Thoughts on water resistance in the shower

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Thread: Thoughts on water resistance in the shower

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  1. #1
    Member jockeys's Avatar
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    Thoughts on water resistance in the shower

    preface: I'm a mathematician by training, so I decided to treat this as a physics problem. This may or may not have any bearing on real life

    I’ve heard it said that the water comes out of your shower at sufficient speeds that it can hit your watch and exceed the pressure rating, causing water to enter the case. Let’s examine the physics behind this. For measurement purposes, I’m measuring my Casio MTG1000. The buttons are about 3mmX7mm which is 21 square millimeters. Slightly less since it’s not a perfect rectangle, we’ll call it 20mm^2 for convenience.
    This watch is nominally rated at 20 bar of pressure resistance. That is equivalent to 2 million Pascals, or 2 megapascals. For the sake of discussion, we’ll assume that the buttons are a weak spot and are only good for 10 bar, or 1 megapascal. We can ignore atmospheric pressure here because there is ostensibly air inside of the watch as well, negating that, so we only need to calculate instantaneous delta-P to figure out of that 10 bar button rating is met. 10bar is equal to 1 million pascals, which is in turn equal to one million newtons of force per square meter, which is in turn equal to one Newton of force per square millimeter.
    A droplet of water is nominally 1 milliliter in free fall. 1ml of water has about 1 gram of weight under ideal conditions, sometimes a little less. If we assume that water leaves the shower head traveling at 10m/s (about 22 miles per hour, a very high estimate unless you are showering with a pressure washer). 1 gram x 10m/s ^2 = 0.01 newton.
    Thus, the most force one drop of falling water can impart on the button is 0.01 newton per 20 mm^2 or 0.0005 pascals of force. In order to hit 10 bar, we would need: 10 bar = 1 million pascals = 0.0005 pascals per drop x some number of drops. Doing the math, it turns out that we would need around 2 BILLION drops of water hitting the same button at the same time. Clearly, this is impossible as the button is very small. Alternately, for one drop to generate 1 million pascals of pressure:
    1 million pascals = (20 newtons)/20mm^2 and so for a 1gram drop of water to generate 20 newtons of force: 20 newtons = 1 gram x velocity^2 so velocity must be 141.421 meters per second, which is about 316.35 miles per hour (which some pressure washers can indeed manage).

    Conclusion: showering with a watch, even if it is only rated to 3bar, should be perfectly safe unless you are using a pressure washer. Even if your buttons are smaller, (let’s say a hundred times smaller), you are still orders of magnitude away from being able to breach it with a shower, unless the manufacturer lied about the pressure resistance.


    Thoughts?

  2. #2
    Member lysanderxiii's Avatar
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    Re: Thoughts on water resistance in the shower

    While I agree with your supposition, there are two false assumptions:

    1) The pusher must be rated at the same over-pressure level as the entire case. The case-case back and case-crystal joints cannot be able to withstand 20 atm, while the pusher only 10 atm. If this were true, under a static overpressure, the weak point (the 10 atm joint) would fail, and the entire watch would leak at 10 atm.

    2) The area of the pusher is not the area in question, but the area of the gasket-to-case seam. But, this seam is invariably covered, or underneath the pusher itself, so there is never any direct impingement of water on this seam, rendering the water velocity immaterial.

  3. #3
    Member Erik_H's Avatar
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    Re: Thoughts on water resistance in the shower

    You are not the first person to question water resistance. However, I think you are looking at it in a too simplified mathematical way, only considering droplet pressure. I suggest you look up the following standards:

    ISO 2281 Water resistant watches standard.
    ISO 6425 Divers watches standard.

    That being said, I did shower with a 3 ATM watch for years with no problem. And I am sure our friends from the Divers Watches Forum here on WUS has a lot more to say.
    Erik_H
    Member NAWCC Chapter 149

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  5. #4
    beshannon
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    Re: Thoughts on water resistance in the shower

    causing water to enter the case.
    At which point? A sealed button, edge of the crystal, the edge of the caseback?

    Also, the physics behind the movement of a drop of water is not nearly the biggest or only problem.

    Temperature causes a variety of materials to expand and contract.

    Soap can cause seals to dry out.

  6. #5
    Member AAWATCHES's Avatar
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    Re: Thoughts on water resistance in the shower

    WOW

  7. #6
    Member lysanderxiii's Avatar
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    Re: Thoughts on water resistance in the shower

    Quote Originally Posted by beshannon View Post
    Temperature causes a variety of materials to expand and contract.
    The coefficient of thermal expansion of any rubber compound is an order of magnitude larger than that of steel or glass. An increase in temperature would only cause the rubber to expand and seal more tightly.

    Even so, under the temperature differences normally encountered by watches (and those that can be survived by the people wearing them) thermal expansion is negligible.

    And, if you are talking about degradation of rubber compounds under elevated temperatures, that does not occur until you get into the +300F range.

