The Right Bag!

INTRODUCTION
These notes are intended for users of air lift bags who may not be trained in their use. We make no apology for the basic nature of some of the information and ask experienced users to bear with us if we are stating the obvious. As manufacturers we are continually being surprised by the level of knowledge in the behaviour of flexible buoyancy underwater, and hope these notes go some way towards a simple and straight forward explanation.
BASIC CONCEPTS
As ambient pressure increases with water depth a flexible buoyancy unit will decrease in volume as it goes deeper because the air inside becomes compressed and occupies a smaller volume. Conversely, as it rises, the air inside will experience a lesser surrounding pressure and expand to provide a greater volume.
As the amount of buoyancy (lift) is equal to weight of the volume of water displaced, it follows that the amount of lift varies in proportion to the change in depth.
In other words, a lift bag going down is compressed and provides decreasing buoyancy while a bag coming up expands and provides increasing buoyancy.
TYPES OF BAGS
There are four basic types of flexible buoyancy units with fundamental differences Open Bottom Bags. (Parachute Bags)
These are designed to dynamically lift objects from depth and rise up through the water column dumping excess air from the open bottom as they ascend. They are rigged so that only a single point fixing is required and because of this they remain upright and stable regardless of the angle of the load. The larger bags can be towed at low speeds on the surface in moderate sea states and excess air can be released from a manually operated dump valve in the corners of the bag. On smaller open bottom bags air is released by a diver pulling a lanyard at the hem of the bag.
Type I - Enclosed Lifting Bags (Elongated and Cylindrical pontoons)
Sometimes referred to a Fully Enclosed Lifting Bags these units have the sole purpose of decreasing the weight of a static underwater object or providing buoyancy to objects on the surface. They are not designed or suitable for dynamic lifting from any depth. Their rigging requires multiple fixing points at set spacing and they must be kept close to horizontal at all times. Their capacity to dump excess air through Pressure Release Valves (PRVs) is limited and any outside pressure variation must be carefully controlled.
Type II - Enclosed Lifting Bags (Pillow Bags)
These units have the purpose of decreasing the weight of a static underwater object or providing buoyancy to objects on the surface. They are the bag of choice for most salvage companies. They are designed or suitable for dynamic lifting from up to a 60 foot depth. Their rigging requires a single fixed point at the bottom of the bag and they can be rigged horizontally or in a flat position. Their capacity to dump excess air through Pressure Release Valves (PRVs) is not as limited has the Type I enclosed bags.