## 3 Results
### Description of the EHD Thrusters  
This section presents all the devices built as well as the equipment used as power supply and the test procedure. Figure 3 shows the ASA 3D printed structure. The layer height used was of 0.1 mm with a 100% infill factor and a tolerance around 12 \(\upmu\)m. The cathode, the anode, the high voltage cable for the connections and the sealing areas with epoxy resin are also presented. Figure 4 illustrates the method for measuring thrust using a precision balance. The process of measuring the thrust is straightforward; the thruster should be connected to the power supply and then, place it on the balance and tare the system to zero, to set the initial conditions. The thrust in g will appear as a negative value with an accuracy of 0.01 g.  
In previous research [15; 16], high voltage was usually applied to the thruster either by means of a flyback DC-DC converter or a relatively simple power supply, [17]. While these options are undoubtedly adequate, none of them allow for an accurate current measurement since in all cases, an external current probe and, as in [17], a Fluke(tm) 116 multimeter must be used. In this study, the thrust measurement setup is very similar to the one presented above but improved by the use of a high voltage laboratory, easing the current measurement and increasing the accuracy.  
Figure 3: Detailed view of the thruster.  
Figure 2: General view of the thrusters.  
In all experimental tests a Baker(tm) D30R was used as power supply. This instrument is designed to perform DC Hipot, Surge, Dielectric Absorption (DA) and Polarization Index tests. Figure 5 presents the setup during one of the thrush measurements, a screenshot of the voltage and current measurements is shown in Figure 6. Voltage is variable from 0 to 30 kV, although some previous tests were done at lower voltages, the results presented in the next sections were all obtained at 30 kV.  
In addition to the advantages of variable voltage and immediate current measurement, the instrument is equipped with a protection system that reduces or prevents the occurrence of arcing. When the current growth is too fast, the voltage application is deactivated. This prevents damages in the test specimen. The technical specifications of the instrument are summarised in Figure 5, where it can be seen that the levels of accuracy in current measurement cannot be compared, for maximum current values below 1 mA, with the use of a current probe and a multimeter. Figure 6 shows voltage, current and insulation resistance during a test.  
Figure 4: Single-configuration asymmetrical capacitor ready for the measurement of thrust.  
Figure 5: Baker(tm) D30R Hipot, surge, DA and PI tester connected to the thruster during a test.  
In order to carry out the experimental measurements with the maximum precision, environmental conditions under which they took place were controlled. For this purpose, a measurement station designed to be installed in an UAS to analyse environmental pollution was used. This piece of equipment measures humidity, temperature, and the concentration of multiple gases that appear as pollutants disolved in the air: CO, CO\({}_{2}\), SOx, NOx, and hydrocarbons. Each series of measurements was taken with virtually identical values for all variables. Figure 7 shows a photograph of the instrument. The following section shows a brief analysis of the electric field and potential distributions in the single and dual thrusters.