MEASURING OF WATER CONTENT

FIELD

[0001] The invention relates to a method, a dewatering element, a coupling element and a measuring device for measuring water content in a wire and a web in a former section of a paper machine.

BACKGROUND

[0002] Water content may be measured in a former section of a paper machine by using microwave radiation having a frequency band of 1 GHz to 300 GHz. Measuring may be performed, for instance, such that inside a mul- tifoif shoe there is placed a microwave sensor that forms a microwave field extending partly up to the wire and the web to be measured and propagating bound to the multifoil shoe partly in the wire and in the web. Permittivity of the sensor material is allowed to be at most equal to that of the wire and the water therein, because otherwise total reflection would inhibit measuring. For instance, silicon carbide is a suitable sensor material (ε _r = 5.6). Because the sensor is inside the multifoil shoe, there is a slight distance between the sensor and the wire. Because the high relative permittivity of water in the wire and the web with respect to other substances contributes to the propagation rate of microwaves, it is possible to measure the amount of water in the wire and the web. This solution is disclosed, for instance in EP 1624298.

[0003] Measuring involves a plurality of problems, however. Because of high frequency only a small portion of radiation used in the measuring extends up to the wire and the web, which reduces accuracy of measuring. This problem is further made worse by the gap between the sensor and the wire. In addition, to arrange permittivity of the sensor material to be clearly lower than that of water makes the measuring frequency rise to the microwave range.

BRIEF DESCRIPTION

[0004] The object of the invention is to provide an improved method, a dewatering element, a coupling element and a measuring device. This is achieved by a method for measuring water content in a former section of a paper machine by radio-frequency electromagnetic radiation on an object to be measured, which comprises at least a wire and a web, in which method there is measured at least one electromagnetic radiation characteristic dependent on

the water content of the object to be measured. The method further transfers with at least one partly metal-coated dewatering element, which is in contact with the object to be measured, has permittivity higher than 75 and serves at the interface between the measuring device unit and the object to be measured both structurally and operationally as a coupling element, radio-frequency electromagnetic radiation of less than 1 GHz between the object to be measured and the measuring device unit for measuring water content in the object to be measured.

[0005] The object of the invention is also a dewatering element, which is located in the wire section of the paper machine and is in contact with the object to be measured that comprises at least the wire and the web. The dewatering element is connected by radio-frequency electromagnetic radiation from the object to be measured to a measuring device unit measuring the water content, the dewatering element is configured to act at an interface between the measuring device unit and the object to be measured both structurally and operationally as a coupling element whose relative permittivity exceeds 75, and the surface of the dewatering element is partly metal-coated, and the dewatering element is configured to transfer radio-frequency electromagnetic radiation of less than 1 GHz between the object to be measured and the measuring device unit for measuring the water content in the object to be measured.

[0006] The object of the invention is also a measuring device, which comprises at least one dewatering element, which is located in the wire section of the paper machine and is in contact with the object to be measured that comprises at least the wire and the web. The at least one dewatering element is connected by radio-frequency electromagnetic radiation from the object to be measured to a measuring device unit measuring the water content, the at least one dewatering element is configured to act at an interface between the measuring device unit and the object to be measured both structurally and operationally as a coupling element whose relative permittivity exceeds 75, and the surface of the dewatering element is partly metal-coated, and the at least one dewatering element is configured to transfer radio-frequency electromagnetic radiation of less than 1 GHz between the object to be measured and the measuring device unit for measuring the water content in the object to be measured.

[0007] The object of the invention is still further a coupling element, which is connected to the measuring device unit that is configured to perform water content measurement in the wire section of the paper machine on the object to be measured, which comprises at least the wire and the web, by means of radio-frequency electromagnetic radiation. The coupling element is located at the interface between the measuring device unit and the object to be measured such that the coupling element is in contact with the object to be measured. The coupling element is configured to transfer radio-frequency electromagnetic radiation of less than 1 GHz between the object to be measured and the measuring device unit; the relative permittivity of the coupling element is higher than 75; and the surface of the coupling element is partly metal- coated.

