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What is a Rebreather?
As open circuit (SCUBA) divers we rely upon filling our lungs with air from a
pressurized gas tank with every inhalation and then exhausting that volume of
air into the surrounding water with every exhalation. As we descend deeper the
water pressure around us increases and the volume required to fill our lungs
increases; in other words, our air supply depletes quicker the deeper we dive.

A rebreather recycles rather than vents the exhaled breath. The exhaled breath
passes into a closed loop, where it is pushed through a chemical absorbent
(scrubber) to remove the carbon dioxide, and returns through the other side of
the loop for the diver to re-breathe, hence the name "closed circuit rebreather"
or CCR.

How do they work?
The basic principle behind every rebreather is the replenishment of metabolic
oxygen consumed with every breath. Your body requires a certain amount of
oxygen to sustain it, which it then burns to produce energy for the body to use.
Oxygen consumption is not constant, however, as it is actually dependent on
the rate and intensity of our activity.

Metabolic consumption of oxygen is practically independent of depth, compared
to the volume of air that you breathe which increases as diving depth increases.
Your workrate is the primary factor affecting the amount of oxygen your body
consumes, therefore whatever affects your workrate (water temperature, trim,
physical conditions, currents etc)will indirectly affect your oxygen consumption.
This then is how rebreathers work, by adding a portion of oxygen or oxygen
enriched gas into the breathing loop to compensate for the oxygen that is consumed,
instead of replacing the entire lung volume each breath as open circuit does.
 
What is a Semi Closed Rebreather?
An SCR is a Nitrox Rebreather. A typical 50% Nitrox supply will flow into the loop at a preset rate of 7.4 liters per minute. This rate is approximately 20% greater than the oxygen you will be metabolizing, so you will be venting this 20%, which is why it’s called Semi Closed Circuit. Your flow rate will need to be increased, as you lower the Nitrox supply gas percentage. This is accomplished with preset orifices. Your Nitrox gas oxygen percentage, that you will be breathing is dependent on the supply Oxygen percentage, the flow rate, and your metabolic rate. This is called your Inspired Oxygen percentage, FiO2, and can be anywhere from 5% to 25% less than your supply oxygen percentage. A SCR for all practical purposes, is like diving a Nitrox gas, is self maintaining, and other than monitoring, does not require any action from the diver or electronics.