## 2 Materials and Methods
### Measurement Facility for Flow Rates from 1 mL h\({}^{-1}\) to 100 mL h\({}^{-1}\)
#### 2.2.5 Measurement Uncertainty  
Both measurement facilities are based on dynamic weighing as reference, which is a special type of the gravimetric method. However, most flow generators or devices under test generate and measure volume flow rate. The mass flow rate of a liquid is measured by taking the mass measured by the weighing scale as a function of time. The volume flow rate is then determined as the mass flow rate divided by the temperature-dependent density of the test liquid. Since the density of the test liquid must be known, most tests are carried out with water according ISO 3696 standard [18] or ultra-pure and degassed water, as water is one of the best determined liquids [19]. In addition, efforts are made to keep the ambient temperature as constant as possible and thus the density as constant as possible. This not only has a positive influence on density, but also on the entire process. Ideally, the measurements should be carried out in a climate-controlled room where the temperature is kept at (20 \(\pm\) 0.5 \({}^{\circ}\)C) and the relative humidity between 45% and 60%. It should at least be ensured that the room is temperature-controlled. The absolute temperature plays a subordinate role, it is more important that the temperature can be kept constant at \(\pm\)1.0 \({}^{\circ}\)C over time. In this case, especially because of the weighing scale, the relative humidity should not be less than 30% and not more than 80%.  
Weighing scales have the great advantage that they are relatively stable over time and therefore need to be calibrated less frequently. Traceability is routinely performed by the National Laboratory for Mass at RISE. As the calibration intervals for analytical balances are relatively long with one year, the linearity is regularly checked with the internal control weights of the balance. There are many references [14, 15, 20, 21, 22] that address the measurement uncertainty budget of the dynamic gravimetric method in microflow. All these approaches are very similar. In addition to the influencing variables mentioned, other influencing variables associated with the dynamic weighing method (e.g., evaporation, buoyancy), the measurement setup (e.g., dimension of the weighing vessel (beaker) and the tubing) and ambient conditions (air pressure, air density, humidity) are also taken into account. As part of the EMPIR JRP 18 HLT08 MeDDII, a technical report [23] was prepared which shows in general terms, using an exemplary flow point in the microflow range, how the measurement uncertainty of the dynamic gravimetric method can be calculated.  
If these guidelines are applied, the following measurement uncertainties as a function of flow rate are obtained for the Nanoflow test facility at RISE (Table 2). The expanded measurement uncertainty of the Microflow test facility is stated as U\((k=2)\leq 0.5\)% over  
Figure 6: Long-term test of evaporation over 10 days while the beaker was filled with 10 mm water and 5 mm oil film and the needle was immersed.  
the entire flow range. In a recently completed inter-comparison [24] within the EMPIR JRP 18 HLT08 MeDDII in the flow rate range from \(5\,\mathrm{nL}\,\mathrm{min}^{-1}\) to \(1500\,\mathrm{nL}\,\mathrm{min}^{-1}\), the given values were confirmed, at least for the lower flow rate range of the Nanoflow test facility.