## 3 Results
### Fully Distributed Tracking of Pressure Waves in Pipes  
For the second set of experiments, where a scenario of pressure waves in pipes was modelled by inducing gas explosions on one end of the pipe, a fully distributed sensor configuration was chosen to capture the full profile of the propagation. The pulse length was 30 ns for all experiments of this type. Figure 9 shows the acquired data after igniting a gas mixture of 4.2% vol. propane in air at an initial overpressure of 6 bar from one end of the pipe. Raw data exhibits two different signal progressions with differing propagation velocities as indicated by dashed lines to guide the eye. All signal features are visible in the spatial range between bins 174 to 252 corresponding to a range of 53 m of applied fiber. To discard regions possibly influenced by the loose fiber loops the range of from bin 179 to 247 is used for data evaluation. The faster component may be attributed to structure-borne acoustic waves travelling inside the pipe material after the explosion, while the slower component should correspond to the pressure wave inside the medium. The signal of the pressure wave is clearly dominant here in accordance with a larger dynamical circumferential strain change than the one caused by the structure-borne waves in steel. However, the dashed lines with a slope of \(\pm 5050\) m/s matching the expected acoustic velocity reflect well the observed signals in advance of the pressure wave. To analyze the slower component in greater detail, differentiation with a larger temporal shift is used as a filter. Automatic identification of signal onset reveals some points that are attributed to other signal features (i.e., structure-borne waves) and are discarded for further processing of the data. A linear fit of the remaining points yields a velocity of the pressure wave of \((380\pm 10)\) m/s. The average velocity determined by the two conventional pressure sensors is 406 m/s and is compatible with valued derived by DAS measurements. Figure 10 shows filtered fiber-optically acquired data along the pipe after igniting a gas mixture of 28.5% vol. hydrogen in air at to different values of initial overpressure. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & **Sensor** & **Position** & **Pulse** & **Evaluation Procedure** & **Velocity/ (m/s)** & **Uncertainty/ (m/s)** \\ \hline \multirow{4}{*}{5} & quasi & & & differentiation and & \\  & distributed & & & summation, signal onset (automatically) & 5320 & 180 \\ \cline{3-6}  & & & & differentiation and & \\  & & & & summation, correlation (absolute maximum) & 5040 & 170 \\ \cline{3-6}  & quasi & & & (automatically) & & \\  & distributed & & & & differentiation and & \\  & & & & summation, correlation (mean over relative & 5080 & 170 \\  & & & & maxima) (automatically) & & \\ \hline \multirow{2}{*}{6} & quasi & & & differentiation and & & \\  & distributed & & & summation, signal onset (automatically) & 5710 & 210 \\ \hline \multirow{2}{*}{7} & quasi & & & differentiation and & & \\  & distributed & & & summation, signal onset (automatically) & 5510 & 200 \\ \hline \multirow{2}{*}{8} & quasi & & & differentiation and & & \\  & distributed & & & & summation, signal onset (automatically) & 5510 & 200 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._Figure 10: Temporal excerpt of filtered data (differentiation with temporal shift of 4) acquired using a fully distributed fiber-optic sensor configuration after igniting a gas mixture of 28.5% vol. hydrogen in air on one end of a closed pipe at different initial pressures.