## 4 Discussion  
### Drop Diameters  
Measuring drop diameters in a spray is a very complex task that has been addressed in different ways and exploiting different techniques and physical principles. Unfortunately, each measurement technique may produce different results. As an example, a study aimed at comparing four different methods for measuring droplet size distributions (image analysis, custom-made stroboscopic imaging, PDPA, and laser diffraction), reported that the larger the droplets, the bigger the differences are between the results [38]. Another study [35] focused on 17 bibliographic references about the results obtained by testing reference nozzles, pre-screened for laboratory purposes, reported a wide range of absolute measurements: when median values of \(D_{v50}\) increased from about 140 to about 395 \(\upmu\)m, the interquartile range was on average 24% of the median values (from 20 to 33%). Despite this wide range of results, nozzle classification in terms of spray quality was identical in 73% of the cases, and this quite uniform classification confirms the usefulness of the reference nozzles.  
The results obtained in this study by applying the liquid immersion method almost always agree with manufacturer information (Albuz catalog 2016) about spray quality based on VMD. In fact, ATR 80 orange nozzles are classified as Very Fine (VMD < 159 \(\upmu\)m) at pressures greater or equal than 0.5 MPa (the measured VMD was 147 \(\upmu\)m at 0.5 MPa and lower for higher pressures); TVI 8002 nozzles are classified as Ultra Coarse at 0.5-0.7 MPa (VMD > 553 \(\upmu\)m, measured VMD = 624 \(\upmu\)m at 0.5 MPa), Extremely Coarse at 1.0 MPa (484 \(\upmu\)m < VMD < 553 \(\upmu\)m, measured VMD = 457 \(\upmu\)m) and Very Coarse at 1.5 MPa (386 \(\upmu\)m < VMD < 484 \(\upmu\)m, measured VMD = 415 \(\upmu\)m); finally, AVI 11003 nozzles are classified as Extremely Coarse at 0.3 MPa (measured VMD = 506 \(\upmu\)m) and Coarse at 0.5 MPa (326 \(\upmu\)m < VMD < 386 \(\upmu\)m, measured VMD = 449 \(\upmu\)m). These results will be further investigated in future researches by also measuring the drop size distributions produced by the reference nozzles used to define the boundary regions, as recommended by the standard ISO 25358:2018 [39].  
No other measurement techniques were used to compare the results. However, [PERSON] et al. (2007) [35], using a PDPA system, reported for the ATR 80 orange nozzle at 0.3 MPa diameter values on average 15% greater than the actual ones (VMD = 191 \(\upmu\)m vs. 174 \(\upmu\)m), whereas [PERSON] et al. (2017) [17] reported, for the same nozzle at 1.0 MPa,  
Figure 9: Comparison between measured and theoretical distribution for the AVI 11003 nozzle. (**a**) Histograms and theoretical densities; (**b**) Empirical and theoretical cumulative distribution functions.  
a VMD value of 95 \(\upmu\)m measured with a laser diffraction system compared with 138 \(\upmu\)m obtained in this research. In both cases, despite the discrepancies in absolute results, the nozzle classification remained unchanged. In the same paper [17], a VMD of 606 \(\upmu\)m was reported for the TVI 8002 nozzle at 1.0 MPa (vs. 457 \(\upmu\)m obtained in this research), outside of the bounds of classification of the nozzle as Extremely Coarse. These results confirm the need to use the reference nozzles for classification purposes.