Editing Diving weighting system (section)

==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====
[[Glossary of underwater diving terminology#buoyancy control|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.<ref name="fund2006-33-35" />

The scuba diver generally has an operational need to control depth without resorting to a line to the surface or holding onto a structure or landform, or resting on the bottom. This requires the ability to achieve neutral buoyancy at any time during a dive, otherwise the effort expended to maintain depth by swimming against the buoyancy difference will both task load the diver and require an otherwise unnecessary expenditure of energy, increasing air consumption, and increasing the risk of loss of control and escalation to an accident.<ref name=CMASISATxManual /><ref name="Knedlik" /> Maintaining depth by finning necessarily directs part of fin thrust upwards or downwards, and when near the bottom, downward thrust can disturb the [[benthos]] and stir up silt. The risk of fin-strike damage is also significant.<ref name="Hammerton 2014" /> 

A further requirement for scuba diving in most circumstances, is the ability to achieve significant positive buoyancy at any point of a dive.<ref name="Knedlik" /><ref name="worksafe" /><ref name="Media diving CoP" /> When at the surface, this is a standard procedure to enhance safety and convenience, and underwater it is generally a response to an emergency.

The average human body with a relaxed lungful of air is close to neutral buoyancy. If the air is exhaled, most people will sink in fresh water, and with full lungs, most will float in seawater. The amount of weight required to provide neutral buoyancy to the naked diver is usually trivial, though there are some people who require several kilograms of weight to become neutral in seawater due to low average density and large size. This is usually the case with people with a large proportion of body fat. As the diver is nearly neutral, most ballasting is needed to compensate for the buoyancy of the diver's equipment.<ref name="BSAC manual" />

The main components of the average scuba diver's equipment which are positively buoyant are the components of the exposure suit. The two most commonly used exposure suit types are the [[dry suit]] and the [[wet suit]]. Both of these types of exposure suit use gas spaces to provide insulation, and these gas spaces are inherently buoyant. The buoyancy of a wet suit will decrease significantly with an increase in depth as the ambient pressure causes the volume of the gas bubbles in the neoprene to decrease. Measurements of volume change of neoprene foam used for wetsuits under hydrostatic compression show that about 30% of the volume, and therefore 30% of surface buoyancy, is lost in about the first 10&nbsp;m, another 30% by about 60&nbsp;m, and the volume appears to stabilise at about 65% loss by about 100&nbsp;m.<ref name="Bardy2005" /> The total buoyancy loss of a wetsuit is proportional to the initial uncompressed volume. An average person has a surface area of about 2&nbsp;m<sup>2</sup>,<ref name="Gallo 2017" /> so the uncompressed volume of a full one piece 6&nbsp;mm thick wetsuit will be in the order of 1.75 x 0.006 = 0.0105&nbsp;m<sup>3</sup>, or roughly 10 litres. The mass will depend on the specific formulation of the foam, but will probably be in the order of 4&nbsp;kg, for a net buoyancy of about 6&nbsp;kg at the surface. Depending on the overall buoyancy of the diver, this will generally require 6&nbsp;kg of additional weight to bring the diver to neutral buoyancy to allow reasonably easy descent  The volume lost at 10&nbsp;m is about 3litres, or 3&nbsp;kg of buoyancy, rising to about 6&nbsp;kg buoyancy lost at about 60&nbsp;m. This could nearly double for a large person wearing a two-piece suit for cold water. This loss of buoyancy must be balanced by inflating the buoyancy compensator to maintain neutral buoyancy at depth.
A dry suit will also compress with depth, but the air space inside is continuous and can be topped up from a cylinder or vented to maintain an approximately constant volume. A large part of the ballast used by a diver is to balance the buoyancy of this gas space, but if the dry suit has a catastrophic flood, much of this buoyancy may be lost, and some way to compensate is necessary.<ref name=CMASISATxManual /><ref name="BSAC manual" />

Another significant issue in [[Scuba diving#Open-circuit|open circuit scuba]] diver weighting is that the breathing gas is used up during a dive, and this gas has weight, so the total weight of the cylinder decreases, while its volume remains almost unchanged. As the diver needs to be neutral at the end of the dive, particularly at shallow depths for obligatory or safety [[Decompression practice#Decompression stops|decompression stops]], sufficient ballast weight must be carried to allow for this reduction in weight of gas supply. (the density of air at normal atmospheric pressure is approximately 1.2&nbsp;kg/m<sup>3</sup>, or approximately 0.075&nbsp;lb/ft<sup>3</sup>) The amount of weight needed to compensate for gas use is easily calculable once the [[Glossary of underwater diving terminology#free gas volume|free gas volume]] and [[density]] are known.

Most of the rest of the diver's equipment is negatively buoyant or nearly neutral, and more importantly, does not change in buoyancy during a dive, so its overall influence on buoyancy is static.

