Taking a $15 Casio F91W 5km underwater | Hacker News Reader

Taking a $15 Casio F91W 5km underwater

watchesofespionage.com

359 pointsnnnnico91 comments

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.

Where did you get the data for the tides?

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/

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

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

And this approach actually works for inland waters since NOAA predictions are accounting for geographic effects. (e.g. the tide in Seattle is drastically different in timing and magnitude from the Pacific Coast)

NOAA publishes the harmonic coefficients for each tide station, so if you hard code those constants into your program you should be able to generate accurate tide predictions. I looked into it a bunch before it started to feel less fun and more like “real work”.

https://tidesandcurrents.noaa.gov/about_harmonic_constituent...

Did you publish your simple solution anywhere?

Not yet. I’ll clean it up someday. But not today.

Looking forward! I hope to work on something similar soon too, the API solution seems helpful.

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

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.

If they update F91W to include better backlight, then good riddance. Their newer models (like A700W) have single-LED backlight, and it's excellent (probably better than EL, but I haven't used EL in a while). The watch is very thin too, unlike EL models.

My son has the newer LED model, and the EL looks better. The EL watch is equally thin too.

Before EL they were using incandescent bulbs. So they're going forward to LED.

Ahh! So there is some improvement at least.

I fixed this on my A158 (basically a silver F91) by replacing the backlight spreader. Super simple mod and only costs about $12. Granted that's close to the cost of the watch itself.

https://www.etsy.com/listing/1448973768/back-light-spreader-...

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

Wow I'm impressed at the developer experience. A reset button, flash with `make install`, woa. I've programmed plenty pro embedded systems that had substantially worse support. Hacky flasher apps, messing around with the power source, definitely no USB connector (on such a small board even). Super cool!

There’s even an emulator that is super helpful when designing new complications https://www.sensorwatch.net/docs/firmware/simulate/focus/

The firmware code quality is good as well. Well documented and easy to extend. It’s clearly a labor of love.

At this point tbf I think this protocol (UF2) that MS collaborated on is the easiest I've seen: https://makecode.com/blog/one-chip-to-flash-them-all.

Just drag and drop, or detect the specific mass storage, or save to location from browser. Super easy.

Which, after checking that Casio mod site...is exactly what they use! So never mind ha ha. But the blog post on UF2 is certainly worth a read; it's just so simple, it's brilliant.

As I read the article this is exactly what I wished the watch could do, and I wondered if there are any around which do it.

I have a free diving watch and always wondered… Why doesn’t it support tides? If even approximately? I wrote a script to check DFO tides once per week and alert me to potentially good diving conditions (cross referenced with the 2 week weather forecast; it’s not super reliable), but I’d love to have a read out of the tide right on my watch.

I guess I could do this with my Apple Watch, but I’m so burned out on that ecosystem.

Casio has a few watches with tide functionality in their G-Shock line. Bit more expensive than the F91W, but still great watches. They’re marketed to surfers, often under the G-LIDE name. Here’s a thread about some of them: https://www.reddit.com/r/gshock/comments/18jpgq5/casio_gshoc...

Garmin Descent series watches have free diving activity profiles and support tides. It kind of "cheats" by downloading tide data from Garmin servers via a Bluetooth connection to your smartphone so it's not doing any tide calculations on the device.

https://www.garmin.com/en-US/p/766516/pn/010-02604-01#specs

I didn't realize you can get these in this price range. Thanks for the heads up!

Ironically, something Casio have been struggled with on their fancies G-SHOCK GBX-100 few years ago. Tides were based on database and not cycles and they were always wrong (contrary to the older, more basic, not connected, tides model). I’m a fan of tides G-Shock, I got one and sold it fast unfortunately… maybe the only time Casio disappointed me.

https://www.watchuseek.com/threads/need-help-gbx-100-tide-gr...

Yeah I own this watch and it is infuriatingly almost accurate for my location, but drifts over the course of the moon cycle. I now just use it as indicative but check other sources if I want the exact high and low tide times. It definitely helps solve the main thing I wanted it for though: glanceable sunrise/set, moon cycle and tide strength (as well as approximate highs and low tide).

