How long can an open circuit diver stay underwater with a cylinder? The amount of time depends on a diver's breathing rate per minute (respiratory minute volume, or RMV) at depth. This is always converted to surface air consumption (SAC).
You will learn how to calculate your personal air consumption rate and how to use that information to estimate how much air you’ll need for a planned dive in the “Management” unit.
When diving open circuit, a regulator supplies air that is equal to or very slightly higher than the surrounding pressure. The deeper you go, the more air you will consume per breath. For example, at 20 meters/66 feet/3 bar, you will breathe three times as much to fill your lungs to their normal volume. Therefore, your air supply lasts only one-third as long at 20 meters/66 feet/3 bar as at the surface. Simply put, if a cylinder lasts 60 minutes on the surface, it will only last 20 minutes at 20 meters/66 feet/3 bar. And in reality, it may last even less time, because at depth you may be a little more excited and breathing a little faster!
The “Management” unit explains this in detail and will explain how to work out the duration of your scuba cylinder for a given dive profile. It’s an easy and straightforward process that will help to prevent you from the stress of unexpectedly running out of gas at depth.