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Air flow sensor can be designed using 2 thermistors. One measuring the ambient temperature and the other is maintained at an elevated temperature. The power required to keep the temperature constant is related to the ambient temperature and the air flow over the thermistor.
Do u have any reference shematic of the same?
Yep, Why we need two thermisters? One is enoug right? Why can't we fix one as variable or fixed value resistor? Other one is thermistor.
Is that makes sence??
You need 2 thermistors because 1 is used to measure the ambient temperature since the power required to maintain the other thermistor at a constant temperature will be highly dependent on ambient temperature.
In order to maintain the 2nd thermistor at constant Temperature, you just have to measure its resistance and heat it up so that its resistance maintains a certain value.
Flatulent is right. The method I described is only usefull if the air-flow is quite low. This method is also not very accurate since humidity and non-linearity are both a factor to consider. But at these rates of flow, measurement using mechanical methods become very expensive or inaccurate. If you are measuring high wind speeds, better go for mechanical.
The method using two thermistors is referred to as Thermal Mass Flow and companies like Endress + Hauser make an exremely accurate instrument unsing this technology, called T-Mass...
It's used in application where the mass flow of a specific gas needs to be measured... As long as the device is calibrated the device can be programmed to any one gas but, it isn't possible to measure more than one different gas with one meter because, the thermal expansion coefficeints of the various gasses arn't always the same, actually they[re never the same...
Once the device has been calibrated and linearized for the specific gas, only moisture can affect its accuracy...
EdnMag , reed electronics articles are full at google do a little search . Basicall you require to heat some thermistor and when air flows over it you measure the resistance ! . But be carefull about ambient temperature you will also require one reference thermistor which in some designs you wont find .
Not all systems work with resistors, if i am correct, the automotive
air flow sensors do not...
they work with a constant heating element and measure the amount of cooling...
A guy once told me, that some of them use a very thin wire, like a light bulb, so when they place a voltage onto it, it starts to glow, and with less or more cooling the resistance of the wire changes, and so they can measure the current flowing through it...
Don't know it for sure, but seems to make sense...
Hope this helps you a little..
It is imperative to understand exactly the environment where the sensor will be used.
There are different characteristics in each environment, and different sensors more appropriate for each one.
In most cases, measuring the diference in temperature (twin thermal elements), one exposed the other don't, is enough to measure how much heat was removed from the exposed, but it is not very linear and depends on ambient effect.
An effective way is by moving a rotor, but requires a certain volume of air to be someway more precise. Sometimes using two different techniques is better.
One should always remember that for the right job there is a right tool, same way here.