The most reliable air-flow sensor that I am aware of is a simple=20 thermistor. You pass enough current through the thermistor that it=20 self-heats to some temperature well above ambient. The resistance=20 changes in a reliable and repeatable fashion as the amount of air=20 passing over the thermistor changes. More airflow results in a lower=20 temperature. Easy to implement and very inexpensive. dwayne At 10:52 PM 8/27/2016, James Cameron wrote: >Thanks for the replies on liquid level sensing. > >Another aspect of the problem is detecting fan failure, which can be >done by measuring current or a rotation sensor, but doesn't actually >tell if the fan is moving air. Blocked intake makes fan spin faster; >it is not moving as much mass, but the same could happen if the >chimney is working well from the heat of the day. > >I'm interested in comments on anemometers, air flow sensors, mass flow >sensors, or pressure sensors relative to ambient. > >The space to work within is a few yards of 3" diameter plastic pipe. > >I'm fine with drilling it, because if I change my mind I can plug it >with kapton tape or silicone caulk. > >The environment can be excessively dirty or wet. Occasionally an >insect gets into the system, lays eggs, and a few days later a swarm >will try to leave. Occasionally the dew point is just right for >condensation in the pipe. > >The fan (at the moment) is a 90mm 12V DC rated at 59 CFM, 3.8 watts, >0.32 amps. The corners get cut off and foam padding added to fit it >into 100mm nominal (4") plumbing pipe. Wish for circular fans. ;-) --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA 780-489-3199 voice 780-487-6397 fax 888-489-3199 Toll Free www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .