Lifting bag
A bag which leaks sufficiently to start sinking will lose volume to compression and become less buoyant in a positive feedback loop until stopped by the bottom.There are three basic ways this can be done:[1] Once a load is lifted off the substrate, it will rotate until the centre of gravity is in the position of lowest potential energy.[7] Installation and removal of propellers is a specialised application where there is usually very little clearance above the load, which is usually at least partly underneath a vessel.and details of the rigging will depend on the precise geometry of the propeller, such as aspect ratio, skew, and number of blades.[8][9][10] Closed lift bags have an overpressure valve to prevent internal pressure from exceeding ambient pressure by more than a set amount (around 10kPa, or 1msw)[1] Closed lift bags are intended for use at or near the surface,[5] as they retain the air even in rough seas.The bag can be attached to the load and when ready, the valve is opened and the diver swims clear.One system operates by pressing on the top or pulling a line attached to the bottom to actuate the spring-loaded valve.This is simple, but there is a risk if the lift bag is too large and cannot be vented fast enough the lift may get out of control and ascend so fast that the bag breaks the surface, capsizes and collapses, losing so much air that it then cannot support the weight of the load, and will then sink back to the bottom.A lift bag which is only slightly larger than needed to support the load will ascend more slowly, and is less likely to capsize at the surface, as excess air will be spilled continuously during the ascent.Advantages of this method are a more controlled lift, the facility to use a larger capacity for initial breakout without risk of a runaway.This will be the last bag, and the divers will be monitoring it most carefully while they are filling it, so will be more likely to react in time to regain control of the lift.Free air volume follows Boyle's law, and is proportional to absolute ambient pressure in bar or ata.Once at the surface, the load may be secured by adding more buoyancy, lifting on board a vessel or other method.
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