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Buoyancy Control- It’s An Inside Job

Diving Medicine

By Dr. David Sawatzky

In the last column I looked at diving fatalities and noted that poor buoyancy control was a significant factor.  Buoyancy control is by far the most difficult skill to master in diving and the easiest one to lose.  Therefore, in this column I will focus on buoyancy control (it nearly killed me twice and did kill a friend of mine) and some of the tricks I have learned over the years as a cave/rebreather instructor and cold water diver.

Neutral buoyancy is achieved by altering your volume so that you displace exactly the volume of water required to equal the weight of you and all your equipment (Archimedes principle).  If the volume of water you displace is less than the weight of you and all your gear, you will sink.  The force pulling you down into the water is exactly equal to the difference in your weight and the weight of the water you displace.

Most people float.  This means that they displace more water than they weigh.  Most of our bodies are water but some parts are heavier than water (bones, teeth, muscles, etc.) while other parts are lighter (the air in our lungs, sinuses, ears, stomach and intestines, and fat).  Fat is the most important factor because the amount of it in our bodies is so incredibly variable.  Simply, thin people sink while fat people float.  The average person needs to add two to six pounds (1-3kg) to be neutral in a swimming pool.

If a person is exactly neutral in a swimming pool, they will float in the ocean.  The reason is that they will displace exactly the same volume of water but salt water is approximately 3 per cent heavier than fresh water, and therefore salt water generates a greater buoyant force than fresh water.  If you displace 175 pounds (80kg) of fresh water, you will have to add approximately six pounds (three kg) to your weight belt to be neutral in the ocean.

The next factor is that the volume of water we displace changes as we change depth.  Neoprene compresses as we increase our depth and therefore displaces less water and we become less buoyant.  This change depends on the thickness of the neoprene and how much neoprene we are wearing.  Most of the change will occur relatively near the surface and it will be slightly different for different types of neoprene.  Remember that total pressure doubles when we go from the surface to 33 feet depth (34 feet fresh water) or 10 meters.  It doubles again when we go from 33 fsw (10 msw) to 100 fsw (30 msw).  It’s the gas bubbles in the neoprene (the air pockets) that compress, not the ‘rubber’, which not only don’t compress, but resist compression.  As a result, the compression of neoprene is less than predicted by Boyle’s Law (P1V1=P2V2).  For divers this change ranges from near zero for a thin neoprene suit worn in warm water to several pounds for a suit designed for use in extremely cold water.  The diver simply adds air to their buoyancy compensator as they descend and removes it as they ascend to maintain neutral buoyancy.  Membrane drysuits do not change buoyancy with depth.

The next factor to be considered is the gas in our bodies.  The volume of gas in our ears and sinuses does not change as we dive.  If it does we will suffer a ‘squeeze’.  The gas in our stomach and intestines compresses with depth and returns to its original volume when we surface.  The volume of this gas is usually too small to notice the resultant change in buoyancy.

The gas in our lungs is a large enough volume that it makes a difference.  If we are neutrally buoyant, we will ascend when we breathe in and descend when we breathe out.  We can also breathe with our lungs nearly empty or with our lungs nearly full.  New divers are almost always anxious and quite concerned about having enough air to breathe.  As a result they tend to dive with their lungs more full than normal.  Therefore, they need to wear more lead to be neutral.  As they gain experience and relax, they will breathe at more normal lung volumes and they will be able to take a few pounds off their weight belts.

Experienced divers will alter their buoyancy by adjusting their breathing.  They will be neutrally buoyant while swimming along the bottom and when they encounter an object or a rise in the bottom they will take a deeper breath so as to rise over it, or they will breath out to go under an obstruction.  Every new rebreather diver I have taught swims right into the object that they should have avoided using this technique.  The problem is that on a rebreather the gas you are breathing in and out is going into the counterlung.  The total volume of gas in your lungs plus the counterlung does not change when you breathe.  As a result, your buoyancy does not change!

The final and most important factor that affects our buoyancy is the gas outside our bodies, in our diving equipment.  The volume of gas in our facemasks is essentially constant and therefore has no effect.  The weight of gas in our dive tanks does matter!  The volume of gas in our buoyancy compensators, drysuits, and for rebreather divers, the volume of gas in the breathing loop is highly variable and has a major effect on our buoyancy.

Eighty cubic feet of air weights approximately five pounds (2.5 kg).  The volume of the tank does not change during the dive and therefore as we breathe the air in the tank and release it into the water, the tank will become lighter.  For a typical one tank recreational dive this change in buoyancy is only two to three pounds (1-1.5 kg) and not really noticeable.  On one technical cave dive I made, I was 25 pounds (11kg) more positively buoyant at the end of the dive than I was at the beginning, just as a result of the weight of gas I had breathed during the five-hour dive.  To be neutral during my decompression stops I had to start the dive 25 pounds (11kg) negative.  If possible, it’s good to have lead weights that you can pick up as the dive goes on.

