Hybrid basis weight sensor

Abstract

A basis weight measurement system for measuring the basis weight of a paper sheet comprising a nucleonic sensor; an infrared measuring device configured for measuring infrared light independently at at least two spectral regions, wherein for at least a first one of the spectral regions the spectrum of light measured is restricted such that the spectral response of the infrared measuring device at a wavelength of 1.954 µm differs from the spectral response of the infrared measuring device at a wavelength of 2.1 µm by at least a factor of 2 when the infrared measuring device is measuring at the first spectral region, and an analysis device comprising a data input interface configured to receive data from the nucleonic sensor and the infrared measuring device and a basis weight determining module configured to determine a basis weight of the paper sheet by combining the data obtained from the nucleonic senor and from the infrared measuring device.

Description

HYBRID BASIS WEIGHT SENSORTECHNICAL FIELD

[0001] Embodiments of the present disclosure relate to a basis weight measurement system for measuring the basis weight of a paper comprising a nucleonic sensor, an infrared measuring device and an analysis device. Further embodiments relate to a method to measure the basis weight of a paper sheet.BACKGROUND

[0002] In paper making, the measurement of the basis weight of a paper sheet provides important data to evaluate the quality of the paper sheet and of a paper production process. Further, adjustments of settings or parameters of a paper making machines may be performed upon the measurement results of a basis weight measurement. The measurement of the basis weight of a paper sheet may be performed using a nucleonic sensor that is based upon the attenuation of a radiative decay products by the paper sheet. Further, infrared light may be employed to determine the basis weight of a paper sheet. Nucleonic sensors are considered to provide accurate absolute values of the basis weight, but have a large amount of noise. In contrast, infrared sensors have a low noise, but provide a less accurate absolute basis weight value. US 5099118A describes the combination of an optical sensor and a nucleonic sensor as a dual sensor to sense the basis weight of paper. With the availability of commonly used isotopes being restricted and radiation protection requirements limiting the range of useable isotopes, there is a desire for improved hybrid basis weight measurement systems that make use of the entire bandwidth of nucleonic and optical sensors and of the information available in different spectral regions.DISCLOSURE OF THE INVENTION

[0003] In the view of the foregoing, the present disclosure is directed to a basis weight measurement system and a method for measuring the basis weight of a paper sheet that comprises a nucleonic sensor, an infrared measuring device configured for measuring infrared light independently at at least two spectral regions and an analysis device todetermine the basis weight of a paper sheet by combining data obtained from the nucleonic sensor and from the infrared measuring device.

[0004] According to an aspect of the present disclosure, a basis weight measurement system for measuring the basis weight of a paper sheet is provided. The basis weight measurement system comprises a nucleonic sensor, an infrared measuring device configured for measuring infrared light independently at at least two spectral regions, wherein for at least a first one of the spectral regions the spectrum of light measured is restricted such that the spectral response of the infrared measuring device at a wavelength of 1.954 pm differs from the spectral response of the infrared measuring device at a wavelength of 2.1 pm by at least a factor of 2 when the infrared measuring device is measuring at the first spectral region; and an analysis device. The analysis device comprises a data input interface configured to receive data from the nucleonic sensor and the infrared measuring device and a basis weight determining module configured to determine a basis weight of the paper sheet by combining the data obtained from the nucleonic senor and from the infrared measuring device.

[0005] According to another aspect of the present disclosure, a method to measure the basis weight of a paper sheet is provided. The method comprises measuring the attenuation of a beam of radiative decay products, wherein the attenuation is caused by a paper sheet, measuring infrared light having interacted with the paper sheet, the infrared light being measured in at least two spectral regions, and outputting a respective sensor signal for each of the at least two spectral regions, wherein for a first one of the spectral regions the spectrum of light measured is restricted such that the sensor signal for the first spectral region is responsive to a wavelength of 1.954 pm differently from a wavelength of 2.1 pm by at least a factor of 2, and determining the basis weight of the paper sheet by combining the data on the attenuation of a beam of radiative decay products and the data on the attenuation of infrared light.NUCLEONIC SENSOR

[0006] According to some embodiments, the nucleonic senor may be a sensor that is based on the attenuation of beam of radiative decay products of a nuclear decay process of an isotope. In particular, the beam of radiative particles may comprise beta-radiation from a decay of neutrons, more in particular beams of electrons or positrons. The isotope maycomprise isotopes of Krypton, Kr, Strontium, Sr or Promethium, Pm, in particular at least one of the isotopes Kr-85, Sr-90 or Pm-147. In some embodiments, the nucleonic sensor may comprise a sensor based on gamma-radiation, neutron beams or X-rays.