The app is kind of clunky and I have had issues with the live GPS tracking of activities but as they're not the main reasons I got it, I don't mind too much.

The author is working on a new version with more features: https://www.crowdsupply.com/oddly-specific-objects/sensor-wa...

I’m excited for the replacement LCD. The stock display has a bunch of restrictions on which segments can be active at once https://www.sensorwatch.net/docs/wig/display/

I found one of these while free diving. Wiped it off and wore it for several years, until I lost it while free diving.

So did I!

Why are these so easy to lose?

*share

The straps tend to break after a few years. Casio G Shock straps last a lot longer, despite being very similar. Not sure if it is because the F91W strap is thinner or made of a less durable material.

The circle is complete

Then something happened which the Casio F91W did not intend. It was found by the most unlikely creature imaginable.

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

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

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

FWIW, a correction is required here. Oil-modding a F91W will NOT make it a dive watch replacement as the video creator claims several times.

A dive watch is designed to be operated at that depth while the weak o-rings around the pushers on the F91W will give up when you use the buttons. Oil modding on it holds up during a dive as long as you'll never push the buttons, so it's more of a neat party trick for show than anything to daily drive.

FYI Casio recently brought out a minimalist series of the F-91W (same watch - just a bit less chrome on the face) e.g. https://www.casio.com/europe/watches/casio/product.F-91WB-1A...

If there is one thing the original is, it’s too flashy. :)

Kidding aside, that looks pretty neat.

Ooh, these are beautiful:

Blue: https://www.creationwatches.com/products/casio-digital-350/c...

Grey: https://www.creationwatches.com/products/casio-digital-350/c...

Black: https://www.amazon.com/Casio-Environmentally-Friendly-Bio-Ba...

White: https://www.amazon.com/Casio-Environmentally-Friendly-Bio-Ba...

Wow, love this. I of course love the classic color lettering (which really is quite tasteful) but I'm going to cop a "minimalist" black style for more formal occasions!

Modified by oil-filling, though.

Yeah but.. it’s just the way of gases and liquids under pressure. Even if you could sustain the pressure with gas it would be an unnecessary implosion risk if it’s pierced. As long as it still functions fully including on the surface, I wouldn’t qualify that as cheating. More like us biological weaklings who need ~1atm can be cheap and skip the liquid/resin because if we accidentally end up in space or the deep sea we generally have bigger problems than checking the time.

easy solution: fill yourself with oil before diving. checkmate, nation-state navies

As depicted in the move "The Abyss". https://en.wikipedia.org/wiki/Liquid_breathing

Also the aliens in the Sylvia & Gerry Anderson TV series "UFO".

I've read that it's possible to breathe oxygenated liquid perfluorocarbons, but something about the idea is just terrifying to me. I think it's the "fluoro" bit specifically that scares me, even moreso than the "liquid" part.

Humans can't really "breathe" oxygenated liquid. Our diaphragms aren't strong enough to move sufficient liquid in and out of the lungs, so it can only work with external mechanical ventilation. This is occasionally used as a salvage therapy for patients hospitalized in critical care but is totally impractical and unsafe for any sort of diving.

In the real world outside of sci-fi movies, any human diving much deeper than about 0.5km will have to be done in an an atmospheric diving suit.

Based on a video of a mouse I saw, I'd describe perfluorocarbon breathing more as you can drown in it and not die.. it definitely did not look pleasant.

I think it would have been interesting to send two down, one oil-filled and one not and see at what depths they break (or don't). The watches are cheap enough that destroying one isn't much of a loss.

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

It breaks

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.

Why does it heat up in the sun?

My guess: higher thermal mass, so over time it can accumulate more heat than a non-filled watch.

I expect it has more to do with thermal conductivity

The back will get hot since the oil improves the heat transfer from the front to the back. The sun will always heat the front, but as long as the heat transfer rate to the back is low enough it won't feel hot - your body will absorb the heat and reach an equilibrium temperature which feels natural.

