PADI Divemaster Certification Practice Exam

If a cylinder of air lasts 40 minutes at 20 metres, how long would it last at 10 metres?

1. 20 minutes

2. 40 minutes

3. 60 minutes

4. 80 minutes

The correct answer is: 60 minutes

The duration a cylinder of air lasts while diving is influenced by the ambient pressure at different depths. At 20 metres, the pressure experienced is approximately three times atmospheric pressure (two atmospheres from the water plus one atmosphere for the surface). If the cylinder lasts for 40 minutes under these conditions, it’s essential to consider how air consumption changes with depth. At 10 metres, the pressure increases to approximately two times the atmospheric pressure (one atmosphere from the surface plus one from the water), which means the diver consumes air at a reduced rate compared to the situation at 20 metres. To calculate the relationship between the two depths, you can use the principle that air consumption is proportional to pressure. Because the pressure at 10 metres is two-thirds of that at 20 metres, the air consumption will be correspondingly lower. Therefore, the air supply will last longer due to the decreased workload on the diver's breathing. If the cylinder lasts 40 minutes at 20 metres, it would logically last longer at 10 metres. By doubling the duration of the cylinder use at two-thirds the depth, the air supply is estimated to last approximately 60 minutes at 10 metres. This understanding of gas laws and pressure dynamics in diving contexts helps