I try to answer where those depth limits come from.
The long-term safety limit for pure oxygen, at the time, was set to 2.0 bar. Hence the 10 meters limit, considering that some nitrogen is always still present in the loop (which was NOT a loop, as the ARO was pendular).
The problem is that I do not have here by hand my original FIPSAS diving manual, written by Duilio Marcante, which I was using during my first course in 1975. It did contain all the references, and I remember very well a "safety curve" for ARO in pure oxygen, which did fix the time limit vs. depth. And it went down to 18 meters, but just for 10 minutes...
At 10m it was 2 hours, if I remember correctly.
I do not have that table by hand, I will search for it.
For long term exposure it was possible to go deeper than 10m, starting with the loop full of air, so NOT flushing the loop before starting the dive.
The idea is that, diving down to AT LEAST 10 m, the gas contained in lungs and loop was squeezing to half, forcing the user (or the automatic oxygen injections system) to inject almost the same quantity of gas. So if one starts with a total volume of 10 liters (full lungs plus loop), at 10m these become just 5 liters, not allowing anymore for a complete inhalation, and you are forced to add some more oxygen, possibly 3-4 liters. At this point the p.p. is still only around 0.6-1 bar. It cannot become much smaller, as this will impede a complete inhalation and you need to inject oxygen. And it cannot grow too much, if not you become too positive and bounce to the surface.
The trick, at this point, is to be very careful not making any bubble. If you loose gas, you loose nitrogen, and slowly the ppO2 will increase, making staying at depth of 15-18m quite dangerous.
We were trained to this advanced technique only in the higher degree courses, not at the first level, where instead we were taught to always flush carefully the loop and the lungs, starting with almost 100% pure oxygen, and to flush the loop during the dive every 15 minutes, and never go deeper than 10m.
The modern Caimano IV SC (Semi-closed) solves the safety problem when breathing a mixture containing Nitrogen thanks to an additional tank filled with Nitrox (usually at 36% oxygen), and a special valve which slowly makes new Nitrox come in the loop, while releasing an equal amount of gas into the water. This way the loop is slowly flushed with new Nitrox, which assumes usually a percentage of oxygen in the loop, which is estimated to be the standard 32%. The flow is truly minimal, so a small tank of 2 liters lasts a couple of hours.
During SC operation, the oxygen automatic injection system must be switched off.
Sorry, I cannot say more about the Caimano IV SC, there are some military secrets (which keep Italian Comsubins way ahead of other similar specialty corps of other nations in the terms of operational capacities).
The long-term safety limit for pure oxygen, at the time, was set to 2.0 bar. Hence the 10 meters limit, considering that some nitrogen is always still present in the loop (which was NOT a loop, as the ARO was pendular).
The problem is that I do not have here by hand my original FIPSAS diving manual, written by Duilio Marcante, which I was using during my first course in 1975. It did contain all the references, and I remember very well a "safety curve" for ARO in pure oxygen, which did fix the time limit vs. depth. And it went down to 18 meters, but just for 10 minutes...
At 10m it was 2 hours, if I remember correctly.
I do not have that table by hand, I will search for it.
For long term exposure it was possible to go deeper than 10m, starting with the loop full of air, so NOT flushing the loop before starting the dive.
The idea is that, diving down to AT LEAST 10 m, the gas contained in lungs and loop was squeezing to half, forcing the user (or the automatic oxygen injections system) to inject almost the same quantity of gas. So if one starts with a total volume of 10 liters (full lungs plus loop), at 10m these become just 5 liters, not allowing anymore for a complete inhalation, and you are forced to add some more oxygen, possibly 3-4 liters. At this point the p.p. is still only around 0.6-1 bar. It cannot become much smaller, as this will impede a complete inhalation and you need to inject oxygen. And it cannot grow too much, if not you become too positive and bounce to the surface.
The trick, at this point, is to be very careful not making any bubble. If you loose gas, you loose nitrogen, and slowly the ppO2 will increase, making staying at depth of 15-18m quite dangerous.
We were trained to this advanced technique only in the higher degree courses, not at the first level, where instead we were taught to always flush carefully the loop and the lungs, starting with almost 100% pure oxygen, and to flush the loop during the dive every 15 minutes, and never go deeper than 10m.
The modern Caimano IV SC (Semi-closed) solves the safety problem when breathing a mixture containing Nitrogen thanks to an additional tank filled with Nitrox (usually at 36% oxygen), and a special valve which slowly makes new Nitrox come in the loop, while releasing an equal amount of gas into the water. This way the loop is slowly flushed with new Nitrox, which assumes usually a percentage of oxygen in the loop, which is estimated to be the standard 32%. The flow is truly minimal, so a small tank of 2 liters lasts a couple of hours.
During SC operation, the oxygen automatic injection system must be switched off.
Sorry, I cannot say more about the Caimano IV SC, there are some military secrets (which keep Italian Comsubins way ahead of other similar specialty corps of other nations in the terms of operational capacities).
