SCR -SEMI-CLOSED &
CCR -CLOSED CIRCUIT REBREATHER TRAINING
A rebreather is SCUBA that allows you to re-breathe the exhaled gas you have already consumed. Unlike Open Circuit SCUBA, you breathe directly from the cylinder and then vent all the gas into the surrounding atmosphere.
When diving with CCR's we only metabolise around 4% of the inspired oxygen, the balance is exhausted along with the CO2. The rebreather captures the exhaled gas and re-uses it by removing the CO2 and re-circulating the oxygen. It also adds fresh gas to the system to keep it at a life sustaining level.
There are three different types of rebreather: SCR Semi-Closed, CCR Fully Closed and Oxygen Rebreathers. The Semi-Closed works on a principle of consistently adding fresh gas to the loop while re-using the exhaled gas. These units also vent a portion of the gas into the atmosphere, and even though this happens the gas supply lasts significantly longer than open circuit.
A Closed Circuit Rebreather is a fully closed unit that doesn't vent any gas and re-circulates all the gas and rather than constantly adding gas the unit monitors the level of oxygen in the unit and only adds oxygen at the time the unit needs it to maintain the preset or desired partial pressure of oxygen PPO2.
An Oxygen Rebreather works on the same principle as CCR but the use of pure oxygen limits their use to 6m in depth.
Once you start diving on a rebreather you many find it very difficult to come back to the dark ages of open circuit diving. The benefits of rebreather diving include:
Increased Gas efficiency - Since gas used is re-circulated, a vast improvement in efficiency is realised. Additionally this efficiency is totally depth independent. CCR's have the best gas efficiency.
Good Weight to Duration Ratio - Since the gas we use with a rebreather is reused, the volume we are required to carry becomes reduced. This translates into smaller, fewer and much lighter weight tanks for support. CCR's have the best ratio (i.e. a 3 litre cylinder, filled with oxygen can sustain the average diver-workload for 400+ minutes).
Little Noise - No more gas whistling through regulators. No more (or very few) bubbles to annoy the fish. Simply put, it is now quiet.
Little Heat Loss - The gas that the diver is breathing is actually warmer than the ambient surroundings due to the CO2 removal reaction. Heat is created in this process.
Reduced Dehydration - The same chemical reaction that makes the gas warmer also adds moisture. This coupled with the fact that the normally moist exhalation of a human being is also trapped within the loop. Together, this provides a comfortably moist inhalation gas.
Modern rebreathers are reliable and safe as long as the user fully understands the unit and it's possible hazards. Each unit requires specific training and it is very important if you are considering diving with a rebreather that you seek training from an experienced rebreather instructor affiliated to a reputable rebreather centre.