Ask an Instructor

As a course director, I get the honor of being one of the people that many in the industry turn to when they have questions about diving that no one else seems to know the answer to.  Usually, these questions are so obscure, unlikely or outlandish that they do not really need to be answered for the safety of other divers.  Sometimes, however, a student or novice diver will have a question that is insightful and thought provoking.  From now on, when one of these crosses my path, I am going to write about it here to share with other divers.

Several days ago, I was running around Key Largo when I got one of these questions via an instructor who talked to an instructor who had a student who had a question.  Did you follow that? 

The question was “When I breathe in, I go up… but the air from the tank just moved into my lungs, so why did it change my buoyancy?”  The air in the scblurry scubauba cylinder just changed locations into the diver’s lungs, so why would this increase the diver’s buoyancy?  

When the diver exhales, the air goes away, into the water and away from the diver, so it makes sense to become less buoyant then. 

Many of you may already know the answer to this, but simply answering the question is not the only reason I am taking the time to write this down.  I want everyone to think about the level that this brand new diver was actually thinking on.  I want to encourage everyone to teach classes that provoke this type of thought from your students.  When was the last time that a student asked a question that you had to stop and think about?  Did you give them an answer that satisfied their question?  Did you have to go find out the answer and get back to them?

The Answer:Balloons

The diver’s lungs are like balloons, when filled with air from the tank, they expand, increasing the volume of the diver’s air spaces, making the diver more buoyant.  A scuba cylinder however, is a nonflexible sealed container, so no matter how much air is pushed into the container, it does not substantially increase in volume.  Every breath from the container will only remove the weight of the air molecules that are breathed.  So there you go, the diver becomes more buoyant due to changes in the human body that occur with each breath, while the tank remains virtually unchanged.

If you have a question that you would like to have answered, please feel free to e mail it to me at


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