Why are ARO units almost extinct?

[Sbiriguda]

Edit: i remember...sanosub

I found it indeed
Autorespiratori ad ossigeno a circuito chiuso -

 

[JMBL]

And they sell to civilians ? According to my experience, rebreather manufacturers sell only to someone already certified or about to do the required training on the machine.

Btw, there's an also O2 only version of the Triton available.

 

[jale]

And they sell to civilians ? According to my experience, rebreather manufacturers sell only to someone already certified or about to do the required training on the machine.
Btw, there's an also O2 only version of the Triton available.

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.

 

[Angelo Farina]

@Sbiriguda, I have a lot of info on AROs, as I work as a consultant (about noise emissions) for SIEL, but the existing NDA impedes me to share everything publicly here. PM me perhaps if you want more info.
I started my first diving course in 1975, and the ARO was the CRESSI mod. 57b. It was the standard scuba system we used all along the course. The 1st degree FIPSAS certification I got at the end of the course enables me explicitly to dive an ARO down to 10m (not 6m).
These were "pendular" AROs, with just one hose, and you had to learn breathing in a peculiar way for avoiding CO2 accumulation and hypercapnia. They were very dangerous, and I would not recommend anyone today using a pendular ARO.
I did use it with my wife during vacations at Isola d'Elba in July 1979, and we did enjoy them immensely. At the time my wife was not yet trained for deep air (she was diving "just" to 40m max and within NDL), and in the afternoon we were doing the "second dive" with the ARO (Cressi), always staying with 10m, for more or less one hour, and enjoying the fact that fish was not scared of the noise, and it was much easier to go close to them...
Later one, in the eighties, we moved to the Technisub "new ARO", which was equipped with a loop (two hoses with one-way valves, as modern CC rebreathers), and an automatic oxygen injection system.
That system, if used properly, did allow to work not in pure oxygen, but with some air in the loop. This allows to extend the operational depth down to 18m, but the risks are severe, as, if you loose some of your precious nitrogen from the mask, the automatic injection system will replace it with pure oxygen, and you risk consequences of too high pp of oxygen (hyperoxia).
Last times I used an ARO system, some 5 or 6 years ago, was a SIEL Caimano IV. They have two versions, the new one also doubles as a semi-closed Nitrox rebreather, which allows to go down to 36m. But using it in SC mode you make some small bubbles, which cause noise, hence you loose one of the best advantage of a CC rebreather, the silence...
For recreational divers, SIEL manufactures a simpler CC ARO, called the Castoro C96:
SIEL Advanced Sea System
This is the model I recommend if you want to try an ARO for shallow water diving.

 

[ginti]

I am curious, about your sources about the depth limits you cited...

Indeed, the way you are using these numbers sounds a bit weird to me... If I remember well (please correct me if I am wrong):
- in rec diving, no more than 1.4ppO2 is recommended;
- in tech diving, 1.6 is ok for deco, assuming that you do not spend too much time at this high environmental pressure (e.g. you can switch to nitrox50% at around 20m, but then you move to shallower deco stop relatively soon);
- for rebreather, I believe I have read somewhere that the recommended ppO2 is only 1.2atm (I think it was a DAN document)
While the reasons for the deco limits are obvious (see table below), I have no idea of the reasons for such strict recommendations about rebreathers. About rec diving, I only can guess that there is no reason to use a high percentage of oxygen, so it makes sense to avoid dealing with oxygen, equipment, etc.

Where did you find those numbers (6, 10 and 15m)? Do they come from the CNS Percentage Exposure Tables, like this one?
http://anaspides.net/documents/scuba_diving_documents/NOAA CNS Percentage Table.pdf

Am I right about the source? Were the limits you found related to some time restrictions, like in the NOAA table?

 

[tbone1004]

AFAIK here in Italy the legal limit for recreational use is 6 meters, some push it to 10 meters more or less which is "tolerated", while it's only in the navy that they commonly use them at about -15 meters. I don't know if they have specific gear or training, or simply push the person to their limits

I do not question your remarks on pure o2 and sorb cost, still I am curious about this. In the Maldives they have for example Maafushi island which is relatively big, it has a hospital, a jail, a psychiatric hospital, etc. etc. for sure pure oxygen must be easy to find there, don't know about the price, but I would expect it to be relatively fair. The coral reef there is ridiculously close to the towns, actually the island itself is an atoll and most of the sorrounding islands are atolls. Perhaps it's the cost of the sorb that makes it not feasible. The limitations of the oxygen rebreathers compared to open circuit instead are very clear, though I believe there is also a price difference in favor of closed circuit

