WO1998044339A1

WO1998044339A1 - Method and arrangement for measuring and controlling consistency

Info

Publication number: WO1998044339A1
Application number: PCT/FI1998/000273
Authority: WO
Inventors: Pekka Jakkula
Original assignee: Valmet Automation Inc.
Priority date: 1997-03-27
Filing date: 1998-03-26
Publication date: 1998-10-08
Also published as: FI971324A0; FI101751B; AU6731798A; FI101751B1

Abstract

The invention relates to a measuring method and a measuring arrangement for measuring particularly consistency with a radiometer which measures the thermal microwave radiation of an object of measurement. The radiometer (101) measures the consistency of the measurement object as a function of the intensity of the microwave radiation (105) using as radiation measuring frequency such a frequency that the intensity of interfering radiation is attenuated to such a low level when penetrating the measurement object (103) that it becomes unessential to the measurement, and the measurement object (103) is thus used to filter down interfering background radiation (108).

Description

METHOD AND ARRANGEMENT FOR MEASURING AND CONTROLLING CONSISTENCY

FIELD OF THE INVENTION

The invention relates to a method for measuring consistency in par- ticular, the method using a radiometer comprising an antenna in the vicinity of an object of measurement for receiving natural thermal microwave radiation, and the radiometer being used to measure microwave radiation transmitted by the object of measurement, the intensity of the microwave radiation being dependent on the consistency of the object of measurement. The invention also relates to a method for controlling a former, the method being used in a paper machine comprising a headbox, a former, a press section, a wire, a paper web being the object of measurement formed on the wire, the method comprising measuring the consistency of the paper web passing from the headbox of the paper machine to the wire and travelling on the wire to the former and further to the press section, and the method using a radiometer comprising an antenna in the vicinity of the paper web constituting the measurement object, for receiving natural thermal microwave radiation, and the radiometer being used to measure the microwave radiation transmitted by the paper web which constitutes the object of measurement, the inten- sity of the microwave radiation being dependent on the consistency of the paper web.

The invention further relates to an arrangement for measuring consistency in particular, comprising a radiometer and an antenna in the vicinity of an object of measurement for receiving natural thermal microwave radiation, and the radiometer being arranged to measure the microwave radiation transmitted by the object of measurement, the intensity of the microwave radiation being dependent on the consistency of the object of measurement.

The invention further relates to an arrangement for former control to be used in a paper machine comprising a headbox, a former, a press section, a wire, a paper web on the wire, and a measuring arrangement arranged to measure the consistency of the paper web passing from the headbox of the paper machine to the wire and travelling on the wire to the former and further to the press section, and the measuring arrangement comprising a radiometer and an antenna in the vicinity of the paper web constituting the object of measurement for receiving natural thermal microwave radiation, and the radi- ometer being arranged to measure the microwave radiation transmitted by the object of measurement, the intensity of the microwave radiation being dependent on the consistency of the object of measurement.

DESCRIPTION OF THE PRIOR ART Accurate measurement and adjustment of the water or solid content, i.e. consistency, is needed in the manufacture of high-quality paper. If the consistency of a paper web is too low, the paper web will not remain homogeneous and much extra energy is consumed in the drying phase. If again the consistency of a paper web is too high, the paper web will not remain homo- geneous, thus impairing the quality of the paper manufactured. The consistency is measured particularly in the former, the adjustment of which is currently generally based on measuring the amount of removed water. This kind of measurement of the water content, or more exactly the consistency, of a paper web is inaccurate e.g. because the removed water contains a lot of air. The measurement can also be based on radioactive transmittance measurement, a drawback of this being that the measurement will include the wire under the web and the water bound thereto. Moisture and consistency meters based on infrared techniques have also been tested in said application, the their problem is that they only measure surface moisture. Microwave technique has also been applied to measuring the consistency of a paper web, the meters usually being installed after the press section before the drying of the paper. At these points there is no longer a wire under the paper web, whereby meters based on paper transmittance can also operate accurately. On the other hand, the former can no longer be controlled after the press section. Mi- crowave methods are based either on a microwave resonator or on an active imaging technique. The problem of microwave resonators is that they are usually disposed at both sides of the web, the wire and the water bound thereto disturbing the measurement. Imaging methods are also based on paper web transmittance, with the transmitter and receiver being disposed at different sides of the paper, or the measurement being carried out by letting the signal be reflected from a reflector installed behind the web. In these cases, too, the wire and the water therein disturb the measurement.

