I want to make an ultrasonic air flow meter. Which Arduino board is suitable for it?

The honest answer is almost any Arduino board will be OK the more important thing here will be the choice of transducers.

Do these Ultrasonic systems offer much greater accuracy than the heated element types like cars use for the MAF sensors?

Kelvin, suppose your tube is 15cm diameter and the spacing between the transducers along the tube is 20cm (sounds OK for  a cooker hood chimney). The path length will be 36cm so the sound will take v. approx 0.36/300 = 1.2mS.

A typical (first one I found with a spec) cooker hood moves 486cubic meters of air per hour = 0.135 per second. The area of the 15cm tube is 0.017 m^2 so the air velocity is 7.6m/s.

Only 20/36 of you path is speeded up by the air moving so the difference in times will be 1.2mS*(15.2/300)*20/36 = 33.7 uS.

If you use a 40kHz transducer each cycle takes 25uS so your full scale sensitivity is 1 sensor cycle time.

You will need to estimate the transit time to within less than 1uS - which will be very very difficult and impossible without additional (to an Arduino) hardware.

Here is a link to a commercial instrument, they use transducers at frequencies from 500kHz to 8MHz.

FLUXUS F601 - The Portable Multi-Functional Flowmeter | FLEXIM

MK

Here's a link to a Maxim chip with lots of info:

http://datasheets.maximintegrated.com/en/ds/MAX35101.pdf

In the first paragraph:

With a time measurement accuracy of 20ps and automatic

differential time-of-flight (TOF) measurement, this

device makes for simplified computation of liquid flow.

Early edge detection ensures measurements are made.........

MK

Good one Michael, It shows that it's the right sensing you should be focusing on rather than the processing.

Kelvin, is there a budget for this I'm sure some of these fine devices are outside a small project budget  ?

and the chip is only 15$ one off, it sounds expensive till you realize it supports the temperature measurement and compensation too which is essential for accurate measurement

That's not bad for what it's doing Peter, Did you find a price for the sensors as well?

I did not, I was assuming that you could use some of the basic transducers from E14 or EBay

I think this is the key think to be looking at for his design the CPU is pretty secondary to the Sensor and electronics sid.

Would be nice if the cheap ones work OK though!

short of getting samples it would not be easy to tell

I can get sample chip from TI without an issue as my business is registered with them, I would just have to get sensors and time