A lower cost alternate to the turbine flowmeter is the paddle wheel flowmeter. This device is somewhat similar to the insertion type turbine flowmeter, but instead of a turbine blade with the flow generating lift forces to cause the rotation, the paddle wheel is perpendicular to the flow and rotates much like an old fashioned steam boat paddlewheel.
These devices are usually inserted into a specially made tee in the flow line. The paddlewheel show in the photo, distributed by Omega, is shown inserted into its flow tee. The PVC fitting is then cemented onto two ¾” PVC pipes. In this size pipe, the meter is capable of reading flows from 1.7 to 34 gallons per minute to an accuracy o 1%.
The sensor can also be used in pipe sizes from ½” to 36” in diameter for a flow range of 1 to nearly 7000 gallons per minute.
Like the turbine flowmeter, these also generate a pulse train generated by the paddles passing an internal pickup. The output is a frequency proportional to t flow measurement devices, this is also dependent on the stability of flow and significant turbulence will cause significant error. In general, the 20 / 10 rule should be followed to ensure that the flow is fully developed at the transducer location.
Credits - Mitchell Cottrell