CHARACTERISTICS OF THE INVENTION

It is accordingly an object of the present invention to provide a method and an arrangement for measuring the microwave radiation originating from an object of measurement, even from underneath the surface, and simultaneously to avoid the interferences inside the object caused by the interfering radiation originating from the wire or from somewhere else in the background and the reflections of the radiation originating from the object itself. This is achieved by a measuring method of the type presented in the introduction, characterized in that the consistency of the measurement object is measured as a function of the intensity of the microwave radiation by using such a measuring frequency or frequency range that the intensity of the interfering radiation through the measurement object is attenuated when penetrating the measurement object to a degree that it becomes unessential to the measurement, and the measurement object is thus used to filter down the interfering background radiation.

The method of controlling a former according to the invention is characterized by measuring the consistency of the paper web constituting the measurement object as a function of the intensity of the microwave radiation by using such a measuring frequency or frequency range that the intensity of the interfering radiation penetrating the paper web is attenuated in the paper web to a degree that it becomes unessential to the measurement, and the paper web constituting the measurement object is thus used to filter down the interfering background radiation, and the measured consistency result being used to control the former to correct the consistency in the paper web.

The measuring arrangement of the invention is characterized in that the measuring arrangement is arranged to measure the consistency of the measurement object as a function of the intensity of the microwave radiation by using such a measuring frequency or measuring frequency range that the intensity of the interfering radiation penetrating the measurement object is attenuated to a degree that it becomes unessential to the measurement, and the measurement object is thus arranged to filter down the interfering radiation originating from the background. The control arrangement of the invention, in turn, is characterized in that the measuring arrangement is arranged to measure the consistency of the measurement object as a function of the intensity of the microwave radiation by using such a measuring frequency or measuring frequency range that the intensity of the radiation penetrating the measurement object is attenuated to a degree that it becomes unessential to the measurement, and the measurement object is thus arranged to filter down the interfering radiation originating from the background, and the measuring arrangement is further arranged to control the former to correct the consistency of the paper web.

The method and arrangement of the invention provide significant advantages. They can be used to measure the consistency of a paper web or a corresponding measurement object by using microwave radiation which originates even from underneath the surface, however, simultaneously avoiding the interfering radiation originating from the wire or other background.

DESCRIPTION OF THE FIGURES

In the following the invention will be described in greater detail with reference to the examples according the attached drawings, in which

Figure 1 shows a measurement procedure and a measuring device, Figure 2 shows the principle of the behaviour of radiation intensity as a function of frequency and the thickness of an object of measurement, Figure 3 shows a radiometer provided with several antennae, Figure 4 shows the placement of measuring devices in a paper machine,

Figure 5 shows measurement of coating slip from a roll, Figure 6 shows measurement of coating slip from a sheet of paper, and Figure 7 shows the change in the brightness temperature of the microwave radiation emitted from the paper web as a function of time with the consistency of the paper web increasing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The solution of the invention is suitable for use particularly in the manufacture and coating of paper, being, however, not limited thereto. Let us first study the theoretical background of the invention. The term depicting the intensity of the radiation is the brightness temperature of the object and it can be presented by the formula (1 ).