I just a fitness band in a f91w or w59 body.

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?

I own legendary GW-5000U. It is amazing to see those cheaper alternatives are as good as 5000U.

I'm wondering is there any other brend except Casio that has watches as amazing as those are.

I think Garmin is doing pretty interesting outdoors models, however I prefer Casio due to simplicity and... nostalgia

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.

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

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

I might be reading into it, but there seems to be a bit of a condescending tone with this comment.

The "oddly specific" website has 191K followers on Instagram[1] and has done interviews with Hodinkee, one of the most well known 21st century watch magazine/blogs. It is not that different from others that hit the first page here on HN.

[1] https://www.instagram.com/watchesofespionage/

You reading a bit to much into it. I did check out their content before commenting, there's also three previous links shared on HN. I was just amazed that they are able to have enough content for something that seems extremely specific.

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.

This video might be of interest to you-

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

A very similar model (A158 — they differ only in the bracelet) breaks at 200 m:

https://youtu.be/G3iMkeF8qmA

https://youtu.be/sep5Tw-55yw

The timekeeping mechanism keeps working fine, though, it's only the display that's busted.

Yep would have loved more on when the watch (unmodified) would actually break and also how you would fill it with oil? There can't be much space inside, at what point does the viscosity of the oil matter? how do you know you've got all the air out?

I'm assuming they are using mineral oil; I've not filled a watch with mineral oil, but I have worked with it. Mineral oil is not particularly viscous; some gentle tapping is probably enough to get all the bubbles out.

Here's a video of a PC immersed in mineral oil with an aquarium bubbler and you can see the bubbles rise fairly quickly:

https://www.youtube.com/watch?v=tUBvWXH1hLs&t=110s

I had a beloved F91W that I used for years. One day, when surfing in Morocco, it gave out and the screen became foggy before showing me an error code and never recovering. I guess I should have gone deeper.

This model doesn't seem to display error codes for anything. I think their failure mode is just to "not work properly anymore".

It's amazing how often this comes up on HN... I'll have to get one.

https://hn.algolia.com/?q=f91w

Is there any reason a watch without moving parts filled without any gas pockets _would_ stop working?

Don't get me wrong, it's a nice demonstration but kinda expected outcome

This or Pebble? What is HNs heartfelt favourite?

> Incredibly, the F91W survives its journey to an official 4,950 meters—an astonishing 16,240 feet—and back.

Findings.

> 4,950 meters under the surface, the pressure is approximately 7,227 pounds per square inch, which is well over three tons pressing on the watch. For context, that’s a Dodge Ram 1500 or a young adult hippopotamus parked on every inch of your F91W. As Americans, we’ll do anything to avoid the metric system, but using scientific terminology, we’re talking about a shitload of pressure.

Appreciate the joke.

The new abl-100, besides more wearable size could have a nice tinkering potential

Is that a real CIA challenge coin? It has what look like strange imperfections.

Looks like it. The imperfections are just glare from the lighting.

I modded mine with olive oil when I bought it. Pretty indestructible.

Awe the Seal kiss, 20,000 Leagues vibe

Any hacks to fix the useless light?

Buy a W-86-1VQES instead.

There is one, but requires another Casio watch to borrow the backlight from.

https://youtu.be/9-jd_7eXACU

n-o-d-e has what he calls a data runner mod which includes simply swapping the stock green light with a brighter white one. I’ve had one for years now, and its still not ideal, but definitely better than the original:

https://n-o-d-e.net/datarunner.html

There are some diffuser mods available too that look like they would make a big difference.

You can buy replacement backlight spreaders off of etsy. I did it to mine and the backlight is now usable..

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.

It seems like actually pressing a watch button at that depth would be quite a feat of precision engineering itself. Are ROV arms typically that precise that it would be possible to see well enough and finely enough control the arm to press the button?

Not sure if the buttons function, but the watch is displaying time in the photos, for 50 minutes of the descent at least.

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