[0007] According to some embodiments, the nucleonic sensor comprises a low activity source, in particular a source with an activity below 0.05, 0.1, 0.5, 1, 2, 5, 8, 10 or 15 GBq. In some embodiments, the low activity source may comprise a Kr-85 source, exemplarily with an activity of 3.7 GBq to 37 GBq, or a Sr-90 source, exemplarily with an activity of 0.037 GBq to 0.74 GBq Employing the low-activity source reduces regulatory restrictions in the use of the sensors and reduces the exposure of staff, in particular during maintenance procedures, or equipment to radiation.

[0008] According to some embodiments, the radiative particles are detected by a detector, in particular by an ionization chamber, by a solid-state detector, in particular by a PIN photodiode, or by a scintillation-crystal based detector.

[0009] Attenuation of a beam of radiative decay products may comprise absorption of radiative decay products, scattering of radiative decay products and reflection of radiative decay products.INFRARED MEASURING DEVICE

[0010] In this application, “infrared” may describe a part of the electromagnetic spectrum that comprises electromagnetic radiation with a wavelength exceeding the wavelength of visible light. In particular, “infrared” may comprise electromagnetic radiation with a wavelength of at least 700 nm, 750 nm, 775 nm, 800 nm, 825 nm, 850 nm, 900 nm, 950 nm or 1000 nm. “Infrared” may comprise “near infrared” light, “mid infrared” light or “far infrared” light. In particular, “infrared” may comprise light with a wavelength of at most 2.2 pm, 2.5 pm, 2.8 pm, 3 pm, 3.5 pm, 5 pm, 7 pm, 10 pm or 20 pm.

[0011] In some embodiments, the infrared measuring device comprises at least one infrared source. The infrared source may be based on a thermal source, in particular a tungsten-halogen lamp, and / or a semiconductor source, in particular an infrared light emitting diode or an infrared laser source. In embodiments, the infrared source is acontinuous wave, cw, light source, and is in particular not a deliberately modulated light source.

[0012] The infrared measuring device may comprise at least one infrared detector. The at least one infrared detector may be based upon a semiconductor detector or a thermal detector. In particular, the at least one infrared detector may comprise an InGaAs detector, an InAs detector, an HgCdTe detector, an InSb detector or a PbS detector. The at least one infrared detector may comprise a single pixel or multiple pixel. A single pixel within a multi-pixel detector may be considered as a detector by itself. In embodiments, multiple infrared detectors may comprise detectors of a single infrared detector type or of different infrared detector types, in particular to cover different spectral regions. In some embodiments, at least one of the infrared detectors is positioned adjacent to the nucleonic sensor.

[0013] The at least one infrared detector may comprise a detector window. The detector window may comprise a material that restricts the spectral range of the transmitted light. In particular, the detector window may comprise a dielectric coating or a material that is only transmissive for a specific spectral range and absorbs or reflects light of other frequencies. In particular a combination of a detector window and a material of an infrared detector may allow to restrict the spectral response of an infrared measuring device to a selected spectral region.

[0014] The infrared measuring device is configured for measuring light independently at at least two spectral regions. In some embodiments, measuring light independently may comprise measuring light of one spectral region without relevant influence of another spectral region, in particular not of another spectral region of interest. In particular, influence of light of another spectral region may be considered as not being relevant if the spectral response of the light of said region does not exceed the spectral response of the selected spectral region by a factor of 2, 3, 5, 8, 10, 20.

[0015] The infrared measuring device is configured for measuring infrared light independently at at least two spectral regions, wherein for at least a first one of the spectral regions the spectrum of light measured is restricted such that the spectral response of the infrared measuring device at a wavelength of 1.954 pm differs from the spectral response ofthe infrared measuring device at a wavelength of 2.1 gm by at least a factor of 2, 3, 5, 8, 10, 20 when the infrared measuring device is measuring at the first spectral region.

[0016] In embodiments, at least one of the spectral regions comprises infrared absorption signals that are correlated with the amount of water being present in the paper sheet. In particular, one of the spectral regions comprises light of a wavelength of 1.954 pm.

