> The purpose of this logger is check the water temp. of a little river
that
> disappear in a cave. Water was traced with fluorescent dyes: it exit from
a
> large spring at about 8 miles from the entrance. The logger will sealed
into
> a plastic ball with ballast and will have to run through unknown submerged
> passages. I haven't any assurance to recovery a single logger at the
spring,
> so I will make some of these units to have a sufficient probability to
find
> at least one of them!!

That sounds like a very tough problem.  Unless these loggers are almost
exactly the same density as the water, they will get caught somewhere in 8
miles.  Probably even then.  How many of these things do you plan on
releasing?  Please let us know how you make out.

If you get none of them back, you could possibly try something that
periodically changes its density from a little below to a little above the
water.  With a little luck it would alternate between being stuck on the
ceiling and on the floor, and maybe move a little downstream each time.  Of
course, that't not easy to do, nor cheap enough to make sufficient
quantities to recover any.

This is off topic, but I'm curious:  Why do you expect the water temperature
to change as it flows thru the cave?  Is the temperature at the spring much
different from the inflow?  Is there geothermal activity in the cave?  Where
is this?


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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