Measuring Lung Volume

I think you could still dive well without arms, sight, or even missing a leg.  But everyone agrees, you definitely want your lungs working optimally.  You'll frequently see questions on forums regarding increasing lung capacity.  To what extent you can do this, and whether it's necessary are both debatable (and we will debate them in future articles).  But in order to know if you've increased lung volume, you must first know what your lung volume is.
Let's lead off by learning the language of the lungs.  A few terms need to be understood.  First is "vital capacity."  Take a big breath.  Now exhale all the air you possibly can.  The volume of air you just exhaled is your "vital capacity."  Try as might, you didn't actually empty your lungs.  A small amount of air called your "residual volume" was still left over.  Combine these two measures (vital capacity and residual volume) and you have your "total lung capacity."  The average adult has a total lung capacity (TLC) of somewhere around 5.5L, made up of about 80% vital capacity and 20% residual volume. But what's yours?

The best way to measure your lung volume would be a spirometer, which is a medical device that none of us have.  What you probably do have, or can get for a couple bucks, are a clear plastic container larger than your lungs, a larger container filled with water, and a plastic tube to make a water displacement apparatus.  Calibrate the clear plastic container by dumping water in a liter at a time and making a mark.  Then, put this plastic box up-side-down in a larger container and fill with water (making sure no air is trapped in your up-side-down box).  Slip one opening of the tube into the up-side-down container, take a big breath, and exhale fully into the other end of the tube.  The contents of your lungs are now trapped in the box you have marked volumes on.  Make sure it's floating level and take a reading- that's your vital capacity.
An even easier method of measuring your vital capacity is to measure the circumference of a balloon you have exhaled into.  Unfortunately, this method is fraught with even more inaccuracies than the previous one.  Generally balloons are not exactly round, and therefore a volume cannot be extrapolated from their circumference.  I have taken the average of multiple circumferences of a single balloon to calculate a volume, and it seemed to compare similarly to volumes measured with the water displacement apparatus.  I also think it's a little harder to blow into a balloon than a plastic tube with little resistance.  Air can also be lost when tying the knot.  Nonetheless, I guess it's still a valid method for it's simplicity.
Residual volumes are a little more difficult to measure.  The simplest way is probably to use figures published in medical journals.  Divide your vital capacity by 4, and that's a good estimation of your residual volume.  I would like to try exhaling all my air at 10m below the surface, leaving me with only my residual volume.  Upon ascending, this volume will double, leaving me with a volume of air available for exhalation (a de facto vital capacity of sorts) equal to my residual volume.  I could then measure it with the displacement apparatus or a balloon.  In fact, I will do that soon.  Then we'll be able to see how it compares with the simple division method, which is all I've attempted so far.
So how does this all pertain to freediving?  Once again take a deep breath.  Now gently exhale, relax, and wait until next week for part 2 of this article.

Don't forget to check out Deeper Perspectives- Episode 5