[spr0cket]

Timur  asked the Naked Scientists:
   
Hi there.

Didn't know where to post this question :-]

A diver's watch can be water resistant up to a certain water pressure, which is often given in Bar or depth in m - at  a defined (water) temperature. The viscosity of water correlates to the temperature of the water.

The higher the temperature the lower the viscosity.

Q 1:  Does the pressure, that is necessary to make water leak into the watch change with the water temperature (the viscosity)? If so, to what amount - say comparing a '200m water resistant' watch at 4°C and at 60°C?

Q 2:  If the surface tension of the water is changed, e.g. by dissolving a detergent, would that have an influence? If so, to what amount?

Thanks in advance for answers or a hint where to post them,

Regards

TT

What do you think?

[Bored chemist]

I don't know how they specify "water resistance" but a change in viscosity or surface tension would only matter if there were holes that water could get in through. In that case I don't think you could describe the watch as water resistant..
In particular, the viscosity would affect how fast water got into the watch. Unless the water resistance test is for a defined time then, in the end; if it leaks, it leaks.

[spr0cket]

viscosity or surface tension would only matter if there were holes that water could get in through

Hi there.
There's not actually holes, I suppose, but at the seams there could be a) a tiny gap between (e.g.) housing and lid or, under a certain pressure the water could 'push aside' part of the gasket. Pressure tests for watches are done, afaik, by pressurising in plain air, then submerging the watch in water and releasing the surrounding air pressure. If pressure has built up inside the watch, then the air bubbles out. So it's actually more of an air pressure test.
If water leaking into the watch is about the water squeezing through a gap (a) the surface tension could affect the pressure afforded to get the water through the gap. If there's a hole in a container filled with water and the hole is small enough for the surface tension to remain 'stronger' than the water pressing against the hole it won't drip through - which might instantly change when adding a detergent breaking up the surface tension. Couldn't the same effect occur at the seams of the watch?
Whether the viscosity of the water changes the pressure needed to 'squeeze' the water through a gap or hole that's small enough for the surface tension to hold the water pressure otherwise... ? But I imagine it might be easier for the water to get it's way through a gasket (b) if it's 'more liquid' (lower viscosity at a higher temperature). Comparable to honey, if you want to get it through a straw. Even if close to impossible at room temperature (if the honey is rather tender), the honey will easily pass the straw when it's heated and 'more liquid'.
Neither the watch maker (who showed me his equipment for testing watches), nor a colleague (who's a physicist) could answer the question... the former pointed to his practical skills excusing his lack of theory and the latter started mumbling something about complexity, frictionless surfaces... started nibbling on a cookie and went away to have a nap. I dare not ask him again... he seemed a bit disturbed by the question and he's thoroughly needed to debug our matlab scripts ;->
regards
spr0cket