Multiple mixes

[RegularScubaDiver]

I have a question about technical rebreather diving. When you go on a deep dive on open circuit (let's say 300ft / 100m) you need multiple gas mixtures for the different decompression stops. When you go on a rebreather, do you use open circuit for the decompression? What mixture do you use for the rebreather?

 

[beldridg]

I think the easiest way to think about this is that under "normal" operating conditions (i.e., not "bailed-out") you stay on the rebreather for the entire dive. Think of the rebreather as a "real-time gas mixing" system.

For example, for a 100m dive, you might use a Diluent of 7/70.

Then you (or your rebreather) add oxygen to a specific PPO2 based on the depth. Let's assume you want a PPO2 of 1.3.

At 100m, you need about 12% O2 to achieve a PPO2 of 1.3. So, the gas you are breathing would mostly be the diluent.

As you start your ascent, you would vent the excess gas that builds up due to the volume increasing and then you (or the rebreather) would add some O2.

If your first stop was at 40m, you would need about 26% O2 for a PPO2 of 1.3. As you go up, the fraction of O2 increases and the amount of Diluent gas in the breading gas decreases in order to maintain a specific PPO2 "set point."

If the rebreather fails, you need to have enough gas available to complete all of your decompression on open circuit -- hence the need to bring along a bunch of regular scuba cylinders filled with appropriate gases.

- brett

 

[inquis]

The rebreather uses a diluent gas with a safe amount of oxygen for the deepest depth, and then mixes additional oxygen "on the fly" for all phases of the dive (including deco).

That said, you must also carry bailout gas (breathed open-circuit) in case the rebreather stops working. The size, quantity, and gas in these additional cylinders depends on the planned profile; but for that 300 ft dive, you'd carry one with the same gas as your diluent (deep bailout) and probably 3 (or more) for open-circuit deco (again, depends on how long you stayed down).

 

[tbone1004]

I have a question about technical rebreather diving. When you go on a deep dive on open circuit (let's say 300ft / 100m) you need multiple gas mixtures for the different decompression stops. When you go on a rebreather, do you use open circuit for the decompression? What mixture do you use for the rebreather?

Assuming the rebreather is functioning properly you would stay on the loop during the entire dive.

There are basically two schools of thought on how to operate the rebreather during the ascent. In either instance you should be carrying enough open circuit gas to make the ascent on OC but also with the right gases. So if I was doing a 300ft bounce dive in open water I would most likely have bottom mix, 21/35 for deep deco, 50%, and 100% for O2. I would switch the diluent on the rebreather to 21/35 at 190ft, to 50% and purge at 70ft to get the helium out, and 100% at 20ft. It keeps you in the gas switch habit and also gets the helium out of your body faster so you can expedite decompression.

Option 1 is the easy way and that is you just vent excess gas out of the loop and add O2 to maintain whatever setpoint you want to run.

Option 2 is what I do and that is switching gases to

 

[RegularScubaDiver]

Thanks for the info. So some rebreathers automatically adjust the percentaje of O2 and other must be adjusted manually right?
What percentage of O2 should you use for the bail out gas? If you have a gas with 50m of MOD and the rebreather fails at 60m you would get O2 toxicity at the gas change .

 

[RegularScubaDiver]

Assuming the rebreather is functioning properly you would stay on the loop during the entire dive.

There are basically two schools of thought on how to operate the rebreather during the ascent. In either instance you should be carrying enough open circuit gas to make the ascent on OC but also with the right gases. So if I was doing a 300ft bounce dive in open water I would most likely have bottom mix, 21/35 for deep deco, 50%, and 100% for O2. I would switch the diluent on the rebreather to 21/35 at 190ft, to 50% and purge at 70ft to get the helium out, and 100% at 20ft. It keeps you in the gas switch habit and also gets the helium out of your body faster so you can expedite decompression.

Option 1 is the easy way and that is you just vent excess gas out of the loop and add O2 to maintain whatever setpoint you want to run.

