Many divers carry lift bags to raise objects to the surface. Typically, divers secure the object to the lift bag along with a line attached to a reel. The lift bag is filled with air and sent to the surface from the bottom. As the lift bag rises to the surface, the expanding excess air can escape through the opening on the bottom. This keeps the lift bag from splitting a seam and dropping the load to the bottom. The reel and line serve as a means of keeping the bag and its load from drifting too far away. Some lift bags are also equipped with a dump valve which allows the diver to ascend with the load and actually trim buoyancy to remain near neutral throughout the ascent. This keeps delicate loads from being left on the surface alone and subjected to wave action.
Divers have also used the lift bag as a means of signaling their location if an ascent is performed away from the anchor line or during drifting deco stops. The lift bag on the surface allows the boat crew to track the diver's position in the water. However, there are problems with using lift bags as marking devices. Without a load on the lift bag, upon reaching the surface, it often flops over and deflates. If this happens, the bag may actually begin to sink and be rendered useless. Also, the shape of a lift bag is not designed for optimal visibility.
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