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 ? may be expressed as ?=?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?

ByJohn Biggs

John lives in Brooklyn and has loved watches since he got his first Swatch Irony automatic in 1998. He is the editor of WristWatchReview.

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