It’s okay to shower with your dive watch.

ESP 01 + BMP280 Dead bug

There’s a fable I once heard on a forum…Reddit – or was it YouTube? – about not showering with your watch. Obviously we’re talking about a water “resistant” watch – with a “dive rating” of 30 meters or more. It goes something like: “The water pressure from the shower falling a few feet is enough to push past the seals.” Similar, I’ve also heard that while swimming: “The pressure created when you swing your arms under the water can also push a watch past its water resistant rating.” This sounds like it might have some truth to it, but we have to get a better idea of the forces acting on (and inside) your watch. Forty years ago manufacturers could say “water proof” on the dial, but that implies that water (or atmosphere) will never get into the case. Nowadays, watches have synthetic seals, are pressure tested at the factory, and are sporting ever higher depth ratings. Yet, to not mislead customers, can only say “Water Resistant”. BUT REALLY?! A 300m dive watch can’t handle a shower? Is this a myth? What’s really going on inside?

About a month ago I was working with an ESP-01 (ESP8266) board on another project and it was on my work bench next to a watch I was working on. At only 25mm long, I thought: “I wonder if this could fit into a watch case? I wonder if they make a really small pressure sensor?” A google search later and I had a BMP280 breakout from Adafruit on its way to me. This is a super accurate barometric pressure sensor that measures in Pascals. It is literally designed to help GPS calculate altitude. For reference 6,985 Pascal is equal to 1.0 Pound (of force) per Square Inch (psi) OR 1.0 lbs/inΒ².

Can you calculate altitude with a pressure sensor? Yep! Our atmosphere doesn’t have a steady density everywhere on the planet. It changes with the weather, temperature, and how high above the ground you are relative to sea level. If you go up in altitude, there is less pressure. If you know the pressure at sea level, and the temperature, you can accurately tell your elevation. In my testing, the atmosphere at my house registered around 14.4 psi – can you figure out my elevation? Here’s a handy calculator for those wanting to learn even more about atmospheric pressure. Even with an less precise measurement like psi – you can calculate I’m around 570 feet above sea level.

Powering the ESP-01: While possible with some expensive super capacitors, the WiFi module draws too much amperage to start up with a coin cell battery. If I wanted some energy density, I was going to need a tiny LiPo battery. Adafruit to the rescue again with this 150mAh (19.75mm x 26.02mm x 3.8mm) Lithium Ion Polymer Battery.

My first build was hasty. After testing I knew I needed pull up resistors on everything to get I2C working. The 1/2 watt resistors I had were just too big to get the device into a watch case. I prototyped it all on a bread board before ordering some SMD resistors. I “dead bug” soldered 4x 3.3k/ohm resistors and my two 330 ohm 1/4 watt resistors had enough space hanging off one side of the board and sensor. For reference SCK = SCL AND SDI = SDA when looking at the BMP280.

Now that I had all the parts, the battery on top of the sensor, on top of the ESP-01 the height was pushing close to 9mm. I was initially trying to squeeze this into a Seiko SKX007 dive watch case, but even with a domed sapphire I only had around 7.5mm in height available. “We’re going to need a bigger boatΒ  -err submarine.” Not to harp on a cliche’ stereotype, but the first manufacturer of giant dive watches that came to mind was Invicta. Don’t worry, I didn’t gut one, I found a “Grand Diver” at 46mm with no movement on eBay for $25. πŸ™‚ Even with an external thickness of 16mm there’s only about 10mm to work with inside. The back tapers, and the crystals* are about 2mm thick each. (*Bonus diplay caseback! πŸ˜€ ) This watch lists a 300m (1000ft) dive rating. Meaning, you could slip past 60m of depth as a recreational diver and fall prey to hypoxia before water would ever push past the seals of this watch, let alone crack the crystal. πŸ˜‰

Ready to dive Captain!

It was at this point I was glad this sensor package included temperature. ACROBOTIC’s video on getting sensor data from the ESP8266 and his corresponding websockets Arduino code available on GitHub made the software side (my least favorite part) pretty easy. After digging into chart.js I was actually able to add a second line graph to report temp and pressure. Testing js code INSIDE the Arduino IDE is pretty impossible, so I used jsfiddle.net to work out any syntax [programmer errors] before copying it to the IDE and flashing it to the ESP8266.

Let’s get testing!

  • First big surprise: Closing the caseback with the crown in, increased the pressure inside the case as much as 0.3 psi! That caseback and the gasket worked as a diaphragm when threaded closed.
  • Nice surprise: With this setup, I was able to detect when the crown was threaded in. This tiny 3mm cylinder of air, pushed in a few mm, was able to slightly raise the pressure in the watch.
  • How Accurate? In one instance, when pressure and temp were steady, I was able to detect that my Air Conditioning had come on. Pressurizing the air in the room slightly and thus inside the case with the crown out.
  • My tests were all done around 70F degrees ambient temp, and 14.4 psi of atmospheric pressure. The sensor measures in Celsius and Pascals – which were converted into sloppier “North American” measurements. Still accurate but less precise. 😐
  • Why precision doesn’t matter here: It takes a LOT of force to blow out a crystal or squeeze past a seal. 100 Pa won’t make a difference, 1 psi might (6.9 kPa). These are temp and pressure measurements that Americans use every day, so it’s easier to understand the effects. Consider 100m of depth is equal to 142psi and 300m of depth is a whopping 427psi.

