New method for measurement of ultra-low liquid flows

Control of liquid flows with flow rates below 1 µl/min (nanoflows) is conducted in e.g. medtech. The accuracy of the flow rate may hence influence the outcome of medical therapies. 

When this project was conceived, gravimetrical methods were the prevailing means to determine the flow rate of liquid nanoflows. Gravimetric methods, i short, relies on weighing of the accumulated flow over a certain time. This type of measurement method is time consuming and also has a lower limit for the measureable flow rate.

This project investigated the possibility to continously evaporate a liquid nanoflow and subsequently measure the resulting gas flow. The evaporation was done in vacuum, i.e. at pressures well below the saturation pressure of the liquid medium at ambient temperature. The gas flow measurement was based on the principle of establishment of a flow-dependent differential pressure over a small circular orifice. By keeping the line pressure below 1 Pa molecular flow could be assumed and together with knowledge of the orifice area simple mathematical flow models were possible to apply to calculate the flow rate from a measured differential pressure.

The project was a cooperation between the National Laboratory for Volume and Flow and the National Laboratory for Pressure and Vacuum.  It proved the principle and the results were published in the peer reviewed scientific journal Measurement.