Backplate and wing

A backplate and wing (often abbreviated as BP&W or BP/W), is a type of buoyancy compensation device (BCD) worn by scuba divers. Unlike most other BCDs, the backplate and wing is a modular system, in that it consists of separable components.

The core components of this system are:
* A harness, which attaches the system to the diver.
* The backplate, a plate normally made from metal which sits against the diver’s back and to which the diver’s cylinder or cylinders are attached.
* An inflatable buoyancy bladder known as a wing, which is sandwiched between the backplate and the cylinder(s).

Configuration

Harness

The standard harness used with a backplate consists of a single piece of Nylon webbing, which passes through slots in the backplate to form closed shoulder loops and an open waist strap, with a weightbelt buckle then securing the waist strap. A second section of webbing forms a crotch strap, running from the bottom of the backplate, between the diver’s legs, and up to the waist strap, which would be passed through a loop at the front end of the crotch strap. This harness is sometimes referred to as a one-piece harness, due to the shoulder and waist straps being made from a single piece of webbing.

The harness typically has D-rings located on the shoulder straps, waist strap and crotch strap, which are used to attach equipment to the diver. A common configuration is a single D-ring located at the front of each shoulder, on the left hip, at the front of the crotch strap and at the rear of the crotch strap. Additionally, a loop of elastic cord is normally attached at the same location as the left shoulder D-ring, and is used to secure the wing’s inflator.

This form of harness may be adjusted to fit different builds of diver by shortening the webbing used to tighten the harness, or installing a new, longer section of webbing to loosen the harness. Once adjusted, some flexibility is still allowed by the positioning of the buckle, which can alter the effective length of the waist strap depending on the position in which it is secured.

Many variations to this basic harness are used by different divers, and these may include:
* Differing number or positioning of D-rings.
* Shoulder straps that cross behind the diver’s neck.
* The absence of a crotch strap.
* Quick release clips on shoulder straps or crotch strap.
* An additional chest strap, which crosses the diver’s Sternum.
* Padding, most commonly around the shoulders of the harness.

Some manufacturers offer alternative harnesses, often marketed as “deluxe” options, which may include the above variations.

Backplate

The backplate is usually constructed from a single piece of stainless steel or anodised aluminium, bent along four lines to form a channel running vertically down the center. The plate is approximately 15” (38cm) long and 10” (25cm) wide. There are two slots cut near each of the shoulders and hips, where the harness passes through, and another slot at the bottom of the plate where the crotch strap attaches. There is also one or more pair of holes cut into the channel to be used for cylinder attachment, with the holes of each pair being spaced 11” apart.

Steel backplates are commonly used when the buoyancy of the diver’s other equipment (primarily cylinders and exposure protection) would require them to wear a weightbelt, as the negative buoyancy of the Steel plate can replace some of this weight. Aluminium backplates are commonly used when the diver would not require a weightbelt (such as when wearing heavy steel cylinders) or when the mass of the backplate must be kept low for air travel. Backplates are occasionally made from other materials, including Titanium and ABS plastic, but their use is quite rare.

Wing

The wing is an inflatable buoyancy bladder, similar to that found in other varieties of BCD, with the exception that it is not contained in or permanently attached to the harness part of a BCD. As with other BCDs, wings possess an inflation valve on a corrugated hose, dump valve and over pressure valve. Wings are usually oval-shaped or U-shaped, and are designed to wrap slightly around the diving cylinder(s) when inflated.

Wings are usually designed to be used with either a single diving cylinder or twin cylinders, although some manufacturers do produce wings that they recommend for both single and twin cylinder diving. Single-cylinder wings are most commonly oval-shaped and are narrow, while twin-cylinder wings are usually U-shaped and wider.

Some wings, known as "bungee wings", incorporate systems that use elastic to constrain the wing when it is less than completely full and accelerate air dumping. These wings will usually have elastic bands wrapping the bladder area, although some designs aim for the same result using an elasticised shell. Proponents of bungee wings often claim that the constriction of the wing increases streamlining and ease of gas dumping. Detractors, however, often claim that the bungee will only increase streamlining if the wing is inappropriately large and that the bungee would forcefully expel gas from the wing in the case of a small puncture. Some detractors also allege that the bungee creates a rough surface, increasing drag, however, in reality it is extremely unlikely that any increase in drag would be significant. While drag is a minor concern, entanglement is a real hazard, for this reason, many technical and cave divers use "bungee-less" wings. Some companies, such as [http://www.omsdive.com OMS] and [http://www.diverite.com Dive Rite] allow the purchaser to choose which style they prefer.

