How about using a mass-air sensor from an EFI vehicle?  Most of these
are the hotwire-type system and actually has a small sub-path thru which
the air is measured.  You would use the whole mass-air sensor as a
sub-path for the wind passing thru the whole cave entrance.  Output
signal is a varying voltage.

If you go with a stock sensor, you can pick these up for next to nothing.

Also, if you decide you don't want to pick out values from a lookup
table, or linear-ize (yes I made that word up) the input signal data with a
formula, you can have a company like Pro-Flow calibrate the meter so
that its output is linear.

Cheers,
-Neil.


----- Original Message -----
From: "marco genovesi" <marco.genovesi@LIBERO.IT>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Friday, April 05, 2002 3:19 AM
Subject: [EE]: Thermal anemometer sensor


> I have to monitor wind flow from a cave entrance. Air flow (in-out) is
> usually fast in Winter & Summer (10-30 ft/sec) but is often very "light"
in
> Spring & Autumn (less than 3 ft/sec) and probably a "cup" anemometer isn't
a
> good choice for this low values, so I'm thinking to a kind of "thermal
> anemometer" circuit (not necessary an "hot-wire" type), simple to build
and
> not too expensive in power requirements (max. 1W) and in accuracy (+/- 10%
> accuracy)
>
> I want use my 16F84 logger to do this job, so the simplest way that I know
> is to measure a frequency with TMR0 (I already have a TMR0 routine to read
a
> conductivity sensor).
> The "ideal circuit" that I dream is a kind of "overheating" sensor (a
> thermistor?) that outputs a frequency, or that I can easily convert in a
> freq. I have searched on the web a sample circuit, but at the moment I
> haven't found anithing (enough simple..).
>
> Have anyone any suggestion for build this circuit?
>
> Thanks in advance
> Marco
>
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