Advantages of a Coriolis sensor
Most MEMS flow sensors are based on a thermal measurement principle. It has been demonstrated that such sensors are capable of measuring liquid flow down to a few nanoliter per minute. Advantages of these sensors are that they are fast and very stable. A disadvantage is that they need to be calibrated for each specific fluid.
A Coriolis type of flow sensor, i.e. flow sensors containing a vibrating tube in which a mass flow is subjected to Coriolis forces, do not have this problem. The Coriolis forces are directly proportional to the mass flow and independent of temperature, pressure, flow profile and fluid properties since Coriolis flow sensors measure true mass flow.
Coriolis flow meters are mostly used for measuring large flow rates (>1 kilogram per hour), since the relatively weak Coriolis forces are correspondingly harder to detect for small flows. In order to gain enough sensitivity to measure ultra low flows below 2 gram per hour, the sensor size and tube wall thickness needs to be minimized to the extreme, which is not possible by conventional machining of stainless steel.
Here MEMS technology comes into play. A process called “surface channel technology”, which we developed in close collaboration with the University of Twente, allows for the fabrication of tubes with 1 micrometer thin Silicon Nitride walls. The choice of material renders these tubes mechanically stable even at this extremely thin wall thickness.