The measurement is based on ultrasonic scanning of the Karman Vortex Street. Vortices occur intermittently around the element in the sensor head and (different to most other Vortex technologies) are scanned ultasonically. This allows low flow measurement starting from 0.4m/s of air/gases. The sensor has no moving parts, a recalibration is not necessary: "simply install and forget about it!"  

Vortex for Biogas

Vortex for corrosive and caustic gases

Vortex in sewage gas waste water treatment