Q & A: underwater electronics

Q:
I am building an acrylic box that is watertight - inside will go some electronics and go underwater (in a swimming pool). I am planning on making the box 1'W x 1'L x 3' high. Out of the top will be a pipe to run some electrical cables to the surface. I plan on placing 2' of the box underwater using some sort of weights or straps. The plastic company said I need to do my research on pressure underwater. Can you help me determine if my 1/4" acryllic walls will hold?
- Bryan (age 37)
Orlando, FL USA
A:

At 2' underwater there's an extra pressure of around 1 psi. On a side that's 1' wide and 2' deep, that gives a pressure of around 1 psi. According to a standard data sheet, thus material should be able to handle up to around 10 psi, although some surface problems start at lower pressure. You'll have a submerged area of 2*144 square inches, with and average pressure around 0.5 psi. That makes a net force of  around 150 lbs. You could try a  3' * 1' 1/4" acrylic sheet with around 150 lbs on it to see how it does.

Usually in engineering anything like this, however, you try to allow plenty of leeway. Things go wrong (weak spots in one sheet, something bumping the box,...) So really you should try a substantially bigger weight. But if I understand those specs right, the acryllic sheets wil be strong enough.

I'd be even more concerned about preventing leaks at the seams between the acrylic sheets. If any develop with electronics inside, there will be big trouble.  If it turns out the the sides are strong enough, they'll bow in substantially, which will tend to rip up the seams.

You'll need to

1) Test for leaks before putting any electronics in.

and

2) Include some auto-switch that cuts off all power to the electronics if any water shows up inside.

and

3) Test to make sure the switch works.

If any of the electronics involve more than say 24V, I'd say it's probably best to forget the project. There are just too many ways something could really screw up badly. If it's all low-voltage stuff (especially under 12V) then I guess at worst you lose some electronics.

Mike W.

(published on 02/01/2016)