Open-Circuit ScubaScuba stands for self-contained, underwater breathing apparatus. It means that scuba divers carry all of the breathing medium they need with them for the duration they are underwater. Open-Circuit scuba may be most easily understood by contrasting it to other types of scuba diving.
In Closed-Circuit scuba there is a total recirculation of the gas supplied to the diver. In Semi-Closed Circuit scuba, there is partial recirculation of gas supplied to the diver. In contrast, the gas supplied during Open-Circuit scuba is not recycled at all. When a diver breathes out, the exhalation goes out to the surrounding water instead of being recirculated somehow.
In Closed-Circuit scuba, oxygen and mixed gas are used as the gas supply for the diver. In Semi-Closed Circuit scuba, mixed gas is used as the gas supply. In contrast, Open-Circuit scuba uses mixed gas and air as the gas supplies, with air being the main gas supply for Open-Circuit scuba divers.
Open-Circuit scuba is the most widely practiced type of diving. Recreational divers definitely tend towards this type of scuba, but scientific, military and public safety divers use it regularly too. The recreational diving community likely tends towards Open-Circuit scuba because it is the simplest, most portable and most inexpensive type of scuba diving. Training for people to become Open-Circuit scuba divers is available around the world.
Open-Circuit scuba does have its disadvantages though. Because none of the gas supply is recirculated, this type of scuba is inefficient. Going for long or deep dives may require the carrying of many gas cylinders to be able to support a diver for a long duration. This not only makes the diver bulky, but the diver experiences a drastic change in bouyancy while he/she is depleting the gas supply. Open-Circuit scuba also has limitations for diving to deep depths or dives where there is low visibility or wired communications needed.
Although Open-Circuit scuba requires fairly simple and easily-maintenable equipment, the possible set of Open-Circuit scuba components that a diver would carry or wear for a normal, recreational dive is: a tank of compressed air or mixed gas, a first stage regulator attached to the tank, a second stage demand regulator and mouthpiece, a face mask that covers the diver's eyes and nose, an extra second stage regulator carried by the diver in case of emergency, depth and air gauges, a bouyancy control device, a weight belt, fins and a wet suit. Additional equipment like a snorkel, dive knife, dive computer, compass or extra tanks of air might be carried as well.
There are risks involved in all kinds of scuba diving. A diver using Open-Circuit scuba with either air or a mixed gas like nitrox runs the risk of overexertion, gas supply depletion, barotrauma, decompression sickness, oxygen toxicity, arterial gas embolism, environmental injury, and of course, drowning. Adequate training, experience, the presence of a dive buddy, and observation of dive tables should minimize the aforementioned risks so that Open-Circuit can be a safe and enjoyable experience.