The reason we wear weights when we freedive or spearfish is to, on the whole, compensate for the additional buoyancy created by the neoprene of our wetsuits.
The colder the water, the thicker the wetsuit, therefore the more weight we have to wear to create the correct level of buoyancy. For freediving the average rule of thumb is to create neutral buoyancy at about 10m, for spearfishing this is often more shallow.
The point of this post is to specifically focus on how, as pressure increases, the wetsuit we wear will get thinner and offer less warmth but critically give us less buoyancy. This is more apparent the thicker your suit is, and was really brought in to the fore as I have started wearing a 9mm wetsuit in the UK (as I get cold diving everyday). To have neutral buoyancy at 10m with this suit i need to wear 7-8kg(compare this to the 3kg for my 5mm and 1kg for my 3mm)! I noticed how rapidly my positive buoyancy turned in to neutral and then negative/freefall. A couple of us started to discuss the ramifications of this in a recreational environment, especially our own safety as instructors, as to be honest none of us had ever really noticed effects as dramatic as this, as we normally wear 5mm or maybe 7mm suits when teaching. Following students to 20 or 30m i realised quite quickly just how heavy i was when deeper than 15m… major freefall!
The difference, in meters, between positive and negative buoyancy in a suit as thick as this is as little as 5m! So thats 5m from being safe to being overly heavy. I get freefall quicker in this 9mm suit than i do in my 3mm suit. The reason for all of this? Boyles law. The suit at the surface offers lots of buoyancy, at 10m its halved in its floatation potential… given how much lead is needed, this means it becomes too heavy. However, in order to not use up all my energy in the first 5m, and actually go down, (and i can kick hard) i have to weight accordingly.
Of course, the opposite is true if you are using a thinner wetsuit. As there is less gas to compress in the suit. So you will see a much steadier and controllable decrease in buoyancy if you are using a suit of 3mm or less, which is safer on the whole.
So, whats the moral of the story? Be extra vigilant and conservative when deep diving with thick wetsuits, your buoyancy control will be affected more than you think!
Ive done a video covering this which ill post below when i get round to it… in the meantime, dive safe and see you in the blue.
FYI… Free immersion in a 9mm suit, to about 15m, with no weight, then returning to surface in a streamlined posture is one of the most fun things you can do when freediving… i got air!