Lifting Buoyancy can be used to raise sunken objects from the bottom. A “lift bag” is the common method, but almost anything that can be attached to the object being recovered and that will safely hold air can be employed. The lift bag is securely attached to the object to be raised, and air is added to the lift bag. The added air fills the bag, displacing water. The increasing buoyancy provided by the displaced water eventually counterbalances the in-water weight of the submerged object, and the lift bag and the object rise to the surface. Several styles of lift bag are commercially available. The simplest is shaped like a small hot-air balloon, open at the bottom so that air can be added easily and so that expanding air can escape if necessary. Lifting devices are described in another chapter where you will also find a discussion of light salvage techniques. Lifting calculations use Archimedes’ Principle. You already know how to find the buoyant force on a submerged object and its in-water weight. Since the inwater weight of the object, not the air weight, is the weight that must be raised, you need only calculate the volume of water that must be displaced to offset the inwater weight. According to Archimedes’ Principle, the volume of water that must be displaced is equal to the weight to be lifted divided by the density of the water. This is the volume of air that must be added to the lift bag. The weight of the air added to a lift bag is so negligible that it can be disregarded. A few words of caution are appropriate in any presentation of lifting. Do not pick up an object and use your buoyancy compensator as the lift bag. If for some reason you drop the object, an uncontrolled buoyant ascent is the likely result. Second, an object may be mired in a mud, silt, or sand bottom. If this is the case, it may take twice the normal amount of lift or more just to free the object from the suction of the bottom. Although the drag of a large lifting device may slow the ascent slightly, an out of control ascent is almost certain when the object breaks loose. Some other mechanism, such as a tidal winch, should be used to free the object. Sample Problems: SI/metric An anchor with an in-water weight of 190 kilograms is resting on the bottom in the ocean. For lifting devices, you have several heavy steel 50-liter containers. Each container weighs five kilograms in the water. How many containers will you need? How many liters of air will be required at depth? Answer: If the containers were weightless in the water, each could furnish lift equal to 50 liters of seawater. However, because of their own in-water weight each container will supply five kilograms less of lift. The effective lift each container can provide is five kilograms less than the volume of the container times the density of seawater. Lift per container = (50 L x 1.025 kg/L) – 5 kg = 46.25 kg Because of the weight of the containers, five containers will be needed rather than four. Four containers can provide only 185 kg of lift. How many liters of air will be required? The total amount of weight to be raised is 215 kg (190 kg + 25 kg). Displacing 215 kg of water will provide enough buoyancy to offset the total weight of the anchor plus the five containers. The volume of air required is equal to the weight of seawater that must be displaced divided by the density of seawater. Volume = 215 kg = 210 L 1.025 kg/L U.S./Imperial Using coated nylon lift bags, which are weightless in the water, what volume of air in cubic feet will be needed to lift an object that weighs 375 pounds in a lake. Answer: We are given the in-water weight; 375 pounds of lift are needed. The volume of air needed is equal to the weight to be lifted divided by the density of the fresh water. Volume = 375 lbs = 6 ft3 62.4 lbs/ft3 In an actual lifting problem, you would probably want to know how much air you would use from your cylinder to provide the lift to raise the above objects. In the above problems, 210 liters and six cubic feet are the air volumes necessary at the depth of the objects. But NAUI Master Scuba Diver 80 Diving Physics
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