Thank you for your interest in improving your knowledge of diving. As always, by viewing this article, you hereby release me from any and all liability. While I will ensure to the best of my abilities, in direct reference to my own experience as a diver, that the information presented below is correct, I am not, nor is any experienced diver, completely infallible. Those who believe they are infallible, can be found on various dry-monuments of long-passed divers around the world – a sobering but truthful reminder.
Better air consumption, enhanced in-water comfort, protection of the beautiful marine environment you’re exploring, improved safety and longevity of your equipment.
With all those benefits, who among us wouldn’t want better buoyancy? (Commercial Divers, we’ll leave you out of this for now, you have your own reasons!)
Thus, let’s dive into this topic.
The PADI Peak Performance Buoyancy Course states that “The BCD is not an elevator”, and it’s absolutely right.
The BCD is there to:
– Keep you buoyant on the surface
– Offset any negative buoyancy at the start to middle of your dive caused by your tank
– Offset any negative buoyancy at depth caused by a wetsuit or compressible exposure protection
Let’s take a moment to look at that second statement, from the perspective of divers using Aluminium Tanks, which, naturally, become lighter(read ‘more buoyant’) as they drop below 90 Bar of tank pressure. Due to this drop, your ‘perfect weighting’, should be at the end of the dive, with an empty tank (50 Bar). At the start of the dive, you should be just slightly negative.
Thus, the BCD, from descent to mid/end of the dive is present to offset the negative buoyancy of your scuba unit until the tank pressure decreases to the point at which it becomes slightly positive.
For wetsuit, or, compressible exposure protection wearers amongst us (anything that is on your equipment to keep you warm and can be compressed by depth to a lower amount of buoyancy – ie. Neoprene or other substance worn), the BCD is also present to offset your increased negative buoyancy during descent.
Take the example of a diver wearing a 5mm wetsuit descending to a 36m dive. He releases buoyancy to descent, then levels off at 5m, with sufficient air in his BCD to offset the negative buoyancy of his tank. Upon descending to 10m, and then further to 15m, 18m, 25m, and down to 36m, the wetsuit the diver is wearing will be compressed by pressure, thereby increasing the negative buoyancy the diver has to compensate for by adding air to his/her BCD.
Remember, we are weighted to be able to descend from the surface, where a wetsuit is at its most buoyant! (And, to be able to hold our safety stop with a potentially 50 bar tank at the end of the dive!)
Now, moving on from these stated facts, for a diver remaining at a single depth for more than a moment, he/she will wish to become neutrally buoyant. This is a ONE-TIME adjustment, and does not need to be repeated until the diver changes depth again – whether, adding air to compensate for decreasing BCD volume during descent, OR, releasing air to control a slight ascent and avoid an uncontrolled ascent.
Thus, returning to PADI’s statement, “The BCD is not an elevator”, we do NOT use the BCD to go up or down, we solely utilize it to maintain neutral buoyancy at a given depth, adding enough air to compensate for the inherent negative buoyancy of our scuba unit – IE, the lead weight we are wearing to compensate for our tank buoyancy at end-of-dive, plus the wetsuit we are wearing, which has, upon reaching depth, been slightly compressed, thus resulting in a lower overall buoyancy and thus requiring more air to remain neutral at each deeper point of the dive.
Upon ascending, you now have two primary factors to consider, whether ascending to the safety stop, or, just ascending to a shallower depth.
A. We need to release air to compensate for the fact that the volume of air in our BCD to keep us neutrally buoyant at depth is now expanding due to the pressure decrease of our ascent.
AND
B. As the pressure decrease increased the volume of gas in our BCD, it is also causing our wetsuit (or gas in a drysuit) to expand, thus resulting in greater buoyancy, and the need to further dump air from a BCD.
All of this becomes pointless if not properly weighted. AND, solely being able to descend at the start of a dive is dangerously pointless if a diver cannot maintain a 5m safety stop at the end of a dive, with 50bar left in his/her tank.
To be continued!
The Diving Source 