Why do cellphone waterproof ratings have a time limit? Example 6 meters for 30 minutes.
In: 54
Water molecules are pushed by other water molecules into spaces where there are less water molecules, i.e. inside the phone. The seals on the phone can push back these water molecules for a while. It’s like wrestling with a guy stronger than you. You can hold him back for a time but eventually you’re gonna get pushed backwards. The water pressure will eventually break the seals allowing water in. The metres part determines how much the water molecules are pushing against the seals.
It’s really difficult to make something completely watertight if it has any sort of openings (like a charging port). It can be done, but the engineering and materials required drive up the cost of production, and therefore the cost to the consumer. You could purchase a pretty much “forever” waterproof phone, but the cost would be significantly higher than whatever you paid for your phone (and would likely not be worth it – most people change phones for reasons other than water damage).
That said, we can make things that will keep water out for a good long while, but water pressure is pretty relentless and it will push water molecules into any crevice, eventually overcoming the strength of whatever is used to keep the water out. The deeper you go, the more pressure there is, and the more the force behind the water pushed into crevices. This is why a water resistance rating is a combination of depth and time – the materials and design of the seal are engineered to last only so long at a certain depth.
A lot of great answers already in the comments that explain the science of water resistance, but none that actually answer the question.
IP (ingress protection) ratings are standardized and have standardized tests to verify the device meets or exceeds the desired rating. For example, IP68 is a common rating for smart phones. The second digit represents water resistance. To have a level 8 water resistance in your IP rating, your device must remain functional after 30 minutes of immersion. Can the device last longer? Maybe! But the testing only requires 30 minutes.
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It’s a procedure called osmosis.
The lower in the water you go, the higher the pressure.
Phones can’t be completely waterproof because they are still made of different components which have very small “cracks” or “fissures” between them.
Since those cracks are so small, it takes water a longer time to enter but the higher the pressure, the easier it will be for the water to enter the nearest low pressure object, that being the phone.
Edit: Osmosis was the wrong term, my bad.