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How to make an AirFlow sensor using thermistor ?

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Advanced Member level 3
Feb 5, 2002
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Air Flow measurement

I need to make a simple AirFlow sensor by utilizing and arrangement of a thermistor in self heating mode. An NTC thermistor is placed in the air flow and a sufficient current is passed through it so that it gets heated to about 80degC. Now the current required to keep the thermistor at this temperature is a function of air-flow.


This will work

This is a simple method that will work if you take care of some problems. One problem is the deposit of chemicals and fibers in the air onto your sensor. This will insulate it and the air flow reading will decline. Another problem is the nonlinearity of the scale factor.

I can take care of the non-linearity by using a lookup table in the micro. The deposits is sure a problem that needs to be looked at. One solution could be to use a smooth metal housing that does not catch debris easily. That could reduce the sensitivity but I gues I can live with that.

Do you have any ciruits or application notes in mind


I do not have any circuit examples. I just know that this method is used to measure weather wind speed.

Can your system be cleaned during some form of scheduled maintenance?

Yes I guess it can be cleaned. Since accuracy is not much of a concern, I think it should not be a problem. The sole purpose of the sensor is to detect whether the ventialation system is working or the filters are choked.

i've seen air speed being measured using a cooling fan
from a cpu .
just don't rememeber where


Simply use ultra-sound to measure airflow !!!

The speed of sound wave is directly relted to air speed

there is a project in EPE 02/2003 (i think ) for air speed measure

and i think u can modify it for your's need :wink:



If you use only one sensor you'll get a big error due to the air temperature.
Usually you take two sensor, one used as you do, the other to get the air temp.
Anyway there are problems in precision if you have "fast changing" of the air flow temperature.
Maybe better but much expensive with ultrasound.
Bye All!

PS: What does it means EPE 01/2003?

anyone can donate any point? thanks..

have a look at this

the idea is very simple. take two very thin parallel wires that are in free air. connect them together on one end and put a circling current through this "U" of wires. Both wires will be heatened because of the current. without any airflow, the temperature on both wires will be almost equal.
when the airflow comes from the side of one wire, the heat is transported with the particle flow to the other wire. thus you have a temperature gradient and the temperature at the wires will be different -> resistance change -> voltage of the middle connection will change like have part of a "wheatstone bridge".
If the temperature of the wire is high enough and the caused temperature difference because of the airflow, then the sensor is quite linear.

(it's used e.g. as velocity sensor for acoustics)

may be it's possible to realize this concept with two thermistors. The absolute temperature (when your gas changes temperature) is compensated. ...

The wire method requires a very accurate measurement of the voltage. Probably a 16bit ADconvertor or better. This makes the circuit expensive. The thermistor method on the other hand is cheap since the thermistor is very sensitive to a slight change in temperature.

I have successfully made the sensor but still without temeprature compensation. My application area is in a controlled room temperatire where variation is maximum from 20degC in winter to 30degC in summer. So if I operate the thermistor at 85degC, I should get a resonable reading.

In the worst case, another thermistor can be used to compensate

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