Option 2 is what I do and that is switching gases to

Oh now I understand, you should carry all the OC gases just in case the rebreather fails at any depth. Thanks for the explanation

 

[tbone1004]

Thanks for the info. So some rebreathers automatically adjust the percentaje of O2 and other must be adjusted manually right?
What percentage of O2 should you use for the bail out gas? If you have a gas with 50m of MOD and the rebreather fails at 60m you would get O2 toxicity at the gas change .

An eCCR will actively add O2 to try to maintain whatever you have chosen as a setpoint. It is particularly irritating on ascent because you have the gas in the loop expanding as you ascent and then it's adding insult to injury by dumping oxygen into the loop. On eCCR's I will change to low setpoint of 0.7 and that way I have an easier time controlling ascent rates. When you get to the stop depth you will manually add oxygen to bring the setpoint up to your chosen decompression setpoint.

An mCCR will be passively adding oxygen during the ascent portion but it's a fairly small amount, certainly not enough to counter the falling ppO2 from the ascent and you would still have to add oxygen manually to bring the ppO2 up to your chosen setpoint.

I do gas switches at the same depth as I would on open circuit. On that 100m dive I would have probably 10/70 in the rebreather for the bottom portion and would probably be running at a setpoint of 1.0 which was calibrated at the surface. When I got to 60m I would switch to 21/35. At this switch I would plug the gas in and perform a diluent flush. This will allow to verify the calibration of the rebreather as the gas should register with a ppO2 of 1.5. If it shows 1.5 then I know that the cells are safe to use up to 1.5 and I will increase my ppO2 accordingly to help expedite decompression.
I would stay on that 21/35 until 20m when I switched to 50% and again I would perform a dil flush. This not only allows verification that the calibration for that high ppO2 is still good *cells tend to start doing weird things as they get warm/wet so as the dive gets longer they can start doing weird thing*, but more importantly than the linearity verification of the cell it also flushes all of the helium out of the loop at that time. Similar to switching to 100% oxygen for standard decompression to have a "perfect" gradient of inspired gas with 0% inert gases to drive the inert gases out of the body, this switch to 50% nitrox at 20m creates a "perfect" gradient against the helium in the body which helps to drive the helium out faster. You do obviously increase you ppN2 at this point so some of your tissues will be ongasing nitrogen but you aren't at 20m for a terribly long time so it's advantageous for decompression times.
Once you switch over and purge to the 50% you will find that the loop is no longer maintaining min-loop volume as your body is pumping out the inert gas from the tissues so you'll be venting gas not only during each depth change but also during the longer decompression stops.
From initial ascent up to the 6m I do not try to maintain a perfect ppO2 and I let it fall down on each change in depth which helps with the vasoconstriction that your blood vessels go through under high ppO2 conditions and impede decompression efficiency. I have found that I feel much better if on ascent I let the ppO2 drop from 1.4-5 down to whatever it falls down to and then slowly bring the ppO2 back up. If I try to maintain a fixed 1.3-1.4 then my lungs feel pretty crispy.

At 6m it gets a little weird and this is a personal preference as well as a gear requirement thing. On 10/70 you have to worry about breathing a ppO2 that will sustain consciousness and I don't like to have the 10/70 regulators anywhere near my mouth because once you get to 3m it gets a little questionable for sustaining consciousness especially if you don't have impeccable buoyancy control but you also can't ascent to the surface so I definitely don't want that gas plugged in. Depending on configuration I will usually leave the 50% plugged in which will allow me to get off of the high ppO2 from the loop and get down to at least 0.8 for an "air break" but what I will typically do is perform an O2 flush to get the loop up to 1.6, and then let the ppO2 naturally decay as my body is pushing out inert gas. Once it gets down to say 1.0 I will flush again and bring it up to 1.6. The closer you get to having your tissues clear the longer it takes for that ppO2 to fall off and that's where I will typically go over to the OC side and get down to 0.8 for a couple of minutes to try to correct the vasoconstriction that high ppO2 will give you. When in a cave I will typically have 32% with me and I prefer to get on that instead of 50% but it all depends on the specific dive profile.