Hacking the Casio F-91W to Handle 1000 Psi

https://dvt.name/2019/06/03/hacking-the-casio-f-91w-to-handle-1000-psi/

by dvt • 7 years ago

198 80 7 years ago

tenpaiyomi• 7 years ago

Very interesting. I'm intrigued to know if anybody with experience knows why there is a common recommendation of warming the mineral oil, versus room temperature which the author did.

michelpp• 7 years ago

No experience here. But my guess is warming the oil makes it less viscous and thus easier for small air bubbles to escape.

semi-extrinsic• 7 years ago

To be precise, warming makes the oil exponentially less viscous, so it can be a really pronounced effect by warming it 30 degrees (depending on the specific oil).

dvt6• 7 years ago

Author here. Would also love to know if any chemist/physicist knows the definitive answer to this :)

Also: why do some watches purposefully leave an air bubble in the watch? Is this an aesthetic choice or is there any benefit/detriment to it?

jacobush• 7 years ago

Maybe a de facto expansion reservoir?

lwhsiao• 7 years ago

My guess would be that in addition to the viscosity, it preemptively compensates for the thermal expansion of the oil, since on your wrist will likely be warmer than room temperature.

Here is another detailed liquid-filled watch guide [0], which talks more about handling thermal expansion, where rather than using an air bubble, he opts for some sort of foam ring. Another person recommends Flourinert with foam rather than mineral oil [1].

[0] https://www.christopherwardforum.com/viewtopic.php?f=5&t=496...

[1] https://forums.watchuseek.com/f21/liquid-filled-citizen-6540...

LeoPanthera• 7 years ago

I am frustrated that he didn't have a way to test it and so has no idea if it actually works.

travisjungroth• 7 years ago

I would bet it did. Here’s an F-91W hydro modded with olive oil taken to 3000 feet.

https://forum.tz-uk.com/showthread.php?330958-The-Little-Wat...

synthmeat• 7 years ago

I was just reading yesterday about another case too - https://forums.watchuseek.com/f2/my-oiled-casio-f-91-dives-d...

dvt6• 7 years ago

Fair point, I'll take it out next time I go diving and report back!

londons_explore2• 7 years ago

If you live near an ocean, just tie the watch to the end of 700m of string, and start lowering it over the side of a boat.

Pull it back again and see if it still works.

Obviously you need a deep ocean, so check a map first.

tapland• 7 years ago

You need a really heavy weight on there as well. Ocean streams and the surface moving will make it not point straight down at all.

TheSpiceIsLife2• 7 years ago

A deep ocean indeed.

You’re probably going to have to go off the continental shelf[1] to get 700m it greater depth.

This will make your plan somewhat less trivial.

1. https://en.wikipedia.org/wiki/Continental_shelf

iammiles• 7 years ago

Looks like a fun little project to do next weekend. Does anyone know where to get a strap like the one shown in the article?

w-ll• 7 years ago

It's just a NATO strap. Very cheap and functional for people that don't like hard watch straps. It seams the color combo is Navy/Crimson/Ivory

Found this on Amazon

https://www.amazon.com/BARTON-Watch-Bands-Crimson-Standard/d...

dingaling• 7 years ago

It's just a NATO strap

Which ironically aren't a NATO standard, but a UK MoD spec.

Pinckney• 7 years ago

Look for an 18mm Nato strap. It looks like Clockwork Synergy makes them with the nylon keeper, if that's important to you.

ctack• 7 years ago

I have one of these watches and really wanted, but decided not to mod it with such a strap as the metal buckles would potentially start to scratch my laptop.

chrisseaton2• 7 years ago

These straps are like ties - you get one in the colour of your school, university, club, or regiment.

ComputerGuru3• 7 years ago

I don't know if I would even remotely trust that not to leak. The same watch is notorious for not having a quality seal and quickly fogs up in the shower. I would rather trade some meters and use straight up petroleum jelly injected into the casing at a high pressure; I'd be much more comfortable wearing that (and, yes, it is technically a liquid even at "freezing" temperatures).

vortico• 7 years ago

Isn't 1000 psi almost a km deep? Not sure if this would work at all.

kijin• 7 years ago

1000 psi would be 700 meters under the water.

There are actual diving watches that are rated for 1km or more. The first image on the Wikipedia page for "diving watch" shows one rated for 3km. Rolex famously attached one of their prototypes to the exterior to the Trieste in 1960, and it survived the trip to the Challenge Deep.

[1] https://en.wikipedia.org/wiki/Diving_watch

Someone• 7 years ago

The petroleum jelly I know of is fairly opaque. Is there any other?

