Originally posted 2011-08-05 00:36:32.
Scuba Buoyancy Masterclass #6
SCUBA BUOYANCY AND BREATHING
by Andy Davis
Breathing plays a critical role in scuba buoyancy control. As we breath, the inflation and deflation of our lungs is continually affecting our buoyancy, but providing the breathing remains steady, slow and controlled it is rarely noticeable on a scuba dive.
The Lungs are Air-Spaces
The human lungs have a relatively large capacity (4-6 litres on average). Of that total capacity, there is a ‘tidal volume’ that varies as we inhale and exhale normally (0.5 litres avg). A set volume of about 1 litre (called residual volume) always remains, as our lungs cannot completely collapse on exhalation.
A full, forced exhalation and inhalation can vary the volume by as much as 5 litres. As we already know, air volume equates to buoyancy, due to the fact that it displaces a weight of water (Archimedes Principle).
For each litre of water displaced, we increase our buoyancy (1 litre = 1kg freshwater / 1.03kg salt water approx). Consequently, we can acknowledge that a 3-4kg buoyancy shift can occur from full exhalation to full inhalation.
A diver can add, or subtract, about 2 kg of buoyancy to compensate for minor variances during their dive. It must be noted though, that few individuals can sustain full tidal volume breathing (forceful, complete inhalation and/or exhalation) for any lengthy period of time.
Scuba Buoyancy Shifts When Breathing
However, there is always a short time delay between actually changing buoyancy and when we feel its effects. This time delay ensures that a diver who is breathing regularly and at a consistent volume will not experience an exaggerated see-saw motion of rising and falling in the water column with each breath.
It is also worth mentioning that during a typical breathing pattern, the diver does not fully inhale to maximum capacity, or exhale down to the minimum capacity of their tidal volume. A much smaller volume shift (.5kg) is normal when breathing normally.
Lung Volume Control
Obviously, when considering buoyancy control, it is easy to identify the positive applications of manipulating our lung volume.
If we are neutrally buoyant and breathing steadily/naturally at a set depth, then any changes to our breathing pattern can be used to implement small adjustments in depth.
If we choose to breathe deeper and slower, then the increased lung volume will increase our buoyancy – raising us in the water. If we choose to breathe slower and shallower, then our decreased lung volume will reduce our buoyancy.
This can be effectively used to allow the diver to temporarily gain/lose buoyancy without having to re-adjust the air volume inside their BCD. The primary uses for this would be:
- To allow the diver to briefly ascend or descend in order to navigate above or below a small obstacle. For instance, as the diver approaches a large coral ‘head’, they choose to breather deeper/slower and rise 1-2m in depth, pass over the obstacle before breathing shallower/slower to descend again and resume their original depth.
- To allow the diver to briefly amend their buoyancy, as a first reaction, to compensate for deviations from ideal neutral buoyancy. For instance, the diver is conducting an ascent but needs to pause and clear water from their mask. At that time, the diver notes that they are still floating slowly upwards (slight positive buoyancy), but does not have the capacity to adjust their BCD because their hands are otherwise occupied with the mask clearance. The diver will opt to breathe slower/shallower to temporarily reduce their buoyancy and arrest their ascent until they have completed the mask clearance and can then dump the necessary air out of their BCD.
- To enable immediate buoyancy compensation if the diver had to pick-up a negative buoyancy object (i.e. buddy hands you a metal torch or reel) or vice-versa if you have to hold a positively buoyant object (such as when deploying a DSMB).
However, the effectiveness of lung control for fine-tuned buoyancy control depends on the divers’ weighting being correct – otherwise, the increased expansion/reduction of surplus air volume in their BCD will cause too significant buoyancy shifts. This will over-exaggerate buoyancy changes beyond the capacity of the lungs to correct and lead to an ascent/descent that requires intervention by adjusting the gas volume of the BCD.
Breathe control and lung volume variability shouldn’t be used as a persistent aid to maintaining a given depth. The diver should always aim to breathe naturally, only using shallower or deeper breathing for brief corrections in buoyancy. At no point should a diver ever hold their breath.
Next article: ‘Buoyancy on Ascent, Descent and at the Bottom’
About the Author
Andy Davis is a RAID, PADI TecRec, ANDI, BSAC and SSI qualified independent technical diving instructor who specializes in teaching advanced sidemount, trimix and wreck exploration diving courses across South East Asia. Currently residing in ‘wreck diving heaven’ at Subic Bay, Philippines, he has amassed more than 9000 open circuit and CCR dives over 27 years of diving across the globe.
Andy has published many magazine articles on technical diving, has written course materials for dive training agency syllabus, tests and reviews diving gear for major manufacturers and consults with the Philippines Underwater Archaeology Society.
He is currently writing a series of books to be published on advanced diving topics. Prior to becoming a professional technical diving educator in 2006, Andy was a commissioned officer in the Royal Air Force and has served in Iraq, Afghanistan, Belize and Cyprus.