Moisture sensor and respective measuring method

Abstract

The invention concerns a moisture sensor (10) for measuring the moisture of materials in the form of dry, liquid or gaseous granulates, or in the form of powders, in at least one conduit arranged at least partially along an axis (X-X) of the moisture sensor (10) through which the materials, whose moisture is to be measured, flow. The moisture sensor (10) comprises at least one capacitor in which the material whose moisture is to be measured flows, wherein the aforesaid capacitor comprises rings made of dielectric material (20) interposed between metal rings (30) mounted coaxially to said axis (X-X) between containment flanges. At least one contactless optical temperature sensor (40, 50) configured to measure the temperature of the capacitor is associated with the moisture sensor (10), in order to calculate a correction to be applied to the capacitance (CX) of the empty moisture sensor (10) due to the thermal expansion of the capacitor following the flow of the materials, whose moisture is to be measured, therein.

Description

[0001] "MOISTURE SENSOR AND RESPECTIVE MEASURING METHOD"

[0002] FIELD OF THE INVENTION

[0003] The present invention concerns a moisture sensor for the industrial field and a respective measuring method . In particular, the present invention concerns a sensor for measuring the moisture of products subj ected to industrial treatments , such as for example materials in the form of dry, liquid or gaseous granulates , or materials in the form of powders .

[0004] KNOWN ART

[0005] As is known, there are apparatuses for measuring the moisture present in many substance samples and which have many applications , in particular in the food, chemical and pharmaceutical industries .

[0006] In some known devices , moisture measurement does not occur in-line during the production process but by sampling and subsequent measuring .

[0007] Such an approach is very complex and requires lengthy times , and the measurement cannot be reliable since many elements can lead to improper sampling .

[0008] Moreover, the sampling is not very representative of the production in progress , since the lab systems can only sample a few grams and since it is not possible to collect a sample at the speci fic point of interest in some production processes , for example in silos or dryers in nitrogenous environments . Systems in which measurement is done in-line are also known .

[0009] For example , document WO 2012 / 004621 illustrates a device for measuring the moisture of materials flowing in the form of dry, liquid or gaseous granulates , or in the form of powders , in at least one conduit arranged at least partially along an axis through which the material flows . The device comprises at least one capacitor through which the material , whose moisture is to be measured, flows . In turn, the capacitor comprises at least two metal rings mounted coaxially to said axis and adj acent to an inner wall of the conduit through which the material , whose moisture is to be measured, flows , and at least one dielectric element having dielectric constant substantially linear with the temperature variation .

[0010] In the case of some types of installation on industrial machines such as inj ection molding machines , extruders , dryers , etc . , a periodic calibration procedure of the equipment , adapted for allowing repeatability and accuracy of measurement , is required .

[0011] In some industrial contexts , calibration also provides for a brief production stoppage since calibration operations are only carried out within certain ambient temperature and moisture values .

[0012] Another problem highlighted is that , when the measuring system is subj ected to temperatures exceeding 50 ° c, mainly due to thermal expansion and permittivity variation of the dielectric material the sensor is composed of , measurement errors are inevitable due to the dimensional and dielectric permittivity variations which af fect the proper value of the measurement .

[0013] The total capacitance measurement is the resultant of the capacitance value of the empty sensor ( capacitor CX ) + the permittivity value of the material crossing it ( CM) + the permittivity due to the relative humidity of the air (HRU) . In other words , the total capacitance ( Ctot ) is given by the following formula : Ctot = CX + CM + HRU .

[0014] From the formula, it is clear that the capacitance value of the empty sensor CX is essential for proper measurement and especially for the repeatability of the measurement . The capacitance dri ft due to the variation of the dielectric capacitance is neither constant nor linear, since depending on many factors , such as the thermal expansion of all the materials composing it , the peculiarity of some installations , where machinery can af fect both thermally and mechanically the structure of the CX sensor, thus generating a dri ft or often hysteresis of the CX capacitance value , forming an even more complex problem to be corrected, even mathematically .

[0015] Due to its structure , the sensor is also subj ected to electromagnetic disturbances deriving from interaction with other machines , which can further alter the value of the signal deriving from the sensor . A metal shield eliminates the problem but adds another one which causes a further temperature increase inside the system, with consequent increased dri ft of the capacitance signal due to thermal expansion of the sensor' s components .

