The Series 641B Air Velocity Transmitter uses a heated mass flow sensor technology. It has four user-selectable ranges from 250 FPM to 2000 FPM with four corresponding metric ranges of 1.25 MPS to 10 MPS. The 641 B provides an isolated 4-20 mA output proportional to the velocity.

The Series 641B's steel sensor allows the unit to be used in dirty air environments. This rugged sensor is ideally suited for quick field cleaning from a simple cloth to a pulse of air from an air source. Proper sensor performance can be maintained in these polluted environments by easily removing dust and debris from the sensor which is problematic for Pitot tubes and other flow sensing transmitters.