Table of contents 1 Why is particle size important?
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| DISTRIBUTION POINTS
While it is tempting to use a single number to represent a particle size distribution (PSD), and thus the product specification, this is typically not a good idea. In nearly every case, a single data point cannot adequately describe a distribution of data points. This can easily lead to misunderstandings and provides no information about the width of the distribution. Less experienced users may believe that the “average particle size” can adequately describe a size distribution, but this implies expecting a response based on a calculated average (or mean). If forced to use a single calculated number to represent the mid-point of a particle size distribution, then the common practice is to report the median and not the mean. The median is the most stable calculation generated by laser diffraction and should be the value used for a single point specification in most cases. Setting particle size specifications Rather than use a single point in the distribution as a specification, it is suggested to include other size parameters in order to describe the width of the distribution. The span is a common calculation to quantify distribution width: (D90 – D10) / D50. However, it is rare to see span as part of a particle size specification. The more common practice is to include two points which describe the coarsest and finest parts of the distribution. These are typically the D90 and D10. Using the same convention as the D50, the D90 describes the diameter where ninety percent of the distribution has a smaller particle size and ten percent has a larger particle size. The D10 diameter has ten percent smaller and ninety percent larger. A three point specification featuring the D10, D50, and D90 will be considered complete and appropriate for most particulate materials. How these points are expressed may vary. Some specifications use a format where the D10, D50, and D90 must not be more than (NMT) a stated size. Example: D10 NMT 20µm D50 NMT 80µm D90 NMT 200µm Although only one size is stated for each point there is an implied range of acceptable sizes (i.e. the D50 passes if between 20 and 80µm). Alternatively, a range of values can be explicitly stated. Example: D10 10 – 20µm D50 70 – 80µm D90 180 – 200µm This approach better defines the acceptable size distribution, but may be perceived as overly complicated for many materials. It may also be tempting to include a requirement that 100% of the distribution is smaller than a given size. This implies calculating the D100 which is not recommended. The D100 result (and to a lesser degree the D0) is the least robust calculation from any experiment. Any slight disturbance during the measurement such as an air bubble or thermal fluctuation can significantly influence the D100 value. Additionally, the statistics involved with calculating this value (and other “extreme” values such as the D99, D1, etc.) aren’t as robust because there may not be very many of the “largest” and “smallest” particles. Given the possible broad spread of D100 results it is not recommended for use in creating specifications involving a statement that 100% of the particles are below a stated size. tải về 7.54 Mb. Chia sẻ với bạn bè của bạn:
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