## 3 Results
### The Static Measurements Experiment  
Figures 2 and 3 show the measured speed of sound and attenuation plots for a verification sample with a mean diameter around 484 nm and 794 nm, respectively. As said before, the apex of the speed of sound (i.e., the frequency where the highest speed of sound measured) is at a different frequency in the two figures. This is because the speed of sound is dependent on the mean diameter of the particles. Each of the five silica-water samples in five different dilutions was measured a few times (typically 4 to 8) with the UNPS. We refer to the samples as sample A until sample E. We refer to their dilutions as 1:1 (the original undiluted sample) until 1:12 (a dilution of 1 part of the original sample and 11 parts water). The 25 averages of these measurements are plotted in Figure 4 (mean particle diameter) and Figure 5 (particle concentration). The standard deviations of these measurements are used to create error bars, spanning the 3\(\sigma\) interval, on top of these 25 averages in both figures. The UNPS results of the mean particle diameter (Figure 4) are verified with the TEM measurements. The TEM measurements were performed on the samples with the highest dilution. The measuredmean particle diameters by the TEM (see Figure 6) were determined through measurement of at least 70 particles per sample. We expect that the silica particles do not grow or dissolve in the samples during dilution. Therefore, the UNPS should measure the same mean particle diameter after each dilution. The fact that the variation for the five dilutions within the samples with mean particle diameters above 200 nm (samples B-E) for the UNPS measurements is small (within 6%) in Figure 4, supports that expectation. Figure 4: UNPS and transmission electron microscope (TEM) measurements of the mean particle diameter of the silica samples. Figure 5: UNPS and drying measurements of the particle concentration of the Silica samples. Figure 6: TEM images of the silica samples. From left to right: sample A, B, C, D, E.  
We expected that the UNPS would have more difficulty measuring sample A. The particles with a diameter around 130 nm in sample A have an apex in the speed of sound that lies outside the measurement range (>250 MHz) of the UNPS. This makes it harder to estimate their size; still, the variation of the UNPS measurements for the different dilutions of sample A is within 12%. The results of the two instruments are in good agreement. In particular, when we compare the UNPS measurements with the verification measurements of the TEM, we see that most measurements are within each other's 3\(\sigma\) range. The average and the standard deviation for the UNPS and TEM measurements plotted in Figure 4 are concentration based. It is more common to report TEM averages and standard deviations on a number base. However, for a more direct comparison with the UNPS we have recalculated the averages and standard deviations in a concentration base. The effect of this recalculation on the TEM averages is, with less than 3%, small. This is due to the fact that samples A-E all have a narrow particle size distribution. We expect that the drying process of the samples needed for the TEM measurements did not have a great influence (<2 nm) on the diameter of the particles. Figure 5 is plotted on a logarithmic scale, such that the particle weight concentrations results at different dilution steps should be easier to relate. Besides the UNPS measurements, the weight concentration was also determined by drying the 1:4 samples (the green columns in Figure 5). The UNPS weight concentration measurements showed good consistency between the undiluted (1:1), the half diluted (1:2) samples, and the mass concentration obtained by drying. Concentrations below 2.5 w % (1:4 until 1:12) were less consistent, although most samples were within a 14% distance from their expected value.