On Sun, 12 Apr 1998 23:04:35 -0600 Gus Calabrese <wft@MAIL.FRII.COM>
writes:
>Alan
>
> I am not sure about the legality of revealing a measurement system
>that
>is copyrighted by the US government.

Of course, the US government can't copyright or patent anything.  All it
can do is try to keep it secret.  Most of the technologies used by the
government were developed by private companies, so there may be problems
there.

But a dropping float river level gauge is a very old idea.  Some early
ones were maintained by the phone company and accessed by telephone.  I
think they worked something like this:  When a call came in, the
mechanism would move the float down until it touched the water.  Then a
motor would slowly wind it back up.  The winding mechanism had a cam
linked to a bell.  Every 6 inches or so wound up, the bell would toll,
which would couple to a nearby telephone microphone.  The caller (a
person) would count rings of the bell and look at a chart for the
corresponding river level.  When the float reached the top home position
again, the motor would stop and the phone line would hang up.

With stepper motors and PIC chips, a much more high-tech implementation
of this technique is possible.  Although some moving parts are required,
all the operations are "digital".  No precise analog sensors or
processing is required.

Ideally, the spool at the top would be designed to wind the cable in one
layer, so the number of turns directly corresponds to a distance.  Using
a couple of turns on a drum with a counterweight on the other end of the
cable could make the winding more reliable but then the other cable with
weight attached has to be kept free of the float cable.  The weight and
some of the cable would be underwater all the time in between
measurements.

The tension in a cable is readily measured by passing it through a
zig-zag arrangement of eyelets.  The top and bottom eyelets are in a line
and held stationary.  The center eyelet is out of line and connected to a
micro switch.  Tension on the cable tries to pull the center eyelet in
line.  The switch is adjusted so the weight of the float will close it
but the weight of just the cable (when the float is floating on the
water) won't.  Another switch may be needed to detect when the float is
in the starting position, all the way up.  It would also be possible to
just operate the motor "up" for many steps to be sure that the float is
all the way up (then the motor will stall, and further steps are
ignored).  But then it isn't possible to double-check each measurement by
verifying that the number of steps "down" are roughly the same as the
number of steps "up".

It would also be possible to detect when the float reaches the water by
fitting electrodes on it, either two on the float or one on the float and
a "ground" somewhere else.  This seems almost as complicated as measuring
the tension, and also prone to fouling.  Another variation would be to
use a sinker, and detect electrically when it touches the surface and
mechanically when it touches the bottom.  This arrangement would be
self-calibrating for the height the mechanism is mounted above the
bottom.



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