Water resistance ratings vary between brands and models, typically expressed in meters (m), bars (bar), or atmospheres (atm or ata). Although scientific measurements differentiate between atm and ata, for standard diving at sea level, atm and ata can be treated the same (differences appear mainly in high-altitude diving).
Pressure Equivalents: 1 bar ≈ 0.987 atm ≈ 14.5 psi ≈ 10 meters in seawater ≈ 1.02 kg/cm²
1 atm ≈ 1.01 bar ≈ 14.7 psi ≈ 33 ft in seawater ≈ 1.03 kg/cm²
1 meter in freshwater ≈ 0.97 bar ≈ 0.445 psi
1 foot in seawater ≈ 0.445 psi
Generally, 1 atm ≈ 1 bar ≈ 1 ata ≈ 10 meters, but this approximation may lead to small errors.
“My Watch Supports 3 ATM, So It’s Safe to Dive to 30 Meters!”
If you’re into diving—say, at the PADI Advanced Open Water (AOW) certification level—you know dives often reach 30 meters. Here’s a hint: those trained in Enriched Air (Nitrox) specialty, or at Divemaster or Technical levels, may notice something off about the claim above.
And you’re right.
At sea level, we’re already under 1 ata of pressure. Every additional 10 meters depth adds 1 ata. So, at 10 meters, you’re under 2 ata of pressure, at 20 meters, it’s 3 ata, and so on. Therefore, a watch rated for 3 ata isn’t actually safe at 30 meters; it reaches its limit around 20 meters.
For watches rated in ata or atm, take note: what’s specified as a water resistance rating might not reflect safe diving depth directly.
A recreational diver doesn’t dive beyond 40 meters, so any watch rated for 5 ATM or above works perfectly fine for them!
PS: As I often say in my posts on water resistance, your watch’s durability is only as good as its maintenance. Regular check-ups are essential for a long and enjoyable watch experience.
