Nitrox and PPO2 in a CCR

[O2BBubbleFree]

Hi all,

I'm still in the dreaming stages, hoping to one day own a CCR, and am trying to learn all I can while I'm saving my pennies...

So here's what I've been wondering lately...

Since the CCR determines your PPO2, what's the advantage of using Nitrox as your diluent, rather than just compressed air?

The only thing I can think is that Nitrox would be a better bail-out gas if doing deco or penetration diving, but otherwise wouldn't air be fine?

Thanks,

 

[RIDIVER501]

Well you are rebreating the air, so each time you run it through a cycle you are using up more and more O2.
Below is a link to O2 deficiency I posted a while back.
But basically by using nitrox you end up getting more use of the gas in your CCR...what percentage you actualy get deliverd to yourself depends on the orifice you are using.... refer to the specs for the particualr rebreather you are using.

O2 Deficiency

Hope this helps

 

[O2BBubbleFree]

Well you are rebreating the air, so each time you run it through a cycle you are using up more and more O2.

But basically by using nitrox you end up getting more use of the gas in your CCR...

Since the loop O2 is being replaced to maintain a PO2 of .21 to 1.4, I assume you're talking about using up the O2 supply?

Since we're talking closed-circuit, only O2 is used up, right? So, putting aside NDLs and scrubber life for a minute, your max dive time would be limited by the total of all O2 in the system, which would be the O2 supply plus the O2 in the diluent. Nitrox is less than 21% O2, so would limit your dive time to less than what air would, for a given O2 supply. So to get the 'most use of your gas,' it seems air would make more sense than nitrox.

What am I missing?

Below is a link to O2 deficiency I posted a while back.


O2 Deficiency

Hope this helps

Not really... It supports the PO2 lower limit, but I don't see how it addresses my question.

 

[RIDIVER501]

nitrox is any N2-O2 mixture. Nitrox for most rebreather is usally between 40% and 60%

while mixed with less then 21% O2 are still nitrox we don't use those mixes in rebreathers

better to take a nitrox course if you haven't taken one yet.

 

[caveseeker7]

Hi 3dent,

Since the CCR determines your PPO2, what's the advantage of using Nitrox as your diluent, rather than just compressed air?

The diver chooses the pO2 setpoint before the dive usually depending on the planned profile. On some units the setpoint can be adjusted UW, too, and on all CCRs the chosen setpoint can be manually adjusted.

Once the diver has chosen the setpoint, the electronics determine the mix (O2/N2 with air) to keep that setpoint.

It is highly unusual to use pre-mixed Nitrox as a diluent in a CCR as you
- have a Nitrox mixing machine on your back
- it costs more than compressed air
- it will reduce your MOD or at least slow you descent until the enriched O2 is consumed.

Nitrox is generally used in SCRs much as it is for OC diving, and for the same reasons, as those rebreathers are mainly gas extenders.

The only thing I can think is that Nitrox would be a better bail-out gas if doing deco or penetration diving, but otherwise wouldn't air be fine?

Nitrox does make a better bailout gas (provided the mix and MOD of the dive plan will work together), and most everybody I know who carries a separate bailout tank & reg uses it.
Those who don't (on recreational profiles) use the diluent supply, which is compressed air. I have yet to see a CCR diver using Nitrox as diluent, although I know of one in Europe somewhere.

Stefan

PS: by Nitrox I mean EAN, as in oxygen Enriched Air Nitrox, meaning a mix composed more than 21% O2 and the remainder N2.

 

[O2BBubbleFree]

It is highly unusual to use pre-mixed Nitrox as a diluent in a CCR as you
- have a Nitrox mixing machine on your back
- it costs more than compressed air

That's exactly what I was thinking. Sometimes I seem to have trouble expressing myself...

I have yet to see a CCR diver using Nitrox as diluent...

My curiosity was raised by pictures of RB divers with Nitrox bottles. I guess they must have been either a. SCRs, b. bail-out bottles, or c. that lone european.

Thanks, Stefan

 

[padiscubapro]

Well you are rebreating the air, so each time you run it through a cycle you are using up more and more O2.
Below is a link to O2 deficiency I posted a while back.
But basically by using nitrox you end up getting more use of the gas in your CCR...what percentage you actualy get deliverd to yourself depends on the orifice you are using.... refer to the specs for the particualr rebreather you are using.

O2 Deficiency

Hope this helps

Using Nitrox on a CCR is a wast of time and limits your depth. For dives where Helium isn't necessary AIR is the norm. I personally always dive with Helium AND usually its a hypoxic mix. The easiest mix being 10% oxygen 50% helium.

lets use a typical 30mdive using AIr or 36%.. lets assume a loop vlume of 8 liters (this is on the high side)
so at 30m thats a total loop volume of 32 liters. filling it with 36% would mean 12.6 l of oxygen and a loop po2 of 1.45 (I'll use this for the setpoint)
if I used 21% thats 6.7l of oxygen, I would need another 6 liters of oxygen to bring the loop up.. The average CCR diver uses approximately 1 l of oxygen per minute so its under 7 minutes of oxygen..

now lets consider the most common size CCR bottles are 3l cylinders, filled to 200bar thats 600 liters of oxygen, the additional 6 l is nothing...
The typical bottle is 10 HOURS of oxygen, which is more than any current scrubber will last. Some divers are using 2l bottles, thats still 400 liters of gas (6.7 hours)

 

[RIDIVER501]

sorry confusing this with a semi...my bad