Tjaart van der Walt wrote:
> How does one measure a river's water level (2.5m change) without
> little contacts that will corrode?

Use little contacts that will not corrode.  Keep them at identical
potential for most of the time and only raise them to make measurements,
or place a sacrificial anode nearby and keep the contacts at least a
little bit on the "right" side of the anode.

> My idea is to put a vertical pipe with a hole in the
> bottom into the water. A speaker (with a plastic
> cone) is placed at the top of the pipe and the [...]

Hmm.  Never thought of peak frequency measurement.  I'd simply bounce a
pulse down the tube and recognise the peak return.  Time for the bounce
is distance.

> BTW what sort of a sensor would be able to measure
> barometric pressure?

Pass.  Last I checked it was expensive.

--
James Cameron                              (james.cameron@digital.com)
Digital Equipment Corporation (Australia) Pty. Ltd. A.C.N. 000 446 800