thumb|right|A bag weight belt and a traditional weight belt
A diving weighting system is ballast weight added to a diver or diving equipment to counteract excess buoyancy. They may be used by divers or on equipment such as diving bells, submersibles or camera housings.
Divers wear diver weighting systems, weight belts or weights to counteract the buoyancy of other diving equipment, such as diving suits and aluminium diving cylinders, and buoyancy of the diver. The scuba diver must be weighted sufficiently to be slightly negatively buoyant at the end of the dive when most of the breathing gas has been used, and needs to maintain neutral buoyancy at safety or obligatory decompression stops. During the dive, buoyancy is controlled by adjusting the volume of air in the buoyancy compensation device (BCD) and, if worn, the dry suit, in order to achieve negative, neutral, or positive buoyancy as needed. The amount of weight required is determined by the maximum overall positive buoyancy of the fully equipped but unweighted diver anticipated during the dive, with an empty buoyancy compensator and normally inflated dry suit. This depends on the diver's mass and body composition, buoyancy of other diving gear worn (especially the diving suit), water salinity, weight of breathing gas consumed, and water temperature. It normally is in the range of to . The weights can be distributed to trim the diver to suit the purpose of the dive.
Surface-supplied divers may be more heavily weighted to facilitate underwater work, and may be unable to achieve neutral buoyancy, and rely on the diving stage, bell, umbilical, lifeline, shotline or jackstay for returning to the surface.
Freedivers may also use weights to counteract buoyancy of a wetsuit. However, they are more likely to weight for neutral buoyancy at a specific depth, and their weighting must take into account not only the compression of the suit with depth, but also the compression of the air in their lungs, and the consequent loss of buoyancy. As they have no decompression obligation, they do not have to be neutrally buoyant near the surface at the end of a dive.
If the weights have a method of quick release, they can provide a useful rescue mechanism: they can be dropped in an emergency to provide an instant increase in buoyancy which should return the diver to the surface. Dropping weights increases the risk of barotrauma and decompression sickness due to the possibility of an uncontrollable ascent to the surface. This risk can only be justified when the emergency is life-threatening or the risk of decompression sickness is small, as is the case in freediving and scuba diving when the dive is well short of the no-decompression limit for the depth. Often divers take great care to ensure the weights are not dropped accidentally, and heavily weighted divers may arrange their weights so subsets of the total weight can be dropped individually, allowing for a somewhat more controlled emergency ascent.
The weights are generally made of lead because of its high density, reasonably low cost, ease of casting into suitable shapes, and resistance to corrosion. The lead can be cast in blocks, cast shapes with slots for straps, or shaped as pellets known as "shot" and carried in bags. There is some concern that lead diving weights may constitute a toxic hazard to users and environment, but little evidence of significant risk.
Function and use of weights
Diver weighting systems have two functions; ballast, and trim adjustment.
Ballast
The primary function of diving weights is as ballast, to prevent the diver from floating at times when he or she wishes to remain at depth.
Freediving
In freediving (breathhold) the weight system is almost exclusively a weight belt with quick release buckle, as the emergency release of the weights will usually allow the diver to float to the surface even if unconscious, where there is at least a chance of rescue. The weights are used mainly to neutralise the buoyancy of the exposure suit, as the diver is nearly neutral in most cases, and there is little other equipment carried. The weights required depend almost entirely on the buoyancy of the suit. Most freedivers will weight themselves to be positively buoyant at the surface, and use only enough weight to minimise the effort required to swim down against the buoyancy at the start of a dive, while retaining sufficient buoyancy at maximum depth to not require too much effort to swim back up to where the buoyancy becomes positive again. As a corollary to this practice, freedivers will use as thin a wetsuit as comfortably possible, to minimise buoyancy changes with depth due to suit compression.
Scuba diving
Buoyancy control is considered both an essential skill and one of the most difficult for the novice to master. Lack of proper buoyancy control increases the risk of disturbing or damaging the surroundings, and is a source of additional and unnecessary physical effort to maintain precise depth, which also increases stress. This may not be sufficient to counteract the buoyancy of dry suits with thick undergarments used in cold water.
Some BCD harness systems include a crotch strap to prevent the BCD from sliding up the wearer when inflated, or down when inverted, due to the weights.
