Diving weighting system
Encyclopedia : D : DI : DIV : Diving weighting system
Divers wear weighting systems, weight belts or weights, generally made of lead, to counteract the buoyancy of other diving equipment, such as diving suits and aluminium diving cylinders.
Providing the weights have a method of quick release, they provide a useful rescue mechanism: they can be dropped in an emergency to provide instant buoyancy which may return the diver to the surface. Dropping weights increases the risk of barotrauma and decompression sickness due to uncontrollable ascent to the surface. This risk can only be justified when the emergency is life threatening. Very often divers take great care to ensure the weights are not dropped accidentally, and many heavily-weighted divers arrange their weights so subsets of the total weight can be dropped individually, allowing for a somewhat more controlled emergency ascent.
The quantity of lead weight required is determined by the overall positive buoyancy of the diver, which depends on the diver's body composition and buoyancy of other diving gear worn. It normally is in the range of 2 kg / 4 pounds to 15 kg / 33 pounds.
The weights are generally made of lead because of its high density and low cost. The lead can be cast in blocks, cast block with gaps for straps or shaped as pellets often named "shot".
Types of system
There are several types of weight carrying and securing mechanisms:- Weight belt : a nylon belt 5 cm /2 inch wide that holds the weights around the diver's waist and is secured by a quick-release buckle.
- *Shot belt : This is a hollow belt made of nylon webbing and filled with lead shot. Unlike shotgun ammunition, each pellet of this shot should have a protective coating to stop sea water from corroding it into powdery lead chloride.
- BCD Integrated weights : are stored in pockets built-in to the buoyancy compensator. Often a velcro flap holds the weights in place. They have handles, which must be pulled to drop the weights in an emergency.
- Weight harness : a belt around the waist with shoulder straps for extra support and security. Often a velcro flap holds the weights in place. They have handles, which must be pulled to drop the weights in an emergency.
- Some rebreathers (e.g. the Siebe Gorman CDBA) have a backpack weight pouch full of lead balls each a bit over an inch diameter. The diver can release them by pulling a cord.
- Non-ditchable weight : in addition to the weight that can be dropped easily ('ditched'), some divers add additional fixed weights to their gear, both to reduce the weight placed on the belt, which can cause lower back pain, and to shift the diver's center of mass to minimize the tendency of the gear to hold the diver in one orientation.
- * Tank weights are attached to the diving cylinder to shift the center of mass backward and upward or downward, depending on placement.
- * Ankle weights, which are typically 1 lb./0.5kg of shot, are used to counteract the positive buoyancy of diving suit leggings, made worse in drysuits by the migration of the internal bubble of air to the feet, and positively buoyant fins. Some divers prefer negatively buoyant fins. The additional effort needed when fining with ankle weights increases the diver's gas consumption.
- * Metal backplates made from stainless steel, which may be used in some Buoyancy compensators, move the center of mass upward and backward.
- * Steel dive cylinders are preferred by some divers, particularly cold water divers who must wear a suit that increases their overall buoyancy, over aluminium cylinders because of their greater density buoyancy, and shift mass upward and backward.
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