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Question possibly to do with atmospheric pressure (but possibly not)

  1. Jul 12, 2010 #1
    I have a question about my watch which has been bugging me for a little while, and I was wondering if anyone could enlighten me as to the physics behind it.

    One day a while back I went out for a run in the rain and some water got into my watch, behind the glass. I only noticed because it had condensed on the glass and was making the watch face blurry. I thought I'd ruined the watch but the next day, the water droplets had vanished, so I thought they'd managed to find their way out of the watch, but no - some time later, they reappeared again. And now, every now and then the glass gets steamed up (but only in the middle), on seemingly random days. Temperature doesn't seem to have much impact (it's steamed up today and it's about 15 degrees, but it also steamed up last week when it was really hot), so I was wondering if it might be something to do with changes in atmospheric pressure?

    I might be barking up completely the wrong tree, but if anyone can explain it to me that would be great.
  2. jcsd
  3. Jul 12, 2010 #2


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    I've seen this a bunch of times, from watches to my phone camera lens to my "waterproof" camera lens.

    I don't know for sure what it is, but would guess it's not atmospheric pressure, but simply temperature differences between the watch and the outside air. Just like breathing hot moist air on your watch causes the water in the air to briefly condense when it hits the colder surface of the watch glass, I think if the interior of your watch is hot compared to the glass surface, the hot moist air inside the watch condenses on the inside surface of the watch, and since it's enclosed, with no airflow, and a perpetually moist atmosphere, it is very difficult for the moisture to evaporate again.

    When it steamed up today (at 15 degrees) the watch was probably taken from a warmer climate (say, indoors) to outside, which caused it to fog, and if it steamed up on a hot day... I don't know, but did you have the watch in an even hotter environment first, like, say, in hot car?

    Next time it steams up on a cold day you could just try mashing your thumb on the glass for half a minute to warm it up and see if it clears it up (just for the short term). It worked on my camera last winter. Also, you can throw your watch in the freezer overnight and then wear it outdoors on a cold day and see if that helps. Not very practical, but maybe an interesting experiment.
  4. Jul 12, 2010 #3
    Thanks for the reply, although I'm not sure it's temperature related. I could understand it if the condensation was just caused by going from a hot place to a cold place, but this seems to a be a day-to-day thing. I wake up in the morning and either it's steamed up or it isn't, and it stays that way for the rest of the day, as far as I've noticed. Mostly it's not steamed up, just occasionally it is.

    I've tried rubbing on it with my finger but it doesn't appear to make much difference. Next time it isn't fogged I might try throwing it in the fridge and see if that does anything... Like you said, would make an interesting experiment.
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