[0008] Preferred embodiments of the invention are disclosed in dependent claims.

[0009] Several advantages are achieved with the method and system of the invention. Because of the frequency selection and the radiation supply method the radiation used in the measuring propagates efficiently up to the wire and the web. This is further improved by fixed coupling to the wire. In addition, high permittivity of the material enables use of low measuring frequency and thus deep penetration into the material to be measured.

LIST OF DRAWINGS

[0010] In the following the invention will be described in greater detail in connection with preferred embodiments with reference to the attached drawings, wherein

Figure 1 A shows the structure of a dewatering element, Figure 1 B shows an alternative structure of a dewatering element, Figure 2 shows behaviour of radiation at an interface, Figure 3 shows a wave-guide-like dewatering element, Figure 4 shows a dewatering element of a coaxial resonator type, Figure 5 shows a dewatering element that operates in the manner of a ring-like slot resonator,

Figure 6A is a top view of a dewatering element having a coupling slot,

Figure 6B is a side view of a dewatering element having a coupling slot,

Figure 6C shows strength of an electric field in the object to be measured,

Figure 7A shows transmission of radiation along the object to be measured,

Figure 7B shows a dipole receiver (or transmitter),

Figure 7C shows a slot transmitter (or receiver),

Figure 8 shows a strip structure in a wire section of a paper machine,

Figure 9 shows a measuring device, and

Figure 10 is a flowchart of the method.

DESCRIPTION OF THE EMBODIMENTS

[0011] In the presented solution the coupling element also serves at the same time as a dewatering element, i.e. a multifoil shoe. One option is that the coupling element is part of the resonator structure that does not radiate (much) to a far field, the electric field as a whole or for major part constituting a near field. Thus the radiation transmitted by the coupling element to the object to be measured does not interfere with other devices or other coupling elements. In that case the coupling element may operate like a resonator, the resonance frequency of which depends on the water content of the object to be measured.

[0012] With reference to Figures 1A and 1B the structure of a dewatering element used in the wire section of the paper machine is now examined. The dewatering element 100 is in contact with the object to be measured 102, which include at least a web 104 and a wire 106. It is possible that on top of the web 104 there is yet another wire. The web 104 may be water-containing wood pulp used for paper and board making. In the former section of the paper machine water is removed from the web 104, and therefore measuring the amount of water in machine direction and in transverse direction to the machine direction is useful. The dewatering element 100 may be coated with metal or other conductive material 108 on all other surfaces except for the surface that is in contact with the object to be measured 102. Other parts in the structure of the dewatering element 100 may be made of ceramic whose permittivity is higher than permittivity of water at the frequency used for measuring. Permittivity may be, for instance, 75 to 100.

[0013] Radio-frequency electromagnetic radiation having a frequency of less than 1 GHz may be supplied through an input terminal 110 to the dewatering element 100 from one edge and received at another edge through a reception terminal 112. In that case radio-frequency radiation is transferred from the dewatering element 100 through the input terminal 110 to the object to be measured 102 and from the object to be measured 102 through the dewatering element 100 back to the reception terminal 112, where- from the radiation is transferred for measuring. Alternatively the input terminal 110 may also act as a reception terminal, and consequently no separate reception terminal is needed. In that case the radio-frequency radiation is transferred from the dewatering element 100 through the input terminal 110 to the object to be measured 102, it reflects from the object to be measured 102 to the dewatering element 100 and therethrough back to the input terminal 110 acting as the reception terminal 110, from which the radiation is transferred for measuring.

[0014] Figure 1B shows a dewatering element 100 which comprises only one combined input and reception terminal 114 in the middle of the dewatering element 100. Thus, the radio-frequency radiation reflects back from the object to be measured and the frequency response of reflection allows measurement of a resonance frequency indicating the amount of water.

