Lift bag
A lift bag is used to lift objects under water and transport them to the surface. This is filled with normal breathing air and, due to the lower density of air , rises to the water surface as soon as the buoyancy force exceeds the attached weight. Depending on the size and design, a separate compressed air cylinder is used, which is only carried for this purpose, or the lift bag is filled using the octopus . The second method is mainly used for smaller lift bags with a buoyancy volume of up to around 100 liters.
The expansion of the air under ambient pressure in the lift bag causes a further increase in buoyancy as the ambient pressure rises, until the lift bag is fully filled. The volume of the filling air, which increases as the ambient pressure continues to rise, escapes freely into the water via the construction that is open at the bottom. The lifting bag cannot burst. It is therefore advisable to select the dimensions of the lift bag in such a way that the load is only lifted when the maximum filling is reached. Otherwise, due to the increase in buoyancy during expansion, there is a risk that the lift bag will gain considerable buoyancy and shoot through uncontrollably to the surface instead of rising slowly and in a controlled manner. This can also have the consequence that the lift bag loses a lot of air on the surface and then sinks again with the attached load at high speed and thereby represents a danger for (e.g. the divers below). To avoid uncontrolled ascents, many lift bags have a simple valve that can be used to vent excess air when ascending.
There are lifting bags of various sizes in use, for example a lifting bag with a volume of 100 l can lift an object with a weight of 100 kg under water. Lift bags are mainly used by fire brigades , the THW and water rescue organizations , but also by private individuals.
In the field of technical diving , lift bags, often referred to there with the English term lift bag , are used as a replacement for the signal buoys that are otherwise often used . Since lift bags have better stability on the surface and a higher load capacity, they are sometimes set at great depths during technical diving in order to use the line, which is unrolled from a spool , as a reference for ascent and a holding line for decompression stops .
