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Casio F91w at 5KM underwater – watches of espionage

Casio F91w at 5KM underwater – watches of espionage

20 comments

·January 31, 2025
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mrweasel

The F-91W is such a fun little watch[1] and people have done the weirdest stuff with it. There's a guide to make the mod on Youtube[3]. There's also the TOTP in a F-91W[2]

1) https://www.youtube.com/watch?v=r6REKCs4-1M

2) https://blog.singleton.io/posts/2022-10-17-otp-on-wrist/

3) https://www.youtube.com/watch?v=jLmAq0epfrI

fnordian_slip

https://www.reddit.com/r/F91Ws_on_NATOs/comments/f9udxl/the_... also deserves a mention here, and /r/f91w for more general stuff.

cbsks

For watch hackers, there is an alternate PCB with programmable microcontroller available for the F91W https://www.sensorwatch.net/

I got one for Christmas and it has been super fun to hack on. I programmed a new face for mine that displays the current tide level, and next high and low tides.

jgrahamc

Yes, it's great fun: https://blog.jgc.org/2022/10/pimping-my-casio-with-oddly-spe...

a-french-anon

Too bad it doesn't support the F105, aka "F91 with a usable light".

MetaWhirledPeas

Casio is phasing out electroluminescent backlighting and going back to a single LED, so I would buy an F105 while you still can. I've heard it said it was to improve battery life and longevity but I've never had a problem with either one.

noja

Where did you get the data for the tides?

cbsks

There’s an algorithm you can use to calculate future tides but it’s complex and I wasn’t sure it would even run fast enough on the watch. I gave up after a few hours and ended up generating an array of high/low tide levels and times for the next few years. NOAA has all the tide data you could possibly want, and an api to grab it.

https://tidesandcurrents.noaa.gov/tide_predictions.html

https://api.tidesandcurrents.noaa.gov/api/prod/

somewhatoff

Casio do have a tide watch, which presumably uses that algorithm:

https://www.greatwatches.co.uk/collections/men/products/casi...

jeffrallen

[delayed]

EVa5I7bHFq9mnYK

So the link to espionage is that a spy diver can dive to -5km wearing that watch and the watch will stay whole. ChatGPT can be really stupid sometimes.

mrweasel

It's actually two separate articles in one, but they had to merge them to make the content work for their oddly specific website.

millitzer

The second half of this article would make a great movie.

eadmund

Modified by oil-filling, though.

OliveMate

I've been stuck down the Casio modding rabbit hole as of late. I knew filling the watch with oil ('hydro-mod') lead to a crisper display with better viewing angles and increased water resistance, but to see a watch with minor splash resistance operate as such depths is insane.

Worth mentioning some drawbacks before you get your precision screwdrivers out. Doing it will make your watch get stupidly hot in the sun, the process can be messy, and sometimes certain mechanisms/features can break as a result of it. Best to check what others have done before you.

aurizon

These watches often have a quartz crystal - the little can would crush and the oil would damp oscillations, so they might have a laser trimmed RC loop - which would be cheaper as well as crush-proof?

ndiddy

Was disappointed that he only brought a modified oil filled watch to 5km underwater. Would have been interesting if he’d have strapped a stock watch next to it so we could see when it would break.

rhd

This video might be of interest to you-

https://www.youtube.com/watch?v=iOT8XU1ss3E (Do Oil-Filled (Hydro Mod) Watches Actually Dive Deeper?)

kali_00

Notably, there was no attempt to operate the watch at such depths. Pressing a side button would be an interesting test, for instance. Many "water resistant" watches, rated to a certain depth are only rated so, given the not inconsiderable caveat of not being able to operated - just looked at. The higher end, more expensive models claiming full waterproof ability don't typically have such functional restriction.

istultus

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