[0015] Figure 2 shows principles used in measurings shown in Figures 1A and 1B. The layer below 202 represents material of the dewatering element and the layer above represents the object to be measured 102. When electromagnetic radiation 200 is directed to an interface of two substances such that the radiation comes to the interface from a substance having a higher refractive index and also a higher permittivity, at a given angle of incidence α the radiation provides total reflection. In order to avoid this, it is possible to use dimensions that do not provide total reflection.

[0016] To avoid total reflection the dewatering element may be dimensioned such that a situation where total reflection occurs is not allowed. In that case the dimension a of the dewatering element is to be sufficiently small, so that rays from the middle of the dewatering element meet the interface at an angle α smaller than the total reflection angle.

[0017] Figure 3 shows a solution in which the dewatering element 100 may act like a wave guide. The exterior 300 of the dewatering element 100, which may produce a short-circuit in the lower part, may be made of metal

or other conductive material, and inside the exterior there is ceramic 302 having permittivity exceeding 75. Field lines of an electric near field may be drawn to start from the centre of the dewatering element 100 and to curve through the object to be measured 102 towards the exterior 300. Thus the water in the object to be measured will affect the resonance frequency to be generated. The dewatering element 100 may work on a non-radiating waveform TM ₀₁ without excluding other waveforms.

[0018] Figure 4 shows a solution in which the dewatering element acts like a coaxial resonator. In this solution there is metal or other conductive material 400 in the middle of the dewatering element. Around the conductive middle part 400 there is ceramic 402 having permittivity exceeding 75. The outer circumference 404 of the coaxial resonator, in turn, is made of conductive material just like the middle part 400. In this solution as weil, field lines of an electric near field start from the centre of the dewatering element 100 and curve through the object to be measured 102 towards the edges. Thus the water in the object to be measured will affect the resonance frequency to be generated.

[0019] The dewatering element 100 may also comprise a radiating slot in a metal plate, which may be metal plating of a circuit board, for instance. A curved slot may constitute a (near) circle, whereby its electric field radiating to a far field is (almost) completely cancelled. The centre line of the slot may form a curve that is linear in sections. The centre line of the slot may also form a curve with continuous curvature like in a non-linear function with continuous derivative. The centre line of the slot may also represent a non-self-crossing arched curve. Figure 5 shows field lines of a near field extending from the slot up to the object to be measured 102. Thus the water in the object to be measured is able to affect the resonance frequency to be generated.

[0020] Figure 6A shows a solution which also employs a slot 600 in metal plating 602. The slot 600 itself is not dimensioned to be a resonator, nor does it radiate, but it connects radio-frequency radiation to the object to be measured. Thus, the slot 600 is shorter than the resonance length.

[0021] Figure 6B shows a dewatering element 100 whose surface that is in contact with the object to be measured comprises a metal plating 602 with slot. Other parts of the dewatering element 100 are of ceramic having permittivity exceeding 75.

[0022] In Figure 6C the arrows indicate the strength of the electric field generated by the dewatering element 100 in the object to be measured 102. The electric field reaches its maximum when the thickness of water layer equals one fourth of the wavelength of the radiation used in measuring. Thus, when the amount of water and consequently the thickness of water layer changes, the maximum wavelength changes. Because the maximum is in resonance, the amount of water may be determined by means of a resonance frequency. The amount of water may correspond to the thickness of 10 mm, for instance. In that case the resonance frequency will be about 833 MHz (= 4 * λ/4 * 380 mm), when permittivity of water is assumed to be 81 , for the sake of simple calculation.

[0023] Figure 7A shows a solution, in which the supplying dewatering element 700 supplies radio-frequency electromagnetic radiation to the object to be measured 102, which radiation is received when the radiofrequency radiation has propagated to a receiving dewatering element 702. The dewatering elements 700 and 702 are supported with support structures 704 and 706, by means of which the dewatering elements 700, 702 may be secured to strips in the wire section. In measuring the radio-frequency radiation may be supplied to the object to be measured 102 at an angle that produces total reflection at the interface between the object to be measured and the air. Thus the radiation will not escape to the environment and consequently will not interfere with other devices. Radiation may also propagate in a substrate form, whereby the radiation propagates partly in the air. The measuring may be carried out as a measuring of phase, attenuation, travel time, resonance frequency or a combination thereof.

