## 2 Materials and Methods
### Vacuuming and Charging the Heat Pipes with Working Liquid  
The amount of working fluid that had to be charged into each tube was set at 36.6% of the pipe volume. This value, as mentioned in Section 2, Materials and Methods, was chosen on the basis of extensive experimental studies on the stability of pressure during the operation of the HP. For this purpose, a copper pipe with an external diameter \(d_{out}\) equal to 6 mm, internal diameter \(d_{in}\) equal to 4.4 mm and an active length \(L_{act}\) equal to 260 mm was used. The entire process of charging the tubes with the working liquid took place in an air-conditioned room, where the ambient temperature was \(T\) = 20 \({}^{\circ}\)C, and the density of acetone at 20 \({}^{\circ}\)C is \(\rho_{a}\) is equal to 0.792 g/cm\({}^{3}\). The volume of the empty pipe charged by the acetone \(V_{a}\) was determined from the following relationship:  
\[V_{a}=0.366\cdot\pi\cdot\frac{d_{in}^{2}}{4}\cdot L_{act} \tag{3}\]  
and a \(V_{a}\) equal to 1.44 mL was determined. Therefore, the mass of acetone \(m_{a}\) to be charged in the pipe was determined from the following relationship:  
\[m_{a}=V_{a}\cdot\rho_{a} \tag{4}\]  
and determined that \(m_{a}\) would be equal to 1.1 g.  
Figure 4: Photo of a heat pipe with a mesh inserted, soldered with a Shrader valve and a plug. To charge heat pipes with the working liquid, a rig had to include:  
* Two fittings equipped with a tee, pressure transducer and shut-off valve;
* Two clamps (self-made);
* A liquid nitrogen vessel made of non-thermally conductive material
* Laboratory-precision balance (model Steinberg SBS-LW);
* Two vacuum pumps. Measuring devices and their parameters are given in Table 2.  
Figure 5 shows the view of the fittings used to flood the heat pipes and control the negative pressure. The fittings consisted of tees, globe valves (model Danfoss BML 6) as well as pressure transducers (models TESTO 549i and TRAFAG DCS6.0 AR). The TRAFAG transducer had a built-in display, while the TESTO 549i transducer was connected to the dedicated TESTO software (version 19.1.13.71843) via the Bluetooth wireless network installed on a laptop computer. The charging of heat pipes was carried out according to a strictly followed procedure:  
1. Assembling the binding on the heat pipe;
2. Vacuuming the pipe for 1 min (the pressure transducer indication must show \(-1\) bar);
3. Weighing the heat pipe on a laboratory scale, saving the weight indication and removing the binding;
4. Charging the heat pipe with 1.4 mL of acetone using a syringe and assembling the binding connector on the heat pipe;
5. Weighing the heat pipe on a laboratory scale and saving the weight indication and connecting vacuum pump hose to the binding;
6. Placing the heat pipe in a vessel containing liquid nitrogen;
7. Filling the vessel with liquid nitrogen; \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Device** & **Measured Parameter** & **Range** & **Accuracy** \\ \hline TESTO 549i & pressure & \(-1\)–\(60\) [bar] & \(\pm 0.5\)\% \\ TRAFAG DCS6.0 AR & pressure & \(0\)–\(6\) [bar] & \(\pm 0.5\)\% \\ Steinberg SBS-LW-3000N & mass & \(0\)–\(3000\) [g] & \(\pm 0.1\) g \\ Czaki T-type thermocouple & temperature & \(0\)–\(300\) [\({}^{\circ}\)C] & \(\pm 0.2\) K \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Technical parameters of measuring devices used during manufacturing and validation of HPs. Figure 5: View of the fittings equipped TESTO 549i pressure sensor and a Danfoss BML 6 globe valve (**left**) and TRAFAG DCS6.0 AR pressure transducer and a Danfoss BML 6 globe valves (**right**).