T_B = T_P, (1 )

where T_B is the brightness temperature, T_P is the physical temperature of the object and η is the emissivity of the object, which again can be generally presented e.g. by the formula (2). 7= ι- r_£ (2)

where I r_E| is the absolute value of variable r_E, which according to the formula (3) is

where μ- is the permeability of loss-free material (air) = μ_r1μ₀, μ_r1 is the relative permeability of loss-free material (air) = 1 , μ₂ is the permeability of lossy material (paper) = μ_r2μ₀. μ_r2 is the relative permeability of lossy material (paper), μ₀ is the permeability of vacuum = 4π x 10^"7 H/m, z. is the permittivity of loss-free material (air) = ε_r1ε₀, ε_r1 is the relative permittivity of loss-free material (air) = 1 , ε., is the permittivity of lossy material (paper) = ε_r1ε₀, ε_r1 is the relative permittiv- ity of lossy material (paper), ε_r1 is the relative permittivity of lossy material (paper), ε₀ is 8.854 x 10^"12 F/m, n is the unit vector of the normal of the reflecting surface, , is the wave vector of the radiation, ω is 2πf and f is the frequency of the radiation.

When applying a microwave measuring technique in accordance with the invention to the measurement of paper, coating material or the like, the following factors caused by the circumstances have to be taken into account. It is preferable to select a measuring frequency at which the measurement object, such as paper, cannot "be seen through". In practice this means that when passing through the measurement object, the microwave attenua- tion has to be so high that all radiation received by the antenna originates substantially from the measurement object, and not from behind it, such as the wire behind the paper web. However, this allows radiation to be originated from inside the measurement object, not merely from the surface. A paper machine environment, for example, or any measurement environment, has many warm, rough objects which radiate at a brightness temperature corresponding to their physical temperature. The measurement antennae have to be so designed and located that the reflection of this radiation from the measurement object, such as paper, to the reception part of the antenna is prevented. Microwave attenuation in the measurement object can be calculated by the for- mula (4) A(dB/cm) = 17.372 ^■ (2π/λ • I ε_r l ^{1 2} • sin(s/2), (4)

where A = microwave attenuation, λ is the wavelength of a microwave in cen- timetres, l ε_rl = js'² _r+£"_r ² , and s is arctan (<s I ε_r).

This means that the dielectric properties of the material to be measured have to be known to be able to use the formula (4). The dielectric properties of water are known and the following values, for instance, can be used in the calculation. When frequency f is f = 10 GHz, the dielectric constants for water are έ_r = 60 and ε_r = 20. When frequency f is f = 24 GHz, the dielectric constants for water are έ_r = 40 and ε = 32. The dielectric properties of paper, in turn, are hardly at all dependent on the frequency because cellulose molecules are large hydrocarbon chains whose polarity is low particularly at high frequencies. The dielectric constants of paper are

- 3 and ε_r = 0.1. The dielectric constants of a paper web having a consistency of

15% can be approximated from the values of water and dry paper. The dependence is non-linear and various models for mixtures have been developed for the calculation thereof. By calculation, the dielectric constant obtained for a randomly distributed needle-like material (paper) with a 15% consistency is as follows. When frequency f is f = 10 GHz, the real dielectric constant έ_r is έ_r = 46.4 and the imaginary dielectric constant ε_r is ε_r = 15. When frequency f is f = 24 GHz, the corresponding values for a paper web are ε_r = 31.4 and ■4 = 24.1. By inserting the values in the formula (4), the following results are obtained: when measuring frequency is f = 10 GHz, attenuation A is A = 4.0 dB/mm and when frequency f is f = 24 GHz, attenuation A is A = 14.7 dB/mm. After the former the paper web usually has a thickness of about 1 mm, i.e. the measuring frequency should exceed 20 GHz in order for the wire under the paper web and the water therein not to affect the brightness temperature to be measured and consequently the consistency measurement. By way of example the brightness temperature difference between the paper web and water could be calculated at frequency f = 24 GHz by the formula (2). By assuming the web to have a physical temperature T_P = 20°C (293 K), the obtained brightness temperature of water T_Bwater = 105.8 K and the brightness temperature of the paper web T_Bpaper = 122.1 K. It is noted that both objects are cold with a distinct difference of about 16 K. The sensitivity needed in the measurement is calculated next. A radiometer's sensitivity can be cal- culate by the formula (5).

where K = 2 (Dicke radiometer), T_s is the system noise temperature of the receiver, B is the bandwidth of the measuring frequency before indication and τ is integration time. The system noise temperature can be calculated by the formula (6)

T_s = T₀ - (F - l), (6)

where F = noise figure of the receiver and T₀ = temperature of the receiver. By assuming the values easily obtainable with the present components: F = 5 dB, T₀ = 293 K, B = 1 GHz and τ = 1 s, ΔT = 0.01 K is obtained, showing that with an one-second integration time a 0.015% resolution power can be achieved.