If not, even with a thin layer, I doubt the result will give you a readable display, as, judging from the red color of the display in the end result, there’s air between the glass and the LCD display in these watches.

ComputerGuru3• 7 years ago

Good point. Petroleum jellies can come in different clarities and with different refractive indices, here's an (expensive) version in the form of refractive index matched gels: https://www.ebay.com/itm/The-refractive-index-Matching-Gel-f... (this one is matched to optical fiber, i.e. very clear).

clouddrover• 7 years ago

The same watch is notorious for not having a quality seal and quickly fogs up in the shower

Casio says the F-91W is only splash resistant, so you shouldn't wear it in the shower:

https://www.casio-europe.com/euro/products/watches/collectio...

If you want one you can wear in the shower try the BD640W or similar:

https://www.casio-europe.com/euro/products/watches/vintage/b...

ComputerGuru3• 7 years ago

That's my point, though. If it can't keep water out, I wouldn't trust it to keep mineral oil in (even if it is more viscous and particles are larger).

jackpeterfletch• 7 years ago

Wait.

Why are people wearing watches in the shower?

ThatsOK• 7 years ago

I've been wearing an F91 for years and everyday I shower with it. Still functioning.

davidzweig• 7 years ago

Me too. Swimming down 10m or so in the sea fogged up the screen though.

pjc50• 7 years ago

How about "potting" it with clear epoxy, or is there some required internal movement?

dewyatt• 7 years ago

It's all digital from what I've seen. It seems like some clear epoxy would work well. Maybe some mineral oil after it cures, just to help fill little gaps (since epoxy tends to be pretty viscous).

EDIT: Although epoxy makes it a bit difficult to change the battery, depending on how you go about it. Maybe some conformal coating on the PCB + mineral oil.

hwillis• 7 years ago

Can do, although you might be better off using a hard conformal compound (black stuff used for potting electronics) where possible. It has a lower viscosity and is less prone to bubbles. I'm not sure how much small bubbles would actually affect it, though.

There's one other good reason to use conformal coating: it has very little change in size when it dries. There are a few spots on the board that would be affected by that, most importantly the load capacitors for the timekeeping fork. Ceramic capacitors have a piezoelectric effect and change shape ever so slightly (they stretch and squash, alternately contracting around their midsection and then expanding). It's not enough to break the solder or the epoxy but if you pot them it changes their apparent capacitance, which will change the frequency of timekeeping. Probably not enough to matter, but still measurable. Large inductors have similar problems and can eventually break loose from epoxy.

gnode2• 7 years ago

I think ideally, you want something which is non-conductive, incompressible, and solid but soft. A solid material will not flow and potentially leak through the seal. A soft material will not fracture, and transmit stress to the components of the watch. In a hard material, fractures might also appear when thermally shocked (such as getting in and out of the water).

I'd suggest a good candidate would be a silicone gel. It can be applied to the watch in liquid form, allowing all air to vacate, then cure into a soft solid once sealed in the watch. It could also be removed relatively easily, if you decided you don't want it in there any more.

pps43• 7 years ago

I did that with extra virgin olive oil a few years back. No leaks since then. I attribute it to higher oil viscosity.

duxup• 7 years ago

Well... trying to hack it.

No confirmation if this works.

gnopgnip• 7 years ago

Mineral oil breaks down latex quickly. What material is used for the seal? Oil expands and contracts with changing temperature. An air bubble will appear soon after if there are no special steps taken to prevent this.

The real advantage of an oil filled face is the greatly increased readability of the watch.

jhoechtl• 7 years ago

True. There is a Liquid you can put on top of eg marmalade which seals it from air. I think it is a from of liquid silicone (mind the final _e !) which I think is not aggressive to rubber.

sq_5• 7 years ago

Did a little research, and it seems like pure silicone oil is much less aggressive to rubber/latex than mineral oil (and you can get it from Amazon pretty cheaply).

Does anyone have any idea what the chemical differences between the two are that affect how they interact with rubber? I’d love to learn more.

swsh• 7 years ago

No idea what the stock seal would be made from.

Nitrile is commonly used in hydraulics as natural rubber doesn't play nice with hydrocarbons.

JamesCoyne• 7 years ago

I decided to switch to a reddish tint after my first failed attempt.

What happened with the first attempt?

dvt6• 7 years ago

The buttons were misaligned with the contact pads so I had to take it apart again.

WheelsAtLarge• 7 years ago

I wish someone would test if it's even water resistant for a period of time in a deep pool. I bet it is. I wish he would at least done the pool test.

If it's waterproof, it's a killer hack for its simplicity and overall cost.

tushar-r• 7 years ago

That watch looks amazing with the red tint and that strap! :-)

sq_5• 7 years ago

This is super cool! Kinda want to try it next weekend...

