Pressure Testing a watch is based on a very simple principle. It is exactly like looking for a puncture in a bicycle tyre. The watch is suspended in the air above the water and put under pressure. Lower the watch into the water and let the pressure our shows any leaks as a stream of bubbles.
But saying that there are a few things to know before starting pressure testing watches.
Pressure Test Shortcomings
This is a list of shortcomings and problems which occur when pressure testing a watch.
The watch exploding in the pressure tester: Yes this can happen. Usually when the watch has a slow leak which you do not spot so the pressure difference between inside the watch and outside is to much and the glass pops off Filling the watch up with water.This is why you never do the test in front of a customer
Air/Water Leaks in but not out: this generally happens on watches with a screw crown (typically expensive watches like Rolex) and it is hard to spot
Sometimes you just cant tell: Watches with many air voids in the casing can hide a leak (especially Casio G-Shock). There are just so many bubbles you just cant tell.
A Leak at low Pressure: If the back does not fit well or a seal is too small the pressure at 3 bar seals the watch and you think the watch is waterproof when it isn’t
Misting of the glass: Sometimes a watch can look like it has let water in and it hasn’t. There are 2 main types of misting.
1): The glass can “steam up” with humidity in the air. Hot humid air gets in the watch then when the weather cools down water is deposited on the glass (as its the coolest surface)
2): The oils (I think) evaporate and deposit themselves on the glass.
The humidity is important to remove as it is water and does rust parts it may come in contact with
Static Test Isn’t the real world: The watch is stationary in the pressure tester and this is not how the watch will be used. Moveing, knocking, and touching pushers and crowns can not be tested. Changes in temperature also affect the water resistancy. A sudden change from the heat of a sunny beach to a cold sea can make a normally waterproof watch leak. You can’t test any of these scenarios.
Waterproof Today Leaking Tomorrow: a seal can fail this fast. Change the time or date, press a button or knock the watch and it could fail. Water is a powerfull enemy and sneaks in through the smallest gap.
Extreme Temperature: I have had 2 watches that have failed when the user has gone in water to “cool off” after sunbathing. The watches pressure tested fine before and after the leak and i am sure it is the sudden change in temperature. For this reason I also do not recommend doing the washing up with any watch. I am also suspicious of some detergents damaging the rubber seals.
Any Extreme Condition: Saunas, Steam, really hot water, freezing cold, Strong Vibrations, etc. are all conditions that the watch was never designed for.
When to pressure test a watch or not is a personal choice that is entirely up to you.
If you guarantee a watch waterproof you have to bare in mind one day you may get it wrong. I guarantee only watches i know I can replace the movement easily and cheaply. I advise the customer about the shortcomings of a pressure test before doing it and then suggest that they test the watch themselves in a swimming pool before going in the sea.