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Steam Machines, Inc. SM1600
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| What is a Rebreather? |  |
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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 more quickly 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. |
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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. (See table below)
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| Activity | Oxygen Consumed
(litres/minute) | Sleeping
| 0.25
| Sitting or at rest
| 0.50 | | Light activity (stroll or easy dive) | 1.00 | | Moderate activity (brisk walk or swim) | 2.00 | | Heavy activity (running or swimming against current) | 3.00 | Very hard exertion, only possible for short periods
| 4.00 to 5.00 |
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| Metabolic consumption of oxygen is practically independent of depth, compared to the volume of air that you breathe which increases as your 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.
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| However, we must also bear in mind that our bodies are partial pressure sensitive, with recognized PO2 limits to sustain life of between 0.16 and 1.6 PO2. Too much or too little oxygen can be bad for you, so how the oxygen content of the breathing loop is controlled, how it is monitored, addition rates, at what depth and what mix, are vitally important. It is also important to note the duration of your exposure to elevated levels of PO2. |
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For example, an open circuit diver using air fills their tank at the surface and it has approx 21% oxygen content, 79% nitrogen or a PO2 of 0.21. When they go underwater as the pressure increases the % of oxygen in the tank remains the same but the PO2 increases. Open circuit is therefore a constant %, variably PO2 system.
A rebreather system such as the PRISM is a constant PO2, variable % system as the unit monitors and controls the oxygen content of the divers breathing mix to give the best mix for the depth. This means typically longer dive times because there is less inert gas dissolved in the divers tissues (it is this inert gas content that requires a diver to decompress) and less weight, as the tanks can be smaller for longer durations. |
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Steam
Machines, Inc
538 Hiwassee Road
Lebanon, TN 37087-9298
Tel. 615-374-0202
Fax. 615-374-9193 |
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