Hi,
There is also another type of non mechanical flow meter, and
that is the measurement of the ability of the flow to cool a
device that is intentionally heated.
The principle of operation is basically simple:
A device that is internally heated by an electrical current will
reach a certain temperature in still air, but in a flow of air
the device will cool off because the air flow sweeps away
some of the heat at the surface. The most common type
of device that uses this principle is probably the ordinary
heat sink, used to cool semiconductors.
Of course there is a little more to it than that, because the
rate of cooling also depends on the air temperature, so two
devices are used: one inserted into the air flow, and the
other not inside the air flow, and the two measurements are
compared to get the final reading. The devices used are
typically some kind of semiconductor whos characteristics
are well known.
Unfortunately i dont have any links right now, but there
may be an app note on the National Semiconductor site
somewhere, or a search on Google might turn something up.