What is there to see underwater?

• Underwater wildlife, such as:
  • Fish - shoals, individuals, ones that hide under sand, ones with big teeth, flat ones, long ones
  • Crustaceans - lobsters, crabs and crayfish are fantastic looking animals
  • Conger eels - scary but not aggressive
  • Shells and molluscs - pretty or ugly
  • Octopus and cuttlefish - colour changing skins and ink jets
  • Tiny animals like nudibranchs (brightly coloured sea slugs) and small crustaceans
  • Sponges, corals, worms - sometimes colourful or wierd
• Wrecks. They are interesting because:
  • You can explore the layout and parts of a ship - often they are very broken up, but you can still see what's happening
  • Wrecks form havens for underwater wild life - shoals of fish, conger eels and crustaceans use wrecks as a habitat
  • Each wreck has a history - often they are tragic involving great suffering
• Unusual geography. Sometimes we dive tall walls, or steep canyons, pinnacles or gullies. You never know what is around the next corner. There is wildlife to be seen if you look carefully. Often we find wreckage too.

How deep do you dive?

It depends on the gas we breathe. The maximums for each type of gas mix are :

• between 6 m and 50m for nitrox mixes
• 50m for air
• 70m for trimix.

It depends on the diver's qualification:

How long do you spend underwater?

That depends on what's down there to see, how much gas you take with you and how far you push yourself. Here are some typical dives:

• a shore dive or a drift dive to a sea bed of 15m. With a full 12 litre cylinder you can probably dive for 45 minutes.
• a 5m tall wreck lying on a flat sea bed at 35m. You could spend 25 minutes looking around it at depths of 25m to 35m then you come up to 6m for a 10 minute decompression stop - total 40 minutes.
• a dive on an underwater slope from 35m to 6m. So, you spend 45 minutes looking around it, starting at 35m but slowly ascending to 6m throughout the dive.

How much oxygen do you use on a dive?

Firstly, we very rarely breathe pure oxygen. On most of the dives in our club we breathe air - that's a mixture of 21% oxygen and 79% nitrogen.

Some people use other gases or mixes to do longer or deeper dives. It is possible to use nitrox (oxygen enriched air), trimix (helium, oxygen and nitrogen) or pure oxygen for decompression (but only in water shallower than 6m because it is poisonous at greater depths).

But to answer the question, how much air you use depends on three things: how deep you go, how much gas you carry in your cylinders and how good your air consumption is:

• because of the increase in pressure of the gas you breath at depth, the deeper you go, the more air you breathe. At 10m you breathe twice as much air as on the surface. At 20m you consume 3 times as much air, at 30m it's 4 times, and so on.
• most divers carry a 12 litre, two 7 litre or a 15 litre cylinder filled to a pressure of between 200 bar (3000 psi) and 300 bar (4500 psi). To calculate the available gas (in litres) multiply the cylinder size (in litres) by the pressure (in bars).
• most divers consume between 12 litres per minute (lpm) and 25 lpm during normally diving activity.

Do I really need diving qualifications?

How far can you see underwater?

Typically, in this country it varies from between 1m or 2m and 10m to 15m with an average of 5m. The best visibility is generally to the west - Cornwall and the Hebrides are good.

Normally we carry powerful torches to help see underwater. Particles suspended in the water reduce underwater visibility by reducing the amount of sunlight penetrating deep water and by reflecting back torchlight.

If you dive near to the mouth of a river estuary or where the sea bed is muddy or sandy, the underwater visibility is reduced. Strong tidal currents stir up the sea bed and human settlements with their pollution also contribute to low visibility.

The underwater visibility in tropical seas can be as high as 30m or 40m.

How far out to sea do you dive?

What is the temperature of the water you dive in?

Type of water	Summer	Winter
English Channel	19oC	5oC
North Sea	15oC	3oC
Western Scotland	17oC	5oC
Red Sea	27oC	22oC
Maldives	28oC	28oC
British inland sites	5-18oC	3oC

Often at inland sites the surface temperature is greater than that at depth. Its warm in summer on the surface but cold at depth all year round.

In the sea around the British Isles the water temperature is fairly similar at all depths - the strong tides ensure the warm surface water mixes with the colder water beneath.

If you dive in the sea in areas where fresh water meets sea water, for example at river mouths and Scottish lochs, you find that fresh water is generally colder, browner and being less dense it floats above the sea water. The boundary between fresh and sea water causes light to shimmer making it seem like you can't focus your vision.

The temperature of the sea water on the south and west of Britain is strongly influenced by the climate of the tropical Atlantic. As our sea water comes to us on the Gulf Stream, there is a couple of months time lag between each British season and its matching sea water temperature - the seas takes along time to warm up in summer but it stays warm into autumn.

How much does the equipment cost?

What sort of issues does the diving community debate?

What is Technical Diving?