Waterproofing The Best Watch Ever Made

The Casio F-91W is probably the most popular wristwatch ever made. It’s been in production forever, it’s been worn by presidents, and according to US Army intelligence it is “the sign of al-Qaeda”. There’s a lot of history in this classic watch. That said, there is exactly one problem with this watch: it’s barely water resistant. [David] thought he had a solution to this problem, and it looks like he may have succeeded. This classic watch is now waterproof, down to 700 meters of depth. If you’re ever 700 meters underwater, you have bigger problems than a watch that isn’t waterproof.

The basic idea of this hack is to replace the air inside the watch with a liquid. This serves two purposes: first, the front glass won’t fog up. Second, liquids are generally incompressible, or at least only slightly compressible. By replacing the air in the watch with mineral oil, the watch is significantly more water resistant.

Filling a watch with mineral oil is done simply by disassembling the watch, submerging it in a dish of mineral oil, and carefully reassembling the watch. Does it work? Don’t know about this watch, but this was done to another classic Casio watch and tested to 1200 psi. That’s a kilometer underwater, and the watch still worked afterward. We’ll take that as a success, although again if you’re ever a kilometer underwater, you have bigger problems than a broken watch.

54 thoughts on “Waterproofing The Best Watch Ever Made

    1. I think you’re on to something. While filling the watch with mineral oil evenly distributes the pressure over the components any components with internal voids are still susceptible to being crushed. I would imagine the quartz crystal can would be one of the first such components to fail under any real pressure.

  1. there may be a problem with positive pressure when the watch heats up. like if you accidentally leave it in the car on a sunny day. the oil will expand and, being in compressible, will put a lot of pressure on the case in the opposite way to what it is designed to handle. my guess is the front ‘glass’ would eventually pop out but i don’t have a feeling for how hot the watch would need to get before that would happen. but its an awesome idea (its how my dive computer is waterproofed) .

    1. I bet it’s the opposite actually, the oil will not expand or contract with temperature but the case will.

      I suspect at high temperature the case would expand while the oil would not and it would want to suck air or water into the watch, and at low temp it would want to expel some oil out because the case will shrink while the oil will not shrink.

      1. As is the case with many fluids, mineral oil has a substantially higher coefficient of thermal expansion than the watch case. In fact, I have tried it in watch before, and the level of the oil noticeably changes between room and body temperature.

        1. Worst thing you can do with your oil-filled SCUBA gauges (besides letting them slam on the deck/groud really hard) is leave them facing up on a sunny day. The oil heats up and pushes its way out a vent, and you get an air bubble that you can never get rid of.

  2. 1. If it is not watertight, then eventually water is going to displace the mineral oil and ruin it.
    2. If this watch has an alarm, I would be surprised if it still works after filling the watch with mineral oil.

    1. It has gaskets that will keep the oil from oozing out when there is no pressure differential, but the case / buttons, etc are flimsy so without the oil supporting it, it leaks at a modest submersion depth.

      1. Does this mean the oil will come out at reduced atmospheric pressure? Commercial aircraft cabins are usually only pressurized to 8,000 feet. If the oil is installed in the watch at sea level…there might be an issue while flying. I can see it now…suddenly a flight attendant sees a thick, clear liquid leaking from your watch and alerts the captain who reports an emergency and diverts the flight to the nearest airport and lands. Then a swat team boards the plane and removes you.

          1. Water does not compress yet we can draw it up a drinking straw by lowering the pressure in our mouth. Would this not be the same? I think it would want to flow in the direction of the lower pressure…but, I am not sure about that.

          2. When you lower the air pressure in the straw, the air pressure on the surface of the liquid in the cup pushes the liquid up the straw into your mouth. You’re moving the liquid, not expanding it.

            What will be a problem is when the oil heats up it will expand and force itself past any seals in the watch. The watch will leak oil all over your arm and shirt.

  3. I’ve had one for a long time…gone swimming, showered multiple times with it and, without having anything done to it, nothing ever happened (dial didn’t freeze/fog up). It works just as well as did when new and that’s watertight enough for 99% of the world population I guess

  4. My main concern with this would be my sleeves. If it was never designed to keep water out, I imagine it’s not going to be exceptionally skilled at keeping oil in..

  5. Interesting. If the basic watch case is not waterproof it is likely to leak oil. Very likely to ooze at a higher altitude than when it was reassembled. I am still tempted to try this for my poor digital food thermometer that gets a surprise rinse but it has no gaskets in the first place.

  6. Have you ever heard the expression “like dissolves like”? That applies to oil and plastic/rubber, which is why you don’t use oil with condoms. I would expect that over time the mineral oil filled watch will fail, especially if it has any rubber bits inside it, like maybe an o-ring or seals in electronic components like caps, crystals, or even the LCD.

    In the second linked article, the author used silicone oil to fill the watch. I’d expect that to last longer than a watch filled with mineral oil. Silicone oil is used to lubricate plastics/rubber because it doesn’t dissolve them.

    1. Not sure about silicone oil not attacking rubber. I used silicone grease on pool filter O rings, and they broke down pretty rapidly. Much faster than not lubricating them at all.

  7. well without any “improvements”, this watch (speaking of the genuine as there is lots of copies) could withstand dives down to 30-35m, repeatedly without any harm … so saying it is not waterproof or tight … maybe it is not certified (hence the WR rating) it still performed very well … (the problem becomes that deep down there it is a little bit harder to read the small display) … still for sub 20e it’s not bad

  8. “…and according to US Army intelligence it is ‘the sign of al-Qaeda’.”

    What does this mean?

    Anyhow, wouldn’t the oil leak out? Also, I take it that when you replace the battery you’d have to do it with the watch submerged in oil?

  9. I filled one of these watches with olive oil, about 6 years ago. My son wore it for the entire period until the battery expired some months ago. I have since replaced the battery and the oil (extra virgin :) and it’s working well. The only drawbacks: the display has a slight greenish tint and the alarm is a little muffled.

    1. I have a Casio Wavecepter from 2006 that I paid $20, some great deal. It syncs up every night, even though I’m in Montreal, and is solar charger so I’ve never changed the battery. I remember changing the battery on a previous Casio, and it was never as water tight.

      So to me, this Wavecepter is the best CSio.

  10. My Casio F-91W regularly goes scuba diving with me to a depth of 30m. It’s gone to 18,000 feet above sea level and goes in the shower everyday. No leaks or fogging – I can’t imagine anyone who needs this watch to be more waterproof.

  11. Wow good to finally know the original :-) I just examined the cheapest watch I got on ebay some time ago and now I see it is an exact clone of this!

    I was actually searching for those 4 button 7 melody watches I remember as a kid and this was nearest one visually. Too bad those 7 melody ones are nowhere to be found as new. They had names like Kessel or Montana but was otherwise quite similar to this casio one. Why those are not worth to be cloned by chinese?

  12. for me when I had these, it was where the small spring loaded pins for the bands clipped in would break out. one good bicycle wipeout or falling from a tree and it was new watch time. ca’s wouldnt stick or hold well enough and the replacement price at the time was about the cost of a whole tube of epoxy.

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