[0017] In embodiments, at least one of the spectral regions comprises infrared absorption signals that are correlated with the amount of fiber, in particular of cellulose, being present in the paper sheet. In particular, one of the spectral regions comprises light of a wavelength of 2.1 pm.

[0018] In embodiments, in at least one of the spectral regions infrared absorption signals that are correlated with the amount of fiber, in particular of cellulose, or the amount of water being present in the paper sheet are not present to a significant level. A level may be considered as significant if the signal intensity, in particular the intensity of the absorbance signal measured by the infrared measuring device, that originates from fiber or water being present in the paper sheet exceeds 0.1 %, 0.5 %, 1 %, 2 %, 3 %, 5 %, 10 % or 20 %. In particular, one of the spectral regions comprises light of a wavelength of 1.835 pm. Thereby, a reference value can be created.

[0019] In some embodiments, the infrared measuring device may comprise at least one wavelength-selective element. An element, in particular an optical element, may be considered as wavelength- selective if the amount of light at two different wavelengths that is transmitted or reflected differs by at least a factor of 2, 3, 4, 5, 8, 10, 20, 50, 100, in particular with respect to a position in space, in particular a detector position.

[0020] In some embodiments, the wavelength- selective element may comprise a suitable material, particularly at least one of a glass material, a semiconductor material or a metal material. In embodiments, the wavelength- selective element may comprise a dielectric coating, in particular at least one of a high-pass, a low-pass, a band-pass coating or a bandblock coating.

[0021] In some embodiments, the wavelength-selective element comprises a dielectric coating with a Gaussian-type transmission characteristic. In particular, at least one dielectriccoating may have a maximum transmission or reflection at a wavelength between 1.934 gm to 1.974 pm, more in particular at a wavelength of 1.954 pm. At least one other dielectric coating may have a maximum transmission or reflection at a wavelength between 2.080 pm to 2.120 pm, more in particular at a wavelength of 2.100 pm. In embodiments, a further optical element may have maximum transmission or reflection at a wavelength between 1.815 pm to 1.855 pm, more in particular at a wavelength of 1.835 pm. At least one of the optical elements may have a half-power bandwidth, HPBW, of 90 nm to HO nm, in particular of 100 nm.

[0022] In some embodiments, the wavelength-selective element may comprise an optical grating, in particular a transmissive or reflective optical grating. The optical grating may be in a fixed position such that a fix, in particular predetermined, relation between a plurality of detectors and spectral regions reflected or transmitted onto each detector is given. In embodiments, the optical grating may be movable, in particular rotatable. Thereby, multiple spectral regions may be measured by a single detector consecutively by a movement, in particular by a rotation, of the grating. In embodiments, the wavelength- selective element may comprise a micro-electro-mechanical-system, MEMS, based filter. In embodiments, the wavelength-selective element may comprise an acousto-optical modulator, also known as acousto-optical deflector. Thereby, the grating parameters may be adjusted to measure multiple wavelengths consecutively with a single detector or to modify the wavelengthdetector assignment.

[0023] In some embodiments, the wavelength-selective element may comprise a beamsplitting fiber, in particular a fiber coupler. In embodiments, a single input port collects the infrared light transmitted by the paper sheet and for each detector, an output port provides light specific for a spectral range.

[0024] In some embodiments, the wavelength-selective element may comprise multiple wavelength-selective elements, for example a combination of an optical grating and a partially transmissive optical window. Thereby, the spectral regions can be separated even more thoroughly.

[0025] In some embodiments, at least one wavelength- selective element has a transmittance of at least 66 %, 75 %, 80 %, 90 %, 95 % or 99 % at a wavelength of 2.1 pmand a transmittance of at most 1 %, 5 %, 10 %, 20 %, 25 % or 33 % at a wavelength of 1.954 pm.

[0026] In some embodiments, at least one wavelength- selective element has a transmittance of at least 66 %, 75 %, 80 %, 90 %, 95 % or 99 % at a wavelength of 1.954 gm and a transmittance of at most 1 %, 5 %, 10 %, 20 %, 25 % or 33 % at a wavelength of 2.1 pm.

[0027] In some embodiments, at least one of the infrared detectors is on one side of the paper sheet and at least one other of the infrared detectors is on another side of the paper sheet. In particular at least one of the infrared detectors measures light transmitted by the paper sheet and at least one other of the infrared detectors measures light reflected or scattered by the paper sheet.

