Hygrometer-based calibration method for syringe pumps providing ultra-low liquid flows

Traceable and accurate measurements are critical in many areas where syringe pumps are used to provide microscale liquid flow. Especially, drug delivery demands for low flow rate calibrations have been increasing, but currently available calibration methods are very limited in range and accuracy at flow rates below 100 µl min-1. This paper presents a new hygrometer-based method for the calibration of syringe pumps providing traceability down to the 0.1 µl min-1 level. In this method, liquid water from the syringe pump under calibration is injected into a porous cloth from which it evaporates and mixes with a dry gas flow. By measuring the humidity, temperature, pressure and flow rate of the gas, the injected water flow rate can be calculated. To test the method, a calibration set-up was constructed and a syringe pump was calibrated at flow rates from 0.1 to 10 µl min-1. A thorough uncertainty analysis carried out as a part of the work shows that a relative uncertainty of 0.4 % (at 95 % confidence level) is achieved at the largest calibration flow rate. The uncertainty increases to 3.3 % towards low end of the flow rate range. The achieved uncertainty level is significantly better than achieved with gravimetric methods so far.