    Quote Originally Posted by beshannon View Post
    Soap can cause seals to dry out.
    No, it cannot.

  8. #7
    Moderator Public Forum John MS's Avatar
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    Re: Thoughts on water resistance in the shower

    Quote Originally Posted by jockeys View Post
    preface: I'm a mathematician by training, so I decided to treat this as a physics problem. This may or may not have any bearing on real life

    Conclusion: showering with a watch, even if it is only rated to 3bar, should be perfectly safe unless you are using a pressure washer. Even if your buttons are smaller, (let’s say a hundred times smaller), you are still orders of magnitude away from being able to breach it with a shower, unless the manufacturer lied about the pressure resistance.


    Thoughts?
    Hmmm...I think I've seen a similar scientific sounding message on this weighty topic before.....

    I would be more concerned about metals and seals being subjected to a sudden temperature change and expanding at a different rate. I would also be concerned gooey substances like soap,, shampooo, conditioner, dead skin, etc., building up in the many tiny crevices of a watch case and bracelet. And of course you won't be washing that wrist much.

    But more to the point, what purpose would be served by wearing a watch while showering??? Counting water drops per minute??

    Makes about as much sense as wearing ones shoes, underwear and a gold necklace while showering!
    Last edited by John MS; August 27th, 2009 at 19:22.

  9. #8
    Member BlackLight's Avatar
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    Lightbulb Re: Thoughts on water resistance in the shower

    I'm an engineer and mechanical designer.

    Some folks say you should not shower with your watch. Some never take it off; swim, shower, sauna…

    Here are my thoughts:
    1. Materials will expand according to the formula:
    dL = (TEC) * L0 * dT, where:
    dL = change in length
    TEC = Thermal Expansion Coefficient
    L0 = length at the reference temperature
    dT = increase in temperature

    2. For SS the TEC varies from 10 to 20.
    3. That means a 2 inch watch case would grow .0005 inch if you take it from -40F to boiling water (~250F jump).
    4. It would of course shrink by the same amount if you decrease the temperature from boiling water to -40F.
    5. A TEC of 30x10-6 in/inoF (for fiberglass re-reinforced PolyPropylene) means it will expand .0015 inch under a temperature jump of 250F.
    6. For rubber it is .00385 inch.
    7. For sapphire the TEC is ~3.5 so it will grow by .00175 inch.

    8. Note though, that the case, the case-back, the crystal and the seals all experience the temperature change, so only the differentials in expansion need to be taken into account.
    9. The seals are probably a polymer or rubber of some sort. That means it is using the material’s compressibility to affect a seal.
    10. Most polymers used in seal applications will seal deformations of between 30 and 50% of the volume. This means if the seal is an O-ring with a section diameter of 0.05”, it can be compressed to 0.035” and be expected to seal a deflection of 0.0105” in the housing.

    11. So if we take the worst case, where the crystal is sealed to the case with a rubber O-ring, and we boil the watch and then drop it into an Arctic climate, the case (and the O-ring seat) will shrink by .0005” in diameter, thus .00025” at the seat, the O-ring will shrink by .00385” and the crystal will shrink by .00175 in diameter, so .000875 at the seat.
    12. This means the gap that the O-ring needs to seal will enlarge by .004975.
    13. The O-ring can cover a gap-change of .0105.
    14. Now at lower temperature (-40F) the rubber (or polymer) O-ring will not spring back as well, but there is also much less liquid water around.
    15. If the process go from cold to hot, the gap will close up with that amount and the seal gets better and better.

    I would say, do not freeze your watch to -40F, but you can shower with it all you like.

  10. #9
    Member PatrickG's Avatar
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    Re: Thoughts on water resistance in the shower

    Quote Originally Posted by John MS View Post
    But more to the point, what purpose would be served by wearing a watch while showering??? Counting water drops per minute??

    Makes about as much sense as wearing ones shoes, underwear and a gold necklace while showering!
    I take my watches into the shower when they start to look dirty or I notice them leaving smudges on my wrist. During a point in the washing and scrubbing process, I'll take my watch off and scrub it up with standard deoderant soap. As I'm washing it, I can see the soap bubbles getting dingy, so it's definitely cleaning out accumulated body oils, dirt, and probably some metal residue that has built up around the wear points in the bracelet. Obviously I only do this with my watches that have metal bracelets.
    This is just a tribute.

  11. #10
    Member Sodiac's Avatar
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    Re: Thoughts on water resistance in the shower

    This question of taking a shower with a watch keeps coming up. Why would anyone take a shower with a watch anyway? Who wants all that icky soap and stuff all over the watch, the watch band, the links (if metal band)...

    Three times to take off the watch: 1) Before a shower or bath. 2) Before you go to bed (although some excuses can be made here). 3) During, uh, extracurricular activities with the partner of your choice.

    I like watches as much as the next guy, but there's a time and a place for everything!

    Sodiac
    Too many watches, only two wrists and never enough time...

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