Let us now study the solution of the invention in greater detail by means of Figure 1. The measuring arrangement comprises a radiometer 101 , an antenna 102, a measurement object 103, in this example a paper web, and a wire 104. The paper web 103 emits natural black body radiation 105 to the antenna 102. The wire 104 also emits radiation 107. Radiation 108 and 106 is also emitted from underneath the wire 104 and from other parts of the environment. The radiation 106 and 108 emitted by the wire 107 and the environment interferes with the measurement performed on the paper web 103. For this reason the radiometer 101 comprises a filter with which the measuring frequency, frequencies or frequency band are selected such that the background radiation 108 emitted from the wire 107 and from underneath the wire is substantially attenuated off when passing through the paper web 103. Consequently the measuring frequency preferably exceeds 20 GHz. More precisely, the frequency or the frequencies are adjusted case-specifically using e.g. the formula (4).

Figure 1 also shows one of the advantageous characteristics of the inventive method. The radiation coming obliquely from behind the radiometer 101 does not affect the measurement if the antenna 102 is installed in the vicinity of the surface of the web 103. Furthermore, the structure of the antenna 102 is such that it prevents harmful background radiation 106 from penetrating the reception part of the antenna and further the radiometer 101. In this case the antenna 102 is e.g. a horn antenna. The antenna 102 is preferably such that it receives best with its middle part with antenna gain decreasing towards the edges. Such a solution further decreases interference, since interference is likely to penetrate the antenna 102 from the edges at which the effect is at its lowest in this manner. The radiometer 101 is calibrated every so often during e.g. production breaks by measuring a reference sample with both antennae. The reference sample is preferably a wire or a body with a cold brightness temperature, such as a metal surface or the like. Between the measurements the radiometer 101 can be preferably switched to measure a reference load being heated, e.g. a matched graphite terminal.

Figure 2 shows how the amplitude of a measurement signal acts as a function of the frequency and the thickness of the measurement object. Figure 2 is schematic and consequently does not present an actual measurement result. At low frequencies the amplitude shows oscillation 202 caused by the interference of radiation emitted from behind the measurement object (e.g. radiation emitted from the wire, since the measurement object can be seen through) and radiation emitted by the measurement object (e.g. a paper web). As the measuring frequency increases, the oscillation substantially stops, and at high frequencies 201 the amplitude remains stable since the measurement object no longer permeates radiation. The inventive method explicitly utilizes the frequency, frequencies or frequency band 201 , with which the amplitude of the radiation remains stable. It can also be seen from Figure 2 that as the thickness d of the measurement object, e.g. the paper web 103, increases, the attenuation of the measurement object also increases, and at lower frequen- cies the amplitude is stable. In other words, the measuring frequency must also be selected according to the thickness d of the measurement object.

The inventive arrangement for measuring consistency preferably comprises at least one antenna 102. The entire width of the web 103 can be measured by sweeping either by moving one pair of antennas or thermome- ters mechanically to and fro over the web or by electronic sweeping according to Figure 3. When one antenna 102 is used, the antenna preferably moves across the measurement object thus allowing measurement of the consistency over the entire width of the surface of the measurement object, or at least at several points. In Figure 1 , arrows by the antenna 102, pointing to both sides, show how the antenna 102 can move. This way the antenna 102 can move across the paper web 103 and measure the transverse consistency of the pa- per web 103.

