In the manufacturing process of paper, a web can be measured by using a sensor unit scanning transversely across the web and a sensor array that is disposed transversely across the web and may be arranged stationary. The characteristics usually measured from a web include moisture content and basis weight. The dry matter content is computationally generated by subtracting moisture content from basis weight. However, in this case, the filler content, i.e. the ash content, fails to be taken into consideration.
Moisture and the chemical pulp content can be measured by means of the attenuation of optical radiation, and the basis weight can be measured by means of the attenuation of β-radiation. Measurements are often made by measuring moisture by means of the transverse sensor array and by measuring both moisture and basis weight by means of the sensor unit traversing across the web. Since the sensors of the traversing sensor unit can be calibrated when the sensor unit is transferred outside the edge of the web, the measurements of the sensors of the traversing sensor unit may be considered accurate. The sensor array, in contrast, whose sensors are not able no move outside the edges of the web, can be calibrated only during web break. At the same time, the sensors of the sensor array are always subjected to dirtying, for example.
However, a drawback of a sensor unit traversing across a web is that during one complete scanning, the web has usually had time to move as much as over a kilometre. Accordingly, measurements from the same transverse location are obtained at extremely long intervals. In addition, for compensating for random deviations, the measurements of a traversing sensor often have to be averaged for instance across four to ten measurements. However, because of the traversing, cross direction measurement results are not close to each other, but far apart in the machine direction, wherefore the cross direction profile cannot actually be measured with a traversing sensor.
Although a transverse sensor array does measure the web more densely, its measurement results are more inaccurate, since the results of the sensors cannot be calibrated. As components of a large number of sensors, usually components of different qualities have to be selected compared with a traversing sensor array having a smaller number of sensors, and, consequently, for instance financing reasons do not cause restrictions in the selection of components. Overall, this results in the incapability of making the measurement accurately both in the cross direction and in the machine direction.
Attempts could be made to solve the problem by measuring the same characteristic in the same manner with both a traversing sensor unit and a sensor array. The same characteristic may be moisture, for example. This way, the moisture measurements of a sensor array can be calibrated with the accurate moisture measurements of the traversing sensor unit, and the direct transverse moisture profile can be measured densely in the machine direction. Generally speaking, all the desired characteristics could be measured with both a traversing sensor unit and a stationary sensor array.
Problems are associated with this solution, too. The placement of all the desired sensors both in the traversing sensor unit and in the stationary sensor array renders the measurement extremely expensive and complicated.