This brief article explains the differences between lift bags and marking bags
and the appropriate usage for each type of bag.
Many divers carry lift bags to raise objects to the surface. Typically, divers secure the object to the bag along with a line attached to a reel. The 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 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. 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 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.
Marking bags have become a popular alternative to lift bags for the purpose of identifying a diver's location. By design, marking bags extend above the surface of the water much higher than a lift bag, greatly extending their visibility range. Marking bags are designed to be sent to the surface from depth with no load. In a manner similar to a lift bag, the marking bag is inflated at depth and allowed to rise to the surface. Since there is no load on the bag, upon reaching the surface the bag will flop over. Then the diver takes up the slack on the reel, and the bag is held upright for extended visibility. To keep the marking bag from deflating when it reaches the surface, the opening on the marking bag has a built-in restricter to reduce the amount of air that can escape from the bag. The restriction could cause a problem because as the bag rises, the rapidly expanding air needs to be adequately vented. Therefore, an overpressure valve is installed in the bag to relieve the air pressure that cannot escape through the restriction.
Marking bags should not be used as lift bags. The majority of the positive buoyancy of a marking bag would be needed to raise the object, causing most of the marking bag to remain underwater, resulting in very poor visibility. The overpressure valve is another reason marking bags should not be substituted for lift bags. Unlike lift bag dump valves, marking bag overpressure relief valves cannot be regulated on ascent to balance a load.
Using a lift bag or marking bags requires skill to be deployed and managed properly.
Individuals desiring to use these devices should seek instruction.