PADI Divemaster Certification Practice Exam

How long will a cylinder of air last at the surface if it lasts 20 minutes at 20 metres?

1. 30 minutes

2. 40 minutes

3. 50 minutes

4. 60 minutes

The correct answer is: 60 minutes

To determine how long a cylinder of air will last at the surface compared to its duration at depth, we need to understand the concept of pressure and its relationship to air consumption while diving. When diving at a depth of 20 metres, the pressure increases significantly compared to the surface. The pressure at this depth is approximately three times greater than at the surface: one atmosphere at the surface plus an additional two atmospheres from the water depth. As a result, the diver's air consumption rate is also increased under this higher pressure. If a cylinder lasts for 20 minutes at a depth where the pressure is three atmospheres, it means the diver is using air at a rate that is three times higher than at the surface. To find out how long it will last at the surface, you can multiply the time it lasts at depth by the difference in pressure. Here, if the cylinder lasts for 20 minutes at 20 metres, at the surface (1 atmosphere), the duration would be calculated as follows: 20 minutes (time at depth) multiplied by 3 (to account for consumption rate decrease at surface) equals 60 minutes. This calculation demonstrates that the cylinder of air, when utilized at surface conditions, will last significantly longer because the diver is