[0028] In some embodiments, infrared measurement signals may be formed from a ratio of signals measured at different spectral regions. In particular, ratios relative to a reference spectral region, exemplarily a spectral region including light at a wavelength of 1.835 pm, may be formed. The ratios may be standardized against detector signals without a paper sheet present, in particular against detector signals in which an air gap is between sensor and detector. The standardization may be performed at regular time intervals, in particular every 20 minutes, every 45 minutes, every 60 minutes, every 90 minutes or every 120 minutes, or after predefined events, exemplarily after maintenance breaks, production disruptions, or unexplained or unusual signal changes. In embodiments, the ratio of the value of the reference spectral region over the value of the spectral region that is correlated with water content is logarithmically correlated with the basis weight of the paper sheet. In some embodiments, the ratio of the value of the reference spectral region over the value of the spectral region that is correlated with cellulose content is logarithmically correlated with the basis weight of the paper sheet.ANALYSIS DEVICE

[0029] In some analysis devices, the data input interface configured to receive data from the nucleonic sensor and the infrared measuring device comprises standard electronic communication interfaces, particularly industry standard interfaces, exemplary USBinterfaces, serial port or parallel port interfaces. In some embodiments, the data received via the data input interface may be raw data, in particular raw data from the nucleonic sensor and the infrared measuring device, more in particular raw data from the at least one infrared detector of the infrared measuring device. In embodiments, the data received via the data input interface may be pre-processed data, in particular data that has been processed within the nucleonic sensor or the infrared measuring device. Pre-processing may comprise at least one of averaging data, removing outlier data points, filtering data, smoothing data, compressing data, calculating parameters from raw data, in particular calculating a transmission or reflection value, removing background noise or associating sensor or combining detector data with a position in space.

[0030] The basis weight determining module configured to determine the basis weight of the paper sheet by combining the data obtained from the nucleonic senor and from the infrared measuring device may be configured to combine data from single spectral regions of the infrared measuring device, in particular from different detectors or different measurement configurations, with data obtained from the nucleonic sensor. In particular, data from single spectral regions may comprise data correlated with water in the paper sheet or data correlated with cellulose in the paper sheet. In embodiments, the basis weight determining module may be configured to combine the data obtained from the nucleonic sensor and from the infrared measuring device in their full bandwidth.

[0031] In some embodiments, measurement configurations of the nucleonic sensor and of the infrared measuring device are controlled by the analysis device, in particular measurement configurations for the at least two spectral regions. In some embodiments, the nucleonic sensor and the infrared measuring device may be placed on a moveable holder. The movement parameters of the moveable holder may be controlled by the analysis device.

[0032] In some embodiments, a profile of a signal measured at a wavelength of 1.954 pm, particularly of a signal measured at a spectral region that incorporates a wavelength of 1.954 pm, after interacting with the paper sheet and a profile of the signal measured at a wavelength of 2.1 pm, particularly of a signal measured at a spectral region that incorporates a wavelength of 2.1 pm, after interacting with the paper sheet is recorded. Interacting with the paper sheet may comprise a transmission, reflection or scattering of light.

[0033] In some embodiments, the basis weight determining module combines at least one of the profiles of the signal measured at a wavelength of 1.954 pm after interacting with the paper sheet with the signal measured at a wavelength of 2.1 pm after interacting with the paper sheet to generate a profile of the basis weight of the paper sheet.

[0034] In some embodiments, the profile of the signal measured is recorded by moving at least part of the basis weight measurement system, in particular by moving the nucleonic sensor and the infrared measuring device, along the paper sheet in at least one direction, in particular from one edge of the sheet to another edge of the sheet. In some embodiments, the paper sheet is moved along the basis weight measurement system in at least one direction. In some embodiments, at least part of the basis weight measurement system is moved along a direction perpendicular to a direction of movement of the paper sheet.

[0035] In some embodiments, the analysis module, in particular the basis weight determining module, is configured to complement the measured data to create a profile of the entire paper sheet based on the data measured. In particular, regions of the paper sheet which have not been measured may be interpolated from other regions of the paper sheet measured.

[0036] In some embodiments, the data of the nucleonic sensor is averaged in regular intervals, in particular for every paper sheet. In embodiments, the data of the nucleonic sensor is averaged in regular time intervals, in particular every 10, 20, 30 seconds, every minute, every 2, 3, 5, 10, 15, 30 or 60 minutes.