Figure 3 shows a preferred embodiment of the invention comprising a plurality of antennae 302. In this case the consistency of the measurement object can be measured from the entire surface at several points by fixed an- tennae 302. The antennae preferably form a chain, the chain at least approximately being perpendicular to the travel direction of the paper web. Since the web can move e.g. 1.5 km/min, the measurement has to be carried out rapidly. The antennae 302 are coupled to the radiometer 101 by cables and switches 301 , which are preferably electronic switches, such as PIN diode switches. The switches 301 switch the antennae 302 preferably one by one to the radiometer 101 to measure the consistency of the measurement object 103. The antennae 302 also preferably comprise a pre-amplifier and a mixer each having a local oscillator, for decreasing the measurement signal frequency. A measurement signal with decreased frequency can be moved more easily to the radiometer 101. When such a mixer solution is used, the switches 301 are not necessary, since switching the antennae 302 to the radiometer can be controlled by the pre-amplifier and/or the mixer. The achievable resolution in the lateral direction of the web depends on the size and number of the antennae 302. In practice the minimum diameter of one antenna is about 2 to 3 cm and the number of the antennae 302 between 100 and 500. The number of the antennae 302 is also affected by the fact that sweeping becomes slower as the number of antennae increases, since each antenna has to be switched to the receiver until an adequate measuring accuracy is achieved. The measuring arrangement is calibrated at times, e.g. during production breaks, by measuring a reference sample with each antenna. The reference sample is preferably the wire or a body with a cold brightness temperature, such as a metal surface or the like. Between measurements the radiometer 101 can be preferably switched to measure a reference load being heated, e.g. a matched graphite terminal. Figure 4 shows the installation site of the radiometer 101 at the paper machine. In the inventive solution, this part of the paper machine typically comprises a radiometer 101 , a headbox 401 , an upper wire 402, a lower wire 403, a thermometer 404, a press section 405 and control means 406. The radiometer 101 measures the moisture content, i.e. the consistency, of the paper web after the former before the press section 405 and/or after the press section 405. The measurement can also be carried out after the drying. For the measurement frequencies the different measuring points have a different predetermined limit frequency, designated by reference 203 in Figure 2. After the former, when the moisture content of the paper web is 85%, i.e. the consistency is 15%, the lower limit frequency of the measurement is about 20 GHz. After the press section 405, the moisture content of the paper web being about 50%, i.e. the consistency 50%, the lower limit frequency of the measurement is about 30 to 40 GHz. After the drying, the moisture content of the paper is below 10%, i.e. the consistency exceeds 90%, and the lower limit measuring frequency is in the order of 100 GHz. Pulp having a consistency of about 1 % is applied from the headbox 401 , which controls the uniform quality and consistency of the paper web, between the suction rolls of the former. A former is the part of a paper machine where cylindrical former rolls supporting the wire are used. The former rolls comprise vacuum-operated suction boxes for drying the paper web on the wire. In the former, water is sucked from the pulp so that the consistency of the paper web at the measurement point is about 15%. In the former control arrangement, the control means 406 adjust the suction power (SUCK) to a suitable level on the basis of the consistency measurement, such that the consistency in the paper web can be maintained at the desired level. Implementing the measurement by sweeping the entire width of the web also allows the suction to be adjusted in the transverse direction of the machine. It is also possible, but not necessary, to control the operation of the headbox on the basis of the consistency measurement. Information on the actual physical temperature of the paper is preferably needed in the measurement to serve as a support for the measurement of the microwave radiometer 101 in order to be able to remove the temperature dependence of the measurement. The easiest way to measure the temperature of the measurement point is without touching the measurement point by the thermometer 404, which is preferably an electronic meter, such as an infrared thermometer.

