Circulation time measurements with miniature radio frequency identification devices

A novel flow follower method has been developed to measure circulation times in stirred vessels employing miniature radio frequency identification devices (RFIDs). The flow followers were miniature RFIDs encapsulated in small plastic spheres (radio pills) ballasted so as to achieve neutral buoyancy. The RFID is inductively powered by an interrogation signal from an antenna so that it does not require a power supply of its own and now followers much smaller than conventional radio pills can be produced. Characteristic results obtained in a small stirred vessel are presented to demonstrate the suitability of the technique for circulation time measurements in fluid mixing studies.