[0016] Another problem highlighted is the heating, often over 100 ° C, of electronic components . The PCB (Electronic Board) of the sensor must be mounted as close as possible to the electrodes of the sensor, this mainly to avoid parasitic capacitances due to contact impedance . The terminal capacitance must fall within a narrow window between 130 fF ( femtoFarad) and 170 fF and has a very weak, i f possibly negligible , temperature dependence .

[0017] An obj ect of the present invention is therefore to make a system which allows to correct the capacitance variation of the moisture sensor due to thermal expansion .

[0018] A further obj ect of the invention is to make a system which allows to correct the variation of the dielectric constant of the dielectric material interposed between the electrodes .

[0019] Last , but not least , obj ect of the invention is to achieve the aforesaid result in a practical and economical way .

[0020] BRIEF SUMMARY OF THE INVENTION

[0021] These and other obj ects are achieved by means of a moisture sensor and a method for measuring the moisture of materials in the form of dry, liquid or gaseous granulates , or in the form of powders , in at least one conduit arranged at least partially along an axis of the moisture sensor through which the materials , whose moisture is to be measured, flow .

[0022] The moisture sensor comprises at least one capacitor in which the material whose moisture is to be measured flows , wherein the aforesaid capacitor comprises rings made of dielectric material interposed between metal rings mounted coaxially to said axis between containment flanges , characteri zed in that the aforesaid moisture sensor is associated with at least one contactless optical temperature sensor configured to measure the temperature of the capacitor in order to calculate a correction to be applied to the capacitance of the empty moisture sensor due to the thermal expansion of the capacitor following the flow of the materials , whose moisture is to be measured, therein .

[0023] The method for measuring the moisture comprises the following steps :

[0024] - prearranging, inside the conduit , a moisture sensor comprising at least one capacitor which comprises dielectric material rings interposed between metal rings mounted coaxially to an axis of the sensor between containment flanges , wherein at least one contactless sensor is associated with the moisture sensor ( 10 ) ; measuring, through the moisture sensor, the temperature of the capacitor ; and calculating a correction to be applied to the capacitance of the empty moisture sensor due to the thermal expansion of the capacitor following the flow of the materials , whose moisture is to be measured, therein .

[0025] An advantage of the invention is that it allows the capacitance of the empty sensor, due to both thermal expansion and the variation of the dielectric constant of the dielectric material interposed between the electrodes , to be automatically corrected .

[0026] The use of the contactless sensors provides several advantages , among which non-intrusiveness , resistance to electromagnetic interference , ability to operate in hostile environments and possibility to perform measurements remotely .

[0027] Further characteristics of the invention can be deduced from the dependent claims .

[0028] In the rest of the present description and in the accompanying claims , the expression "capacitance of the empty moisture sensor" refers to the capacitance of the sensor when it is not crossed by any material .

[0029] BRIEF DESCRIPTION OF THE FIGURES

[0030] Further characteristics and advantages of the invention will become clearer from the reading of the following description provided by way of example and without limitations , with the aid of the figures depicted in the accompanying tables , in which :

[0031] - figure 1 illustrates a top view of moisture sensor, according to an embodiment of the invention; figure 2 illustrates a view, according to the section A-A of figure 1 , of the moisture sensor ; and figure 3 illustrates an axonometric view of the sensor of figures 1-2 . DETAILED DESCRIPTION OF THE FIGURES The invention will now be described with initial reference to figure 1 , which illustrates a moisture sensor, according to an embodiment of the invention, generally denoted by the numerical reference 10 .

[0032] Moisture sensors of this type are known and are generally composed of some metal rings 30 and some rings made of dielectric material 20 , wherein each metal ring 30 is interposed between two rings made of dielectric material 20 so that to form a structure of a ring-shaped electrical capacitor having a substantially cylindrical extent along an axis X-X ( figure 2 ) .

[0033] According to an embodiment of the invention, the dielectric material with which the rings 20 are made is PEEK (polyether ether ketone ) , i . e . a high-performing semicrystalline thermoplastic material .

[0034] As is known, PEEK has high resistance to heat and chemical agents and is certainly a good electrical insulator provided with low thermal expansion .

[0035] In the moisture sensor 10 of the invention, the metal rings 30 and the rings made of dielectric material 20 are assembled between two ring-shaped containment flanges (not depicted for simplicity) by using assembly screws .

[0036] The moisture sensor 10 of the invention measures the moisture in-line in the processes in which materials flowing in the form of dry, liquid or gaseous granulates , or in the form of powders , are treated .

[0037] The flow of these materials occurs inside the substantially cylindrical structure of the moisture sensor 10 in the direction of the axis of figure 2 .

