User:Pbsouthwood/Low visibility diving

Low visibility diving is underwater diving where the visibility through the water is limited to distances which affect the ability of the diver to perform standard procedures, or the dive task. Some recreational agencies group low visibility with night diving, as some of the procedures are common to both, but mere lack of ambient light is not normally considered a low visibility situation by technical of professional divers, as vision is easily restored by artificial lighting when the water is clear. The more widespread meaning implies low water clarity.

Low visibility
Underwater visibility is a term which generally predicts the ability of some human or instrument to detect an object in that environment, commonly expressed as the estimated or measured distance at which an object of high contrast, like a Secchi disk or light can be discerned against a neutral background. It usually refers to horizontal visibility unless specified.

Low visibility is defined by NOAA for operational purposes as: "When visual contact with the dive buddy can no longer be maintained."

DAN-Southern Africa suggest that limited visibility is when a "buddy cannot be discerned at a distance greater than 3 metres."

Turbidity
Suspended particlulate material like silt, clay or other light or fine sediments affect the visibility underwater in much the same way as fog effects visibility in air. Loss of visibility caused by particles depends on the density, type, and amount of sediment suspended in the water. fine sediment will be suspended easily and remain in suspension for a long time, and in large quantities will reduce the visibility to nearly zero almost immediatelely. Larger denser particles are not as easily suspended and settle out more quickly.

Organic particles, algal blooms.

Salinity Gradients (Haloclines) Temperature Gradients (Thermoclines)

Dissolved organic and inorganic materials. Hydrogen sulfide, tannins,

Lack of light (night diving, penetration diving) Equipment problems - mask fogging

Reasons for low visibility diving
For professional divers, the task may happen to be in an area of temporary or permanent low visibility, and though some tasks cannot be done in low visibility, such as fish counts, other work can be done by feel if necessary. Public safety diving and police diving are often done in low visibility. Underwater archaeology often involves work which stirs up bottom deposits, which may take considerable time to disperse.

Recreational divers may dive in low visibility for training, because there is no convenient site with better visibility, or because there is some other attraction to the site, such as in muck diving, or looking for Megalodon teeth in turbid rivers. Technical divers may find themselves in low visibility due to silt-out during cave or wreck dives, and have to manage the problem correctly to survive.

Muck diving
Muck diving is recreational diving over a loose sediment substrate which causes low visibility, but is the habitat for interesting organisms found nowhere else. These organisms have in some cases evolved to be well camouflaged in this habitat, and are an attraction particularly to underwater photographers and naturalists. The low visibility can also sometimes allow divers to get closer to the animals.

Sudden changes
Tidal exchange, weather conditions, usually due to wave action, also sometimes upwellings, runoff of rainwater, rivers in flood, divers disturbing silt, algal blooms (usually not too fast )

Hazards
Buddy separation, navigation problems, depth monitoring, reading instruments, communication confusion, disorientation, panic.

Working
Effects on diver efficiency, task loading of maintaining buddy contact

Communications
Rope, light, and touch signals

Through-water voice communication

Navigation
Use of Guidelines

Avoiding siltout
Finning techniques that reduce the risk of disturbing silt

Precautions to enable reliable navigation in low or zero visibility

Dealing with sudden loss of visibility
Finding the way out of an overhead environment

Reducing disorientation - which way is up

Training and certification
Entry level recreational divers are not usually trained to manage low visibility unless that is the prevailing condition where they learn to dive. Additional training is often needed. Training programs such as night diving are offered for those who need or want the basic skills of diving in low visibility. Cave diving training concentrates on avoiding and managing siltout and exiting the overhead in zero visibility as a survival skill.

Professional divers are generally expected to be able to deal with low visibility without special training, so the basic skills are included in entry level training, but the skills require some practice to allow the diver to work at reasonable efficiency, when the work can actually be done min low visibility. This may require the diver to be able to do some tasks by feel. Familiarisation and practice with tools and equipment out of the water can make a large difference to efficiency. This may include doing the job blindfolded for practice. Some work simply cannot be done effectively if conditions are too poor.