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... Let's say you take a breath at the surface and then descend. Your lungs are diminished in size by the increased water pressure, but the amount of gas molecules is about the same. That increases the pressure inside the lungs.

Ahhh, yes. I stand corrected. Pressure within the lung must increase to counteract ambient water pressure. This will increase the pp of all gasses relative to the tissues which will mean some ongassing of N2. However, since the supply of N2 (molecules) is fixed, the lung-tissue system will equalize quickly considering most breath-hold dives are brief (compared with scuba). The system will offgas just as quickly. I don't think the N2 loading is a major factor and can be ignored in free dives.

Going back to the op's comment regarding should he do a free dive with his DC set to air or nitrox mode, my answer remains the same: no. Set it to bottom time mode. Although, some ongassing is evident in a breath-hold dive the DC will make the assumption that N2 pressure in the lungs is relatively constant for a particular depth rather than decreasing. This increases residual N2 loading and will result in lower dive times on subsequent scuba dives.

 

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Ahhh, yes. I stand corrected. Pressure within the lung must increase to counteract ambient water pressure. This will increase the pp of all gasses relative to the tissues which will mean some ongassing of N2. However, since the supply of N2 (molecules) is fixed, the lung-tissue system will equalize quickly considering most breath-hold dives are brief (compared with scuba). The system will offgas just as quickly. I don't think the N2 loading is a major factor and can be ignored in free dives.

Going back to the op's comment regarding should he do a free dive with his DC set to air or nitrox mode, my answer remains the same: no. Set it to bottom time mode. Although, some ongassing is evident in a breath-hold dive the DC will make the assumption that N2 pressure in the lungs is relatively constant for a particular depth rather than decreasing. This increases residual N2 loading and will result in lower dive times on subsequent scuba dives.

I agree completely that all but extreme free-divers can ignore problems with N2 loading unless they mix it with scuba.

But I take exception on the point about the limited mass of N2 being a major factor. I found a reference somewhere that said a 70kg person has about 1 liter of N2 absorbed in the body at the surface. I'm assuming that it would take not more than another liter to saturate the body with N2 at 2ATM (an upper bound assuming unlimited time). With only a couple minutes at depth (adding a time constraint) we are probably talking a rather small fraction of .1 liters added. Since a "deep breath", as would be used by freedivers, is at least 3 liters of air, the available mass of N2 will make almost no difference in loading during that couple minutes (that is, you can treat PPN2 as a constant).

Just playing around with tables, I think a free diver who does a couple minute dive every 10 minutes to 30 ft is probably finishing the hour in pressure group B. If he does that for a few more hours, he would probably be in the neighborhood of PG E. Neither are any consequence to that diver unless he then dons scuba and sets off for a 50 minute dive to 60 feet. And I don't believe any of those parameters are extreme. They might even be a little conservative for a good abalone diver.

 

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...But I take exception on the point about the limited mass of N2 being a major factor. I found a reference somewhere that said a 70kg person has about 1 liter of N2 absorbed in the body at the surface. I'm assuming that it would take not more than another liter to saturate the body with N2 at 2ATM (an upper bound assuming unlimited time).

The fixed supply is a major factor in two ways: (1) limits the amount of N2 absorbed by the tissues and, (2) reduces the rate of N2 flow compared with scuba.

Going back to the idea of pressure drops and flows, you need a pressure drop between the lungs and tissues to get flow. At 2 atm at constant depth the N2 in the scuba divers tissues are rising from 0.79 atm toward 1.58 atm. If the diver stays long enough a particular fast tissue will saturate at 1.58 atm. For the free diver his tissues are also rising from 0.79 atm but the lung ppN2 is dropping at the same time. If the free diver could stay long enough his tissue will saturate to 0.79 + ((1.58 - 0.79) / 2) = 1.18 atm. This takes care of (1) above. Assuming a constant flow resistance the flow rate is determined by the pressure drop. Since the pressure drop is decreasing faster in the free diver as opposed to the scuba diver his flow rate is also lower.

Another issue is how far can you take the lung volume down. As N2 flows out to the tissues the amount of gas available in the lungs is decreasing. This lowers the pressure and with it the volume.

Just playing around with tables, I think a free diver who does a couple minute dive every 10 minutes to 30 ft is probably finishing the hour in pressure group B. If he does that for a few more hours, he would probably be in the neighborhood of PG E.

I don't think it's wise to extrapolate from the tables since the tables are designed for a diver with a non-fixed source of N2.

---------- Post added August 21st, 2014 at 04:24 PM ----------

I thought of a way we can use the tables. Let's fudge the tables for a 99 ft (4 atm) 10 minute dive for our free diver. First, let's calculate the saturation level based on the fact that our N2 supply is decreasing with time. 4(0.79) = 3.16 atm. Our actual sat level will be 0.79+((3.16-0.79)/2) = 1.98 atm. To use the tables we can approximate the equivalent depth (ED) by taking the ratio of the N2 loading times the depth. Our ED = 99(1.98/3.16) = 62 ft. From the PADI tables we round up to 70 ft. and round the time up to 12 minutes we get PG C.

For 10 minutes at 33 ft we get an ED of 25 ft. (Does 25/33 = 62/99? No. Everything needs to be calculated for each depth). The table starts with 35 ft. and 10 minutes so this dive stays at a PG of A. If you string enough of these together with minimum SI you might get into B.

An important point that AWAP brings up is the initial lung volume of 3 liters and with it the question: can the lungs supply the volume of 4(1) = 4 liters for our free diver saturated at 100 ft? The answer is no because it exceeds the initial lung volume. Actually, the full 3 liters isn't available because we can't take the lung volume down below what the volume is with 1.98 ppN2. However, not all tissues need to ongass. If we limit our tissues to the fastest 1 compartment then the supply will be sufficient and our analysis is reasonably accurate.