# Development and Field Testing of a Wireless Data Relay System for Amphibious Drones
## 2 Materials and Methods
### Drone Mounters for the Water Quality Monitoring System  
The number for the developed water quality sensors (Figure 4) was produced using a fused deposition modeling (FDM) 3D printer (EnderTM-5 plus; Shenzhen Creality 3D Technology Co., Ltd., Shenzhen, China) with a reinforced polylactic acid (PLA) filament (PolymaxTM PLA; Polymax LLC, Shanghai, China). The reinforced PLA filament was selected for reproducible output without sensitive care for the desiccation of filament, thermal equilibration during printing, etc. The 3D-computer-aided design (CAD) data files for the 3D printer were made using software (FreeCAD version 0.19; the FreeCAD team, 2021). The STL format 3D-CAD files were processed using slicer software (Creality Slicer ver. 4.8.2; Shenzhen Creality 3D Technology Co., Ltd., China) for the 3D printer. The wall thickness was set as 2 mm (10 lamination layers accumulated at 0.2 mm intervals)  
Figure 3: Water quality monitoring modules and output data. (**a**) Modules for the SplashDroneTM 3+ in a data stability test; (**b**) sensor electrodes for the SplashDroneTM 3+; (**c**) modules for the SplashDroneTM 4 in a data stability test; (**d**) received data displayed on a laptop PC. Its components have been numbered as follows: 1, sensor boards for the SplashDroneTM 3+ system; 2, sensor electrodes for the SplashDroneTM 3+ system; 3, controller for the SplashDroneTM 3+ system; 4, controller for reference temperature measurement with a continuous power source; 5, rechargeable batteries; 6, data display (laptop PC); 7, pH sensor; 8, EC sensor; 9, DO sensor; 10, temperature sensor (reference); 11, temperature sensor; 12, pH 4 buffer solution in a screw cap; 13, EC 1.41 dS m\({}^{-1}\) standard solution in a screw cap; 14, aerator; 15, controller for the SplashDrone 4 system; 16, XBeTM S2C board; 17, sensor boards for pH/EC/DO; 18, sensor board for temperature; 19, I°C connector hub; 20, rechargeable battery (for demonstration of a minimized installation into the system mounter). for water-tight vessels. Support material and raft-type bottom adhesion were used in the printing. For stable adhesion of the bottom of printing materials to the printing stage, stick glue was used on the surface of the printing stage. The respective temperatures of the printing nozzle and the printing stage were set as 230 \({}^{\circ}\)C (503 K) and 60 \({}^{\circ}\)C (333 K). The printing speed was set as 60 mm s\({}^{-1}\). The infill ratio of the support material was 5%. The ratio of most of the moment parts was 20%. The ratio was increased to 100% for parts used under mechanical stress (e.g., the lid and legs of the monuter). Each part of the monuter was printed separately and was fitted together after printing by a (formerly patented) LegoTM block-like joint structure (6 mm diameter). After removing the support/adhesion materials, the exposed wall of each vessel was coated with a silicone resin sealer (Bath BondTMQ; Konishi Co., Ltd., Osaka, Japan) for waterproofing. Nitrile butadiene foam rubber (10 mm thickness, approximately 30 in the Asker-C hardness) with closed cells was used as the gasket for sealing spaces around sensor cables in the lid of the water-tight vessels. A wedge for each cable was made on the gasket surface using a triangular bar file. After fitting the cables in the wedge of the gasket, a thin (1 mm thickness) spongy stick tape was placed on the cables. Then, the gasket was pressed with the lid of the water-tight vessel by screwing. These simplified procedures of sealing were chosen for vessels above the draft line of amphibious drones on water, in order to avoid exposing high-impedance sensors to additional noise (e.g., pH glass electrode sensor; susceptible to cable splicing (cutting and reconnection) for installing waterproof connectors) and to widen choices of external sensors. Variation in the connectors of sensors to the sensor regulators is expected to necessitate numerous waterproof sockets if the sockets are to be installed on the vessels.