Scuba Buoyancy Masterclass Series #4
PERFECT SCUBA WEIGHTING
by Andy Davis, PADI TecRec, ANDI, BSAC and SSI Technical Diving Instructor
Divers seeking perfected scuba buoyancy can achieve precise and correct weighting through the application of several assessment methods. As the diver progresses in experience, whenever they change equipment or whenever they change diving environments (salt/fresh water) they should re-assess their weighting. During initial stages of diver development, changes in breathing patterns and increased relaxation underwater often also allows a slow progressive reduction in weights.
Our aim is to carry sufficient weight to allow us to maintain neutral buoyancy at any stage in the dive, without carrying excess weight that has to be off-set by adding extra air to our BCDs. At the very most, we need sufficient weight to allow us to maintain neutral buoyancy at the end of the dive, when our exposure suits re-gain buoyancy on ascent and we have depleted a significant weight of gas from our scuba cylinders. This is essential if we are to maintain safety stops and conduct a slow, controlled ascent from that stop to the surface.
There are three primary methods for estimating, assessing and confirming our weighting requirements:
1) Basic Weighting Guidelines
-Estimate- We can follow a series of basic calculations, which incorporate the buoyancy of our exposure suit and body size to provide a rough estimate of our weighting needs (see below). This allows us to set-up a weight belt in advance of our dive.
2) Pre-dive Weight Check
-Assess- Once we actually enter the water, we can confirm that our estimation was correct. This check ensures that we have sufficient weight to descend, but are not carrying surplus, unnecessary weight. It is a simple, quick check that can be conducted before any dive. To carry out this check, conduct the following steps:
- At the surface, take a deep breath and hold it.
- Cross your ankles to prevent any instinctive fin movement (upwards propulsion).
- Keep your arms against your body to prevent any instinctive sculling.
- Hold your LPI vertically upwards and release all the air from your BCD.
- You should now float at approximately eye-level in the water.
- Exhale the breath that you were holding.
- You should now begin to slowly sink.
- If not, then add/remove weight until you do float at eye level whilst holding a normal breath and can sink when you release that breath.
- Once this is achieved, add 1-2kg of weight to account for air use during the dive.
3) Shallow-Water Weight Check
-Confirm- Whilst the pre-dive weight check gives us a fair idea of our weighting requirement, it does not account for air use during the dive. It is well worth conducting an accurate check that absolutely confirms your weighting requirement under these circumstances. To conduct a weight confirmation, follow these steps;
- Attempt to hover neutrally buoyant at 3-5m depth, whilst breathing normally.
- Deplete the air in your cylinder to 30bar.
- Slowly reduce the weight that you are carrying (small weights are best, for fine tuning).
- When you reach a stage that requires breathing adjustment (shallow breathing) to maintain your stop depth, then you have reached your absolute minimum weighting.
Once you confirm your minimum weighting requirements, you can be confident that you will have enough weight for your initial descent, and enough weight to hold a safety stop with minimum air remaining in your cylinder. Nothing more, nothing less. Your progress towards effective buoyancy now has a strong foundation and will be considerably easier.
PADI – BASIC WEIGHTING GUIDELINES
Basic weight estimation for an average build diver in salt water — decrease weight for lean body type and increase for heavier.
In order to compensate for an expected loss of weight, caused by the consumption of air from the scuba cylinder, it is also necessary to add 2-4lbs of weight to the total amount calculated.
(Adventures in Diving Manual, PADI, 2001, p.213)
For accurate details of scuba cylinder buoyancy change: please see the Luxfer Scuba Cylinder Buoyancy Specifications.
Next article: ‘Trim and Positioning’