Home |  Scuba Women |  More About Diving |  Pictures |  Links |  Books
More About Diving

History of Diving
Dive Equipment
Frequently Asked Questions

Frequently Asked Questions

To top of page What does scuba mean?

SCUBA is the abbreviaton for Self-Contained Underwater Breathing Apparatus. This abbreviation was used for the first dive gear which allowed a diver to descend independently, i.e. without an air supply from the surface. Scuba gear was invented in 1943 by Jacques-Yves Cousteau. All modern diving equipment is based on this first prototype by Cousteau.


To top of page How deep can you dive?

The depth limit for recreational scuba diving depends on the diver's training. Most organisations prescribe the following maximum depths: 18 meters for novice divers (Open Water Diver) and 30 meters (with an absolute maximum of 40 meters) for advanced divers (Advanced Open Water and up).

These depth limits have been determined based on the pressure changes that occur under water: the deeper you go, the higher the pressure. This increasing pressure has various influences on the human body: it will cause an increasing risk of decompression illness and will reduce the ability to react. In short: the deeper you go, the greater the risk of accidents.


To top of page What is deep-sea diving?

When talking about diving, many people use the term "deep-sea diving". Recreational diving doesn't have anything to do with deep-sea diving; the "deep sea" starts at about 500 meters, a depth at which a recreational diver with conventional dive equipment would never be able to survive. Deep-sea diving is always done with submarines or special suits that can withstand the enormous pressure that occurs in the deep sea.


To top of page What kind of air or gas does a dive tank contain?

A dive tank contains compressed air, not oxygen. Compressed air is exactly the same as the ambient air we normally breathe, but it is compressed to enable the diver to take as much air with him under water as possible. Ambient air consists of roughly 80% nitrogen and 20% oxygen.

You will, however, often encounter tanks filled with 100% oxygen on, for example, dive charter boats, but these are only used in emergencies for injured divers (for example, divers with decompression illness).


To top of page What is caisson disease?

Caisson disease is usually called decompression illness. Decompression illness can occur when you stay under water for too long and/or if you ascend too quickly after a dive.

During a dive, a diver breathes compressed air, which consists of 80% nitrogen and 20% oxygen. Oxygen is processed by the human body, but nitrogen isn't, which means that all the nitrogen you inhale, you will have to exhale again at some point to get rid of it. Normally, this isn't a problem, but when diving you encounter differences in pressure under water: the deeper you go, the higher the pressure. And the higher the pressure, the more nitrogen the human body can absorb.

So, after a diving for a while, the body has absorbed a large amount of nitrogen. During the ascent, the pressure decreases and the body needs to get rid of the surplus nitrogen. If you ascend too quickly, the nitrogen is released in the form of bubbles, and this is called decompression illness. This effect can be compared to opening a bottle of soda: as soon as you open the bottle, the pressure drops and the carbon dioxide in the soda is released all at once in the form of bubbles.

The effects of decompression illness depend on the area in the body where the bubbles form; if the bubbles reach the heart or the brain, this can be fatal. Decompression illness can be treated by having the patient breathe 100% oxygen (which will help release the nitrogen) and by bringing the patient back under pressure in a special recompression chamber.


To top of page What is nitrox?

Nitrox is commonly used as a synonym for enriched air. Enriched air is air that has had some oxygen added. Nitrox refers to any nitrogen-oxygen gas mix, including blends that have less oxygen than air has.

Recreational scuba divers usually breathe normal air, which consists primarily of two gases: nitrogen (79%) and oxygen (21%). Enriched air is air that has been "enriched" with oxygen. Therefore, enriched air always contains more than 21% of oxygen, most commonly between 22% and 40%. This higher percentage of oxygen allows you to make longer dives than when breathing normal air.

However, because of the higher percentage of oxygen, enriched air has a higher potential for oxygen toxicity than normal air. Therefore, when diving with enriched air, adherence to depth limits is extremely important. When diving with an enriched air mix with a high oxygen percentage you cannot dive as deep as with a mix with a lower oxygen percentage.


To top of page Are sharks dangerous?

In films like Steven Spielberg's Jaws, sharks are depicted as vicious killing machines. In reality, however, only 21 of the 400 species of sharks are considered a threat to humans and only 4 of these have proven to be a serious risk. Swimmers, snorkelers and scuba divers are therefore at minimal risk. In fact, more people are killed by lightning, bee stings and farm animals than by sharks every year.

Worldwide, less than 100 shark attacks occur each year, of which only 15% are fatal.

The best way to avoid a shark attack is not to swim in waters known to be frequented by dangerous sharks.


To top of page What is a rebreather and which types are there?

A rebreather is a system in which the air you exhale is processed (carbon dioxide is filtered out and oxygen is added), so that you can inhale it again. In regular diving equipment, all exhaled air vanishes into the environment and cannot be used again.

There are two types of rebreathers: fully closed circuit (FCC) and semi-closed circuit (SCC).

Fully closed
A fully closed rebreather will keep the partial oxygen pressure at the same level during the whole dive, regardless of depth. This will give you a lot more bottom time. To achieve this constant pressure, the system mixes air (or trimix) with 100% oxygen. If, for example, the partial oxygen pressure is set to 1.3 bar, the mixture you inhale will be nearly 100% oxygen at 5 meters; at 40 meters, it is nearly 27% oxygen. A fully closed system will also allow you to use the gases more efficiently: you only use what you need.
Fully closed systems are to be used if you don't want to produce any air bubbles, if you want to make deeper dives with longer bottom times, and if you want to use trimix (this is because helium is very expensive and because for a fully closed system you will only need a small tank).

When using a semi-closed rebreather, the partial oxygen pressure will vary throughout the dive and a constant flow of nitrox will be injected into the circuit. The surplus nitrox will be dumped by the system (which is why you see air bubbles emerge about every five breaths you take). Diving with a semi-closed rebreather is almost the same as diving with nitrox; you can even use the same tables. The higher the oxygen percentage in the mixture, the less forceful the flow of nitrox, and therefore the longer the gases will last. However, with a nitrox mixture with a high oxygen percentage, you cannot dive very deep.

Special thanks to Pim van der Horst, Pims Tek Diving


To top of page What is freediving?

Freediving is the oldest diving technique in existence and was used for the first time thousands of years ago as a way to collect food or treasures. The freediving technique was later used in military actions; Italian and French soldiers used it during the Second World War to descend to staggering depths on one breath of air to place mines.

The first official freediving world record dates from 1969 and was set by a marine diver in the United States: Robert Croft dived to 75 metres on one breath of air. In 1976, Jacques Mayol from France was the first to exceed 100 metres.

Nowadays, there are three freediving categories:

  • Constant balast, where the diver must wear a weight belt while descending and returning by fin power.
  • Variable balast, where the diver can jettison the weight before ascending.
  • No-limits, which allows the diver to attach an unlimited amount of ballast to an underwater "sled" fixed to a line to speed the descent.

The deepest dives are made in the latter category. The current world record was set on 18 January 2000 by Francisco "Pipin" Ferreras from Cuba: 162 metres. Together with the Italian Umberto Pelizzari, he is the best-known freediver at the moment. The women's world record (130 metres) was set by Audrey Mestre from France on 19 May 2001. Audrey Mestre attempted a new world record to 165 metres on 12 October 2002, but unfortunately she didn't survive this attempt.



Home |  Scuba Women |  More About Diving |  Pictures |  Links |  Books