English (United Kingdom)

”New shives” and new calculation procedure at measurement of shives

Freeness, shives, fibre length distribution and brightness are well established parameters for quality control at production of mechanical pulp. They are all measured on-line with L&W Pulp Tester. The fibre morphology analysis available in the laboratory analyzer L&W Fiber Tester is also used in L&W Pulp Tester.

In L&W Fiber Tester different fibre properties are measured in a detailed way. Apart from fibres the pulp consists of ray cells, fibre debris, fibrils, vessel cells and shives etc. In L&W Fiber Tester a dedicated software module analyses vessel cells. Measurement and classification of vessel cells are relatively straight forward in virgin hardwood pulps. Most wide objects are vessel cells. The degree of difficulty increases considerably at measurements on mixed pulps, especially on recycled pulp. Because of that, the tools in L&W Fiber Tester, to measure vessel cells have been refined during recent years, in order to separate vessel cells from other objects. The same technique is applied to measure also these other objects in the pulp. Measuring shives was of special interest.

The limitation of measuring shives, with L&W Fiber Tester, lies with the limitation of the statistics. The amount of shives in the measurement is limited by the total sample amount, even though L&W Fiber Tester handles bigger amounts of samples compared to many other laboratory fibre analysers. Interesting however is the detailed analysis at low consistency in L&W Fiber Tester that finds considerable more narrow shives than what is measured in other commercial optical based fibre and shive analysers.

Detailed analysis or good statistics
You probably want both best possible statistics and highest accuracy in your measurements. But in practice all measurements are compromises between different demands. In L&W Fiber Tester the statistic is somewhat limited with respect to measurement of shives. But the degree of details and the ability to measure narrow shives are unique. The recently released on-line pulp analyzer, L&W Pulp Tester, includes a new developed more conventional shive analyzer. This shive analyzer measures on a bigger amount of pulp than other systems, with unique statistics and repeatability as a result. L&W Pulp Tester includes both the new shive module as well as a fibre morphology module, which is the same module used in L&W Fiber Tester. Good statistic for both “traditional” shives and a detailed analysis of small but more frequent shives is therefore possible.

Calibration of shive measurements in L&W Fiber Tester
One question when vessel cells are measured with camera technology and fibre analysis is how you calibrate the instrument. Against which instrument do you calibrate? Are there model particles that can be used? When camera technology and image analysis are used there is a unique possibility to actually see what is measured and registered and this can be used as a calibration method.

A well developed technique in L&W Fiber Tester selects and saves images of all objects wider than the fibres and classifies them as vessel cells or not. Images of all wide objects, both classified as vessel cells and not vessel cells, are saved. The images can then be monitored together with the measured classification parameters. The calibration is done by modifying the selection criteria relative to the included and excluded object data respectively, until you are satisfied with the classification. It is an iterative process with visual estimation of the actual images.

New procedure calculates length and width for objects with complicated structures
In the fibre analysers from Lorentzen & Wettre the fibre length and width are calculated from the fibre perimeter and fibre surface. This technique was developed, already at an early stage, by STFI for L&W STFI Fibermaster. The method requires a good image processing that creates an ideal image of the fibres before the calculations. An equation system with two equations and two unknown variables is solved and the result is calculated length and width. The advantage with this method is that all pixels in the image are used in the calculations. The resolution in the calculated fibre width is much higher with this method compared to what you expect from the pixel size itself. If the pixel size is 10 µm the resolution can be around 0.2 µm. The method works fine on long and slender objects like fibres.

A = b × L
P = 2 × b + 2 × L

Formula: The length L and width b of a rectangle formed model fibre are calculated from this equation system. A=fibre surface, P=fibre perimeter.

For short and wide objects like vessel cells or more complicated structures like many shives another method for calculation of length and width is to prefer. Different alternatives have been tested at Lorentzen & Wettre. Recently it was decided to use a rectangle as a model also for these objects, but the calculation procedure has to be completely different. Following calculation procedure will be applied for the future:

1. Mass centre of the object is calculated
2. The direction through the centre of mass for the main surface angular moment is calculated
3. The object is approximated with a rectangle with the same centre of mass, direction of main axis and main surface angular moment as the object.
4. The length of the rectangle is used

The width of the object is calculated as the surface of the object divided with the calculated length of the rectangle. 

This principle for calculation has shown to be perfect for vessel cells from southern hardwoods and is a good approximation also for other vessel cells and for shives. Classification of shives was the main reason for development of a new calculation procedure.