[0037] In some embodiments, averaging the data of the nucleonic sensor may comprise applying a filter, in particular an exponential filter, before averaging the data. In embodiments, the filter is applied digitally on the data, particularly on the entire bandwidth of the data measured.

[0038] In some embodiments, the data of the infrared measuring device is normalized by the data, in particular by the average of the data, of the nucleonic sensor. Thereby, the information provided by the nucleonic sensor, which has a high absolute accuracy but also a comparatively high noise, can be combined with the data from the infrared measuring device, which has a comparatively low absolute accuracy but a lower noise. Therefore, insome embodiments, the nucleonic sensor data, in particular the averaged data, defines the absolute range in which the basis weight of the paper sheet can be found and the data from the infrared measuring device describes the overlaying variations of the basis weight throughout the profile of the paper sheet.

[0039] In some embodiments, the basis weight measurement system further comprises an an X-ray sensor, in particular an ash sensor. The data measured by the nucleonic sensor and the data measured by the infrared measuring device may be augmented by data obtained by the ash sensor, particularly by data, exemplarily a profile, indicative of the ash content of the paper sheet. In some embodiments, an ash content of the paper sheet may be provided from a distributed control system, DCS, or may be an estimate, particularly based on a particular grade. In some embodiments, the ash content data may be provided by another measurement device that is separate from the basis weight measurement system and provides information on the ash content to the basis weight measurement system, in particular to the basis weight determining module.

[0040] In some embodiments, the response of the infrared measuring device, particularly of the at least one infrared detector, may be zeroed at one edge of the paper sheet. As the sensor is moved across the paper sheet in at least one direction, differences in the response of the infrared measuring device are measured and recorded, in particular in the form of differential measurements. In embodiments, at least one average of data points of the infrared measuring device is generated. In embodiments, differential measurements for one spectral region, in particular for a spectral region correlated with water in the paper sheet and differential measurements for another spectral region, in particular for a spectral region correlated with cellulose in the paper sheet, may be combined, in particular added, to provide a differential measurement of the basis weight of the paper sheet. In embodiments the differential measurement of the basis weight of the paper sheet may be combined with, in particular added to, the average basis weight obtained from the nucleonic sensor. Thereby, an absolute basis weight measurement can be performed that incorporates advantageous features of the nucleonic sensor and the infrared measuring device, in particular providing an improved resolution.MISCELLANEOUS

[0041] The basis weight measurement system may be part of a distributed control system, DCS, that controls the operation of a paper-making machine. In particular, the basis weight measurement system may exchange information, particularly information on the basis weight of the paper sheet or information relevant for the determination of the basis weight of the paper sheet, with the DCS or other parts of the DCS. The DCS may provide information on parameters of the paper sheet, in particular on the composition of the paper sheet, or information on the operation of the paper making machine, in particular on the speed of the paper sheet passing in machine direction, to the basis weight measurement system.

[0042] The basis weight measurement system may be part a paper-making machine. In particular, the basis weight measurement system may be configured to be integrated to a control system of a paper-making machine. In some embodiments, information from the basis weight measurement system may be provided to an operator of a paper-making machine, in particular along information provided by other parts of the paper-making machine.BRIEF DESRICPTION OF THE DRAWINGS

[0043] The accompanying drawings relate to embodiments of the disclosure and are described in the following:FIG 1 schematically illustrates a basis weight measurement system for measuring the basis weight of a paper sheet according to the embodiments described herein;FIG 2 schematically illustrates an embodiment of the basis weight measurement system of Fig. 1 in more detail;FIG 3 schematically illustrates an alternative embodiment of the basis weight measurement system of Fig. 1 in more detail;FIG 4 schematically illustrates an alternative embodiment of the basis weight measurement system of Fig. 1 in more detail;FIG 5 schematically illustrates an alternative embodiment of the basis weight measurement system of Fig. 1 in more detail;FIG 6 schematically illustrates the basis weight measurement system of Fig. 1 with additional features;FIG 7 schematically illustrates a measurement track of at least part of the basis measurement system of Fig. 1;FIG 8 schematically illustrates an exemplary near-infrared absorption spectrum of water and cellulose;FIG 9 schematically illustrates data obtained by the basis weight measurement system for measuring the basis weight of a paper sheet according to the embodiments described herein.DETAILED DESCRIPTION OF EMBODIMENTS

[0044] Reference will now be made in detail to the various embodiments of the disclosure, one or more examples of which are illustrated in the figures. Generally, only the differences with respect to individual embodiments are described. Each example is provided by way of explanation of the disclosure and is not meant as a limitation of the disclosure. Further, features illustrated or described as part of one embodiment can be used on or in conjunction with other embodiments to yield a further embodiment. It is intended that the description includes such modifications and variations.

