I've found that the 'pretty accurate flow rate meters' require large flows and
are expensive.  But it was some time ago when I looked for them, and I only
looked at those suited for high flow rates.  Perhaps a bellows type flow meter
would work here...

If you don't mind going slowly you could always use a parastaltic (sp?) pump to
pump a known quantity in and out of the ballast tank.  Put a pressure sensor on
the outside and inside, and use the pressurized air to make the job easy for the
pump.

-Adam

Dale Botkin wrote:
>
> On Mon, 16 Oct 2000, Andy Shaw wrote:
>
> > Just a quick note here. The way I like to run my subs is not to spend ages
> > ballasting them so that taking on a full tank of water takes them to a
> > neutral buoyancy (which is what most people do). Instead what I would like
> > to do is to automate this part so that I can just do a simple pond side
> > set-up of taking on water until the sub is neutral then store the value
> > associated with this state in eeprom. Then I just command the sub to take on
> > the same amount of water when I dive. I currently do this open loop (timing
> > based) but this tends to be a bit hit and miss as the batteries go down...
>
> Just a thought:  There are available pretty accurate flow meters, as used
> in automotive applications to sense fuel flow.  Couldn't you use one of
> these (or even a home-brewed one using a paddle wheel and slotted disk or
> whatever) to measure how much water you pump into and out of the ballast
> tank?
>
> Dale
> ---
> The most exciting phrase to hear in science, the one that heralds new
> discoveries, is not "Eureka!" (I found it!) but "That's funny ..."
>                 -- Isaac Asimov
>
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