# Design and Construction of a Low-Cost Test Bench for Testing Agricultural Spray Nozzles
## 2 Materials and Methods
### First Experimental Spraying Tests  
These trials were carried out to test the functionality of the whole system and its sensitivity to pressure changes. No proper experimental design was defined, and results were considered as qualitative only. Future experiments and trials will be carried out and presented in other scientific papers, comparing several types of nozzles and testing under the same conditions the reference nozzles used to define the boundary regions of spray quality as recommended by the ISO/FDIS 25358:2018 standard [55]. Spraying tests were carried out with an ATR80 orange hollow cone nozzle (Albuz, France) at the working pressures of 0.3, 0.5, 1.0, and 1.5 MPa, and spraying as test liquid clean water with the addition of red Ponceau (Novema Srl, Torino, Italy) as a soluble coloring agent at the concentration of 2 g/L. In these first trials, surface tension of test liquid was not measured. Temperature and relative humidity of the ambient air were measured by using a thermohygrometer (HD 8901 model from Delta Ohm, Padova, Italy). The three Petri dishes that were used to sample the drops had a diameter of 55 mm, and the distance between their centers was 195 mm (Figure 5). Nozzle speed during the tests was set to 1.5 m/s. Figure 4: User interface: (**a**) tab used to manage the spray trials and set the working parameters; (**b**) tab used to monitor the sensor signals and to stop the system in case of emergency. Figure 5: Positioning of the Petri dishes (PD) on the table of the test bench (sizes in mm). Five milliliters of silicone oil (AR200 from Sigma-Aldrich, Milano, Italy) were deposited in each Petri dish by using a pipette (Mettler-Toledo, Milano, Italy), taking care to avoid bubble formation. Dynamic viscosity and volumetric mass of the silicone oil were 200 mPa s and 1050 kg/m\({}^{3}\), respectively. After sprays, Petri dishes were photographed in situ by using a Nikon D5500 DSLR (digital single-lens reflex) camera equipped with a macro lens (Nikon Micro Nikkov AF-S 60 mm f/2.8 G ED) and an electronic flash (Neewer 48 Macro LED Ring Flash). To avoid involuntary movements, the camera was remotely controlled via the qDslrDashboard Version 3.5.3 application ([[http://dslrdashboard.info](http://dslrdashboard.info)]([http://dslrdashboard.info](http://dslrdashboard.info))) running on an Android tablet. Spraying tests were repeated three times, so nine photos were taken for each working pressure. The images were saved as high-quality JPEG files with a resolution of 6000 \(\times\) 4000 pixels. The red color of the test liquid allowed for an easy recognition of the droplets during the subsequent image analysis (Figure 6). Images were calibrated by taking photos of a \(10\times 10\) mm grid pattern engraved on a glass disc, placed in correspondence of the three Petri dish positions, as far from the camera focal plane as the drops. The resulting scale factor ranged from 188.0 to 189.0 pixel/mm for the three Petri dish positions. All images were analyzed with the ImageJ software, release 1.52a [56]. The use of this software for the spray droplet image analysis is well documented in the literature [44; 45]. After conversion in 8-bit grey scale, images were segmented. The selected threshold value was obtained, on the basis of a careful inspection of the images, by increasing the \"default\" threshold value prompted by ImageJ by 20% on average. A threshold value directly linked to the grey level of the image as described in Sanchez-Hermosilla and Medina [57] was not exploited in this case because it was specifically studied to analyze water sensitive paper images, which are different from actual images, mainly due to the lighting conditions. In future trials, when other types of nozzles will be tested, a specific study will be dedicated to the segmentation process, and it will be checked whether the criterion adopted in this study for the ATR80 hollow cone nozzle is still valid.