While it is possible to calculate the required ballast given the diver and all his or her equipment, this is not done in practice, as all the values would have to be measured accurately. The practical procedure is known as a [[Glossary of underwater diving terminology#buoyancy check|buoyancy check]], and is done by wearing all the equipment, with the tank(s) nearly empty, and the buoyancy compensator empty, in shallow water, and adding or removing weight until the diver is neutrally buoyant. The weight should then be distributed on the diver to provide correct trim, and a sufficient part of the weight should be carried in such a way that it can be removed quickly in an emergency to provide positive buoyancy at any point in the dive. This is not always possible, and in those cases an alternative method of providing positive buoyancy should be used.<ref name="Knedlik" /><ref name="worksafe" /><ref name="Media diving CoP" />

A diver ballasted by following this procedure will be negatively buoyant during most of the dive unless the buoyancy compensator is used, to an extent which depends on the amount of breathing gas carried. A recreational dive using a single cylinder may use between 2 and 3&nbsp;kg of gas during the dive, which is easy to manage, and provided that there is no decompression obligation, end-dive buoyancy is not critical. A long or deep technical dive may use 6&nbsp;kg of back gas and another 2 to 3&nbsp;kg of decompression gas. If there is a problem during the dive and reserves must be used, this could increase by up to 50%, and the diver must be able to stay down at the shallowest decompression stop. The extra weight and therefore negative buoyancy at the start of the dive could easily be as much as 13&nbsp;kg for a diver carrying four cylinders. The buoyancy compensator is partially inflated when needed to support this negative buoyancy, and as breathing gas is used up during the dive, the volume of the buoyancy compensator will be reduced, by venting as required. The inconvenience of additional weight and managing the gas required to compensate for it in a dive that goes according to plan is the price that must be paid for the ability to decompress after an emergency which uses up most of the gas. There is little value in having enough gas to avoid drowning if the diver is killed or crippled by decompression sickness instead.<ref name=CMASISATxManual /><ref name="Cudel 2021" />

Examples:
* The common 80&nbsp;ft<sup>3</sup> (11 litre, 207 bar) cylinder carries about {{convert|6|lb|kg}} of air when full, so the diver should start the dive about {{convert|6|lb|kg}} negative and use about 1/10&nbsp;ft<sup>3</sup> (2.7&nbsp;L)of air in the BCD to compensate at the start of a dive.
* A twin 12.2 litre 230 bar set carries about {{convert|6.7|kg|lb}} of Nitrox when full, so the diver should start the dive about {{convert|6.7|kg|lb}} negative and use about {{convert|6.7|liter|cuft}} of gas in the BCD at the start of the dive.
* A twin 12.2 litre 230 bar with an 11 litre 207 bar deep deco mix and a 5.5 litre 207 bar shallow deco gas will carry {{convert|10.7|kg|lb}} of gas, and while it is unlikely that all will be used on the dive, it is possible, and the diver should be able to remain at the correct depth for decompression until all the gas is used up.
<!-- * A twin 15 litre 230 bar set with trimix bottom gas, with four 11 litre 207 bar sling cylinder for travel gas, deep bailout, deep and shallow decompression gas, would carry about {{convert||kg|lb}} of gas, which would require a minimum BCD volume of {{convert||liter|cuft}}. -->

====Optimum weighting====
<!--target for redirects [[Optimum scuba weighting]], [[Correct scuba weighting]] -->
Optimum weighting for scuba allows the diver to achieve neutral buoyancy at any time during a dive while there is still usable breathing gas in any of the cylinders carried, using the least amount of ballast. Deviations from this optimum either make the diver buoyant while there is still usable breathing gas, which is a disadvantage in emergencies where decompression stops are required, or make the diver more negatively buoyant than necessary at the start of the dive with full cylinders, necessitating more gas in the buoyancy compensator for most of the dive, which is more sensitive to buoyancy changes with change in depth, and may make a larger buoyancy compensator necessary. These disadvantages can be compensated by skill, but more attention and effort is required throughout the dive.<ref name="CMASISATxManual" />

====Surface-supplied diving====
In [[surface-supplied diving]], and particularly in [[saturation diving]], the loss of weights followed by positive buoyancy can expose the diver to potentially fatal [[Decompression illness|decompression injury]]. Consequently, weight systems for surface-supplied diving where the diver is transported to the worksite by a [[diving bell]] or [[Decompression practice#Diving stage|stage]], are usually not provided with a quick-release system.

Much of the work done by surface-supplied divers is on the bottom, and weighted boots may be used to allow the diver to walk upright on the bottom. When working in this mode, several kilograms beyond the requirement for neutralising buoyancy may be useful, so that the diver is reasonably steady on the bottom and can exert useful force when working.

The [[Diving helmet#Lightweight demand helmets|lightweight demand helmets]] in general use by surface-supplied divers are integrally ballasted for neutral buoyancy in the water, so they do not float off the diver's head or pull upwards on the neck, but the larger volume [[Diving helmet#Free-flow helmets|free-flow helmets]] would be too heavy and cumbersome if they had all the required weight built in. Therefore, they are either ballasted after dressing the diver by fastening weights to the lower parts of the helmet assembly, so the weight is carried on the shoulders when out of the water, or the helmet may be held down by a [[Glossary of underwater diving terminology#jocking strap|jocking strap]] and the harness weights provide the ballast.

