
Hacking the Casio F-91W to Handle 1000 Psi - dvt
https://dvt.name/2019/06/03/hacking-the-casio-f-91w-to-handle-1000-psi/
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_Microft
_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](https://en.wikipedia.org/wiki/E%C3%B6tv%C3%B6s_rule)

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

~~~
_Microft
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...](https://en.wikipedia.org/wiki/Critical_point_\(thermodynamics\)#/media/File:Phase-
diag2.svg) [1]
[https://en.wikipedia.org/wiki/Critical_point_(thermodynamics...](https://en.wikipedia.org/wiki/Critical_point_\(thermodynamics\))

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ComputerGuru
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).

~~~
pjc50
How about "potting" it with clear epoxy, or is there some required internal
movement?

~~~
hwillis
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.

------
gnopgnip
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
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_
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.

~~~
ncmncm
Good point.

Mineral oil, contrary to the remark in TFA, is 100% organic. Unlike olive oil,
though, it's saturated. Does that matter? It's not exposed to air, so no.
_But_ olive oil goes cloudy when cold.

The watch is unlikely to have latex in it, but quite likely to have silicone
parts and seals that might soften in silicone oil.

~~~
dvt
> Mineral oil, contrary to the remark in TFA, is 100% organic.

Thanks for the correction! From what I was reading, olive oil goes rancid (and
acidic; but mineral oil does not).

~~~
ncmncm
Olive oil, like any unsaturated oil, may go rancid on exposure to air. But in
a sealed watch there will not be enough air.

("Organic", outside of agriculture, means "composed of carbon compounds". In
practice, that usually means compounded by something alive, because life has
been at the carbon for a long, long time.)

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

~~~
londons_explore
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.

~~~
TheSpiceIsLife
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](https://en.wikipedia.org/wiki/Continental_shelf)

~~~
lb1lf
-Off the shelf, or, if you happen to live in a country designed by Slartibartfast, find a fjord.

The view from my office window includes a fjord more than 500m deep; a couple
of hours drive to the south, you can get down to 1,300m/4,000ft or so a few
hundred meters from the shore.

Edit: Xylakant beat me to it!

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maltenuhn
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...

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Theodores
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?

~~~
dvt
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.

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WheelsAtLarge
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.

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

~~~
Jerry2
> _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_explore
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.

~~~
Jerry2
I have no idea how tightly those things are sealed. Given some of the watch
tests in this thread (the olive oil one), it surely can withstand the positive
pressure but I'm not certain it could withstand negative pressure. If oil got
in there, that oscillator would never keep the correct frequency again.

Hopefully someone does some tests.

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tenpaiyomi
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.

~~~
dvt
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
Maybe a de facto expansion reservoir?

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JamesCoyne
> _I decided to switch to a reddish tint after my first failed attempt._

What happened with the first attempt?

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

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jacknews
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/](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?

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iammiles
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
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...](https://www.amazon.com/BARTON-Watch-Bands-Crimson-
Standard/dp/B00ZO6GA8Q/)

~~~
dingaling
> It's just a NATO strap

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

~~~
chrisseaton
I don't think these even are NATO straps - they don't fold back on themselves
like the issue ones do, and these have an extra fabric flap that issued ones
don't.

This is a genuine 'NATO' issue strap:

[https://www.cwcwatch.com/collections/m-o-d-
spec/products/cwc...](https://www.cwcwatch.com/collections/m-o-d-
spec/products/cwc-nato-watch-strap-1)

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sq_
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
The entire watch is 10$.

~~~
sq_
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.

~~~
dvt
The o-ring is pretty flimsy (you can see it in a few pictures), but some have
done this mod and have worn their F-91Ws without any leakage for 5+ years, so
I expect it to be "good enough."

~~~
sq_
Good to know. Thanks for the info and for writing the original post about
this!

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

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

~~~
sq_
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.

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duxup
Well... trying to hack it.

No confirmation if this works.

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mattmoose21
Does anybody know what dyes he used?

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johnnycab
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.

