## 2 Materials and Methods
### Measurement Facility for Flow Rates from 1 mL h\({}^{-1}\) to 100 mL h\({}^{-1}\)
#### 2.2.2 Operating Principle  
The measurement set-up (Figure 3) was comparable to the previously presented test facility (Figures 1 and 2). The difference is that, due to the higher flow rates, a weighing scale with a higher capacity was used. A Mettler Toledo XPR205 (ultra microcomparator balance) with a capacity of 220 g and a readability of 10 ug (5 decimal places) was selected. As before, the balance was operated with the aid of a PC and a USB connection via MT-SICS interface commands. For measurements over a long measurement period, the scale was read out at 1 Hz, and for short measurement periods, at about 10 Hz. Data acquisition and control of the scale were also carried out via LabVIEW. The required flow rate was generated using the ultra-high precision Cetoni syringe pump or the ultra-high precision syringe pump Chemyx Nexus 3000, which is also available (Table 1). For the lower flow rates, the Cetoni pump was used, and for the higher flow rates, the Chemyx pump. Onemain reason is that the syringe holder of the Cetoni pump can only take syringes up to 50 mL and the Chemyx pump can firstly take much larger syringes and secondly also two syringes at the same time. In the simplest configuration, the ultra-high precision syringe pump Chemyx Nexus 3000 delivered the set flow rate directly to the DUT or to a secondary standard (Figure 3). The secondary standard (reference flow meter) was a mini Coriolis Flow Meter (CFM) CORI-FLOW M12 from Bronkhorst with 1/8\" OD Swagelok connections installed on a solid metal block with vibration dampers to ensure zero point stability. On the metal block, at the same distance upstream and downstream of the CFM, there are two needle (shut-off) valves (Swagelok SS-ORS2), which are used when determining the zero point of the CFM. Finally, there is a pressure transmitter (Swagelok PTI-S-NG100-12 AQ) on the block connected via a t-connector (Swagelok SS-200-3) and a tube fitting reducer (Swagelok SS-400-R-2) to measure the inlet pressure. The CFM has a flow rate range from 0 g h\({}^{-1}\) to 200 g h\({}^{-1}\) and is calibrated against the weighing scale at regular intervals. Subsequently, the DUT (or the CFM reference setup) is connected to the needle, which is inserted into \begin{table}
\begin{tabular}{c c c} \hline  & **Chemyx Nexus 3000** & **Cetoni neMESYS 290N** \\ \hline Drive mechanism & Stepper motor & Stepper motor \\ \hline Step resolution & 0.0120 microns & 0.0168 microns \\ (Advance per step) & & \\ \hline Flow rate range & 0.012 nL min\({}^{-1}\) & 0.006 nL min\({}^{-1}\) (10 \(\mu\)L syringe) \\  & to 500 mL min\({}^{-1}\) & to \\  & (manufacturer information) & 150 mL min\({}^{-1}\) (50 mL syringe) \\  & & with gear box 29:1 \\ \hline Syringe size & 140 mL; 300 mL (with & \\  & self-made & \\  & holder) & holder) \\ \hline No. of syringes & 2 & 1 (multi-systems available) \\ \hline Readout / handling & Configuration of the initial & Configuration of the initial \\  & values (pump and syringe) via & values (pump and syringe) \\  & keypad or software. & exclusively via software. \\  & Information during operation & Output \\  & only via display. & of the (current) readings via \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison of the two ultra-high precision syringe pumps used as flow generators. Figure 3: Schematic drawing of a typical measurement setup (Microflow test facility) for the calibration of a flow meter as DUT. In this case, the drawing shows the CORI-FLOW M12 from Bronkhorst, which was used as a secondary standard. the weighing vessel (beaker), which is placed on the weighing scale. The beaker had an OD of 53.15 mm and a wall thickness of 0.11 mm, which results in an ID of 52.93 mm. The height of the beaker was around 135 mm, resulting in a usable volume of about 250 mL. The container weighed about 11 g, which means a usable weight of the weighing scale of about 170 g when pre-filled with water (22 g, 10 mm) and oil layer (15 g, 8 mm) to prevent evaporation.