## 2 Materials and Methods
#### 2.1.2 Operating Principle  
The measuring principle of the test facility is based on the dynamic weighing method, also known as the dynamic gravimetric method [8; 9; 10] (Figures 1 and 2). For the measurement, a Mettler Toledo XPR10 (ultra microcomparator balance) with a capacity of \(10.1\,\mathrm{g}\) and a readability of 1 \(\upmu\)g (6 decimal places) was used. The weighing scale was read with the aid of a Personal Computer (PC) and a Universal Serial Bus (USB) connection using Mettler Toledo Standard Interface Command Set (MT-SICS) interface commands. For measurements over a long measuring period (basal mode, long-term delivery), the weighing scale was read-out with 1 Hz and, for short measuring periods (bolus mode, quick-acting delivery, dynamic flow rates), with around 10 Hz. The data acquisition and control of the weighing scale was realised using LabVIEW. The desired flow rate was generated by means of a high precision syringe pump (CETONI neMESYS Base 120 controller and neMESYS Low Pressure module 290N) and calibrated syringes of different sizes and volumes. The syringe was connected to a 3-way valve (Swagelok SS-41 GXS2). By changing the position of the 3-way valve, it was possible  
Figure 1: Schematic illustration of a typical measurement setup (Nanoflow test facility) for the calibration of a flow meter as DUT. Figure 2: Photo of a typical measurement setup (Nanoflow test facility) for the calibration of a flow meter as DUT. to either fill the syringe with the liquid to be tested from a glass flask or to carry out a measurement. Normally, the dosing unit of the Cetoni syringe pump has a 2-way valve (aspirate, dispense), for this purpose, which can be switched by software. When the valve is electronically switched to the filling position, the valve becomes very warm and transfers the heat to the liquid with which the syringe is filled. It can then take several hours before the liquid has reached room temperature again. For this reason, the manual solution with the 3-way valve was chosen. In the measurement, directly after the 3-way valve, a pressure relief valve (IDEX U-456 with a pressure setting of 100 psi (7 bar) and 1/16\" OD tubing with 1/4\"-28 UNF connections) was installed with the aid of a tee connector (Swagelok 1/8\" union tee SS-200-3). The pressure relief valve was intentionally not installed directly in the measuring line to keep its possible influence on the flow through its cross-section reduction negligible. An additional pressure relief valve is not actually required, as the Cetoni syringe pump has its own emergency stop system if the pressure is too high. For safety reasons, however, a second safety mechanism was installed. From the tee, the measuring line continues to the actual measurement section. The measuring section was placed on a tripod measuring table and consisted of the Device Under Test (DUT) and, in this case, two pressure sensors. The two external pressure sensors (Cetoni) with flat ceramic membrane and a nominal pressure rating of 500 kPa (5.0 bar) were used to measure the pressure upstream (before) and downstream (after) the DUT. The pressure sensors were directly connected to the Cetoni syringe pump by means of a Cetoni Input/Output (I/O) port splitter. After the downstream pressure sensor, the measuring line ends at an infusion needle that protrudes into the weighing vessel (beaker). When preparing the measurements, special attention was paid to the beaker. To minimise the evaporation effects, the beaker was first filled with around 10 mm of water (Chemical Abstract Service (CAS) number 7732-18-5) and then with around 5 mm of paraffin oil. The paraffin oil (CAS number 8012-95-1) has a lower density (0.827 g mL\({}^{-1}\) to 0.890 g mL\({}^{-1}\) at 20 \({}^{\circ}\)C) than the water and thus prevents evaporation on the surface. Our own investigations have shown that, for a visible effect for this setup, an oil film of at least 4 mm is necessary.