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By the end of this topic, you should be able to answer these questions: 1 How do you determine the minimum weight you need for a technical dive? 2 What is the primary hazard of diving negatively buoyant, and how do you manage this hazard? 3 What is the primary hazard of excessive positive buoyancy, and how do you manage this hazard? 4 How do you determine the minimum buoyancy you need on a technical dive? Weight Yourself Regarding weighting, you weight yourself so you can stay at your final stop with near empty cylinders and no deco/bailout cylinders. This gives you sufficient options to deal with gas loss emergencies during a decompression dive. You need to have enough weight to comfortably stay at the stop. Divers scootering their DPVs. You find your weight by wearing all your equipment (no deco cylinders) and doing a conventional buoyancy check with 50 bar/500 psi or less in your cylinder(s). Weight yourself so you float at eye level, or slightly sink. This is the minimum for that gear configuration. In warm water that requires minimal thermal protection, you may be negatively buoyant even with no weight on. This is acceptable; you don’t need any more weight. With full cylinders, you may be substantially heavier. A single cylinder containing air will be around 3.5 kilograms/7.5 pounds heavier than an empty cylinder Doubles are about 7 kilograms/15 pounds heavier. Full deco/bailout cylinder(s) add even more weight. Different gases and fill pressures also affect your overall weight, especially at the start of a dive, as well as how your weighting changes throughout the dive. Don’t assume your weighting remains the same from one dive to the next. Check it frequently, including whenever you dive a new configuration. Diving Negatively Buoyant The primary hazard of diving negatively buoyant is having a wing/BCD failure that makes it effectively impossible to ascend. Your overall weighting is affected by all of your equipment, but especially your backplate (if using one), cylinders and exposure suit (as discussed in the Dive Equipment section). Where possible, minimize excess negative buoyancy by your equipment choices. If you are significantly negatively buoyant, you can manage this by having more than one way of regaining buoyancy control, such as a redundant wing (two bladders) or a wing/BCD and a dry suit (if the amount of gas you would need to use does not exceed manufacturer recommendations for the suit). image placeholder for 730x365 Photo credit Fourth Element Options If you have a wing/BCD failure, but no redundant buoyancy, you need to consider how you will ascend. You may be able to use one or more of the following: Maintain a diving position that allows your wing/BCD to hold some gas. For example, if the hip dump valve has failed, remaining in an upright position will allow your wing/BCD to hold some gas. Drop some weight. You don’t want to risk a rapid ascent, but you may be able to drop small increments of droppable weight at a time until neutral or nearly neutral. Drop unnecessary equipment. If you're carrying equipment you don’t need that is negatively buoyant you may choose to drop it, for example, a suit inflation system or primary light. You may need help from a team mate to do this. Follow a sloping bottom upward or use a fixed ascent line. Gently fin upwards. If you're not excessively negative you may be able to fin to ascend. Take care not to fins so hard you get leg cramps as this will likely complicate the problem. Partially inflate a DSMB or lift bag. Remember that, just like a BCD or dry suit, you will need to exhaust gas during the ascent to control ascent rate. DSMBs and lift bags are not designed as buoyancy devices and this option is unlikely to be endorsed by the manufacturer, so use it with caution and only if other alternatives have been exhausted. Too Much Buoyancy The primary hazard of positive buoyancy is the inability to make required decompression stops, and/or a rapid ascent, leading to a high risk of decompression illness. You manage this risk by: Checking your weight with near-empty cylinders so that you’re adequately weighted at maximum buoyancy. Using non-droppable weight, often called trim weight because it can be placed in a location for optimum body position. Trim weights should be placed where the risk of accidental loss is minimal, such as trim pockets on your harness, keel/v-weights on backmount doubles, weighted backplates or single cylinder adaptors. Distributing droppable weight over more than one location, such as weight pockets on either side of your harness, similar to a recreational BCD. This is often more comfortable and if you do lose weight from one source, it is less likely to be catastrophic. Minimum Buoyancy The minimum buoyancy you need for a technical dive is sufficient buoyancy to float your head comfortably above the surface while wearing the entire kit you need for the dive, including full cylinders. Choose your wing/BCD lift capacity with this in mind. Check buoyancy in water shallow enough in which to stand, first. Don’t forget you use dual bladder wings one at a time – they don’t double your lift.