Here's an excerpt from my buoyancy workshop notes, that may help explain some issues and procedures..
CORRECT WEIGHTING.
Efficient weighting is the foundation of proper buoyancy control. Being over, or under-weighted makes buoyancy control an increasingly more difficult task for the diver. Compensating for incorrect weighting often also requires physical effort – by fining or sculling to maintain a depth. Consequently, any effort involved with conducting weight checks in order to optimize requirements will pay great dividends to the diver when they later seek to fine-tune their buoyancy.
Over-weighting. Many novice divers suffer from over-weighting. This is due to two primary factors. Firstly, some instructors favor over-weighting divers during entry-level training, in an attempt to mitigate the risk of uncontrolled ascent and because they feel it gives the student diver more stability to conduct skills whilst in a static, kneeling position. This is a very short-term outlook that often hampers the divers’ devel opment afterwards. Secondly, many novice divers incorrectly attribute difficulty in descending upon insufficient weighting. In reality, most difficulties encountered with achieving an initial descent in the water are due to improper technique. Regardless of the cause, many novice (and some intermediate-advanced) level divers
find themselves over-weighted – which causes subsequent and unavoidable difficulties with their buoyancy and trim control. An over-weighted diver has to compensate by adding air to their BCD. This causes a dual push-pull effect across their horizontal centre-of-gravity (COG) and prevents effective flat trim. A heavy weight belt below the horizontal COG pulls the lower torso downwards. The air added to the BCD to compensate for that weight pulls the upper torso upwards. This ‘see-saw’ like effect creates a ‘head-up, feet-down’ trim for the diver that increases water resistance and reduces the efficiency of fining.
Carry surplus weights means that surplus compensating air must be added to the BCD on descent. Apart from the impact upon trim (and, maybe, an increased ‘ungainliness’ and ‘ wal lowing eff ect’ caused by the extra air migrating around the BCD as we move about) there is also a major impact on our buoyancy when we ascend. Any air that is contained in our BCDs will expand on ascent. The expansion is proportional to the constant reduction of pressure as we get shallower. More air equals more drastic air expansion. This is a major cause of uncontrolled ascents and missed safety stops for novice divers, especially as they reach shallower depths where the air expansion is greatest.
For the record, 1 litre volume of air displaces approx 1kg of fresh water (1.03kg of salt water). This means that to compensate for every 1kg of surplus weight, the scuba diver needs to add 1litre of surplus air to their BCD. Each litre expands constantly as the diver gets shallower. Each litre provides 1kg of positive buoyancy. From 10m to the surface, the volume of air will double:
1 litre becomes 2 litres (2kg positive buoyancy)
2 litres becomes 4 litres (4kg positive buoyancy).
4 litres becomes 8 litres (8kg positive buoyancy).
The over-weighted diver results in having much more air to add on descent and dump on ascent. They also experience much greater fluctuations in their buoyancy during even a small rise or fall in depth during the dive. Their workload is higher and a proportionately larger amount of their attention is required to constantly adjust their buoyancy for every minor depth change.
Why do we carry weights? Divers carry weight to over-come any positive buoyancy that may be inherent in their bodies and equipment. The amount of weight they carry should be sufficient (no more, no less) to overcome that buoyancy. Any surplus weight carried has to be compensated by adding air to the BCD, which is inefficient and unnecessary.
BCDs. The most obvious source of positive buoyancy comes from the air that we put into our BCDs. Naturally, the volume of this air is variable and can (should) be complete expelled when we begin our descents. If we are thorough in venting the air from our BCDs, this factor should not form any part of our weighting calculations. It can be surprising how often novice divers fail to fully deflate their BCDs on descent – which is one issue that leads to a miscalculation of their weighting requirements. When descending, keep vertical and be sure to fully elevate the Low Pressure Inflator hose so that all the air in the BCD is able to descend. It is a common novice mistake to not fully elevate the LPI and, consequently, allow some air to remain in the BCD (even though the LPI deflate button is depressed).