Figure 5 shows an embodiment of the solution of the invention, in which the measurement point is a coating agent, such as coating slip for paper. The slip is usually comprised of water, minerals, talk, kaolin, latex or corresponding materials in different combinations. The measuring arrangement comprises a radiometer 101 , an antenna 102, a thermometer 404, a roll 501 , coating slip 502 on the surface of the roll 501 , coating slip 503 in its container, and a paper 504. The measurement is carried out in the same way as the paper is measured. As the roll 501 rotates, its surface touches the slip 503, which adheres to the surface of the roll 501. The radiometer 101 measures by the antenna 102 the thermal radiation 105 transmitted by the slip 502 on the surface of the roll 501. The result is preferably corrected by a temperature result measured by the thermometer 404 in the control means (not shown in the figure).

Figure 6 also shows a solution in which a coating agent is measured. The measuring arrangement comprises a radiometer 101 , an antenna 102, a thermometer 404, a coating agent 602 and an object 601 to be coated. The coating agent 602 is preferably a coating slip for paper, and the object being coated is the paper 601. Once the paper 601 has been coated with the coating slip 602, the radiometer 101 measures by the antenna 102 the thermal radiation 105 emitted from the coating slip 602. The result is corrected by a temperature result measured by the thermometer 404 in the control means (not in the figure). Figure 7 shows the measurement result for the paper web. The horizontal axis is the time scale and the vertical axis is the temperature scale. The black squares are measuring points. On the even section 71 of the measurement curve, the consistency of the paper web has been kept unchanged, whereby the brightness temperature measured by the microwave radiometer is also stable. On the rising section 72 of the curve, water is sucked from the paper web, whereby the brightness temperature rises. When the amount water can no longer be decreased by suction, the measured brightness temperature settles in range 73. The measurement has been carried out at frequency 24 GHz, solid content in range 71 is about 1% and in range 73 solid content is about 15%. The measurement result corresponds well to the theoretical estimate, although the exact numerical values slightly deviate from the theoretical. As was stated above, the achievable measurement sensitivity can be calculated by the formula (5). It is possible to calculate that a connection according to the following table exists between the integration time and the achievable measurement sensitivity, the consistency measuring accuracy and the sweeping time (100 sensors). In the calculation, the bandwidth is assumed to be 1 GHz and the noise figure of the receiver 5 dB. la/s Mh/K Mt/% Pa/s

1 0.023 0.02 100

0.5 0.032 0.03 50

0.1 0.069 0.06 10

0.05 0.115 0.11 5

0.01 0.230 0.21 1

0.005 0.322 0.30 0.5

0.001 0.690 0.63 0.1

In the table, la/s is the integration time in seconds, Mh/K is the measurement sensitivity in kelvin, Mt % is the measuring accuracy in percent and Pa/s is the sweeping time in seconds. The measuring sensitivity can also be increased instead of increasing the integration time by widening the measuring bandwidth or by reducing the receiver noise. Although the invention has been described above with reference to the example according to the attached drawings, it is obvious that the invention is not limited thereto, but can be modified in a variety of ways within the scope of the inventive idea disclosed in the attached claims.

Claims

1. A method for measuring consistency in particular, the method using a radiometer (101) comprising an antenna (102, 302) in the vicinity of an object (103) of measurement for receiving natural thermal microwave radiation (105 to 108), and the radiometer (101 ) being used to measure microwave radiation (105) transmitted by the measurement object (103), the intensity of the microwave radiation being dependent on the consistency of the measurement object (103), c h a r a c t e r i z e d in that the consistency of the measurement object (103) is measured as a function of the intensity of the microwave radiation (105) by using such a measuring frequency or frequency range that the intensity of the interfering radiation (107, 108) through the measurement object (103) is attenuated when penetrating the measurement object to a degree that it becomes unessential to the measurement, and the measurement object (103) is thus used to filter down the interfering background radiation (107, 108).