However, for most divers the gas in their BCs, drysuits and rebreathers has the greatest effect on buoyancy.   The most important factor is that the volume of this gas will change with depth (P1V1=P2V2).  As a result, the larger this volume of gas, the harder it will be to control your buoyancy.  Let’s consider a simple example.  One diver has gas in their BC that displaces 11 pounds (5kg) of water.  If they are neutral at 100 fsw (30msw) and they ascend to 33 fsw (10msw), this gas will double in volume and they will be 11 pounds (5kg) positively buoyant at 33 fsw (10msw) as a result of this buoyancy change.  They will have to dump one half the gas in their BC to return to neutral buoyancy.  The same change will occur again as they ascend from 33 fsw (10msw) to the surface.

A second diver has no gas in their BC to be neutrally buoyant at 100 fsw (30msw).  When they ascend to 33 fsw (10msw) their buoyancy will not change (as a result of the gas in their BC).  The bottom line is that every diver should learn to dive with the minimum amount of gas in their BC.  The goal is to be exactly neutral near the surface at the end of the dive with the BC completely empty.

Most divers are lazy.  It is easy to dive with a fair amount of gas in the BC.  If you want to descend you simply dump gas and you sink quickly.  If you are this kind of diver and you start to dive using a drysuit, you will get into trouble.  Some gas has to be inside the drysuit as it is the air trapped in the underwear that provides the insulation and keeps you warm.  The more underwear you wear, the more gas will be trapped inside the drysuit.  Most divers dive with much more than this amount of air inside the drysuit.  In a neoprene suit you need a fair amount of air in the suit to move easily.  A membrane suit is far more flexible and it is possible to dive with less ‘surplus’ gas in the suit.  In addition, if you have too much air in the drysuit it is quite possible to end up going for a ride to the surface, feet first.  The solution is to tuck your legs into your chest to force the gas up into the top of the suit and paddle like crazy with your arms to turn head up so that you can dump the excess gas, but this is very hard to do with a lot of gas in the suit.  Learning to dive with minimum gas in the BC and in the drysuit is an important skill.

Rebreather divers have a third, fairly large gas space in the breathing loop.  The amount of gas in the counterlung can be adjusted but the rest is fixed.  Like the BC and the drysuit diver, rebreather divers have to learn to dive with minimum gas in the counterlungs (they should be nearly empty when the diver has taken a breath).

If you are slightly positive or negative, you can counteract the buoyancy by swimming slightly head up or head down.  If your buoyancy is off by more than a few pounds, you will have to add or release gas.  When the volume of gas in the BC, drysuit, rebreather loop is large, the diver has to learn to react to the change in buoyancy before it occurs!  When they start to ascend, they have to be dumping gas or ready to instantly dump gas.  If they are deep and descending quickly, they have to start adding gas long before they want to stop or they will continue to descend, even when they are adding gas.  Most rebreather divers I have taught (including a few very experienced drysuit instructor trainers) have popped to the surface unintentionally while they were learning to control their buoyancy on the rebreather.

I have run out of room for stories of diving fatalities but I want to emphasize one critical lesson learned from them.  All drysuit divers have to adjust the amount of gas in the suit as they change depth.  Many of us also use the drysuit to control our buoyancy and leave the BC empty (it is much easier as we only have one gas volume to adjust).  The problem is that the excess gas in the drysuit can ‘burp’ out the neck seal when we are vertical and we will sink.  A good friend of mine had a rebreather failure, did not have anything else to breathe (a serious mistake), and most likely inflated his drysuit to ascend.  He made it to the surface but then sank and drowned, most likely as a result of air burping out the neck seal of his drysuit.  This was also a factor in one of my near-drowning experiences.  If you don’t use the BC on a regular basis you will not use it in an emergency.

So remember, dive with minimum surplus gas in your gear and practice buoyancy control on every dive.

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2 Responses to “Buoyancy Control- It’s An Inside Job”

  1. stella
    12/02/2013 at 3:31 pm #

    You are totally wrong when you say, thin people sink and fat people float. Iam average weight have always been able to float, was never taught, it just came naturally to me, while my sister who is thinner, can not float.

    • not stella
      27/10/2014 at 10:37 am #

      > You are totally wrong when you say, thin people sink and fat people float.

      > my sister who is thinner, can not float.

      Do you have troubles with reading comprehension Stella?

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