The Casio F-91W is, famously, the only watch worn by Osama and Obama. As a beater it can’t be beat an, as David Tinarenco writes, it’s even better if you put it on a NATO strap. But what about that pesky water resistance?

Tinarenco has a solution: he basically fills the watch with mineral oil, thereby improving the pressure resistance to 1000+ PSI.

Fluids are said to be “virtually incompressible.” Generally speaking, a gas (like air) is compressible: when pressure increases, volume decreases. The compressibility factor β may be expressed as β=−1V∂V∂p where V is volume and p is pressure. A trick to achieve incredible depth ratings has been to submerge the internals of a watch in inert, non-reactive, non-corrosive, non-polar liquids. Mineral oil (a byproduct of petroleum refinement) is a fantastic choice — and this is exactly what I’ll be doing to my Casio F-91W. Theoretically, as water pressure increases around the watch, due to the fact that it’s filled with an incompressible liquid (as opposed to a compressible gas), it can successfully “push back” against the water and it will not succumb to implosion due to increasing pressures.

Obviously this does a few things to the watch including make the backlight unusable and most probably voids the warranty but it’s a great little hack. Interestingly, this has been tried with a number of G-Shocks with similarly spectacular results. Maybe we should cut open our Rolexes next and fill them with olive oil?

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