[0038] Such moisture sensor 10 is adapted for being positioned in a conduit , not shown, in which it is possible to measure the moisture in-line , i . e . arranged according to the axis X-X along which the material , whose moisture is to be measured, flows .

[0039] In particular, thanks to the structure of the sensor described above , the capacitance of the capacitor is af fected by the dielectric permittivity of the material between its plates , which permittivity changes depending on its moisture content of the material to be measured, since water has a much higher permittivity than many solid materials or air, and such phenomenon allows a measurement of the moisture of the material crossing the sensor to be obtained .

[0040] As mentioned previously, the total capacitance measurement is the result of the capacitance value of the empty sensor ( capacitor CX ) + the permittivity value of the material crossing it ( CM) + the permittivity due to the relative humidity of the air (HRU) , i . e . total Capacitance ( Ctot ) = CX+ CM + HRU .

[0041] From the formula, it is clear that the capacitance value of the empty sensor CX is essential for proper measurement and especially for the repeatability of the measurement .

[0042] According to an embodiment of the invention (not depicted for simplicity) , a holder made of a mica-based material , which allows to channel a flow of air through a fan to maintain uni form heat distribution along the entire length of the sensor, was designed .

[0043] According to an embodiment of the invention, the use of a couple of temperature sensors 40 , 50 is provided to measure the temperature of the moisture sensor 10 .

[0044] The temperature sensors 40 , 50 applied to the moisture sensor 10 are visible in figures 1-3 .

[0045] The temperature sensors 40 , 50 are contactless optical temperature sensors , i . e . sensors that , as is known, are able to measure the temperature by detecting the optical properties of the materials , properties which change predictably with temperature .

[0046] According to an embodiment of the invention, it is provided to use at least one first contactless sensor 40 able to measure temperature variations of 0 . 1 ° C and placed in proximity to the moisture sensor 10 to accurately measure the temperature of the dielectric 30 placed between the metal rings 30 .

[0047] The information obtained from the sensor 40 allows to calculate , through a control algorithm, the correction to be applied to the measurement of the moisture sensor 10 due to both thermal expansion and the variation of the dielectric constant over a temperature range from 10°C to 200°C, with variation of less than 2.8fF / °C.

[0048] At least one second contactless sensor able to measure temperature variations of O,1°C and placed in proximity to the moisture sensor 10 to accurately calculate the thermal expansion of the metal rings 30 is further provided.

[0049] The information obtained from the sensor 50 allows to calculate the capacitance variation due to the thermal movement of the metal part of the sensor and to compensate for it adequately through a control algorithm, i.e. by calculating the correction to be applied to the measurement of the moisture sensor 10 due to thermal expansion.

[0050] In this way, in a temperature range from 20°C to 200°C, a capacitance variation from the capacitance value of the empty sensor of no more than + / - 200fF (femtoFarad) is maintained over time.

[0051] The use of the two very accurate (0.1 °C) contactless sensors 40,50 for measuring the electrode and dielectric temperature allows, thanks to appropriate adjustment algorithms, to automatically correct the capacitance of the empty sensor due to both thermal expansion and the variation of the dielectric constant of the dielectric material interposed between the electrodes.

[0052] Moreover, according to an embodiment of the invention (not depicted for simplicity) , a holder made of a micabased material, which allows to channel a flow of air through a fan to maintain uni form heat distribution along the entire length of the sensor, was designed .

[0053] The operation of the moisture sensor for measuring the moisture of materials in the form of dry, liquid or gaseous granulates , or in the form of powders , in at least one conduit according to the present invention is clear for the field technician thanks to what was described above and is , in particular, as follows .

[0054] Firstly, the moisture sensor 10 is positioned inside the conduit . Subsequently, the temperature of the capacitor inside which the material flows is measured through the moisture sensor 10 . Finally, a correction to be applied to the CX capacitance of the empty moisture sensor 10 , i . e . when the moisture sensor 10 is not crossed by any material , is calculated, such correction taking into account the thermal expansion of the capacitor following the flow of the materials , whose moisture is to be measured, therein .

[0055] In particular, the measuring step comprises measuring the temperature of the rings made of dielectric material 20 and the calculating step comprises calculating the correction to be applied to the measurement of the moisture sensor ( 10 ) due to both thermal expansion and the variation of the dielectric constant over a temperature range from 10 ° C to 200 ° C with a variation of less than 2 . 8 fF / ° C .

