On Fri, 26 Dec 2003 12:18:34 -0600, Don Foreman
wrote:

Hot wire aenomometers: check out
http://www.efunda.com/designstandard...res_theory.cfm

Some of this stuff is starting to come back to me.

A direct-heated aenomometer actually depends on the element having
some tempco of resistance w.r.t. temperature. Bead thermistors work
well but tungsten works OK also, with tempco of about 0.45%/degC.

As air movement removes heat, temp drops, resistance drops and
voltage drops (with current drive). This produces an error signal
which can be amplified to raise the current until it nearly makes up
the power lost to the wind with only a small temp change necessary to
produce the error signal necessary to re-establish equilibrium.

Note that such a device is a mass flow meter so it would measure SCFM
rather than CFM Given velocity (hence CFM) of air at higer
pressure would produce higher reading on the anomometer.