# Air flowmeter

Discussion in 'General Electronics Chat' started by aka-rato, Jun 5, 2012.

1. ### aka-rato Thread Starter Member

May 11, 2012
58
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helo my family.

im given a project to make an air flowmeter. The whole idea behind this is to measure the air that flows through the tubes, i have to use air flow sensors and a 2digit display seven seg to display the amount of that flow. I do have the components but i cant figure out the circuit diagram. Please i need ideas, and a help with a circuit diagram will do.

thanx.

2. ### Rbeckett Member

Sep 3, 2010
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Pass a voltage across a thermistor or thermo couple. The difference in temps can be obtained by how many volts it takes to maintain the temp of the sensor in the airstream. The 7 segment display can the use a look up table to determine the flow volume in CFM or LPM. Most modern AFM's use this approach in the intake air stream coupled with a multiple O2 sensor lay out to get efficiency and trim the fuel injectors for better fuel economy. This is a gross over simplification, but that is basically how it works in most modern automobiles and fuel burning engines today.
Wheelchair Bob

3. ### aka-rato Thread Starter Member

May 11, 2012
58
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can i please get a circuit for that.

4. ### BillB3857 Senior Member

Feb 28, 2009
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A bridge circuit with two fixed resistors and two thermistors is common. One thermistor is exposed to the air stream while the other thermistor, although in close proximity to measure ambient temperature, is shielded from direct air flow. The degree of unbalance is an indication of the volume of air flow. Look up "Mass air flow sensor" and you should get a lot of information.

5. ### KMoffett AAC Fanatic!

Dec 19, 2007
2,574
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Four-thermistor bridge (diagonals in the stream) is balanced and has a higher output.

Ken

6. ### KMoffett AAC Fanatic!

Dec 19, 2007
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Google: thermistor bridge air flow circuit

Ken

Apr 24, 2011
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8. ### cork_ie Member

Oct 8, 2011
348
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Hi OP . Is it mass or volume airflow you are looking for?
Approximately what range of airflow are you talking about?

If you are just measuring a relatively small airflow then just use the thermistors in Wheatstone Bridge format as described by the previous posters. Then to amplify your signal please refer to the following application note from Texas Instruments. http://www.ti.com/lit/an/sloa034/sloa034.pdf It is a mine of information.
If you are measuring a large airflow eg 10Litres/sec or more you will probably find it is not very linear or reliable.
I am working with airflow meters in vehicles almost every day of the week All modern types measure the mass of the airflow rather than volume.They do the calculation based on the amount of current required to heat a wire or substrate to keep the Wheatstone bridge in equilibrium, then condition the signal so that the output voltage gives a reasonably linear signal proportional to the actual mass airflow for that particular vehicle.
I remember the older volume type (made by Bosch, and others and a 600 part ) which pushed open a spring loaded vane and operated a potentiometer) Circuit wise it is an easier system to play with. It is a fairly simple calculation to convert from volume to mass and vice-versa from the air density ( which is affected by vapour pressure, barometric pressure, temperature etc.)

9. ### strantor AAC Fanatic!

Oct 3, 2010
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I believe the gas pipelines use an inline orifice of calibrated diameter, and compare the ΔP from one side to the other. Analogous to inserting a shunt resistor inline of power cable and measuring the voltage developed across it to determine current flow through it.

cork_ie likes this.
10. ### cork_ie Member

Oct 8, 2011
348
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Very good thinking.

It's a venturi and the pressure drop would be very simple to measure using something like this differential pressure sensor costing less than \$20-00
http://www.farnell.com/datasheets/29745.pdf

Possibily a sensor with a narrower range would be more suitable.

11. ### aka-rato Thread Starter Member

May 11, 2012
58
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thank u guys for your comments, i hear all the comments the thing is i do have the sensor, microchip and ofcouse the small components resistors etc. but how am i going to bring this together to make a working display of an airflow measured in L/min not PRESSURE. if u can help me with the circuit diagram and the program.

thank u

12. ### cork_ie Member

Oct 8, 2011
348
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A good place to start would be to check the datasheet for the sensor that you have. What is the output v's airflow? Then you will need to drive the display from your output signal.

We understand that it is airflow you require , just that there is more than one way to get it and an easy neat way is to measure pressure drop across a venturi.http://en.wikipedia.org/wiki/Venturi_tube#Venturi_tubes

Dec 19, 2007
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14. ### aka-rato Thread Starter Member

May 11, 2012
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thank u family for ur inputs it realy helped.

15. ### aka-rato Thread Starter Member

May 11, 2012
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can i jst make the air flow to be displayed using pure electronics without writing any program or rather without using pic18f etc, if it is possible which ic to use??? please help

16. ### KMoffett AAC Fanatic!

Dec 19, 2007
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How do you want it displayed?

Ken

17. ### BillB3857 Senior Member

Feb 28, 2009
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You could use a volt meter connected to the output of your amplifier and create a scale for it to represent the air flow.

Apr 5, 2008
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19. ### aka-rato Thread Starter Member

May 11, 2012
58
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the input as 0-5v to be displayed 0-20L/min