Belemnite Diver

For anyone even slightly ‘into’ watches, the Casio F-91W needs little introduction. It’s the quintessential cheap digital watch. Sold into the Billions and seen on the wrist of everyone from Obama to Osama. (In)famous as having been used as a cheap and accurate timer in IEDs, it’s said anyone wearing one during the War on Terror in an American airport had a higher probability of being pulled over for that extra special attention.

Despite all of that however, it’s not a dive watch, and as mentioned in my previous entry - I like dive watches. It’s listed as ‘splashproof’ in the Casio documentation which means good for a rainshower or handwashing, but nothing beyond that. Anecdotally it will survive some depth, but given its internal build, won’t do so for that long.

Usually when things fail underwater it’s due to pressure. The pressure of the water outside will very quickly exceed the atmospheric pressure inside a device and lead to an ingress of water. Dive watches, like dive equipment, use threaded seals and o-rings to ensure this doesn’t happen until you hit 2-300m or even deeper. So how to get a cheap, basic, plastic watch like the F-91W to survive repeated exposure to the deep…? Easy, just replace all the air inside it with something that won’t compress when exposed to pressure. Something like mineral or silicon oil (Silicon as the mineral, not silicone as in the tits). A light, clear mineral oil will also not affect the electronics inside the watch, especially with a digital which has no moving parts, the non conductive oil will mean electricity keeps going where it’s supposed to and nothing slows down.

The process is fiddly, but fairly straightforward. Use a watch screwdriver to remove the back of the watch and soak both pieces in the mineral oil. Gently agitate to remove any bubbles. Once convinced there are no air bubbles, the hard part is assembling under the oil to ensure everything stays together. Screw the back on, ensure the simple o-ring inside is still in position. Remove from oil, wipe everything down and leave it for a few days to check no leaks and no air bubbles. I’ve done two versions - one, I suspect, got returned to the sea at some point sadly. The other I’ve mounted on a 3d printed PETG mount so it can more easily be bungy’ed to my kit. So far it’s survived a couple of >40m dives and some shallow quarry stuff. In theory, this watch should maintain itself down to a considerable depth.