If a scuba cylinder lasts 60 minutes at the surface, how long will it last at 20 m/66 ft?

When considering how long a scuba cylinder will last at depth, it's important to understand how pressure affects the volume of air available to the diver. At sea level, the atmospheric pressure is 1 atmosphere (atm). As you descend, the pressure increases, and at a depth of 20 meters (66 feet), the pressure is approximately 3 atm—1 atm from the surface and 2 atm from the water.

The air consumption rate at depth increases due to this higher pressure, meaning that your air supply is being consumed at a faster rate. Specifically, because the air pressure is three times greater at 20 meters, you are effectively using air three times faster than you do at the surface. As a result, if a cylinder lasts 60 minutes at the surface, it will only last one-third of that time at 20 meters.

Dividing the surface duration of 60 minutes by the factor of three results in approximately 20 minutes of available air at depth. This calculation reflects the fact that deeper diving requires greater awareness of air supply due to the increased rate of consumption.