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Some cell phone cases have a waterproof rating of IP-68, often specifying that they can tolerate immersion to 6 feet for 30 minutes or 3 feet for 60 minutes. My understanding, however, is that a good seal (like an o-ring) would fail near a certain water pressure without much time dependence. So, why the strange time dependence? This makes me wonder about diffusion of water across a poor seal being a factor, but I am guessing instead that these numbers are simply conservative times used in final manufacturer QC checks. Anyway, is there a theoretical predictive model to determine the expected "waterproof time vs depth" scaling?

bobuhito
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    The time limit is probably so that products can be tested for compliance. If the spec said the box had to withstand something for a year, it would take a year to test. – Olin Lathrop Dec 25 '15 at 22:40
  • Do you assume 6 feet for 60 minutes caused liquid entry? If so, I don't have any physical explanation. If not, compliance testing could be shortened and simplified by just quoting 6 feet for 60 minutes. – bobuhito Dec 26 '15 at 08:34
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    Things like dust or hairs in the O ring seal can cause very small failures which would allow a little water in. Over 30 minutes or 60 minutes, it might not add up to a drop, and therefore not be enough water to foul the electronics. Over days of weeks, the inside of the device would probably end up saturated. This is especially important for things like battery covers or charging ports where the user unseals and reseals the device often, usually in dirty environments. – Ethan48 Feb 24 '16 at 14:11

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As the comments suggest, it's generally something the manufacturer is comfortable standing behind & waranteeing. While you're correct that a seal should either hold or breach, in practice a "soft" seal such as an o-ring can be subject to creep under pressure. Given enough time, that deformation will lead to a leak.

FWIW, my advice would be only to wear cheap watches you don't mind losing if you need to keep a watch on while diving; if you're a serious scuba fellow then buy a watch (expensive) rated as immersible at depth indefinitely.

Air
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Carl Witthoft
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