environment 9 can be discriminated against from the moisture measurement. This means that the humidity measurement is to be limited specifically to the immediate sensor environment.

6th shows a diagram with the typical signal curve of a simulated measurement on a textile material 8th and six different humidity levels from air dry (D) to completely soaked (W). A change in resistance of the heating element (ΔR / R) and its temperature change in arbitrary units, which are both directly related (PTC resistance properties), is shown. The time range from 100 ms to 300 ms (Mx) is suitable for this measurement in order to estimate the moisture in the textile.

List of reference symbols

1.

Humidity sensor

2.

structure

3.

Heating element

4th

connection

5.

Control unit

6th

Evaluation circuit

7th

Energy source

8th.

material

9.

Surroundings

10.

11.

Line course

12th

Curve

13th

Curve

Q

Heat flow

t

time

T

temperature

x

Penetration depth

Mx

Measuring range

D.

dry

W.

wet

Claims (12)

Method for determining the moisture content of a moisture-absorbing material (8) with a moisture sensor (1) comprising a heating element (3), which is coupled to a control unit (5) and an evaluation circuit (6), characterized in that the time control and Analysis of the temperature change of the heating element (3) using the finite propagation speed of the heat pulse in the material (8) as well as the different temperature effects due to the different heat capacities depending on the moisture content a distance to the heating element (3) in which the moisture content is measured is influenced by a To determine moisture in thin layers, contact with the material to be examined (8) being sufficiently good to detect the moisture. Procedure according to Claim 1 , characterized in that the control unit (5) acts on the heating element (3) for a predetermined period of time with a defined heating current and the temperature curve, i.e. the time-dependent temperature change, is measured to determine the moisture content. Procedure according to Claim 1 or 2 , characterized in that the temperature change during and / or after the heating current is applied to the heating element (3) by the detection of the temperature resistance of the heating element (3) or a signal from a temperature sensor arranged on the heating element (3) is detected by the evaluation circuit (6). Method according to one of the Claims 1 to 3 , characterized in that the control unit (5) varies the level of the heating current and the duration of the heating current application, the course of the heating current being the same for individual measurements of the temperature. Moisture sensor (1) with a heating element (3) and a temperature sensor for carrying out a method according to one of the Claims 1 to 4th , characterized in that the heating element (3) is surrounded by an electrically non-conductive layer. Humidity sensor (1) Claim 5 , characterized in that a structure (2) of the moisture sensor (1) comprises a flexible circuit carrier. Humidity sensor (1) Claim 5 , characterized in that the structure (2) of the moisture sensor (1) is designed as a film made of a polyimide. Humidity sensor (1) Claim 6 or 7th , characterized in that the structure (2) is perforated or perforated. Humidity sensor (1) Claim 5 or 6th , characterized in that the structure (2) of the moisture sensor (1) is thread-shaped or fiber-shaped. Humidity sensor (1) Claim 5 , characterized in that the heating element (3) is made from a gold foil. Arrangement for carrying out the procedure according to Claim 1 , characterized in that the moisture sensor (1) is incorporated directly into the material (8) to be examined or is arranged adjacent to the material (8) to be examined. Arrangement for carrying out the procedure according to Claim 1 , characterized in that the moisture sensor (1) is embedded in a reference material which is connected to the material (8) to be examined.

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