On Sun, Aug 28, 2016 at 4:36 AM, James Cameron wrote: > Hmm, yes, or pair metal tubes intruding into the flow, upstream has a > DS18B20, downstream a DS18B20 and a heating resistor in some thermal > potting compound. > > A good idea, in theory. Playing the devil's advocate here, how do you calibrate around the random insect infestation, or when they build a nest on your sensors? Depends on the part of the world and seasons, but in some cases, even a little bit of heat is going to attract the little buggers. You may have better luck measuring the _rate_ of heating and cooling rather than static temperature. It would need to be a fairly slow process (10s of minutes?) With no airflow and a specific heat input, you'll get a predictable rise and fall slope. With airflow, the rise will be slower and the fall will be quicker. If it's possible to turn off the fan every once in a while, (daily, weekly?) a cycle with known zero airflow could be used to re-calibrate the rise:fall proportion. With a little effort, it seems possible to build such a sensor in the wall of the pipe, rather than having something protrude into the airflow - less chance of crud collecting on it. The other idea that occurs to me (and I don't like it) is to use a pressure sensor. It seems "Easy enough" to arrange a diaphragm across a chamber ported into the main pipe. With a suitably light/soft membrane and enough area, you should get measurable deflection with the fan running. (I'm thinking latex balloon, maybe 4" diameter?) Then do non-contact sensing with a photo pair on the "clean" side of the membrane. Finding a long-lived membrane that can tolerate the environment you're working in may be a non-trivial task, then you still have to deal with the wildlife. -Denny --=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 .