Decompression meter (dive computer) is a wrist watch-sized device that calculates (and signals) a safe ascent rate on the basis of duration and depth of a dive. Its purpose is the same as the purpose of decompression tables but its advantage is continuous control of pressure of gases in the body, the actual depth and time. The device can also warn divers (for example about missed decompression stops).
Depending on the model of dive computer, it can display:
- decompression status,
- ascent speed,
- water temperature,
- directional information (digital compass),
- pressure of the remaining breathing gas,
- the diver’s oxygen toxicity.
With a dive computer it is not necessary to have a separate watch and a pressure/depth gauge (device that displays the depth and measures the pressure in the water).
Decompression stops are periods of time (stages) that a scuba diver must spend at a particular constant depth in water when ascending. Decompression stops are necessary for all divers who breathe gas at high pressure to avoid decompression sickness. Stops give the bubbles time to leave the diver’s body safely. On rapid ascent, without decompression stops, the air in the blood and tissues increases its volume and leads to injuries. Depending on the dive duration and depth divers may need one single stop or a series of stops. Usually stops are 1 to 5 minutes at the depth of 3-
Decompression tables (dive tables) help divers to define a particular dive profile, breathing gas and a need of decompression stops. They may be in form of printed plastic cards or brochures. The factors which determine the dive profile are depth and duration. In particular, with tables divers know how long they can stay at certain depths without decompression stops (no-stop diving) – no-decompression limit time (NDL).
Commonly used tables:
- navy tables (e.g. US Navy Tables, French Navy Tables),
- scuba association tables (BSAC tables, PADI tables, NAUI tables),
- DCIEM (Defense and Civil Institute of Environmental Medicine) tables.
Tables vary from each other, some of them being simple and some terribly intricate with difficult terminology and hundreds of numbers. But when a diver becomes familiar with the terminology and looks closer, the charts and tables won’t be frightening anymore. It is advisable to become familiar with one set of tables and understand them without bothering about the same terms called differently in other set.
There are basically three types of chambers: decompression chambers, recompression chambers and hyperbaric chambers. These terms are often used interchangeably. In fact, their usage is similar and the difference lies in the way the pressure is produced and controlled. Hyperbaric units are used mainly to treat decompression illness, to allow divers to complete their decompression stops and to train divers to adapt to hyperbaric conditions.
Hyperbaric chamber (hyperbaric oxygen therapy chamber) is a hard shelled pressure vessel where a diver is put under increased pressure to treat gas embolism or decompression sickness. In hyperbaric chamber the hyperbaric conditions (similar to conditions underwater) are artificially reproduced with the use of a gas compressor. Pressure vessels can be made of steel or aluminium. Hyperbaric chambers are usually situated in hospitals and may be small portable (able to treat one patient) or fixed (able to treat eight or more patients). Besides hyperbaric chambers on land, they may be also used underwater (submersible chambers).
In decompression chamber a surface-supplied diver can complete their decompression stops on the surface.
Maximum operating depth is a depth that should not be exceeded when diving. Going beyond that depth is not safe due to the oxygen partial pressure of the gas mix (especially nitrox or trimix). Exceeding the maximum operating depth incurs the risk of oxygen toxicity.
Partial pressure (pp), in simple terms, is the number of molecules of a particular gas per volume of this gas. When the number of molecules increases, the partial pressure becomes higher. In order to calculate the partial pressure of the gas mixture, one has to know the total pressure of the mixture and the fraction of each gas in the mixture. For example, having a gas mixture of 80% nitrogen and 20% oxygen at the surface (ambient pressure = 1 atmosphere), it is easy to calculate that nitrogen pp is 0.8 and oxygen pp is 0.2. However, at a depth of 30 meters with the same gas mixture (ambient pressure = 4 atmospheres), nitrogen pp is 3.2 and oxygen pp is 0.8.