The word "SCUBA" began as an acronym, but it is now usually thought of as a regular word—"scuba". It has become acceptable to refer to "scuba equipment" or "scuba apparatus"—examples of the linguistic RAS syndrome.
The first commercially successful scuba sets were the Aqualung open-circuit units developed by Emile Gagnan and Jacques-Yves Cousteau, in which compressed gas (usually air) is inhaled from a tank and then exhaled into the water adjacent to the tank. However, the scuba regulators of today trace their origins to Australia, where Ted Eldred developed the first mouth piece regulator, known as the Porpoise. This regulator was developed because patents protected the Aqualung's double hose design. It separated the cylinder from the demand valve giving the diver air at the same pressure surrounding his mouth, not surrounding the tank.
The open circuit systems were developed after Cousteau had a number of incidents of oxygen toxicity using a rebreather system, in which exhaled air is reprocessed to remove carbon dioxide. Modern versions of rebreather systems (both semi-closed circuit and closed circuit) are still available today, and form the second main type of scuba unit, most commonly used for technical diving, such as deep diving.
Types of diving
| Type of diving | Classification |
|---|---|
| Aquarium maintenance in large public aquariums | Commercial, scientific |
| Boat and ship inspection, cleaning and maintenance | Commercial, naval |
| Cave diving | Technical, recreational |
| Civil engineering in harbors, water supply, and drainage systems | Commercial |
| Crude oil industry and other offshore construction and maintenance | Commercial |
| Demolition and salvage of ship wrecks | Commercial, naval |
| Diver training for reward | Professional |
| Fish farm maintenance | Commercial |
| Fishing, e.g. for abalones, crabs, lobsters, pearls, scallops, sea crayfish, sponges | Commercial |
| Frogman, manned torpedo | Military |
| Harbor clearance and maintenance | Commercial, military |
| Media diving: making television programs, etc. | Professional |
| Mine clearance and bomb disposal, disposing of unexploded ordnance | Military, naval |
| Pleasure, leisure, sport | Recreational |
| Policing: diving to investigate or arrest unauthorized divers | Police diving, military, naval |
| Search and recovery diving | Commercial |
| Search and rescue diving | Police, naval |
| Spear fishing | Professional (occasionally), recreational |
| Stealthy infiltration | Military |
| Surveys and mapping | Scientific |
| Marine biology | Scientific, recreational |
| Underwater archaeology (shipwrecks; harbors, and buildings) | Scientific, recreational |
| Underwater inspections and surveys | Commercial, military |
| Underwater photography | Professional, recreational |
| Underwater tourism | Recreational |
| Underwater welding | Commercial |
Table of Hand Signals
 |
| http://www.google.com/ |
| No. | Signal | Meaning | Comment |
|---|---|---|---|
| 1. | Hand raised, fingers pointed up, palm to receiver. | STOP | Transmitted in the same way as a traffic police officer’s STOP |
| 2. | Thumb extended downward from clenched fist. | GO DOWN or GOING DOWN | |
| 3. | Thumb extended upward from clenched fist. | GO UP or GOING UP | |
| 4. | Thumb and forefinger making a circle with three remaining fingers extended (if possible). | OK! or OK? | Divers wearing mittens may not be able to extend 3 remaining fingers distinctly. |
| 5. | Two arms extended overhead with finger tips touching above head to make a large O shape. | OK! or OK? | A diver with only one free arm may make this signal by extending that arm overhead with finger tips touching top of head to make the O shape. Signal is for long-range use. |
| 6. | Hand flat, fingers together, palm down, thumb sticking out, then hand rocking back and forth on axis of forearm. | SOMETHING IS WRONG | This is the opposite of OK! The signal does not indicate emergency. |
| 7. | Hand waving over head (may also thrash hand on water). | DISTRESS | Indicates immediate aid required. |
| 8. | Fist pounding on chest. | LOW ON AIR | Indicates signaler's air supply is reduced. |
| 9. | Hand slashing or chopping throat. | OUT OF AIR | Indicates that the signaler cannot breathe. |
| 10. | Clenched fist on arm extended in direction of danger. | DANGER |
Source: http://en.wikipedia.org/wiki/Scuba_diving
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