Electronics > Beginners
Mass flow sensor
ZeroResistance:
To build a mass flow sensor for sensing air flow, I have heard the principle of measuring this with a hot wire, but
1. Would we need 1 hot wire or 2 hot wires inline with the flow, and to measure the temperature of the hotwire would we need a thermistor each for the hot wire?
2. Is it possible to build this with a hot thermistor instead of a hot wire?
Rerouter:
https://www.electroschematics.com/11900/measuring-air-flow/
1. Generally its 1 hot wire and 1 thermistor, due to this I don't think you could get away with a second thermistor, generally the wire is stupidly thin to keep the power draw low.
mikerj:
The hot wire in a MAF is the sensor itself, it has a positive temperature coefficient so resistance increases as it gets hotter. These typically work by having two wires, one to measure the temperature of the air and the second, heated one to measure flow. They are wired in a Wheatstone bridge and the circuit maintains a fixed temperature difference between them by increasing or decreasing the hot wire current. This cancels out the temperature of the air, and the heater drive current required to do this is the output of the sensor.
Platinum wires are used as they are fairly linear and provide very fast response. Thermistors could be used but are horribly non-linear, so compensating ambient temperature would be more difficult. A better solution for a DIY sensor would be to use the Vf of a PN junction in a diode or transistor which has a linear, predictable temperature coefficient.
max_torque:
Why wouldn't you use an OTS MAF sensor? They cost buttons, and are very accurate and robust ? For learning reasons sure, play around yourself, but if you actually want to measure mass air flow, just buy an existing device......
Simon:
--- Quote from: max_torque on May 05, 2019, 11:03:05 am ---Why wouldn't you use an OTS MAF sensor? They cost buttons, and are very accurate and robust ? For learning reasons sure, play around yourself, but if you actually want to measure mass air flow, just buy an existing device......
--- End quote ---
Ditto, it does not sound like something that is worth putting the efort into designing if and off the shelt unit will do the job. Many of the available sensors can be removed from the tube or you can buy just the sensing head.
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