Imagine you need to reclaim a heavy and bulky object from the bottom of the ocean and raise it up to the surface, such as an outboard engine. How would you do it?
This section explains what a lift bag is, how it works, and how to use the two different kinds of open or closed dive lift bags when salvaging valuable items underwater.
Standard salvage bags are available in many different sizes, from 25kg up to 1000 kg.
The bigger the bag, and the bubble of air held inside, the more weight the device can hoist.
Each litre of gas forced inside a lift bag will lift up to one (1) kilogram in weight.
Lift bags are available in red, high-visibility yellow, blue, or white, and manufactured to include several types of attachments and strapping.
Consider using a lift bag with an overpressure relief valve to slow down the ascent rate. Even so, scuba divers must avoid using the primary second stage regulator to inflate the teardrop shaped balloon. It is safer to use the alternate air source (octopus).
In simple terms, the buoyancy created by this airtight diving device can haul objects from underwater via several strong straps.
Containers that have an opening at the bottom allow excess air to spill out and escape as the gas expands. This is an important feature because it stops the bag seams from splitting.
For this reason, open lift bags tend to be 'balloon' shaped to distribute the volume air "vertically" - not horizontally. In other words, the opening should always be at the bottom.
Here's the thing:
As a result, you cannot overfill a parachute lift bag. That's why divers use them as salvage lift bags if they expect large pressure changes. Plus, it's easier to invert the bag (capsize it) to help avoid runaway lifts.
The configurations and shapes of closed bags varies. But, camel lift bags tend to be horizontal or vertical cylinders.
Yet, the most important feature is the overpressure valve. This safety device helps to prevent the internal pressure exceeding the ambient pressure, by around 10 kilopascal (kPa), or 1 metre sea water.
Thus, divers use them most often at the surface (or near it) because they don't lose air in turbulent seas. In some cases, you can use a cover to convert an open bag to a closed version.
Pro Tip: The short tutorial video [7:29 seconds] presented by 'PADI IDC Roatan' demonstrates how best to brief and teach the in-water use of scuba diving lift bags.