Instructor Questions

Why can you dive for such a long period of time with a re-breather?

That is a great question! As you may know rebreathers have one or two small tanks of compressed gas. The re-breathers have this small gas supply because like the name says you rebreathe the same gas over and over. The trick is that the CO2 is removed and oxygen added as needed.

To explain this you have to take a look at the human respiratory system. An average breath admits about 350 ml of new air to the mix of air in our lungs already (about 2500 ml). This air in our lungs is composed of oxygen, nitrogen , carbon dioxide and water.

Oxygen diffuses from the air in the lung to the circulatory system and then into the cells where it is used in oxidation of carbohydrates (we will call this metabolism). This results in the release of CO2 and energy.

Carbon dioxide, CO2, diffuses from the cell to the circulatory sytem and then to the lung. Here through respiration the body is rid of excess CO2 and water.

So how does a re-breather work?
It has a closed circuit loop where no volume of gas is lost (like when we breath out on open circuit, the bubbles float up). A large portion of the gas breathed out continues to have oxygen in it.

The CO2 is filtered out with a CO2 scrubber material. The water is trapped at a low point in the loop. The oxygen is sent back to be breathed again.

So you are asking, isn't the oxygen level going down as it is used in metabolism? Yes, indeed it is. The beauty of the re-breather is that more oxygen is added as needed! This is a very small amount as we breath 21% O2 and our supply is 100% oxygen! Think of it as getting almost 5 times as much as you would use in a similar sized air (21%) cylinder of gas.

You can adjust the mix you breath during the dive if you desire. You will continue to accumulate nitrogen, but this is dependent on the mix you breath. It is like having an adjustable nitrox tank. Even better, you can dive trimix if you have the equipment and are trained.

So decompression will have to be done just like on open circuit.

Next question. Do you produce more carbon dioxide the deeper you go?

Another good question! The simple answer is that CO2 production is a byproduct of metabolism. Not of depth or more accurately with higher PCO2.

But, as you dive deeper your body is working harder, if doing nothing other than breathing as the gases become more dense with an increase in partial pressure.

More work equals more metabolism. More metabolism equals more CO2 production.

The quandry with CO2 is that an excess can lead to bad things. Things like shortness of breath, headache, difficulty concentrating and eventually to unconsciousness!

Last question. Do you use gas four times as fast at 4 ata in a closed circuit system as you do in a open circuit system?

The simple answer is that what ever system you are on be it open circuit or closed, as the absolute pressure increases, the volume of gas decreases. (once it comes out of the cylinder)

Since you your lung capacity does not vary with depth or pressure, you still have to fill the lung up with each breath.

So for instance, if you breath .5 cubic feet of air per minute on the surface you will breath 1 cubic foot at 2 ata, 1.5 cubic feet at 3 ata and 2 cubic feet at 4 ata.

The lung capacity remains the same but the volume of gas you inhale decreases (as pressure increases volume decreases... some gas law that escapes me) so that the volume on the surface is not enogh to fill that lung if that gas gets compressed. You must have more gas to create the same size as that on the surface to fill the lung.

It does not matter if you are on closed circuit or open circuit. If you go deeper, you use more gas.

If this is not clear please let me know. I will be happy to explain futher if I am able!