Gasket Contact Pressure for Water Tight Design 2

[umrchi]

I came across a question about designing a resin gasket for a water proof watch, and I have a question about it. Does anyone know how I can get how much contact pressure between the gasket and the watch back cover is needed so that the watch is water-proof under certain hydrostatic pressure?

Can I assume as long as the contact pressure between the gasket and the back cover is higher than the water pressure, it is safe?

Thanks

 

[gerhardl]

Interesting, and maybe I am wrong (others?) but on speculation:

1. A watch sealing protecting from outside to inside against water pressure could be either for back lid, front cover or time adjustment screw. The last the most difficult, possibly both for rotary and axial movement.

(Alternatively: all pushbuttons digital and 'put into a rubber sack' : contact pressure: zero!)

2. Thightness would depend on several factors:
a) geometrical form (length of sealing area, whether outside pressure would contribute to extra pressure by acting on larger area than inside and elasticity in sealing giving larger contact pressure for sealing (against stem)
b) Precision of sealing and stem measurements (roughness, form and clearing/fitting between the two components)

3. Thightness to be dropthight (no leakage at all), that is no water molecules able to pass.

Result:
c) Contact pressure must be sufficiantly higer than outside pressure minus inside pressure to compensate for outside variations (caused by practical use, movements and variations in water density) and inside variations (possible compression of air inside watch caused by compression of total watchcase by outside pressure).

d) Or contact pressure might possibly(?) be lower if fitting is so thight that contact area 'spillroom' under all outside/inside practical possible pressure conditions are smaller than a water molecule.

 

[HDS]

Look in the parker O-ring handbook. You need enough pressure to have the gasket close off all the gaps water could get through.

 

[umrchi]

Thanks gerardl. The watch water tight design was previously proven and now there a rechargeable battery is added to the design so the back lid is being put on and off more often.

That comes down to a risk that the user may not tighten the back lid far enough to cause water leak. So I need to look into the lid sealing contact pressure.

And yes, I think the outside pressure would help. But I don't know if the water tension going to drive a crack between the seal and the back lid interface (see attached picture). And I would want to see what pressure would stop it the crack from growing into a leak.

 

[umrchi]

@HDS:

Thanks for the tips. I have referred to parker O-ring Handbook page 4-18 for face sealing for the design. It is just not knowing the exact contact pressure getting me nervous. May be I just worry about too much as long as I follow the hand book.

 

[HDS]

Parker has information about compression force which changes from size to size but it most goes by %compression which does not change as much.

 

[umrchi]

@HDS:

I find it now and I think I can actually live with it without knowing the actual contact pressure between the seal and the back lid.

Thanks.

 

[IRstuff]

When I still had a watch, rated for 50 m, I changed the battery about once every 5 yrs. Of course, much depends on how much power your watch is sucking up.

Note that while contact pressure and all that is good, one rarely changes the battery in the same environment in which it was initially installed. You might want to be more concerned about dirt and lint getting into the seal area than the contact pressure.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[umrchi]

@IRstuff:

The watch has an intercom system and some other voice reminder functions, so the rechargeable battery, or the battery as of today's need to be recharged/replaced somewhere between 3mo to 6mo, depending on actual usage.

I do worry about the situation you mentioned, and we are still working on a fault-proof design so that user error can be reduced or eliminated.

 

[unclesyd]

A miniature O-ring of the proper compound is the way to go as this is what's currently being used. Checkout the first link.
I have used both Apple Rubber and Precision for miniature gaskets and O-rings..

http://www.rnhorological.co.uk/gaskets.htm

http://www.applerubber.com/

http://www.precisionassoc.com/

 

[umrchi]

Thanks unclesyd