Friday, 2 November 2012

Underwater Lift Bags

When an object is immersed in water, it feels lighter. In a cylinder filled with water, the action of inserting a mass in the liquid causes it to displace upward. In 212 B.C., the Greek scientist Archimedes discovered the following principle: an object is immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object. This became known as Archimede's principle. The weight of the displaced fluid can be found mathematically. The fluid displaced has a weight W = mg. The mass can now be expressed in terms of the density and its volume, m = pV. Hence, W = pVg.

It is important to note that the buoyant force does not depend on the weight or shape of the submerged object, only on the weight of the displaced fluid. Archimedes principle applies to objects of all densities. If the density of the object is greater than that of the fluid, the object will sink. If the density of the object is equal to that of the fluid, the object will neither sink or float. If the density of the object is less than that of the fluid, the object will float.

A lifting bag is an item of diving equipment consisting of a robust and air-tight bag with straps, which is used to lift heavy objects underwater by means of the bag's buoyancy. The heavy object can either be moved horizontally underwater by the diver or sent accompanied or even unaccompanied to the surface.

Lift Bags are the most commonly used tool for recovering submerged objects and are available in a wide variety of sizes and shapes. Copy rights reserved to/Subsalve USA Commercial Lift Bag
It is advisable to select a lift bag with an appropriate capacity for the task at hand. If the lift bag is grossly oversized a runaway or otherwise out of control ascent may result. Commercially available lifting bags may also incorporate dump valves to allow the operator to control the bags buoyancy during ascent.
If a single bag is insufficient, multiple bags may be used, and should be distributed to suit the load.

The shape of an open lifting bag should distribute the volume in a vertical rather than a horizontal direction so that the open end of the bag always remains underwater. If the open end reaches the surface, air will escape from the bag and it may sink.
The simplest version are two-sided bags, either joined round the edges or foldes and joined along two sides. Webbing straps may be stitched to doubler patches which are then glued or welded to the bag on light duty bags, but on large and heavy duty bags there are usually strips of bag material bonded to the bags which form flat retaining tubes for the webbing which is threaded through the tubes and may be withdrawn for maintenance and inspection. heavy duty open bags are generally conical with a domed top or a reversed truncated cone top, and may have several straps from the lifting point at the bottom, through the guide tobes on the sides, to a crown ring of webbing or steel at the top, to spread the load evenly over the fabric of the bag.

Safe use during lift bag operations requires advanced training and expertise.

All divers should be away from the object while the lifting operation
Horizontal movements should occur along the bottom if possible

 Lift Bags Safety

When planned a dive involving the use of liftbag divers should consider the following;                                                      

- Safe use during lift bag operations requires advanced training and expertise
The lift bag can suddenly and dangerously tangle lines and regulator hoses causing a fatal situation to evolve.  

- A diver can also lose control of the lift bag during ascent.

- A lift bag runaway may rise out of the water and spill its air dropping the obtained object back to the bottom. 

- Whenever you lose control over the liftbag, let it go, back off and keep eyes on the liftbag.

- A liftbag should be inflated from an independent air source which is not attached in any way to the diver.

- Leaks in lift bags, causing loss of buoyancy and sinking of the load after lifting. The load may then sink increasingly rapidly as the air in the bags compresses, and may be a hazard to divers below or working on the load at the surface, or the load may be lost. 

- Never use your BCD for lifting !!!

Last but not least, a liftbag can be a handy piece of equipment but it requires very high skills of buoyancy and sufficient diving experience.To guarantee your own safety it is recommended to practice it with a qualified diving instructor.

1 comment:

  1. I would like to thank you for the efforts you have made in writing this article. I am hoping the same best work from you in the future as well. Thanks...
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