Why are ARO units almost extinct?

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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).
 
I'll add that the growth of full CCR market that doesn't have the limitations of a pure O2 system has captured a big chunk of that specific market. So there is this one model of rebreather that has a 6 meter depth limit, or twenty other models that can do 100 meters. And the ones that can do 100 meters will do 6 meters just fine as well. Not much of a sales point for a 6 meter unit when open circuit will let you hit ~50 meters easy enough.

Military, yes that is a niche market. They have a lot of specialty gear for just the right situation.
 
So you get a surplus unit cheep and add some pieces

I designed this one, it cost less than a factory open circuit Apeks second

BOV DSV for rebreathers

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my mate designed this one

Sensor House for Rebreather

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maybe one of these Gompleman valves

Constant mass flow

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and just go diving, to a bit under 100m
 
and then there's the pure genius of Dave Sutton
and his spare air

World smallest CCR by Dave
Very nice. Exactly the Haas method we were using 40 years ago, but with the added safety of a ppO2 meter!
Just an addition. The procedure we were using (starting with both lungs and counterlung full of air) was considered safe for diving in the range 10 to 20m. During ascent, the loop and lungs had to be emptied and flushed to pure oxygen when coming above 10m, for avoiding the risk of hypoxia.
 
Yes Sanosub sells to civilian but frankly the price is a bit high. Second hand market is better.
As for the Triton, the one you are linking is not really the O2 only version as you can add Dil at the ADV. It is a Triton with Monox only without a computer. They used to sell this version but, even if it is on the site, I wonder if they still sell it.

Well, you're right about the dil, but the unit is fully operational without it, within O2 diving limits of course. Yes, they still sell it : a guy did his training with me last July. He preferred to use a non hard wired computer.
 
Well, you're right about the dil, but the unit is fully operational without it, within O2 diving limits of course. Yes, they still sell it : a guy did his training with me last July. He preferred to use a non hard wired computer.
All units are fully operational without dil (maybe not the Poseidon :)).
Good to know they are still selling the "original" bare set-up. Is this unit come with the CE marking? Merci
 
Nice "Triton inspired" unit :wink:

So instead of stuffing just any plumbing in and around a wine cooler

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You make it look recognisable so no one needs wonder about it's pedigree

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This had been done before by some UK diving chaps.
Joking on: Using a ppo2 monitor removes the fun :) :)

Everything has been done before including Daves more than twenty years ago worlds smallest rebreather
When did your UK diving chaps do what they did and what did they did and who are they that did it
I have read Hans Haas rebreather exploits and emulated some, also many years ago, have to learn somehow



ARO



and here's the rEvo inception


"Build it and you can go"
 
Very nice. Exactly the Haas method we were using 40 years ago, but with the added safety of a ppO2 meter!
Just an addition. The procedure we were using (starting with both lungs and counterlung full of air) was considered safe for diving in the range 10 to 20m. During ascent, the loop and lungs had to be emptied and flushed to pure oxygen when coming above 10m, for avoiding the risk of hypoxia.



You are very detailed in presenting your experiences, an excellent contributor to the board Angelo Farina
maybe not so much with those bloody Y valves
and remember, it's the lazy ones that usually kick up the biggest stink

Molte grazie
Emilio.
 

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