2. A method for controlling a former, the method being used in a paper machine comprising a headbox (401 ), a former, a press section (405), a wire (104, 402 and 403), a paper web being the measurement object (103) formed on the wire, the method comprising measuring the consistency of the paper web passing from the headbox (401 ) of the paper machine to the wire and travelling on the wire to the former and further to the press section (405), and the method using a radiometer (101 ) comprising an antenna (102, 302) in the vicinity of the paper web constituting the object (103) to be measured, for receiving natural thermal microwave radiation (105 to 108), and the radiometer (101 ) being used to measure the microwave radiation (105) transmitted by the paper web which constitutes the measurement object (103), the intensity of the microwave radiation being dependent on the consistency of the paper web, c h a r a c t e r i z e d by measuring the consistency of the paper web constituting the meas- urement object (103) as a function of the intensity of the microwave radiation (105) by using such a measuring frequency or frequency range that the intensity of the interfering radiation (107, 108) penetrating the paper web is attenuated in the paper web to a degree that it becomes unessential to the measurement, and the paper web constituting the measurement object (103) is thus used to filter down the interfering background radiation 107, 108), and the measured consistency result being used to control the former to correct the consistency in the paper web.

3. A method as claimed in claim 1 or 2, characterized in that a predetermined or higher frequency (203) is used as the lowest measur- ing frequency.

4. A method as claimed in claim 1 or 2, characterized in that when using the method in a paper machine comprising a headbox (401), a former, a press section (405), a wire (104, 402 and 403), a paper web constituting the measurement object (103) formed on the wire, and the method being used to measure the consistency of the paper web passing from the headbox (401) of the paper machine to the wire and travelling on the wire to the former and further to the press section (405), the consistency of the web is measured after the former before the press section (405).

5. A method as claimed in claim 1 or 2, characterized in that when using the method in a paper machine comprising a headbox (401), a former, a press section (405), a wire (104, 402 and 403), a paper web constituting the measurement object (103) formed on the wire, and the method being used to measure the consistency of the paper web passing from the headbox (401) of the paper machine to the wire and travelling on the wire to the former and further to the press section (405), in addition to or instead of the measurement after the former, the consistency of the web is measured after the press section (405).

6. A method as claimed in claim 2, 4 or 5, characterized in that the calibration of the measurement is carried out by measuring the radia- tion of the wire (104).

7. A method as claimed in claim 1 or 2, characterized in that the antenna (102) or the antennae (302) are shaped horn-like or the like to enable the antenna (102) or the antennae (302) to prevent the background radiation (106) from reaching the antenna (102) or the antennae (302).

8. A method as claimed in claim 1 or 2, characterized in that the measurements are carried out by one antenna (102) which moves and measures the measurement object (103) from several different points.

9. A method as claimed in claim 1 or 2, characterized in that the measurements are carried out from several points of the measure- ment object (103) by several antennae (302) forming a chain over the measurement object (103) and coupled to the radiometer (101) one by one for the measurement.

10. A method as claimed in claim 1 or 2, characterized in that the physical temperature of the measurement object (103) is measured from one or several points to remove the temperature dependence of the mi- crowave radiation measurement.

11. A method as claimed in claim 10, characterized in that the temperature is measured by an infrared thermometer (404).

12. A method as claimed in claim 1 or 2, characterized in that the measurement object (103) is a paper web.

13. A method as claimed in claim 1, characterized in that the measurement object (103) is a coating agent (502, 602) whose consistency is preferably measured from a coating roll (501) or from the surface of a coated object (601).

14. A method as claimed in claim 3, characterized in that the selected measuring frequency preferably substantially exceeds 20 GHz.

15. An arrangement for measuring consistency in particular, comprising a radiometer (101) and an antenna (102) in the vicinity of an object (103) of measurement for receiving natural thermal microwave radiation (105 to 108), and the radiometer (101) being arranged to measure the microwave radiation (105) transmitted by the measurement object (103), the intensity of the microwave radiation being dependent on the consistency of the measurement object (103), characterized in that the measuring arrangement is arranged to measure the consistency of the measurement object (103) as a function of the intensity of the microwave radiation by using such a measuring frequency or measuring frequency range that the intensity of the interfering radiation (107, 108) penetrating the measurement object (103) is attenuated to a degree that it becomes unessential to the measurement, and the measurement object (103) is thus arranged to filter down the interfering radiation (107 to 108) originating from the background.

