Lift bags are a tool that every more experienced diver knows how to use, but despite the impression that "it is just a balloon", working with them carries real risks. In this blog we will try to show you how to use lift bags underwater safely and sensibly.
How does a lift bag work?
A lift bag uses Archimedes' principle, which says: "A body immersed in a fluid loses as much weight as the weight of the fluid it displaces." In other words, the goal is for the amount of displaced water (or sea) to be greater than the total weight of the object you are lifting.
The buoyancy formula: FB = ρ × g × V. Where:
- ρ (density of the fluid),
- g (gravity),
- V (volume of the displaced fluid).
With many materials, for example concrete blocks, simply immersing them in the sea reduces their weight by almost half, because their density is only slightly greater than the density of water.
How do you calculate the lift you need?
- Measure the volume of the object (in litres).
- Convert the volume into kilograms (1 L = 1 kg of water).
- Subtract that "lost weight" from the real weight on dry land.
- You get the actual lifting force you need.
- From that you know what lift bag volume you need.
Note: The lift bag you have may not be enough, or it may be too big, which is even more dangerous.
The 10% rule, theory and practice
In theory, a lift bag should have 10% more lift than needed, to prevent a sudden ascent and the "catapult effect". In reality, though, hardly anyone has a 1675 litre lift bag when they need exactly 1523.
That is why this rule often remains just theory. In the field, adaptation and experience are the key.
What happens during the lift?
The shallower the lift bag goes, the more its volume grows. The air expands, the lift increases, and the object you are lifting can suddenly "take off", which is very dangerous. If the object breaks the surface explosively, it will almost certainly fall back to the bottom afterwards.
Another problem is the cohesive force between the object and the seabed. A large concrete block on sand can create such strong resistance that even 7 tonnes of force does not help, until you undermine the block or move it with another tool (an airlift, for example).
Safety when filling a lift bag
A lift bag is always filled from a separate cylinder, never from your main one!
Why?
- You could run out of air.
- The regulator can freeze, which leads to a freeflow or a blockage.
The ideal is to fill directly from the cylinder (without a regulator) and with a hose that has a lower flow rate. Regulators used for this purpose often get damaged, so the direct method is safer and faster.
A controlled lift, easier said than done
Lifting smaller objects with a lift bag is feasible. Bigger, large or very large ones? Hard. That is where more complex systems come in, with several lift bags, steps planned out in advance and a lot of experience.
Types of dive lift bags
1. Open lift bags
The most common, cheaper and available in various sizes. They have:
- A dome for filling with air
- A quick dump valve
- Rings for attachment
- On the larger ones, also a valve for fine venting of air
They are divided into:
- Deep water ones (teardrop shaped, taller than they are wide)
- Shallow water ones (wider than they are tall, for work in harbours and shallows)
Larger models have reinforcements and straps so that the force is distributed evenly, as the dome material does not tolerate high stress on a small point.
2. Closed lift bags
Excellent for use inside sunken ships, they can be secured and filled more precisely.
- Shaped like a "sausage"
- They have quick couplings and pressure valves
- They are filled exclusively through regulated pressure
- They are much more expensive and more complex to use
To finish
Working with lift bags is not a game. With the right preparation, a good calculation, suitable equipment and a lot of care, they can be an extremely useful tool. But if you use them casually, they can very quickly become a danger to you and your team.
Work smart, not just hard.