The traditional [[Standard diving dress|copper helmet and corselet]] were generally weighted by suspending a large weight from support points on the front and back of the corselet, and the diver often also wore weighted boots to assist in remaining upright. The US Navy Mk V standard diving system used a heavy weighted belt buckled around the waist, suspended by shoulder straps which crossed over the breastplate of the helmet, directly transferring the load to the buoyant helmet when immersed, but with a relatively low centre of gravity. Combined with lacing of the suit legs and heavy weighted shoes, this reduced the risk of inversion accidents.<ref name="Mark V" />

===Trim===
{{main article|Diver trim}}
[[File:Diver swimming with head up trim.png|thumb|Diver trimmed with weight far towards the feet: The static moments of buoyancy and weight cause the feet to rotate downwards, and the thrust from finning is then also directed downwards]]
[[File:Diver trimmed level.png|thumb|Diver with weight and centre of buoyancy aligned for level trim: The static moments of buoyancy and weight keep the diver horizontal, and fin thrust can be aligned with direction of motion for best efficiency]]
 
Trim is the diver's attitude in the water, in terms of balance and alignment with the direction of motion. Optimum trim depends on the task at hand. For recreational divers this is usually swimming horizontally or observing the environment without making contact with benthic organisms.<ref name="CMASISATxManual" /> Ascent and descent at neutral buoyancy can be controlled well in horizontal or head-up trim, and descent can be most energy efficient head down, if the diver can effectively equalise the ears in this position. Freediving descents are usually head down, as the diver is usually buoyant at the start of the dive, and must fin downwards. Professional divers usually have work to do at the bottom, often in a fixed location, which is usually easier in upright trim, and some diving equipment is more comfortable and safer to use when relatively upright. 

Accurately controlled trim reduces horizontal swimming effort, as it reduces the sectional area of the diver passing through the water. A slight head down trim is recommended to reduce downward directed fin thrust during finning, and this reduces [[silting]] and fin impact with the bottom.<ref name="fund2006-35-37" />

Trim weighting is mainly of importance to the free-swimming diver, and within this category is used extensively by scuba divers to allow the diver to remain horizontal in the water without effort. This ability is of great importance for both convenience and safety, and also reduces the environmental impact of divers on fragile benthic communities.<ref name="Hammerton 2014" />

The free-swimming diver may need to trim erect or inverted at times, but in general, a horizontal trim has advantages both for reduction of drag when swimming horizontally, and for observing the bottom. A horizontal trim allows the diver to direct propulsive thrust from the fins directly to the rear, which minimises disturbance of sediments on the bottom, and reduces the risk of striking delicate benthic organisms with the fins. A stable horizontal trim requires that diver's [[centre of gravity]] is directly below the centre of buoyancy (the [[centroid]]). Small errors can be compensated fairly easily, but large offsets may make it necessary for the diver to constantly exert significant effort towards maintaining the desired attitude, if it is actually possible.<ref name="CMASISATxManual" /><ref name="fund2006-33-35" />  

The position of the centre of buoyancy is largely beyond the control of the diver, though some control of suit volume is possible, the cylinder(s) may be shifted in the harness by a small amount, and the volume distribution of the buoyancy compensator has a large influence when inflated. Most of the control of trim available to the diver is in the positioning of ballast weights. The main ballast weights therefore should be placed as far as possible to provide an approximately neutral trim, which is usually possible by wearing the weights around the waist or just above the hips on a weight belt, or in weight pockets provided in the buoyancy compensator jacket or harness for this purpose. Fine tuning of trim can be done by placing smaller weights along the length of the diver to bring the centre of gravity to the desired position. There are several ways this can be done.<ref name="McCafferty and Seery 2014" />

Ankle weights provide a large lever arm for a small amount of weight and are very effective at correcting head-down trim problems, but the addition of mass to the feet increases the work of propulsion significantly. This may not be noticed on a relaxed dive, where there is no need to swim far or fast, but if there is an emergency and the diver needs to swim hard, ankle weights will be a significant handicap, particularly if the diver is marginally fit for the conditions.

Tank bottom weights provide a much shorter lever arm, so need to be a much larger proportion of the total ballast, but do not interfere with propulsive efficiency the way ankle weights do. There are not really any other convenient places below the weight belt to add trim weights, so the most effective option is to carry the main weights as low as necessary, by using a suitable harness or integrated weight pocket buoyancy compensator which actually allows the weights to be placed correctly, so there is no need for longitudinal trim correction.

A less common problem is found when rebreathers have a counterlung towards the top of the torso. In this case there may be a need to attach weights near the counterlung. This is usually not a problem, and weight pockets for this purpose are often built into the rebreather harness or casing, and if necessary weights can be attached to the harness shoulder straps.