Oh yeah, temperature affects pressure.

When I first got this prototype together, I didn’t have the temp sensor reporting. I noticed immediately that, just sitting on the desk, the pressure was rising in the case. First a few hundredths of a psi, and then almost half a psi (0.5) in a few minutes. It was increasing continuously. The only explanation for this must be that the wifi package with cpu and battery was getting hot. The only way to show this, was to report temperature along with pressure. Sure enough, my hypothesis was correct; as the electronics warmed up, the pressure went up.

Shockingly cold!


Plunging the watch with sensor package into a cup of ice water yielded some interesting results. Right off the bat the pressure plummeted, and in 32F (0C) ice water, the pressure reduced to as low as 13psi. As low pressure here is a vacuum; -1.4 psi of pressure was now sucking on those seals from inside the case. Outside temperature changes make a big difference on the pressure recorded inside the watch. Warming the watch up to 100F increased the internal pressure by as much as 3psi!

Recreational Diving

This is a pressure vessel that will safely hold 100psi of air pressure. It will help us to simulate 70m of depth into the ocean. For those interested, here is the depth calculation. 70m is 10m past where recreational divers would go, so this is a decent real world test of watch water resistance.

Pressurized, only a 0.3psi change is recorded inside! Yes, 100psi is attempting to sink into the watch, but the seals; crystals; crown; and watch case are preventing this from happening. The seals and crystals are squeezing in ever so slightly, resulting in the 0.3psi increase in pressure inside the watch. This is exactly how “dry” pressure testers work. Check out this Hodinkee article – Under Pressure: A Look At Rolex Water Resistance Testing

“The dome is sealed shut and air pressure inside the chamber is increased to the desired level, up to 10 bar, which is equivalent to about 100 meters of water pressure. As the air pressure increases, any leaks in the case will allow air to infiltrate the watch itself and deflect the crystal upwards slightly. The probe that is resting on the crystal detects this deflection and transmits a digital readout on the front of the machine, both in micrometers of deflection and a simple β€œPass” or β€œFail” verdict based on set criteria that differ depending on crystal type.”

Time for a shower…

Close enough to strip paint…

This is my gas powered 2800psi pressure washer way too close to a 300m dive watch. I’m using the white nozzle here to simulate a proper soak. I was only able to change the pressure inside the watch 0.14psi or about half of our 70m dive of 0.3psi. If water was somehow able to seep into the case we would have seen a much higher change in pressure, and the pressure wouldn’t revert back to where it was when we stopped hitting it with the spray. If taking more than 2000psi of shower to the case is like diving 35m, then I’m sure that your 50psi home shower will not hurt your dive watch. 😐

I tried to simulate pressure from taking an actual shower, and pressure from swinging my arm in a tub of shallow water – but nothing registered. I was excited to film some footage at the lake, but besides temperature, I couldn’t detect a pressure change at all. For reference here; 1m (3ft) of water is only 1.42psi, which is a lot less than 2000. πŸ˜‰

If your seals are bad, your watch isn’t water proof… Don’t blame the shower.

Just as 100psi is kept out of the watch, -1.4psi to 3.0psi is kept inside. If your seals are bad, it will be catastrophic to jump into freezing cold water. The pressure inside the watch will fall with temperature, and actually suck water in.

I would argue that taking a shower with a watch that has bad seals is actually a tiny bit better than jumping into a cold pool. The watch warms up slightly with your hot shower and air is pushed out. Until it isn’t…

If you find yourself in this panic situation (wearing a questionably sealed vintage watch in the shower by accident). Immediately take the watch off and dry it thoroughly. Then place it in another room where it’s cool and dry. As I showed, when the watch cools off from the shower, the pressure inside will reduce and could suck in atmosphere. You don’t want this to happen in the 90% humidity from the bathroom.

But I take cold showers, and I like to ski.

Arguably problematic is opening your watch up (unscrewing your crown) at altitude. If you’re at 10.5psi of atmosphere (9,000ft) and you head back to sea level (assuming temp on your wrist remains the same) you could have a -4psi vacuum inside the case now. Jumping into the cold shower could reduce that even more. Is that the pressure combination that pushes the seals past their limit? This now seems much more plausible than “the water hitting the case did it.”

  • Myth busted. Go back to showering with your dive watch if you want.
  • Make sure your crown is screwed in all the way. Check it before water activities.
  • Probably should avoid setting the time in extreme environments.
  • These are purpose built tool watches. The moment you second guess their ability, they become jewelry. πŸ™
  • Have a watch older than 10 years that would be a shame if water got into it? Get the seals replaced and get it pressure tested.

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