[0056] As an alternative or in addition, the measuring step comprises measuring the temperature of the metal rings 30 and the calculating step comprises calculating the correction to be applied to the measurement of the moisture sensor 10 due to thermal expansion, so that in a temperature range from 20 ° C to 200 ° C, a capacitance variation from the capacitance value of the empty sensor of no more than + / - 200 fF is maintained .

[0057] Obviously, a technician of the field can make further changes and variations to the present invention, all thereby comprised in the protection scope of the invention, as defined by the following claims , for the purpose of meeting contingent and speci fic needs .

Claims

CLAIMS1. Moisture sensor (10) for measuring the moisture of materials in the form of dry, liquid or gaseous granulates, or in the form of powders in at least one conduit arranged at least partially along an axis (X-X) of the moisture sensor (10) through which the materials whose moisture is to be measured flow, the moisture sensor (10) comprises at least one capacitor in which the material whose moisture is to be measured flows, wherein the aforesaid capacitor comprises rings made of dielectric material (20) interposed between metal rings (30) mounted coaxially to said axis (X- X) between containment flanges, characterized in that the aforesaid moisture sensor (10) is associated with at least one contactless optical temperature sensor (40,50) configured to measure the temperature of the capacitor in order to calculate a correction to be applied to the capacitance (CX) of the empty moisture sensor (10) due to the thermal expansion of the capacitor following the flow of the materials, whose moisture is to be measured, therein .

2. Sensor according to claim 1, wherein the at least one contactless optical temperature sensor (40) is positioned in proximity to the capacitor to accurately measure the temperature of the rings made of dielectric material (20) .

3. Sensor according to claim 2, wherein the temperature measurement obtained from the contactlessoptical temperature sensor (40) allows to calculate the correction to be applied to the measurement of the moisture sensor (10) due to both the thermal expansion and the variation of the dielectric constant over a temperature range from 10°C to 200°C with a variation of less than 2.8fF / °C.

4. Sensor according to claims 1 to 3, wherein the at least one contactless optical temperature sensor (50) is positioned in proximity to the capacitor to accurately measure the temperature of the metal rings (30) .

5. Sensor according to claim 4, wherein the temperature measurement obtained from the temperature sensor (50) allows to calculate the correction to be applied to the measurement of the moisture sensor (10) due to thermal expansion, such that in a temperature range from 20°C to 200°C, a capacitance variation from the capacitance value of the empty sensor of no more than + / - 200fF is maintained .

6. Sensor according to any one of claims 1 to 5, wherein the contactless optical temperature sensor (40, 50) is configured to measure temperature variations with an accuracy of at least 0.1°C.

7. Sensor according to any one of claims 1 to 6, wherein the moisture sensor (10) comprises a holder made of a mica-based material, with which a fan is associated to facilitate uniform heat distribution along the entire length of the sensor.

8. Sensor according to any one of claims 1 to 7, wherein the rings made of dielectric material (20) are made of PEEK (polyether ether ketone) , such that to give the moisture sensor (10) high resistance to heat and chemical agents, as well as good electrical insulation properties.

9. Method for measuring the moisture of materials in the form of dry, liquid or gaseous granulates, or in the form of powders in at least one conduit, the method comprising the following steps:- prearranging, inside the conduit, a moisture sensor (10) comprising at least one capacitor which comprises rings made of dielectric material (20) interposed between metal rings (30) mounted coaxially to an axis (X-X) of the sensor between containment flanges, wherein at least one contactless sensor (40,50) is associated with the moisture sensor ( 10 ) ;- measuring, through said moisture sensor (10) , the temperature of the capacitor; and calculating a correction to be applied to the capacitance (CX) of the empty moisture sensor (10) due to the thermal expansion of the capacitor following the flow of the materials, whose moisture is to be measured, therein .

10. Method according to claim 9, wherein said measuring step comprises measuring the temperature of the rings made of dielectric material (20) and said calculating step comprises calculating the correction to be applied tothe measurement of the moisture sensor (10) due to both the thermal expansion and the variation of the dielectric constant over a temperature range from 10°C to 200°C with a variation of less than 2.8fF / °C.

11. Method according to claim 9 or 10, wherein said measuring step comprises measuring the temperature of the metal rings (30) and said calculating step comprises calculating the correction to be applied to the measurement of the moisture sensor (10) due to thermal expansion, such that in a temperature range from 20°C to 200°C, a capacitance variation from the capacitance value of the empty sensor of no more than + / - 200fF is maintained.

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