## 2 Materials and Methods
### Measurement Facility for Flow Rates from 1 mL h\({}^{-1}\) to 100 mL h\({}^{-1}\)
#### 2.2.4 Design Considerations  
Flow Generation (Syringes)  
A syringe consists of a hollow, cylindrical barrel in which a movable piston, the syringe plunger, can slide back (aspiration) and forth (injection). The flow rate of the syringe is determined by the internal diameter (ID) of the syringe respectively by the piston area or plunger tip. The pressure also depends on the piston area of the syringe beside the outlet opening. A smaller piston area and/or a smaller outlet opening cause a higher pressure than a larger one for the same amount of force. The ID of a syringe is not constant and therefore flow and pressure variations occur. To minimise these flow and pressure fluctuations, a set of syringes was modified at RISE CentralWorkshop (Figure 4) so that the plunger can move freely without contact with the inner wall of the syringe. In this configuration, the plunger tip does not seal against the inner wall, but the sealing is made at the syringe inlet with the aid of an O-ring. In this case, the outer diameter (OD) of the plunger determines the flow rate. The OD of the plunger was measured at RISE National Laboratory for Length and Dimensional Metrology with micrometre resolution. The pressure fluctuations due to friction at the sealing surface improved only minimally, for an exact volume determination the spatially resolved determination of the OD of the plunger is easier than the spatially resolved measurement of the ID of the syringe. Water Collection (Evaporation)  
Measurement uncertainty studies on syringe pumps and insulin pumps have shown that evaporation is one of the biggest factors influencing measurement uncertainty [14,15]. The influence of evaporation increases at the lower flow rates. For this reason, special attention was paid to this influencing factor when designing the test facilities. To keep the influence of evaporation as small as possible, various methods have been investigated before the actual measurements. The AAMI technical information report (TIR) TIR101:2021 [16] gives recommendations to minimise evaporation. A sufficiently thick layer should be used over the test solution, which has a lower density than the test liquid, is immiscible and is non-evaporative, such as liquid paraffin or mineral oil. Alternatively, an evaporation trap can be used. An initial step was to fabricate an aluminium evaporation trap at RISE Central Workshop (Figure 5). Figure 4: **Left**: Typical construction of a syringe which works according to the piston principle. **Right**: Modified syringe that works according to the displacer principle. In the first case, the ID of the barrel determines the flow rate; in the second case, the OD of the plunger. Figure 5: (**Left**) Glass filter that can be placed in the beaker; (**Centre**) self-made evaporation trap; (**Right**) evaporation trap and beaker on the scale. The first method examined was the METAS method described by [PERSON] et al. [17]. Here, a purpose-made glass filter was inserted into the beaker and the outlet needle was positioned about \(20\,\mathrm{\SIUnitSymbolMicro m}\) to \(50\,\mathrm{\SIUnitSymbolMicro m}\) above the glass filter. The capillary force sucks the water into the filter before a droplet can form on the surface. The water flows through the glass filter and is collected either in water-absorbing foam or directly in the beaker. Depending on the flow rate, glass filters with different porosities are available. The experiments carried out have shown that the evaporation rate with this method was in the range of the lowest flow rate despite the use of the evaporation trap. Although it is possible to correct the measurement results for this value, the evaporation rate itself was not satisfactory. The second method studied was the approach when the needle is immersed in water. Here, the forces on the scale were reduced during filling, but in contrast to the previous method, there is the disadvantage that the buoyancy of the needle must be taken into account as the water level in the beaker rises.