[0045] Fig. 1 schematically illustrates a basis weight measurement system 100 for measuring the basis weight of a paper sheet 1. The system 100 comprises a nucleonic sensor 2, an infrared measuring device 3 and an analysis device 4. The nucleonic sensor 2 is positioned adjacent to the infrared measuring device 3. The analysis device 4 comprises a data input interface 5 and a basis weight determining module 6. Data from the nucleonic sensor 7a is transmitted to the data input interface 5 of the analysis module 4. Data from the infrared measuring device 7b is transmitted to the data input interface 5 of the analysis module 4. From the data input interface 5, the data is transferred to the basis weight determining module 6 of the analysis device 4.

[0046] Fig. 2 schematically illustrates an embodiment of the basis weight measurement system 100 of Fig. 1 in more detail. The infrared measuring device 3 comprises an infrared light source 31, a beam splitting fiber 34 with the beam splitting fiber input 33 facing the paper sheet 1 and the infrared light source 31. The beam splitting fiber 34 comprises an infrared transparent material. The infrared measuring device 3 further comprises a plurality of infrared detectors 32a, 32b, 32c which are connected to the output ends of the beam splitting fiber 34.

[0047] Fig. 3 schematically illustrates an embodiment of the basis weight measurement system 100 of Fig. 1 in more detail. The infrared measuring device 3 comprises an infrared light source 31, a plurality of dielectric mirrors 35a, 35b and a plurality of infrared detectors 32a, 32b, 32c. Each of the dielectric mirrors 35a, 35b reflects a predefined part of theelectromagnetic spectrum, in particular a spectral region, and transmits a predefined part of the spectrum, in particular another spectral region.

[0048] Fig. 4 schematically illustrates an embodiment of the basis weight measurement system 100 of Fig. 1 in more detail. The infrared measuring device 3 comprises an infrared light source 31, an optical grating 36 and a plurality of infrared detectors 32a, 32b, 32c. The infrared light emitted by the infrared light source 31 and transmitted through the paper sheet 1 is diffracted by the optical grating into several beams with different beams of light having different wavelengths. The light diffracted by the optical grating 36 is measured by the plurality of infrared detectors 32a, 32b, 32c. The optical grating 36 may be configured to be rotated, particularly automatically rotated, in some embodiments.

[0049] Fig. 5 schematically illustrates an embodiment of the basis weight measurement system 100 of Fig. 1 in more detail. In addition to the embodiment of Fig. 3, the infrared measuring device 3 further comprises an additional infrared detector 32d that is positioned on the same side of the paper sheet 1 as the infrared light source 31. A suitable optical element 35c, in particular a dielectric mirror, reflects infrared light reflected from the paper sheet 1 to the infrared detector 32d.

[0050] Fig. 6 schematically illustrates an embodiment of the basis weight measurement system 100 of Fig. 1 with additional features. In addition to Fig.1, the basis weight measurement system comprises an X-ray sensor 8. The data from the X-ray sensor 7c is transmitted to the data input interface 5 of the analysis module 4.

[0051] Fig. 7 schematically illustrates a measurement track 700 of an embodiment of the basis weight measurement system 100 along the paper sheet 1. At least the nucleonic sensor and the infrared measuring device are moved along the paper sheet 1 perpendicular to the direction of the movement of the paper sheet through the paper machine. Thereby, the measurement track resembles a zigzag path along the paper sheet 1. The measurement track 700 comprises a plurality of paper sheet scans 701, that comprise the movement of at least the nucleonic sensor and the infrared measuring device from one edge of the paper sheet 1 to another edge of the paper sheet 1.

[0052] Fig. 8 exemplarily illustrates the absorbance of water 801 and of cellulose 802 in a near infrared spectral region. At a wavelength close to 1.9 pm, the absorbance of water 801 has a local maximum 811. Further, at another wavelength close to 1.9 pm, the absorbance of cellulose 802 has a local minimum 812. At a wavelength close to 1.8 pm, the absorbance of cellulose 802 is similar to the absorbance of cellulose 802 at the spectral position where the absorbance of water 801 has a local maximum 811.

