At 03:58 PM 24/06/98 +1000, you 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
>
>





Firstly I am assuming that the range you are after is 2.5m, not a change
from x to x + 2.5m

Ah, it all sounds so simple! There have been many attempts to derive an
ultrasonic depth gauge, most do work, however the current drain is high. You
may wish to look at the ones made by Kodak. As for your idea of a plastic
pipe and speaker, yeah, it will work but with vary limited success due to
variations in temperature, atmospheric temperature drumming in the pipe due
to wind and other general activities on the water eg boating, yet alone the
problems that you will have with keeping the pipe inlet unclogged and wave
action.

If the money is coming out of your own pocket, then you may wish to look at
other methods. The first question is do you have mains power, if so then a
simple float on a cheep optical encoder will do the job (You can purchase
quadrature shaft encoders, but they need a well or bubbler system to
operate). If you want to have more fun, then look at a silicon strain gauge
that Motorola make, with this you could make a vented differential pressure
transducer (Hint hint for the barometric one, note that there are some cheep
voltage type sensors here that you could use to and mount directly on a
PCB!). But the ultimate way to measure the depth is with a pressure
transducer (Vented) these can be obtained from Transmetrics or Druck, in
either submersible or dry.

Effectively you are after a whether station, if you have the bucks to spend,
then look for "ALERT" equipment on the WEB (Around $3000, and it will log
the data to, only some not all!), this is the cheapest form of remote radio
based stuff (Will also measure rainfall (Bonus)). Other than that there are
a few companies that make the sensors and data loggers that you're after try
Unidata


Dennis
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Dennis Plunkett: Embedded Hardware, Software design
NEC Australia
ph 03 9264-3867

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