Frequently Asked Questions

Below is a list of questions and answers that many people ask about SCUBA diving....

How long can you stay down for?

The Dive Time is restricted not only by the amount of air in the cylinder (and how quickly you breath!), but also by the maximum depth during the dive.

Most notably, Professor Buhlmann published tables outlining Dive Times against Depths. Essentially, the deeper you go the less time you can stay there for. This is because more Nitrogen is dissolved in the body at depth and at a faster rate. Shallow depths allow times which tend to be more restricted by the available air.

The table breaks the times down into two sections; No-Stop Dives and Stop (Decompression) Dives. The former dives allow the diver to simply ascend to the surface (at the prescribed ascent rate of 15m/min), whilst the latter requires the diver to stop and wait at certain depths on the ascent to allow the extra Nitrogen to be released before proceeding. Decompression dives obviously require extra air, safety precautions and planning! They should only be attempted by the more experienced diver.

Of course dive times can also be restricted by the divers' tolerance to cold water! Most dives last between 30 mins to an hour.

What causes the bends?

The Bends is just one symptom of what is called Decompression Illness or DCI. DCI is caused by Nitrogen coming out of solution in the blood and tissues at an uncontrolled rate. The Nitrogen bubbles then cause blockages and damage to various parts of the body. Most seriously in the lungs, heart and brain.

Nitrogen goes into solution on compression, i.e.. on descent, and comes out of solution on decompression, i.e.. ascent. The level of Nitrogen dissolved depends not only on depth but time, as the body takes a while to fully saturate at a given depth. This is all perfectly natural, so long as precautions are taken in ascending at a rate (15m/min) that off gasses the Nitrogen at a rate the body can cope with.

What happens if you come up too fast?

The common assumption is that this would give you the Bends. This is true in most cases, but not totally (refer to What causes the bends?). The biggest problem associated with fast, i.e. ballistic ascents, is a Burst lung. Caused by the lungs' air expanding at a rapid rate. Therefore, even in emergencies the ascent rate should be monitored. In the event of a fast ascent the diver should exhale hard to prevent lung damage.

How deep can you go?

As this is sports diving and until we receive very advanced training, we are diving on air or nitrox rather than mixed gases, and the BSAC limit is 50m. Having said that, most dives fall into ranges short of the maximum...

Depth Type
0-15m Shore and / or reef dives
15-30m Most well dived wrecks
30-45m More intact wrecks

Depth is usually increased with experience and training, therefore the maximum range allowed by the branch has been specified for Ocean Divers (20m) and Sports Divers (35m).

How much air do you have?

This depends on what cylinders you can carry! Cylinders, like all things, come in various sizes and specifications. The capacity is specified by internal volume (water capacity) in litres and the working pressure in Bars (Atmospheres). Multiplying these figures gives the total air capacity. Sometimes cylinders are specified by their total capacity in cubic feet (rounded to nearest integer), e.g..

litres Bars Litres Cubic ft
3 232 696 25
10 190 1900 67
200 2000 71
228 2280 81
232 2320 82
12 232 2784 98
15 232 3480 123
300 4500 159

Cylinders can also be paired as Twin-sets or smaller (3L) Pony Bottles can be strapped on. Usual twin-set arrangements use 10 or 12 litre cylinders. Anything larger would give someone a hernia!

Cylinder weight depends not only on size, but also the material used in manufacture. Aluminium cylinders tend to be heavier than steel ones due to the extra thickness required for strength.

The effective air available is greatly dependant on the depth (or ambient pressure) at which the air is breathed. The amount of air in a breath increases to match the water pressure, but the same amount of oxygen is metabolised as at the surface, therefore a lot of oxygen is simply exhaled!

As a guide though; a 12L, 228Bar cylinder is usually adequate for up to an hour at 20m depth. This equates to a breathing rate of 15L/min @ the surface.

How far can you see underwater?

How long is a piece of string? It all depends on where and when you dive... In the UK, the rule is that the visibility (The viz) gets worse as you get nearer the narrow part of The Channel - Kent being the worst!

The viz is also affected by the weather - wind and storms churn the seas up and especially with a silty seabed, takes a week or so to clear again. Lost of rain can cause muddy river run-off.

Another factor is the time of year, not only for weather patterns, but like the countryside, the sea experiences changes due to the seasons. In spring the algae blooms and even though the water itself may be quite clear, the amount of algae hanging around in the water again reduces the viz.

If the viz is below 1m (horizontally) then many divers wouldn't bother to get wet! The norm' is somewhere between 1 and 5m. On a good day the viz can be as much as 20m.

Do you use spearguns?

The simple answer is no! Spearfishing is a separate sport, using snorkel gear to hunt for specified fish to gain points. It is deemed unsporting (not to say dangerous) to spearfish whilst diving with SCUBA. Most fish are inquisitive and will usually stay or even follow a diver at quite short distances.

Also bear in mind that most divers don't want a Buddy with a loaded weapon, flapping around near them!

What's the difference between BSAC and PADI?

PADI courses are usually run by professional instructors affiliated to a dive shop, they are therefore run as a commercial enterprise, rather than a non-profit making club. Their charges reflect this.

Other differences are :

  • PADI do not initially train for decompression diving.
  • PADI do not normally condone diving below 30m.
  • Rescue training is not an integral part of the Open Water or Advanced Open Water diver courses.
  • The Shops' Diving Club, if one exists, is separate from the training courses.
  • Training in open water usually entails one instructor, assisted by one or more Divemasters, to a group of trainees, rather than the two man Buddy System.
  • All level of courses are paid for separately.

Text originally extracted from Brighton BS-AC's Web Site