Water entering in the container while at 30m depth

  1. Hi,

    It may not be the best title chosen but here is the question. If I have one container that has two compartments, one has a battery pack built from several alkaline cells and the second compartment has some pcb, wiring etc. The two compartments are separated each other with a plug, no communication port between the two. My container is supposed to be waterproof. If I drop this container in the water at 30m depth and water enters in the battery compartment, do I expect to have pressure built up in the unit when brought to the surface ? When the container is opened up, is there a safety hazard because of this possible pressure building in the unit ? There should be no chemical reaction between alkaline batteries and salt water that builds up gases ( I think). Thanks for help.
     
  2. jcsd
  3. Generally speaking, if the water can get in, the water can get out too. I don't see how you'd have a buildup of pressure in a container that isn't airtight, or even watertight for that matter. That said, it's always possible, if you had a slow leak, left the container underwater a long time, and then brought it back up quickly, the compartment would still be pressurized.

    Off the top of my head, I think that's about 4atm of pressure you're dealing with, so it isn't insignificant.
     
  4. sophiecentaur

    sophiecentaur 14,140
    Science Advisor
    Gold Member

    If there's a small leak, working over a long time, then the container could end up 3/4 full of water and a rapid depressurisation could be a problem. If you make the container big enough then that need not be an embarrassment and it may be best to operate at ambient pressure, rather than at 1 Bar. You could always keep the batteries in a waterproof bag inside the container - again with a large surplus of air in it - about four times the total air volume of all the small spaces between and around the batteries, so the bag will not rupture.
    You need to be you sure that the gland which takes the cables from the battery into the sealed electronics box can withstand the 3 Bar excess pressure but that shouldn't be a problem (i.e. fairly cheap). 30m is not very excessive.
    It may be worth pointing out that the maximum dynamic pressure you may need to deal with may be somewhat higher than just the 3 Bar static excess (all watch manufacturers point this out when warning about the 'real' depth that their 30m 'waterproof' watches can handle).
     
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