## 1 Introduction  
The application of Plant Protection Products (PPPs) is affected by many factors, among which drop size is the most important as it affects biological efficacy [1], environmental pollution [2, 3], and operator safety [4, 5, 6]. In fact, the correct spray spectrum is capable of ensuring the required dose on the target, minimizing drift and ground losses, and reducing dermal and inhalation worker exposure. A research of [PERSON] et al. (2016) [7] aimed at assessing spray deposit and canopy penetration of nozzles with different spray qualities in an out canopy, showed that droplet number densities were inversely related to the droplet size produced by the nozzles, yet coverage was increased more by application volume rate than droplet size. Another study by [PERSON] et al. (2014) [8] showed that drop size also affects spray retention, i.e., the overall capture of spray droplets by plants on initial or subsequent impact, so influencing efficacy, economic losses, and environmental contamination. The most known example of the environmental impact of pesticide application related to drop size is spray drift: small droplets are more easily deflected out of the target area by wind action [9]. The adoption of low-drift nozzles [10, 11, 12], sprayer calibration by means of test benches [13; 14; 15], and direct or indirect drift measurement [16; 17; 18; 19], are common practices to reduce spray drift.  
To know the drop size distribution of nozzles under ordinary working conditions is, therefore, of great interest to guide the most appropriate choice in relation to target structure, liquid properties, and environmental conditions. Drop size measurement is based on several principles, both intrusive and non-intrusive.  
Intrusive measurement techniques include the image analysis of the drop stains on water sensitive papers (WSPs) and the liquid immersion method. WSPs are semi-rigid papers, with one side covered with a special yellow film that turns deep blue when hit by a drop of water [20]. The analysis of stains by means of image processing software, under some limitations, allows sizing the drop diameters [21; 22; 23; 24; 25]. In the liquid immersion method, droplets are collected on Petri dishes containing lightly viscose liquids, such as Vaseline, light mineral oil, and silicone oil, which cause drops to form spherical shapes. They are immediately photographed or observed with a microscope, allowing drop counting and size measurement [26; 27].  
Non-intrusive methods include high speed imaging (HSI) [28; 29; 30], laser diffraction (LD) [31; 32; 33], and Phase Doppler Particle Analyzers (PDPA) [34; 35; 36]. HSI and PDPA methods also allow measuring of drop velocity.  
Each measurement technique gives results that are significantly different, depending on measuring protocol, settings, and type of measuring equipment [35; 37; 38], so much so that the international standard ISO 25358:2018 [39] recommends basing nozzle classification on comparison with reference nozzles.  
In this study, a low-cost custom-made system that allows measuring of the drop size of agricultural spray nozzles under ordinary working conditions was used. The design of the equipment was mainly based on the ISO 5682-1 standard [40], which exploits the liquid immersion method. Three nozzles were tested, all from Albuz (France): an orange hollow cone nozzle ATR 80 (European color code), an air induction flat spray nozzle AVI 11003, and an air induction hollow cone nozzle TVI 8002. The ATR 80 and the TVI 8002 nozzles were tested at four pressure values: 0.3, 0.5, 1.0, and 1.5 MPa; the AVI 11003 nozzle was tested at 0.3 and 0.5 MPa. For each test condition, the usual spray parameters were computed: the arithmetic mean diameter, the Sauter mean diameter, the volumetric diameters \(D_{v10}\), \(D_{v50}\), and \(D_{v90}\), and the Relative Span Factor (RSF). Finally, the theoretical drop size distribution that best fitted the experimental data was devised and the effects of spray pressure were analyzed.