I would be floored if that island had pure oxygen. The odds of them having pure oxygen in the Maldives are so close to 0 it is not even worth investigating. There are several ways to remove oxygen from the air. In most large scale production sites, they use a distillation process. This creates pure nitrogen, oxygen, argon, xenon, neon, hydrogen, etc. They can remove each of the gases individually at very high purity levels. These systems are very large, very expensive to operate, and make gas on a massive scale. The technology that I can almost guarantee is used on that island is a process called pressure swing adsorption. These systems can make oxygen out of environmental air by extracting it through molecular sieves. Unfortunately, Argon is a VERY similar gas to Oxygen and both gases are extracted from the environment producing something that is essentially 95% Oxygen and 5% Argon. For open circuit breathing, this is not an issue in the slightest because the levels of Argon aren't high enough to cause gas density issues or narcotic effects. In a CCR though, particularly an O2 only unit, the argon builds up continuously in the loop every time the addition valves are opened. Argon is very dense, highly narcotic, but is also an inert gas that can build in your body. In an O2 breather fed with O2 from a PSA machine, you have to perform a loop flush every 10-20 minutes depending on work load to prevent the buildup of argon in the loop. The military does it all the time, but it's not something taught in any rebreather course for civilians.
I use a PSA machine to generate oxygen in my garage for blending nitrox, but when I fill the breather bottles, they come out of "pure" O2 from the gas suppliers.

 

[Sbiriguda]

Thanks all for your posts now things are much clearer...

 

[Sbiriguda]

Where did you find those numbers (6, 10 and 15m)? Do they come from the CNS Percentage Exposure Tables, like this one?
http://anaspides.net/documents/scuba_diving_documents/NOAA CNS Percentage Table.pdf

I am quoting these numbers just based on my memory, and please note I don't dive with rebs (not yet at least). Anyways, in some countries of Europe like Croatia closed circuit rebs are prohibited for recreational use (someone correct me if I am mistaken). In some other countries like Italy they are allowed (though quite uncommon nowadays) but with limitations. 6 meters should be the depth for which these devices are approved in Italy. Everybody knows that "you can go a little deeper" and some go as deep as -10 or similar, there are also some kind of breathing techniques to do that. Few out of the military go deep like -15 and it is prohibited by the law to civilians (I think, not 100% sure)

 

[jale]

I would be floored if that island had pure oxygen. The odds of them having pure oxygen in the Maldives are so close to 0 it is not even worth investigating. There are several ways to remove oxygen from the air. In most large scale production sites, they use a distillation process. This creates pure nitrogen, oxygen, argon, xenon, neon, hydrogen, etc. They can remove each of the gases individually at very high purity levels. These systems are very large, very expensive to operate, and make gas on a massive scale. The technology that I can almost guarantee is used on that island is a process called pressure swing adsorption. These systems can make oxygen out of environmental air by extracting it through molecular sieves. Unfortunately, Argon is a VERY similar gas to Oxygen and both gases are extracted from the environment producing something that is essentially 95% Oxygen and 5% Argon. For open circuit breathing, this is not an issue in the slightest because the levels of Argon aren't high enough to cause gas density issues or narcotic effects. In a CCR though, particularly an O2 only unit, the argon builds up continuously in the loop every time the addition valves are opened. Argon is very dense, highly narcotic, but is also an inert gas that can build in your body. In an O2 breather fed with O2 from a PSA machine, you have to perform a loop flush every 10-20 minutes depending on work load to prevent the buildup of argon in the loop. The military does it all the time, but it's not something taught in any rebreather course for civilians.
I use a PSA machine to generate oxygen in my garage for blending nitrox, but when I fill the breather bottles, they come out of "pure" O2 from the gas suppliers.

Yes, you must be right about the supply of oxy in the Maldives.
Regarding the flushing of the loop, civilian are also doing it

 

[jale]

I am curious, about your sources about the depth limits you cited...

Indeed, the way you are using these numbers sounds a bit weird to me... If I remember well (please correct me if I am wrong):
- in rec diving, no more than 1.4ppO2 is recommended;
- in tech diving, 1.6 is ok for deco, assuming that you do not spend too much time at this high environmental pressure (e.g. you can switch to nitrox50% at around 20m, but then you move to shallower deco stop relatively soon);
- for rebreather, I believe I have read somewhere that the recommended ppO2 is only 1.2atm (I think it was a DAN document)
While the reasons for the deco limits are obvious (see table below), I have no idea of the reasons for such strict recommendations about rebreathers. About rec diving, I only can guess that there is no reason to use a high percentage of oxygen, so it makes sense to avoid dealing with oxygen, equipment, etc.

Where did you find those numbers (6, 10 and 15m)? Do they come from the CNS Percentage Exposure Tables, like this one?
http://anaspides.net/documents/scuba_diving_documents/NOAA CNS Percentage Table.pdf

Am I right about the source? Were the limits you found related to some time restrictions, like in the NOAA table?

I know the question is for Angelo but I still can say that oxygen limits have changed with time.
As an example, not so long ago, the limit for rec diving was 1.6...now it is 1.4.