[0053] Fig. 9 exemplarily illustrates the data of the nucleonic sensor 902, the data of the infrared measuring device 903 and the average of the data of the nucleonic sensor 912. In Fig. 9, the average of the data of the nucleonic sensor 912 is set as a zero-point for the variation of the data of the infrared measuring device 903.

Claims

CLAIMS1. A basis weight measurement system for measuring the basis weight of a paper sheet comprising:- a nucleonic sensor;- an infrared measuring device configured for measuring infrared light independently at at least two spectral regions, wherein for at least a first one of the spectral regions the spectrum of light measured is restricted such that the spectral response of the infrared measuring device at a wavelength of 1.954 pm differs from the spectral response of the infrared measuring device at a wavelength of 2.1 pm by at least a factor of 2 when the infrared measuring device is measuring at the first spectral region,- an analysis device comprising- a data input interface configured to receive data from the nucleonic sensor and the infrared measuring device and- a basis weight determining module configured to determine a basis weight of the paper sheet by combining the data obtained from the nucleonic senor and from the infrared measuring device.

2. The basis weight measurement system according to claim 1, wherein in at least one of the spectral regions, absorption bands correlated with water and cellulose in the paper sheet are not present to a significant level.

3. The basis weight measurement system according to any of claims 1 or 2, wherein the infrared measuring device comprises- a continuous wave infrared light source,- a plurality of infrared detectors- at least one wavelength- selective element.

4. The basis weight measurement system according to claim 3, wherein at least one ofthe wavelength-selective elements has a transmittance of at least 66 % at a wavelength of 1.954 gm and a transmittance of at most 33 % at a wavelength of 2.1 pm.

5. The basis weight measurement system according to any of claims 3 or 4, wherein at least one of the wavelength-selective elements has a transmittance of at least 66 % at a wavelength of 2.1 gm and a transmittance of at most 33 % at a wavelength of 1.954 pm.

6. The basis weight measurement system according to any of the preceding claims, wherein a profile of the signal measured at a wavelength of 1.954 pm after interacting with the paper sheet and a profile of the signal measured at a wavelength of 2.1 pm after interacting with the paper sheet is recorded.

7. The basis weight measurement system according to claim 6, wherein the profile of the signal measured at a wavelength of 1.954 pm after interacting with the paper sheet is combined with the profile of the signal measured at a wavelength of 2.1 pm after interacting with the paper sheet in the analysis device to generate a profile of the basis weight of the paper sheet.

8. The basis weight measurement system according to any of the preceding claims, wherein at least one of the wavelength-selective elements comprises a dielectric coating.

9. The basis weight measurement system according to any of the preceding claims, wherein at least one of the infrared detectors is on one side of the paper sheet and at least one other of the infrared detectors is on another side of the paper sheet.

10. The basis weight measurement system according to any of the preceding claims, wherein the data of the nucleonic sensor is averaged in regular intervals, in particular for each paper sheet scan, particularly wherein averaging comprises applying a filter to the nucleonic sensor data before averaging the data, in particular applying an exponential filter.

11. The basis weight measurement system according to claim 10, wherein the data of theinfrared measuring device is normalized by the average data of the nucleonic sensor.

12. The basis weight measurement system according to any of the preceding claims, further comprising an X-ray sensor, in particular an ash detector.

13. A distributed control system comprising a basis weight measurement system according to any of the preceding claims.

14. A paper-making machine comprising a basis weight measurement system according to any of claims 1-12.

15. A method to measure the basis weight of a paper sheet comprising- measuring the attenuation of a beam of radiative decay products, wherein the attenuation is caused by a paper sheet,- measuring infrared light having interacted with the paper sheet, the infrared light being measured in at least two spectral regions, and outputting a respective sensor signal for each of the at least two spectral regions, wherein for a first one of the spectral regions the spectrum of light measured is restricted such that the sensor signal for the first spectral region is responsive to a wavelength of 1.954 pm differently from a wavelength of 2.1 pm by at least a factor of 2,- determining the basis weight of the paper sheet by combining the data on the attenuation of a beam of radiative decay products and the data on the attenuation of infrared light.

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