in fact i've a couple of Qs which are all related to providing the
solution looked for

i'm building a sealed (water tight)an expandable underwater platform
and i need to grow its size from 50cms by 50cms by 5.6cms to 100cms by
100cms by 7.8cms - therefore going from 14 lts displacement to 78 lts
by my calculations an increase of 62 lts - surface area going from
6120 sq cms to 11560 sq cms

Q1 - what is the work in hp or watts to expand the platform - displace
the water assuming air pressure inside remains constant (platform has
outlet to adjacent wall

Q2 - same as above but the internal air pressure changes as a result
of having a fixed quantity prior to sealing at sea level (it’s a free
standing unit) The weight of the unit will always exceed the displaced
water and will also be secured to the bottom of pool) - will the
change in internal air pressure due to the expansion and contraction
make any difference to the relative force required to displace the
water?

thanks much
tkmn

---

Clarification of Question by tankman-ga on 13 Feb 2003 13:51 PST

ommited to mention the depth 
- the platform will sit at 10 meters 
- will it make much difference if its fresh water or sea water?

Hello tank man

 Sea water makes about 2 % difference. But you need to specify size of the pool.
Or is it at sea?

thnx hedgie - i understand the water density between fresh and sea to
be 2 to 3% - but given the work to be done, will that have a linear
effect on the work to do the displacement?

given pressure is a function of depth not area -does the size of the
pool make any difference!

You are welcome tankman

 Sure calculation would have to be done. 
 That was not an answer,just a comment.
 No work was done, no links, no cost to you.

 
The power (watts) will depend mostly on the pump you will use;
Bigger the pump, more Watts, the sooner the WORK will be done.

What you want IMHO is the WORK which has two components:

1) work which can be estimated as buyoancy * depth , which is linear
in density and you can surely calculate it yourself,

once you  think a bit and brush up on units:
Usually, we measure energy or work in Joules, not Watts :-)
http://cator.hsc.edu/~kmd/caveman/projects/motor/

Notice, as you are pushing the IMMERSED tank down. The level of water
is not changing. You imagine you build it on the shore and push it down,
OK? The WORK is same, no matter how you do it, right?

2) The second is a bit more tricky: Your tank is floating, and you
immerse it.  In a narrow pool, the water level will rise. How much? 
Volume/A. A is area OF THE POOL. In the Sea, A is infinity and second
 component is simple. If finite, a bit more complex.
right?

Hi hedgie,

as you can appriaciate physics is far from my strong point - and
having to get my head around the calculations is what i'm trying to
avoid!

bit more background - the expandable platform will be sunk in the
pool, where it will electro-mechanically be made to expand and shrink.
so i need to know how much work (J as you well say) is needed once it
is in place (submerged) - not really interested in the work to place
it at the bottom.  another variable, which i will deal with later, is
the speed at which i need it to open and close the platform (ie power
- Watts/sec)

re Displacement - interested in the work effort required to expand the
platform per first origial Q. (closing will be easier since the
pressure will assist). we know the displacement of the expanding
platform, so here's where i'm not getting it - we have a pressure of
1kg/cm^2 at 10m and the surface area is going from 61k cm^2 to 11.5k
cm^2 with a vol growth of 62lts - what will it take to make this
happen - that's all i'm looking for for $10- (including the
description of the calculation)

not interested in the change/distribution in pool level 
tnkmn

Hi tankman

 We are making progress.That's good.
 But the question cannot be well answered
until it is well formulated. We are Ok on Joules,
we still need to be careful on power: power is measured 
in J/s = Joules per second, that's Watts, not W/s.
( That's OK. Students often have problem with that. )

NOW, think please : weather YOU are 
"interested in the change/distribution in pool level"
is subjective and not relevant here. Physics is objective.
 What I am trying to make you to see, is that the formula you seek has two
terms.  The second (which tends to be smaller) has A in it.

 I understand that you will expand your tank under water. 
 I want you to see the WORK W is a function of the 'state of the
 system', which in your case is given by V = volume of the tank
and Depth.  The work of expansion you seek is easiest to calculate
as W(D, V1) - W(D,V2) . That's the work of expansion from V1 to V2
at the same depth. 
Do you know what I mean by buoyancy ?

 http://www.geocities.com/wetnwilddivers/archem.htm

 Researchers in general, (and me in particular) are wary of clients
 who argue instead of thinking. 

The point is, it takes more WORK in a narrow pool, then in the ocean,
(since the water get pushed higher).
Do you se that?

Answering your Q in reverse

Drawing an analogy between a hydraulic system and a “pool vs ocean”
would have made the penny drop as one of the considerations!!

As an artist I’m looking to build something which has form and a
certain function – and don’t wish to be bothered by a bunch technical
detail – preferring to pay money for the answer to what is a rather
simple Q for someone in the field. So, your pedantic approach is
aggravating – making anyone argumentative.

If I wanted an education I’d  1- go to school; 2- buy the appropriate
books and spend sometime figuring it all out or 3- placed a more
general Q on an educational Q&A site.
I’m looking for a straight answer with pointers as to what the
issues/variables to consider are. As such, I care little about
figuring it all out – I’ve got other things to think about. Can you
answer this simple Q - Care to make $10 – or not?  If you aren’t I
must therefore conclude you are on some educational power trip – are
you a failed teacher?

Thank you tank man for clarification of what you want.

  I do not feel question is well defined for me to work on,
  so I am leaving it to other researches.

 Good luck with your project.

hedgie