Weight harness
A weight harness usually consists of a belt around the waist holding pouches for the weights, 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 or to remove the weights when exiting the water. A weight harness allows the weights to be comfortably carried lower on the body than a weight belt, which must be high enough to be supported by the hips. This is an advantage for divers who have no discernible waist, or whose waist is too high to trim correctly if a weight belt is worn. These advantages may also be available on some styles of integrated BC weights. A weight harness may also incorporate a crotch strap or straps to prevent weight shift if the diver is in a steep head down posture.
Clip-on weights
<!-- target for redirect Clip-on weight -->
thumb|upright=0.5|Clip-on trim weight on harness webbing (front view showing D-ring)thumb|upright=0.5|Lead clip-on diving weight by Draeger with bronze spring clip, c. 1980
These are weights which attach to the harness directly, but are removable by disengaging the clip mechanism. They can also be used to temporarily increase the weight of a conventional weight belt. Various sizes have been available, ranging from around 0.5 to 5 kg or more. The larger models are intended as ditchable primary weights, and are used in the same way as BCD integral weights or weight harness weighs, but clipped to the backplate or sidemount harness webbing, and the smaller versions are also useful at trim weights.
Backpack weight pouch
Some rebreathers (e.g. the Siebe Gorman CDBA) have a pouch containing lead balls each a bit over an inch diameter. The diver can release them by pulling a cord.
Fixed weights
Surface-supplied divers often carry their weights securely attached to reduce the risk of accidentally dropping them during a dive and losing control of their buoyancy. These may be carried on a weight belt with a secure buckle, supported by a weight harness, connected directly to the diving safety harness, or suspended from the corselet of the helmet. Heavily weighted footwear may also be used to stabilise the diver in an upright position.
In addition to the weight that can be dropped easily ('ditched'), some scuba divers add additional fixed weights to their gear, either to reduce the weight placed on the belt, which can cause lower back pain, or to shift the diver's center of mass to achieve the optimum position in the water.
- Tank weights are attached to the diving cylinder to shift the center of mass backward and towards the head or feet, depending on placement.
- V-weights are long, narrow, weights which are carried in the groove between twinned cylinders. They may be carried singly or as a pair. Traditionally wedge sectioned lead castings, but also found in solid cylindrical format and as long narrow webbing weight pockets filled with shot.
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Notes
Derivation of formula for equivalent apparent weight in water.
:Density = mass/volume, ρ = m/V so m = ρ × V
:Buoyancy in water: B = (ρ - ρ<sub>water</sub>) × V × g, where g = gravitational acceleration at earth' surface
:For two objects of different densities but the same buoyancy in water: B<sub>1</sub> = B<sub>2</sub> so (ρ<sub>1</sub> - ρ<sub>water</sub>) × V<sub>1</sub> × g = (ρ<sub>2</sub> - ρ<sub>water</sub>) × V<sub>2</sub> × g (g can be dropped from both sides)
::therefore: V<sub>1</sub> = V<sub>2</sub> × (ρ<sub>2</sub> - ρ<sub>water</sub>) ÷ (ρ<sub>1</sub> - ρ<sub>water</sub>)
:Also, for the same two objects in air (ignoring the buoyancy of the air): m<sub>1</sub> = ρ<sub>1</sub> × V<sub>1</sub> and m<sub>2</sub> = ρ<sub>2</sub> × V<sub>2</sub>
::by substitution: m<sub>1</sub> ÷ m<sub>2</sub> = (ρ<sub>1</sub> ÷ ρ<sub>2</sub>) × ((ρ<sub>2</sub> - ρ<sub>water</sub>) ÷ (ρ<sub>1</sub> - ρ<sub>water</sub>))
::so: m<sub>1</sub> = (ρ<sub>1</sub> ÷ ρ<sub>2</sub>) × ((ρ<sub>2</sub> - ρ<sub>water</sub>) ÷ (ρ<sub>1</sub> - ρ<sub>water</sub>)) × m<sub>2</sub>
:And the same works with SG in place of density: m<sub>1</sub> = (SG<sub>1</sub> ÷ SG<sub>2</sub>) × ((SG<sub>2</sub> - SG<sub>water</sub>) ÷ (SG<sub>1</sub> - SG<sub>water</sub>)) × m<sub>2</sub>
:And since SG<sub>water</sub> = 1: m<sub>1</sub> = (SG<sub>1</sub> ÷ SG<sub>2</sub>) × ((SG<sub>2</sub> - 1) ÷ (SG<sub>1</sub> - 1)) × m<sub>2</sub>
:Substituting values for 1 kg lead, iron gives: 1kg lead × (7.87/11.34) × ((11.34-1)/(7.87-1)) = 1.044kg iron