Making a 'Hydro modded' Casio F-91W Dive Watch

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.

On Dive watches.

On Dive watches.

One of my few (lol) kit obsessive tendencies is for dive watches. For other divers its torches, for some its masks (wait I have that one too), for me after my general diving kit, its watches. I like a nice watch, be it a dive watch or a dress watch.

A Shearwater Petrel with cable for a rebreather like the SF2 is around £1400 at the time of writing. So two of those will be £2800 which is still considerably less than £3680 which is the current price of the Tudor Pelgos FXD allegedly used by the French Marine Nationale.

You can get a nice Doxa as a dive watch with a very nice pedigree for a little bit more or around the same price as one Shearwater, but of course it only tells the time and doesn’t do GF calculations, gas tracking, decompression maths or anything else you can do with a decent dive computer, most can also tell the time.

Therein, as the bard tells us, lies the rub. The fact I can buy a very sexy dive watch that looks nice and tells the time but otherwise is fairly useless when compared to a dive computer that’s certified to the same depth seems to render the watch a bit pointless.

So why do I love dive watches? Firstly it’s simply the only piece of male jewellery I can tolerate and I’ve worn a watch since I was a child so I miss not having one on my wrist. For dive watches specifically, it harkens back to an earlier time. When true underwater pioneers had to rely on a timepiece to track air and decompression. The sense too of owning a beautiful item that is itself a tool and able to tolerate the very extremes of places I’m ever likely to go and far beyond. I will shortly be able to dive to 60m (legally) and to have a piece of kit that can go to 10 or even 100 times that amount and withstand the pressure and still function is rather remarkable. Which is great for me - but still doesn’t explain why MN get Tudors! Incidentally the only watch in my collection I’ve ever dived with is the $50 (or £50 basically) Casio ‘Duro’ the nato strap on which was nearly the same cost as the watch.

I have recently managed to get a Scurfa MS25 (the 2025 limited edition dive watch). I also own the MS24. These British micro-brand dive watches have their own developing dive pedigree as the company is owned and run by a working saturation diver. So when people moan about watches (Omega I’m looking at you) having a Helium escape valve - this guy actually needs one. I was intending to take a sexy photo of the MS25 at 40+ m on a wreck… but see my previous post, so that will have to wait.

Its all a bit futile.

Its all a bit futile. Don’t worry if you’re reading this, it’s not a cry for help and instead merely an observation. For a large and growing number of reasons, I’m increasingly dissatisfied with social media. Yet despite this, the desire to share interests with like minded people is surely one that is common from football fans to classic music fans and SF2 rebreather owners with an interest in British wrecks. Misery, after all, loves company. Social media is entirely at the mercy of the algorithm but this means it’s actually extremely hard to engage and share with other people beyond clicking a ‘like’. Blogging I will admit is very 2002 (happier times!) but at least it’s my own authentic voice, my own findings and areas of interest and I’m happy if someone wishes to reach out. Until then, I write for my own satisfaction and the hope that one day, someone, somewhere, googles something and the hateful AI will remember this small diving blog and throw up something that may assist another diver, 3d printer or miserable overworked person who wishes social media was actually social.