## 3 Measurement Results
### Further Research
#### 3.2.2 Flow Meter (Dynamic Calibration)--Sensirion SLG64-0075  
There are a variety of microfluidic applications that require fast dynamic response and precise control of flow. In this case, the dynamic flows are often measured with flow meters. Flow meters are usually calibrated statically but later used under dynamic flow conditions. When designing the calibration facilities, however, care was taken to ensure that calibrations are also possible under dynamic conditions. At these low flows, Thermal Mass Flow Meters (TMFM) or Coriolis Mass Flow Meters (CFM) are very commonly used. Both types of flow sensors are available without (flow meter) and with control valve (flow controller). In the first case, the dynamic flow profile must be generated by a flow generator; in the second case, the control valve itself can initiate changes in the flow rate.  
Figure 11 shows the measurements with the TMFM Sensirion SLG64-0075. Here, the dynamic flow profile described in Table 3 was generated with the help of the Cetoni pump.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Start Time** & \begin{tabular}{c} **Measuring** \\ **Period** \\ **min** \\ \end{tabular} & \begin{tabular}{c} **Volume Flow** \\ **nL min\({}^{-1}\)** \\ \end{tabular} & \
\begin{tabular}{c} **Volume** \\ **\(\mu\)L** \\ \end{tabular} &
\begin{tabular}{c} **Volume** \\ **Volume**\(\mu\)L \\ \end{tabular} \\ \hline
0 & 30 & 1000 & 30 & 30 \\ \hline
30 & 30 & 2000 & 60 & 90 \\ \hline
60 & 30 & 2500 & 75 & 165 \\ \hline
90 & 30 & 1500 & 45 & 210 \\ \hline
120 & 30 & 500 & 15 & 225 \\ \hline
150 & 30 & 1000 & 30 & 255 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Investigated dynamic flow profile containing step changes in flow rate.  
Figure 10: Measurement results of the Bronkhorst flow meter for a set flow rate of 1000 nL min\({}^{-1}\). The red dots show the high-resolution (10 Hz) flow rate values from the Bronkhorst flow meter (DUT) and the blue curve shows the mass increase recorded by the weighing scale (1 Hz) in the same time frame.  
The software of the Cetoni pump offers the possibility of conveniently programming such dynamic flow profiles. The green curve in the plot shows the flow rate from the Cetoni pump, the blue curve, the flow rate from the flow meter (DUT) and finally the red curve, the flow rate values from the scale. In this example, the time-resolved weight data of the balance were used to determine the flow rate by means of a moving average over ten values.  
It turns out that both the flow meter and the scale can follow the generated dynamic flow profile remarkably well. For both devices, the time for the change from one to the other flow point is below 1 s. If static and dynamic calibrations are compared for the same total volume, both measurements are well within the stated measurement uncertainty range. In further investigations, the times between the flow changes were extremely shortened from 30 min to 30 s and dynamic measurements (\(n=10\) repetitions) were compared with static measurements. As in the previous case, the agreement was within the measurement uncertainty range of the flow facility, which indicates a good performance of the flow meter.