Seeing a lot of commenters wondering if the seals on the watch will manage to keep water out/oil in. Anyone know if it's possible to replace the o-rings or anything like that?

godzilla82• 7 years ago

The entire watch is 10$.

sq_5• 7 years ago

Yeah, I’m aware. I wouldn’t really care if the watch itself died, but leaking oil would be a bit annoying and making things that should fall apart last longer is often a fun challenge.

hoseja• 7 years ago

You could pull vacuum over the oil bath to positively coax all the air bubbles out of the watch.

Jerry22• 7 years ago

You could pull vacuum...

Unfortunately, that would also destroy the LCD and quite possibly the crystal oscillator (not sure how strong the metal can enclosure of those things are). Battery wouldn't survive it either but you could easily take it out and insert it after the vacuuming.

londons_explore2• 7 years ago

The metal can for the oscillator surely doesn't fill with oil, and would therefore crush at 1000 psi?

Filling this with mineral oil might actually reduce rather than increase its pressure withstanding ability.

_Microft2• 7 years ago

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.

The likely explanation is that heating lowers the surface tension [0] and viscosity of the oil so that it can creep easier between the parts. I also considered heating for a problem where a liquid had to enter very narrow cracks.

[0] https://en.wikipedia.org/wiki/E%C3%B6tv%C3%B6s_rule

oeuviz• 7 years ago

Could it also dissolve solved air contained within the oil, as it happens by boiling water?

_Microft2• 7 years ago

The liquid should be able to hold less gas at higher temperatures, yes.

What you observe with boiling water though is not solved gases leaving the water but actually water turning into steam. Steam is exactly the same stuff as water, just a lot less dense.

Actually if you increase temperature and pressure enough, there will be a point at which it makes no sense to differentiate between water and steam anymore [1]! Which is a fun thing: you can turn water into steam without boiling it. Raise the temperature and pressure enough while keeping it liquid, reduce temperature while keeping the pressure up and then reduce the pressure as far as you like, as long as it doesn't make the water condensate.

Voilà, steam without boiling!

What is happening there is taking a detour around the critical point instead of crossing the blue line (which is where water boils) in the following graphic[0]:

[0] https://en.wikipedia.org/wiki/Critical_point_(thermodynamics... [1] https://en.wikipedia.org/wiki/Critical_point_(thermodynamics...

Theodores• 7 years ago

How does the piezo buzzer get affected by this?

Is the alarm louder or quieter?

Is the tone any different?

I have no plans for deep sea diving any time soon, however, I do like getting up in the mornings. Would an oil filled Casio help me in this practical regard?

I also wonder about how fragrant the oil could be, if you used some oil based perfume, could the watch that is not waterproof be made to smell nice, if only slightly so, but mitigating the odour of bacteria that accumulate on the back of the watch when tightly worn?

dvt6• 7 years ago

The buzzer is definitely more "muted" but still beeps! I'm not sure if a fragrance would "keep." I used odorless mineral oil so I can't give much insight.

johnnycab• 7 years ago

I like the F91W because it refuses to die, keeps good time, has as an alarm and a hour minder. However, to repurpose it into some kind of frankenwatch to behave as a diving watch, is counter-intuitive to the fact that, it will lose most it's useful features, freely admitted to by the author in the article.

maltenuhn• 7 years ago

Before this became the de factor standard in tech dive computers, advanced divers would often do the same and fill their units with mineral oil.

For anyone interested in how this can go wrong, Raising the Dead - a book about scuba diving at its limits - has a passage about a rescue diver repairing his dive computer in the field, re-filling the oil, and noticing that he'd lost a drop or two in the process, with a resulting tiny air bubble...

gnode2• 7 years ago

I wonder whether any mechanical watches could cope with this technique (and maintain any semblance of accuracy).

sq_5• 7 years ago

I'd imagine that any mechanical watch in which the mechanism wasn't designed with the viscosity of oil in mind would be completely incapable of accuracy if one were to add oil.

jacknews• 7 years ago

What's the problem with the LED, does the liquid filling cause TIR off the watch-face/external air boundary? If that's not the problem, you could try side-mounting the led, as here: https://www.instructables.com/id/Watch-LED-Light-Mod/

Does the oil leak out of the buttons when they're pressed?

How about the buzzer? I think it's attached to the back of the watch so shouldn't be affected too much?

Someone mentioned improved readability. I assume underwater that is definitely the case, as you no longer get TIR reflections off the back of the watch-face/internal air boundary at steep angles, but how is readability affected in air?

mattmoose21• 7 years ago

Does anybody know what dyes he used?

Hacking the Casio F-91W to Handle 1000 Psi

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