[0024] Figure 7B shows a solution, in which on the surface or in the vicinity of the surface of the dewatering element 700 there is at least one di- pole 720 for transmitting radiation. The elements 722 of the dipole 720 may be of metal. A corresponding dipole structure is also suitable for a receiving dewatering element 702. This solution allows transmission of TE-waveform which propagates in a substrate form in the object to be measured 102. When using a plurality of dipoles a transmission direction may be defined by suitably phasing the radiation from the dipoles 720. Correspondingly, it is possible to define a direction of reception.

[0025] Figure 7C shows a solution, where on the surface of the dewatering element 700 there is metal plating 740 comprising at least on slot

742. A corresponding slot structure is also suitable for a receiving dewatering element 702. This solution allows transmission of TM-waveform which enables total reflection at the interface between the object to be measured 102 and the air. When using a plurality of slots a transmission direction may be defined by suitably phasing the radiation from the slots 742. Correspondingly, it is possible to define a direction of reception.

[0026] Figure 8 shows strips holding the multifoil shoes, i.e. the de- watering elements. Strips 800 to 806 may be more than one and they are located successively in machine direction. Each strip comprises at least one de- watering element 100. When measuring is performed horizontally, in the direction of the strip, with dewatering elements, it is possible to receive information on water content in the web in transverse direction. When measuring is performed vertically, using the shaded dewatering elements, it is possible to receive information on water content in the web in machine direction. When water content is measured in machine direction, it is also possible to determine a change in the water content of the object to be measured and thus it is possible to receive information on the effectiveness of dewatering in the wire section.

[0027] Figure 9 shows a measuring device. Each dewatering element 100 is connected to a measuring device unit 900 that includes a source of radio-frequency electromagnetic radiation. The measuring device unit 900 supplies radio-frequency radiation to at least one dewatering element 100, wherefrom radiation propagates to the object to be measured 102. The object to be measured 102 affects at least one characteristic of the radio-frequency radiation (phase, frequency, strength, delay, resonance, etc.). At least one de- watering element 100 receives radio-frequency radiation from the object to be measured 102 and forwards the radio-frequency radiation to the measuring device unit 900, by which at least one characteristic of the radio-frequency radiation may be measured. The measuring part of the measuring device unit 900 may measure water content in the object to be measured 102 on the basis of the measured characteristic.

[0028] An electric near field cooperates thus efficiently with the wire 106, the web 104 and the water therein, because the dewatering element 100 comes into contact with the wire 106. The resonance frequency of the dewatering element 100 depends on the wire 106, the web 104 and the amount of water, of which only the amount of water varies. Correspondingly, when the

measuring method of e.g. Figure 7 A is used, the value of phase, attenuation, travel time depend on the amount of water. The measuring device may scan over a desired frequency band and search for a resonance frequency, or the resonator relating to the dewatering element may automatically seek its way and lock to its resonance frequency according to its characteristics, which are affected by the wire, the web and the amount of water, for instance.

[0029] Figure 10 is a flowchart of the method. In step 1000 with at least one partly metal-coated dewatering element, which is in contact with the object to be measured and whose permittivity is higher than 75 and which serves at the interface between the measuring device unit and the object to be measured as a coupling element both structurally and operationally, there is transferred radio-frequency electromagnetic radiation of less than 1 GHz between the object to be measured and the measuring device unit for measuring water content in the object to be measured.

[0030] Even though the invention is described above with reference to the examples of the attached drawings, it is apparent that the invention is not restricted thereto, but it may be modified in a variety of ways within the scope of the accompanying claims.