Lift bags can be used to recover objects to the surface. Although referring to a lift bag, any suitable container, spare BCD, drum, bucket, etc. may be used.

"Air will displace its own volume of water, generating up thrust (buoyancy)"

The volume of an air-filled container must create more buoyancy than the weight of the object in water, in order to lift it.

Due to the air expanding on the ascent the lift bag needs to be able to release the excess air. Failing this the lift bag may burst. Another effect of the expanding air is the increase in the rate of the lift. This may result in the object 'shooting' out of the water - a dangerous and uncontrolled event.

To safely control a lift, the lift bag should be slightly bigger than what is required.

What size bag?

Determine the 'out of the water' weight of the object.

Alternatively calculate the volume of the object and multiply it by its relative density to calculate the weight.

Next deduct from this figure the weight of the water displaced, to find its actual submerged weight.

Example:

Lift a concrete block - 1m x 1m x 0.5m from 20m below the surface.

Volume of the block:  1 x 1 x 0.5 = 0.5m³

Density of concrete:    2 323 kg/m³ (from table above)

Weight of the block:    2 323 x 0.5 = 1 162kg

Block displaces 0.5m³ water, which weighs 500kg. Thus the submerged weight of the block is:

1 162kg - 500kg = 662kg.

A lift bag of 700 to 750kg should be adequate.

How much air will this bag require? (approx.)

Block is at 20m, thus absolute pressure is 3 bar. Assume using a 700kg bag.

A 700kg bag will need 700 liters of air at surface (1 bar) to fill.

Thus: 3 bar x 700 liters = 2 100 liters air. This equates to a fully charged (200 bar) 12 liter cylinder to fill this bag.

Tips

Disclaimer: The examples and concepts were taken from various materials to give some idea of lifting objects underwater. Only fully qualified and experienced divers should attempt such a lift.