Re: Measuring pressure with no flow

Hi David 

This looks to be an interesting project.  

Air, as a compressible fluid can cause some problems in an application such
as this. At some depth, the air bladder you are using to control depth may
be completely compressed so you will lose control.  Water on the other hand
is for all practical purposes incompressible and will give you very precise
depth control.

Maybe I don't completely understand your question, but a simpler approach
might be to simply measure the pressure of the water outside the sub and
control the depth by comparing the rate of change of this outside pressure.
For example, if the pressure is increasing, you are submerging, if the
pressure is decreasing, you are surfacing, if the pressure is not changing
you are steady at some depth.  

As you know, pressure varies with depth. Pressure will increase 1psi for
every 2.31 ft increase in depth.

Submarines control their depth through the use of ballast tanks that can be
fully or partially filled with water or air.  When the tanks are full of
water, compressed air is used to empty them and the submarine rises.

What you really need to concern yourself with though is buoyancy and stability.

Buoyancy is the tendency of a fluid to exert a force on a body placed in
the fluid.  This force is defined by Archimede's principle: 

"A body in a fluid, whether floating or submerged, is buoyed up by a force
equal to the weight of the fluid displaced."

This principle is defined mathematically as:

                                Fb=GfVd

Where 
                                Fb=buoyant force
                                Gf=specific weight of the fluid
                                Vd=volume of the fluid displaced


Stability is another area where you may run into trouble.  For a submerged
body to be stable, the center of gravity must belocated beloth the center
of buoyance, but for a floating body the reverse must be true.

Sensors:

Simple pressure sensors can be fabricated.  A good reference for these
circuits might be the Forrest Mimms Enginnering notebooks.  There are also
some model rocket altitude sensing PIC applications available on the web
that you might be able to adapt.

If you have the budget to buy sensors, Omega Engineering, www.omega.com,
has a multitude of pressure sensors.

I hope this is helpful.
Greg