PADI Divemaster Certification Practice Exam

If you fill an open bottomed lift bag halfway with air at 20 metres, what will happen as you ascend?

1. Air will escape the top of the bag

2. Air will bubble out the bottom of the bag

3. The bag will implode

4. The bag will remain unaffected

The correct answer is: Air will bubble out the bottom of the bag

When you fill an open-bottomed lift bag halfway with air at a depth of 20 meters, the behavior of the air inside the bag as you ascend is determined by the principles of buoyancy and pressure. As you ascend, the pressure surrounding the bag decreases. The air inside the bag, which is under higher pressure at 20 meters due to the water column above, will expand as the external pressure drops. Since the bottom of the bag is open, the expanding air will seek refuge in the path of least resistance, which is to escape out of the open bottom of the bag. This leads to the formation of bubbles that vent from the bottom since the lift bag cannot hold a fixed volume of air with fluctuating pressure. This is different from the other options provided. For instance, the bag does not implode because it is not sealed; rather, it is designed to allow air to escape. The bag remains unaffected in terms of its structure, but the dynamics of the air inside alter significantly during the ascent. Therefore, the correct answer reflects the principles of how gases behave under changing pressures, highlighting that air will bubble out the bottom of the bag as a result of ascending.