You can bounce an ultrasonic wave at it without a pipe, or use an air
bubbler and measure the air pressure change, use a float attached to a
string and a variable resistance, use 2 stainless steel electrodes (Highly
inaccurate as condictivity of water varies greatly in a river), there are
thousands of ways to measure fluid level, check out an industrial catalog
of sensors.

On Wed, 24 Jun 1998, James Cameron wrote:

> 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
>