Exposure Suits. Another major source of positive buoyancy is the divers’ exposure suit . Neoprene wetsuits contain a volume of trapped air, which creates an amount of buoyancy relative to their size and thickness.. This has to be balanced with sufficient weighting to allow descent. For the record, an average 3mm 1-piece wetsuit possesses less than 1kg of buoyancy whilst uncompressed. A 5mm 1-piece is approx 2-3kg buoyant. A 7mm 1-piece is around 6.5kg buoyant. Add a hood, vest and gloves to a 7mm suit and you’ll have in the region of 8.5kg of uncompressed buoyancy. The same is true of a drysuit, except that the air volume of that suit is variable and adjustable by the user. It is worth bearing in mind that the air volume
trapped within an exposure suit is governed by the same pressures that effect the air in our BCDs. As we descend on a dive, increasing ambient pressure crushes the air within the exposure suit and reduces its buoyancy (and its thermal properties!). As we ascend shallower, the ambient pressure reduces and the material in the exposure suit re-expands with a consequent increase in buoyancy. Our concern with weighting for an exposure suit is primarily aimed at off-setting its buoyancy at, or near, the surface. As we descend, the reduction in the suit’s buoyancy will require off-setting by the addition of air to our BCDs (this should be the main reason why we add any air to our BCD underwater at all).
Scuba Equipment. The rest of our scuba kit will add, or reduce, some buoyancy depending upon the individual properties of that specific equipment. We have air-spaces inside our masks that are positively buoyant. We carry metal clips, torches and reels that are negatively buoyant. Our BCDs may contain foam backplate padding and other materials that contribute to our over-all positive buoyancy. Items such as fins may be either positively or negatively buoyant depending on their material construction. It is worth bearing these factors in mind, when we are purchasing our scuba kit.
Air Consumption. We also need some weight to compensate for the air that we use from our cylinders during the progress of the dive. Depending on the material, design and size of the scuba tank, the weight of the air that we consume can be as much as 2-3kg.
The weight of air is (depending on pressure, temperature and humidity) approximately 1.2 grams per litre. To calculate your approximate weight of air, simply multiply the tank capacity (i.e.
11 litres for an AL80 cylinder) by its working pressure in BAR (i.e. 200 Bar).
11litres (AL80) x 200 bar = 2200litres.
2200litres x 1.2 grams per litre = 2.64 kg (total weight of air in the cylinder)
To calculate the air that you use, simply subtract the reserve (i.e. 50bar) from the initial calculation.
11lites (AL80) x (200-50) bar = 1650litres used.
1650litres x 1.2 grams per litre = 1.98 kg (total weight of air consumed)
As we use this air, we become more buoyant. We have to add weight to offset for losing this weight of air by the end of the dive.
Scuba Cylinders. Scuba cylinder buoyancy is determined by the weight of water that they displace, compared with the actual weight of the cylinder, plus the weight of any gas contained inside. You can research the relative buoyancy (empty and full) of your cylinder online (see the cylinder specification tables on the scubatechphilippines.com website).
Steel cylinders are typically very negative, and will remain negative even when the air inside has been consumed. In contrast, aluminum cylinders are less negatively buoyant. They are typically negatively buoyant at the beginning of the dive, due to weight of the gas they contain. As that gas is consumed, they can become positively buoyant. In most instances, a typical AL80 cylinder (aluminum cylinder commonly used by dive centres for rental) requires 2kg of weight to off-set cylinder buoyancy at the end of the dive, when reserve air capacities are reached (50bar/500psi).
Assessing our weight requirements. Divers 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 Appendix B). This allows us to set-up a weight belt in advance of our dive.
2) Pre-dive Weight Check (Assessment). 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:
a. At the surface, take a deep breath and hold it.
b. Cross your ankles to prevent any instinctive fin movement (upwards propulsion). c. Keep your arms against your body to prevent any instinctive sculling.
d. Hold your LPI vertically upwards and release all the air from your BCD.
e. You should now float at approximately eye-level in the water.
f. Exhale the breath that you were holding.
g. You should now begin to slowly sink.
h. 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.
i. Once this is achieved, add 1-2kg of weight to account for air use during the dive.
3) Shallow-Water Weight Check (Confirmation). 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;
a. Attempt to hover neutrally buoyant at 3-5m depth, whilst breathing normally.
b. Deplete the air in your cylinder to 30bar.
c. Slowly reduce the weight that you are carrying (small weights are best, for fine tuning).
d. 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.
