Flow meter for high-purity and aggressive liquids

We describe a new kind of flow sensor for liquids, which is based on a balance between hydrodynamic and magnetic forces. A floating body, containing a permanent magnet, is kept at constant position in a tube by an external solenoid. The current in the solenoid required to compensate the hydrodynamic force by a magnetic force is used as a measure for the flow rate in the tube. An integrated viscosity and density measurement allows an autocalibration, if liquids of unknown density or viscosity have to be measured. The only material in touch with the liquid is teflon, which allows measurements of highly aggressive media or such with high requirements to hygiene or purity. The prototype covers a range of 20–250 ml/min and has a repeatability of ± 0.5 % full scale and an accuracy of ± 1 % full scale. The simple and modular design should lead to production costs which are significantly lower than those for other devices while offering the same performance.