16. An arrangement for former control to be used in a paper machine comprising a headbox (401), a former, a press section (405), a wire (104, 402, 403), a paper web on the wire, and a measuring arrangement arranged to measure the consistency of the paper web passing from the head- box (401) of the paper machine to the wire and travelling on the wire to the former and further to the press section (405), and the measuring arrangement comprising a radiometer (101) and an antenna (102) in the vicinity of the paper web constituting the measurement object (103) for receiving natural thermal microwave radiation (105 to 108), and the radiometer (101) being arranged to measure the microwave radiation (105) transmitted by the measurement object, the intensity of the microwave radiation being dependent on the consis- tency of the measurement object (103), characterized in that the measuring arrangement is arranged to measure the consistency of the measurement object (103) as a function of the intensity of the microwave radiation by using such a measuring frequency or measuring frequency range (201) that the intensity of the radiation (107, 108) penetrating the meas- urement object (103) is attenuated to a degree that it becomes unessential to the measurement, and the measurement object (103) is thus arranged to filter down the interfering radiation (107, 108) originating from the background, and the measuring arrangement is further arranged to control the former to correct the consistency of the paper web (103).

17. An arrangement as claimed in claim 15 or 16, characterize d in that a predetermined or higher frequency (201 ) is used as the lowest measuring frequency.

18. An arrangement as claimed in claim 15 or 16, characterized in that the arrangement is arranged to be used in a paper machine comprising a headbox (401), a former, a press section (405), a wire (104, 402 and 403), a paper web on the wire, and the arrangement is arranged to measure the consistency of the paper web passing from the headbox (401) of the paper machine to the wire and travelling on the wire to the former and further to the press section (405), the measuring arrangement being arranged to measure the consistency of the web after the former before the press section (405).

19. An arrangement as claimed in claim 15 or 16, characterize d in that the arrangement is arranged to be used in a paper machine comprising a headbox (401), a former, a press section (405), a wire (104, 402 and 403), a paper web on the wire, and the arrangement is arranged to measure the consistency of the paper web passing from the headbox (401) of the paper machine to the wire and travelling on the wire to the former and further to the press section (405), the measuring arrangement being arranged to measure the consistency of the web after the press section (405) in addition to or instead of the measurement after the former.

20. An arrangement as claimed in claim 16, 18 or 19, charac- terized in that the measuring arrangement is arranged to be calibrated by measuring the radiation of the wire (104).

21. An arrangement as claimed in claim 15 or 16, characterize d in that the antenna (102) or the antennae (302) are shaped horn-like or the like to enable the antenna (102) or the antennae (302) to prevent the background radiation (107, 108) from reaching the antenna (102) or the antennae (302).

22. An arrangement as claimed in claim 15 or 16, characterize d in that the measuring arrangement is arranged to carry out the meas- urements by one antenna (102) which moves and measures the measurement object (103) from several different points.

23. An arrangement as claimed in claim 15 or 16, characterize d in that the measuring arrangement is arranged to carry out the measurements from several points of the measurement object by several antennae (302) forming a chain across the measurement object (103), and the measuring arrangement comprises switches (301) arranged to switch the antennae (302) to the radiometer (101) one by one.

24. An arrangement as claimed in claim 15 or 16, characterize d in that the measuring arrangement comprises a thermometer (404) for measuring the physical temperature of the measurement object (103) and for removing the temperature dependence of the microwave radiation measurement.

25. An arrangement as claimed in claim 24, characterized in that the measuring arrangement is arranged to measure the temperature by an infrared thermometer (404).

26. An arrangement as claimed in claim 15, characterized in that the measurement object (103) is a paper web.

27. An arrangement as claimed in claim 15, characterized in that the measurement object (103) is a coating agent (502, 602), and the measuring arrangement is arranged to measure the consistency preferably from a coating roll (501 ) or from the surface of a coated object (601 ).

28. An arrangement as claimed in claim 15 or 16, characterize d in that the measuring frequency preferably substantially exceeds 20 GHz.