The Casio F-91W is one of the most iconic watches of the past few decades. It’s notoriously cheap, reliable, and retro; in fact, it’s probably the only watch that was worn by both Obama and Osama. At just under $10, it’s the perfect daily beater watch. And in the age where you need to charge your Apple Watch every day, the Casio’s battery can last up to a decade. Still, though, it has a few downsides:

The resin strap is liable to break

The watch is barely water resistant

The former is easily rectified by switching to a vinyl or leather NATO strap; but what about the latter? Is there any way to turn the F-91W into a deep sea diver?

The Magic of Fluid Incompressibility

Fluids are said to be “virtually incompressible.” Generally speaking, a gas (like air) is compressible: when pressure increases, volume decreases. The compressibility factor \(\beta\) may be expressed as

$$\beta = -\frac{1}{V}\frac{\partial V}{\partial p}$$

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.

Step 1: Preparing the watch

First, we take off the stock resin straps and wipe off the watch of excess oils and debris.

Step 2: Unscrew the back plate

We next unscrew the back plate and remove the watch internals, also removing the white tape on the back of the battery compartment. All parts are wiped carefully with with rubbing alcohol.

Step 3: Fill bowl with mineral oil

Some sources suggest heating the mineral oil to ~50° Centigrade, but I’m not exactly sure what the reasoning for that might be, so I stuck with room temperature. For some extra flair, I added a fat-soluble colorant which will give the viewport a nice bluish tint and set it apart from a stock F-91W.

Step 4: Submerge

Slowly submerge all watch parts in the mineral oil. Ensure that there are no air bubbles in either the watch chassis or electronic internals. Leave watch submerged while mixing slowly for a few minutes to make sure there are no air bubbles.

Step 5: Re-assemble the watch

Re-assemble the watch by securing the back-plate via the Phillips screws while submerged in the oil. Make sure no air bubbles accidentally form while screwing in the screws.

Here, you’ll notice I decided to switch to a reddish tint after my first failed attempt.

Step 6: Extract watch and clean

Remove the assembled watch from the oil mixture and wipe it down, cleaning any excess oil. Add a fancy strap, and et voila!



Final Words

If you decide to use colorant, as I did, keep in mind this will effectively reduce visibility to zero and it will be very difficult (but not impossible) navigating the murky oil solution by touch alone. Also, keep in mind that after this procedure the F-91W’s nigh-useless LED light will be even more useless due to the oil’s refractive index. Finally, even though mineral oil is “virtually incompressible,” it’s still technically compressible. Professional dive watches filled with oil will often have a flexible chassis that will allow for around a \(\pm0.5\%/1000\text{psi}\) wiggle room in volume to account for this compression at high ambient pressures.

So what kind of pressure can my little Casio handle? I don’t have any way of testing, but a similar experiment (on a Casio G-Shock) allowed for pressures of up to 1200PSI, so I would expect similar numbers out of my watch. Another enthusiast did this experiment on an F-91W with olive oil. However, olive oil will oxidize, so if you’ll give this a try, I would suggest a synthetic compound.

Overall, this was a fun little project and not a bad way to spend a lazy Sunday.