Dwyer TFP-GV - Manual (Page 4)

F-20-TFP-G, pg. 4 of 27
2. Product Overview and Principle of Operation
TFP Series Flow Meters and Flow Sensors for gases are capable of
measuring volumetric flows as low as 20-100 ml/min or as high as 100-
500 l/min. Highly repeatable results are achieved using a patented Pelton
Turbine Wheel flow sensor design. This proven design minimizes zero drift
while maintaining fast response and linear outputs with virtually no
maintenance.
Patented technology measures flow using a miniature turbine wheel similar
in size to a U.S. dime (16 mm diameter, 0.75 mm thick). The micro-turbine
wheel is supported on a very small sapphire shaft that is held in position by
two sapphire bearings.
As flow passes through the flow sensor, a precision machined nozzle directs
the gas onto the very small teeth of the micro-turbine wheel. This causes the
wheel to spin at a speed proportional to the volumetric flow rate.
The micro-turbine wheel has alternating white and black sections evenly
spaced on one side of the wheel. An infrared light beam is directed onto the
wheel. As the wheel rotates the infrared beam is reflected off each white
section. The reflected beam is detected by a phototransistor that converts the
reflections into electrical pulses. As the wheel spins faster the pulse rate
increases. Processing circuitry provides analog and/or pulse output that are
linearly proportional to the volumetric flow rate
When the wheel stops (under zero flow conditions), no pulses are
generated. Consequently, zero drift is not possible and zero adjustments are
never required.
Every unit is supplied with a calibration certificate detailing the results
obtained during calibration. Units are calibrated using AIR as the reference
media. Flowing gases with different physical characteristics may affect the
calibration.