low cost air flow

Thermal dispersion may be a good solution for your application.

http://www.magnetrol.com/us/html/products.asp?family=10

Would need to know more including the velocity, desired accuracy, and your definition of low cost to be shure.

thank you, i was not familiar with this company.

You are very welcome. My intention was to provide reference material for the technology.

Several companies manufacture similar instruments.

This technology inherantly provides mass flow measurement so that correction for the density effects of pressure and temperature are not necessary for a known gas.

thank you again, i am familiar with the technology, as we are working with such a device at the moment. the problem was that all the units i found were limited to 120 degC, proccess temperature. that is the first one i see which is over this limit.

You could drill a small hole in the pipe wall, connect this via a few foot of pipe (to allow for cooling) to a U tube manometer...you will have to work out the calibration yourself against some known points.

It certainly won't interfere with the flow, but will doubtless require some experimentation.

( I would expect the venturi effect to predominate resuling in a negative pressure)

Del

Thank you for the suggestion, two points are stopping me: 1. i can not afford doing the study at this point (time wise). 2. the static pressure in my system is very low (few mBars) which will require a very sensitive (and expensive) sensor.

Suggest you re-read my post...

A U tube manometer costs nothing.
The static pressure is irrelevant.

Del

And another thing
1" of water = ~ 2.4 mbar

Therefore a simple home made U tube manometer will easilly measure the pressure.

Del

Don't forget that a single point will only give you the pressure. Some sort of orifice, pitot tube, or even a right angle bend, between two points will give you a pressure differenial that can be easily related back to flow. This type of arrangement to both sides of Del's manometer is easy and cheap to construct and install.

Put a free-running Aluminium propeller(fan) with SS axle running on Teflon /Sintered bronze bushes with a LED focussed lightbeam interruption by rotating propeller blade/s.

Rest is Electronics -- pulse Counter/Ratemeter/Calibration outside the flow Pipe.

If you want a low cost solution (And I mean < 100 USD), you can probably forget about any industrial mass flow meter.

The U tube proposal is not a bad idea. You could replace this classic concept by a electronic differential pressure sensor, which is used for HVAC applications. You can buy very sensitive sensors for less than 100 USD. Check electronic component distributors, for instance www.digikey.com, www.farnell.com or www.sensortechnics.com for these kind of sensors. Keep in mind that the rangeability will not be very high, but it will work fine for monitoring around a setpoint. Search for "ultra low differential pressure sensors".

If you have the budget for an industrial quality solution (>700 USD), you could consider the Magnetrol device. Kurz Instruments (they offer thermal mass flow probes for high temperature) or Sierra offer alternatives.