What is the absolute pressure at 20 meters/66 feet of salt water?

To determine the absolute pressure at a depth of 20 meters (66 feet) in salt water, it's essential to understand how pressure increases with depth underwater. For every 10 meters (approximately 33 feet) of salt water, the pressure increases by about 1 atmosphere (about 1 bar).

At sea level, the atmospheric pressure is approximately 1 bar (or 1 atmosphere). Therefore, when you descend to a depth of 20 meters, the pressure contributed by the water is about 2 additional bars (since 20 meters is twice 10 meters). So, you add the atmospheric pressure to the pressure due to the water:

1 bar (atmospheric pressure at the surface) + 2 bar (pressure from the 20 meters of water) = 3 bar (absolute pressure).

Thus, the absolute pressure at 20 meters of salt water is 3 bar (or 3 atmospheres). This is the reason the answer reflects 3 bar/ata, which encompasses both the atmospheric pressure and the pressure due to the depth of water above you. Understanding this principle allows divers to accurately assess the pressures they will experience at various depths.