Another variety, "dual bladder wings", contain a second, redundant bladder and inflation assembly, with the second bladder being intended for use in the event of the primary bladder failing, either through a puncture, or through a non-functional inflation valve. Many technical divers may choose a dual bladder wing in order to have backup redundancy in the remote possibility of primary bladder failure.

Cylinder attachment

When a backplate is used with a single cylinder, a single tank adapter, or STA, is usually employed. The STA is a small metal structure that bolts on to the backplate on the outside of the wings, contains two camstraps, and accommodates the cylinder.

In some instances, the STA may be omitted, and the camstraps threaded through the wing and backplate. Occasionally, the wing or backplate will contain a built-in STA or may be manufactured with a slight channel in the central ridge which allows the single cylinder to locate.

Twin cylinders are usually attached to the backplate via bolts, passing through the cylinder bands. An alternative is to use two sets of camstraps and extra slots in the backplate and wing. This arrangement will allow the convenient attachment of independent cylinders of almost any size without the use of cylinder bands.

Some rebreather divers fit backplates to their rebreathers. The exact method of attachment varies between users and rebreather models, and may include modification to the rebreather or the use of a customised backplate. Some rebreathers are designed specifically for use with backplates.

Ancillary equipment

The backplate and wing itself is an extremely minimalistic system, however it does facilitate the addition of other equipment. Ancillary equipment, commonly attached to the backplate and wing includes:
* Stage or decompression cylinders.
* “Tow behind” diver propulsion vehicles, which utilize a tow-cord, attaching to the crotch strap D-ring of the harness.
* Pockets.
* Knife sheaths.
* Lights/Torches, and light/torch battery canisters.

Much of this equipment is difficult or impossible to attach to many makes or other styles of BCD.

Differences from other styles of BCD

The predominant type of BCD currently used is the jacket style BCD.

The backplate and wing differs from the jacket style primarily in the way that the functions required of a BCD (attachment to diver, buoyancy control and attachment to cylinder(s)) are performed by distinct components, rather than a single unit. The most significant effects of this division are the shifting of the buoyancy bladder from the diver’s chest to their back and the modularity of the system, allowing buoyancy cells, harnesses and plates to be interchanged as needed. Furthermore, the buoyancy of a backplate is often significantly negative, especially when the plate is made from Stainless Steel, and so can act as a replacement for some of the weight that would otherwise be worn in a weightbelt. Ancillary features that would often be present in jacket BCDs, such as pockets or weight integration, are not found in the core system of a backplate and wing, but can be added as additional components if desired.

Other types of BCD exist which bear more of a resemblance to backplate and wing BCD. Rear inflation BCDs are similar in design to jacket BCDs, with the exception of the buoyancy cell location. In a rear inflation BCD, as with a backplate and wing, the cell is located behind the diver’s back, and is of similar shape to a wing. However, a rear inflation BCD usually does not possess the modularity of a backplate and wing, although some models do allow buoyancy cells to be substituted.

Softpack BCDs are another style closer still to backplate and wing BCDs. Softpacks, like backplates, are designed to be modular, and are often marketed towards technical divers. A softpack consists of a padded, semi-rigid section that serves that same purpose as a backplate, and uses a harness that is either replaceable, like a backplate harness, or permanently fixed. Softpacks may be used with the same models of wings that backplates are used with. The primary differences between these and backplates are the lack of a rigid plate and possible non-separability of the softpack and harness.

A minimalist form of softpack harness sometimes known as the "Capepac" comprises a set of webbing straps much like that of the backplate, but with a webbing strap in place of the plate. This strap may be formed by stitching or threading through sliders a double layer of webbing with slots between the layers through which the cambands pass and the wing is sandwiched between harness and cylinder. There is no need for a plate as the cylinder forms the rigid part of the assembly. This arrangement is best suited to single cylinders, and can be made very compact and light for travelling. In some cases a stabiliser plate may be included at the base of the vertical strap, and the harness shoulder and waistband straps thread through this as is done on the backplate.