Maker Anthony Douglas has designed a thermistor-based anemometer sensor, "using some of the same physics" as behind hot-wire versions — but at a lower cost and with increased sensitivity, he claims.
"You can actually read up a lot of stuff about these hot wire-like thermistor based anemometers, but where's the stuff you can actually use? The cheapest unit I could find was $25 CAD plus shipping from Europe," Douglas explains of his reason to go homebrew for his latest sensor. "This device works great and is $0.50 except it still needs calibration."
Anemometers are designed to measure wind speed and, in some cases, direction, and are typically created by putting vanes on a central spindle and measuring how quickly the spindle rotates in the breeze. For more compact readings, "hot-wire anemometers" work by heating a wire and measuring its heat loss — the faster the wind speed, the faster teh rate of heat loss. Rather than a simple wire, you can also use a thermistor as a heater — measuring its temperature using a second thermistor.
"It gets slightly complicated because the resistance of the thermistor changes with the temperature, and thus so does the current that flows through it, the voltage drop across it and thus the amount of heating power," Douglas explains of the issues in building your own. "There are a number of other complications, like radiant heat from the heated thermistor raising the temperature of the unheated one. One more complicated issue is that the heated thermistor drops a lot in resistance when you heat it up, and so if you aren't careful you can overheat it."
After some trial-and-error experimentation, Douglas was able to build something which proved stable in various weather conditions — except if it's cold enough for the thermistor to "go out," he admits, which would require the ambient temperature to rise above a set minimum for the heated thermistor to restart.
There's also the issue of calibration. "I was planning to just use my little cheap vane anemometer right behind it," Douglas explains, "write a program and some manual recording of anemometer readings to collect data at a bunch of points and just use a least squares fitting calculator to give me a polynomial I could use to fit the data, to go backwards from thermistor delta T, and the heating power to the air velocity. A function of A and B to Y."
"Then," Douglas continues, "I realized I don't really need to do that either and don't really have time, also the most important region for me can't be measured by my crummy vane anemometer anyway, and it's accuracy is kind of suspect. All I actually need anyway is a device which is repeatable and gives output which is the same for velocity at both directions i.e. symmetrical. I don't even need to know the actual velocity of the air quantitatively. I just need to make sure it stays the same and is the same in both directions, and is the same between devices, or close enough."
While Douglas hasn't yet solved the calibration problem, he's published his project to Hackaday.io in the hope "someone else can take it further some day, or maybe I will have a need to do so and will do that." For now, the sensor will be used as a "precise but wildly inaccurate flow sensor."