## 5 Discussion  
The reproducibility of particle size and surface area measurements relies primarily on using representative samples of the bulk material and setting appropriate measurement conditions. If properly maintained, the SAS instrument has a slight advantage over the wet laser light diffraction equipment in that it examines larger samples, which provides a better representation of the bulk powder. Besides the sample size, by an automation of the analysis, operator errors are almost excluded, and the reproducibility and repeatability are markedly improved. On the other hand, the merit of laser light scattering is the capability to measure the size of individual particles. Consequently, the particle size distribution of the powder can easily be determined. Care is needed, however, in interpreting the particle size analysis data when the powder is agglomerated or irregularly shaped. Although the two instruments used in this study operate on different principles, they generate similar results for spherically shaped particles. Using the Fisher Number (\(d_{FN}\)) as a baseline, the D[3,2] diameters obtained from laser light scattering differ by less than 9% for the 316L-A and YTF-HY2 powders. These small differences may originate from a relatively high proportion of splats and welded satellites (see Figure 3a), aggregates of firmly bonded particles (see Figure 4a), misshapes developed during powder fabrication [25], and other uncertainty associated with measuring procedures. It has to be noted that the acceptance criteria for central values of the distribution (i.e., D50, D[4,3], D[3,2] evaluated by laser light diffraction require deviation from the expected value <10% or even <20% when measuring below 10 \(\mathrm{\SIUnitSymbolMicro m}\)[26]. In the case of the most spherical 316L powder, the Sauter diameters produced by both methods are virtually identical (see Figure 3 and Table 2). It is noteworthy that the 316 L powder bed porosity achieves the lowest value of 40.7%. This may improve the accuracy of permeability testing because the bed becomes more uniform as the porosity decreases [13]. The D[4,3] and D50 values have not been taken into account in this analysis. They indicate markedly larger mean particle sizes, which is typical for volume-based distributions. Cobalite CNF has rough surface configuration (see Figure 4b) and shows the widest particle size distribution among the tested powders (Span = 2.07). According to its producer, the powder is composed of multi-phase agglomerates of individual particles finer than 1 \(\upmu\)m [27]. As is evident from Table 2, the ultrasonic energy applied during the laser light diffraction measurements has a negligible effect on the disintegration of bigger particles seen in Figures 3f and 4b, mistakenly regarded as agglomerates. It becomes obvious that the powder consists of porous, fine-grained, predominantly rounded particles, bigger than 1 \(\upmu\)m. The SAS data indicate that Cobalite CNF is characterized by the highest specific surface area (see Table 2). Due to its spongy structure, developed during the milling of loosely sintered metallic cases received from the reduction of fine oxide precipitates [28], the \(S_{M}\) figure obtained from laser light diffraction and \(d_{FN}\) generated by the SAS are both highly underestimated. It is worthy of notice that the 72.7% bed porosity is the highest among the tested powders. At high levels of porosity, the pore texture is non-uniform because internal pores in bigger powder conglomerates provide markedly smaller channels for flowing air than the inter-particle porosity. Large capillaries have disproportionately high rates of flow, which suppresses the effect of the small capillaries, thus leading to errors [13; 29]. As seen in Figure 3c,d, both Hoganas iron powders show irregular particle shapes. The deviation from sphericity provides room for errors in particle sizing and specific surface area measurements.