Well, I have received a lot of useful suggestions for the anemometer...

First, thanks to John Dammeyer for his schematic: the "airflow" section
seems quite simple. Also the Linear Technology Hot-wire circuit suggested by
Philip Starbuck seems to have a good sensitivity, especially on low flow
rate (the problems that I see are  voltage to pulses conv. and the 15V
supply).
A mass-air sensor from an EFI vehicle can be another idea but where I live
isn't too easy find cheap samples of these sensors. Olin, I don't know if
impellers work fine at these low velocities. Yes, at a first time I thinked
to a self-made and very "light" cup  anemometer (the sensor was a simple
led+phototransistor as frequency counter: very cheap and easy to interface
wit 16F84). But i think that isn't so easy to make an unit (really, i will
need 4-5 of these ) sensible to 0,5 ft/sec and so strong to endure at 10
mt/sec or more.
In fact, the cave that I'm studying have a very variable airspeed at the
entrance: also in the mid-seasons (Autumn, Spring) when the air in/out from
cave entrance is usually low, a deep low in barometric pressure can generate
speeds over 30 mph for a lot of hours. Probably I have to use, together, the
two kinds of flowmeters..

At the moment, however, I'm starting to build the circuit suggested by Bob
Blick ("heated" transistor): it seems cheap and  simple, 5V supply (as my
logger) and with frequency output.  I hope it will have enough sensitivity
for my use.

Thanks to all!
Marco

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