Dipl.-Ing. Gert Beckmann, Retsch Technology GmbH Dipl.-Ing. Johannes Jüngling, Süd-Chemie AG

A new means of evaluation give a different dimension to measuring bulk material by means of digital image processing: be it filter sand or abrasive grains - higher performance software and hardware offer economically interesting possibilities for complying with the increasing demands made of manufacturing, conveying and processing dispersive solids. The latest development in this field is the Camsizer® measuring device with the evaluating software Advanced. It is now possible with one single measurement to analyse all desired particle sizes and shapes. Its special feature is the fact that analysis results are compatible with traditional screen analysis. This is a breakthrough in industrial particle measurement technology because this makes it significantly easier to substitute the Camsizer measuring device for screen machines that were previously used for quality assurance. This means that the user can drastically reduce the amount of time and effort required for particle analyses while having access to a wide variety of new information with practical benefits.


An Introduction and the Principle of Measurement

It is not only traditional screen analysis that helps ascertain the particle size distribution of bulk material and granulates, but also a variety of optical measuring techniques. A number of material properties are not only determined by size distribution, but also by particle shape. This is the reason why quality assurance departments in companies in the bulk material industry frequently demand both a fast and accurate determination of the particle size and a simultaneous analysis of the particle shape. These are demands that can be fulfilled by means of digital image processing via matrix cameras.
The Camsizer from Retsch Technology takes advantage of this optical measuring principle. Its spectrum of particle sizes for particle analysis ranges from 30 µm to 30 mm. The diverse analytical capabilities of the Camsizer also offer you the possibility of evaluating bulk material and granulates with reference to their shape in order to monitor quality specifications with little effort or to define for the first time. By streamlining this process, it is possible to save a significant amount of costs in the fields of raw materials, production and finished products, for instance by lowering the reject rate. It is now possible to reliably assess particle size and particle shape in a quantitatively correct, user-friendly fashion in a short period of time with a high degree of accuracy and reproducibility. This was previously only possible with a great deal of staff and working time. In addition, the measurement is done without contact and therefore non-destructively, meaning that it is ideal for sensitive materials. Furthermore, if what the software has to interpret as particle size x is suitably defined, the results are compatible with the screen analysis that is now predominant.

The Camsizer takes advantage of the principle of shadow projection. Particles fall vertical to the optical axis between a surface light source and two digital CCD cameras and are recorded by it in fast sequence so that the entire bulk material flow can be analysed. The extremely short exposure time guarantees that the projection surfaces of the particles can be ascertained very precisely in the digital images to be evaluated.

Figure 1: The Camsizer Measuring Device and a Digital Snapshot
(to view image please see pdf-file)

The particle shape is measured at high resolution in various directions. This detailed determination of particle shape supplies the basis for various definitions of particle size (for instance, lengths, chords, surfaces) that may be used for evaluation.


Size Analysis: Compatibility is What the Customer Demands

Especially, particle size is something that we have been able to define most recently so that it corresponds to the comparison between particles and the screen mesh made in screen analysis. This option supplies the user with results compatible to screen analysis. For example, rice particles as shown on the right-hand side in Figure 1 can pass the screen meshes with their front side as a result of the screening movement. They are therefore measured in their smallest extension (projection). In contrast, the Camsizer measures particles in a random projection and from that it calculates such things as the equivalent diameter of a circle with the same area. This is the reason why the values for the equivalent diameter defined in this fashion are greater with this measuring technique than with screening. In the past, the user had to simply accept this systematic deviation between the particle definitions when switching to the Camsizer measuring principle.
In order to compensate for this deviation between the methods of analysis, it is possible to specify a new means of evaluating the readings to the software. The particle diameter is then no longer determined from a determination of the projection surface, rather by ascertaining characteristic dimensions in the particles detected. This, in turn, is specified by the shape of the particle. If, for example, the cumulative curve Q3 is applied through the widths of the particles, it is possible to show a traditional screen result directly.

Figure 2: A Comparison of the Screen Curve and Camsizer Measurement
(to view image please see pdf-file)

Figure 2 shows the screen curve of raw material pressed forgings for producing plastics. The red curve shows a measurement from a Camsizer where the particle size was ascertained based upon the particle width. This definition of the particle diameters represents the screening almost perfectly.
Since the size distribution of the Camsizer technique consists of up to 1,000 supporting points, the user receives additional information on the distribution of particles of the sample between the supporting points of the previous screen results that can be seen as the reference points of the black curve. We have also shown a Q3 curve (in green) to illustrate the progress in compatibility that you receive with a particle definition through the circle of the same surface (surface measurement). This evaluation of the mean particle projections has been shifted into the course range as expected. In this case, it is only possible to represent screening results by shifting the curve in accordance with the specific material. However, this involves a significantly greater amount of effort and a greater number of errors sources.
In sum, the new Camsizer Software Advanced provides the user with the following benefits:
- the results of traditional screen analysis anchored in a number of standards may now be directly and almost perfectly represented.
- as a result of the high-resolution representation of the distribution sum, there is also information between the screen cuts.
-if longitudinal dimensions are used, it is also possible to substitute other established measuring techniques (such as microscopy)


Form Analysis: An Example from Practice Demonstrates Additional Benefits

Apart from determining the size distribution of bulk material samples, the Camsizer simultaneously ascertains the particle shape of the material to be sampled precisely and reliably. The detailed evaluation of the projection surfaces of individual particles makes it possible to state special form characteristics such as the absolute length, roundness and length-to-width ratios. These reference quantities are issued for specific size classes or as a mean for the overall sample. Statistically securing the values received may be stated directly based upon the number of particles detected. These results are shown both in table and in graphic form. Furthermore, it is possible to convert the data for transferring it to common office software.
You can see the special benefits, for example, when measuring a difficult sample of extruded rods (refer to Figure 3). The measuring task was to determine the following characteristic dimensions: the absolute length of the rods XFe max, the particle width Xc min and the exterior width X Fe min.


Figure 3: A Camera Image of Rods Particles and their Dimensions
(to view image please see pdf-file)

The evaluation in Figure 4 shows that the width of the rods rises only slightly in the range of 2 mm to 15 mm over the length of the rod. The temporal change in the curve for particle width may be looked upon as a direct measure for the wear and tear of the extruded sections. It is possible to recognise malfunctions by means of form analysis, thus making it possible to initiate service work at an early stage. The longer the rods become, the more clearly formed the arch described by them becomes. This is shown by the rise in the exterior width. This makes it possible to draw conclusions on the material behaviour or den extruder sections such as soiling.

Figure 4: The Exterior Width and Particle Width through the Particle Length
(to view image please see pdf-file)

The benefit of combining particle size and particle shape analysis can be seen from this sample in the comparison of particle width and volume distribution Q3 as a function of particle length.

Figure 5: Particle Width and Q3 Curve through the Particle Length
(to view image please see pdf-file)

While it is not possible to see from the Q3 curve whether particles can be allocated to the dust content by means of particle fracture, this can be quickly and unambiguously ascertained from particle shape analysis. The representation of the curves shows that particles with a width below 2 mm represent fracture or dust.


Summary and the Look Ahead:

The excellent measuring quality of the Camsizer provides the user with a measuring technique in connection with innovative software features in the field of particles analysis that is capable of preparing detailed particle shape analyses and high-resolution size distribution within a very short period of measuring time. And, if desired, the compatibility to screen analysis may be retained without having to take the detour of mathematical adjustment algorithms such as aligning readings for specific materials by means of fitting. Furthermore, its robustness guarantees that it can be applied in industry without any problems. The next step we are planning is developing on-line versions of the Camsizer for continuous production monitoring.