# Detection of Pneumatic Conveying by Acoustic Emissions
## 5 Results and Discussion
### Effect of Different Signal Acquisition Methods  
To realize an on-line measurement system, data processing time should be short. While the experiments were conducted for fixed conditions, the movement of particles was still disordered. Therefore, the obtained data should be representative of smaller data volumes, as required by a real-time system. Two different probe types were compared. The first one was of the T-type, shown in Figure 3, and the second was the wire-mesh type (Figure 4). When the T-type probe was installed in the system, since the sensitive area is concentrated, only part of the particles would have contact with the T-type probe. Together with the uncertainty of the flow type, the uncertainty of particle impact would increase and unpredictable. The wire-mesh probe does not pose this same challenge. Without increasing the occupation of the cross-sectional area, the wire mesh covers the entire cross-sectional area, while not significantly affecting the pipe's cross-sectional volume. Thus, the wire-mesh probe can be effectively increasing the number of particle impacts.  
Figure 5 presents typical raw AE signals acquired simultaneously by two AE sensors attached on both the T-type and wire-mesh probes. This signal corresponds to a particle mass flow rate of 16 g/s and a particle size of 550 \(\upmu\)m. The peak amplitude of the AE signal obtained by the T-type probe is larger than that recorded by the wire-mesh probe. Figure 6 presents the samples retrieved from the raw AE signals, for both probes, and each graph column was collected at the same time. All data in Figure. 6 were extracted randomly from the entire raw signal. One can see that the signals for the T-type fluctuate more than those from the wire-mesh probe. Figure 7a shows the root mean square (RMS) of AE signal for the two probe types. Even though the signal peak-amplitude for the T-type probe is higher, the RMS value for the wire-mesh probe is slightly greater than for the T-type probe. Figure 7b shows that the relative error for the wire-mesh probe deviates less from the zero line, which  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Conveying Velocity (m/s)** & **Particle Mass Flow Rate (g/s)** & **Particle Loading (\%)** & **Particle Size (\(\upmu\)m)** \\ \hline
21 & 6 to 16 with an increment of 2 & 0.006–0.016 & 550/250/180/150 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Experimental test conditions.  
Figure 3: Parameters of the T-type probe (dimensions in mm).  
means that the randomly selected signals acquired by the wire-mesh probe are more representative of the sample. Therefore, it can be concluded that the AE signal is more stable and representative for the wire-mesh probe than for the T-type probe. Figure 8 plots the RMS average values (for all four repeated tests) of the AE signals with respect to particle mass flow rates.  
Figure 4: Flange mounted wire-mesh probe (dimensions in mm).  
Figure 5: Raw acoustic emission (AE) signals obtained by the T-type probe and wire-mesh probe.  
Figure 6: Raw AE signals selected at random and divided into samples.  
It is worth noting that as mentioned, the AE signal RMS obtained by the wire-mesh probe is higher than for the T-type probe, and this value gap increases with a growing particle mass flow rate.