Deep dive gas mixer?

[tbone1004]

not quite.
Trimix optimum blends are determined in the following way. This is basics, but it's close
Set your O2 % based on your chosen PO2 at your max depth, typically 1.3 or 1.4
Nitrogen based on PN2 of 3.2 or lower at max depth
Top rest off with nitrogen.

On a breather you set your dil for whatever that max depth is if you are custom blending, standard gasses make it a bit easier, then you just choose the right gas for that depth.
The ratio of nitrogen to helium is fixed, but the O2 percentage can go up.
If you're using 12/70, that is obviously hypoxic at the surface, but to keep the PO2 up, at say 100ft, 4ata, the breather will have the following mix based on a setpoint of 1.3

32.5% O2-gets you the PO2 of 1.3
Remaining percentage is 67.5 and the ratio of helium to nitrogen is 3.9/1
53.7% Helium
13.8% Nitrogen

What it can't do is change the ratio of helium to nitrogen, but that doesn't really matter, you set your PO2 for the setpoint you choose, and since your maximum PN2 is already set, you will always have less narcotic effects from the nitrogen than you would normally experience unless you are at max depth.

The technology does exist for the diver to put a tank of helium, nitrogen, and oxygen on the breather, and have it custom mix, but the cost of the sensors to do it reliably is not worth the nonexistent benefit of it.

 

[Bobby]

So if you have 10/60 trimix (diluent) the RB can change this on the fly to something like (30/30)??

No and yes. If you are running 10/60 then you have 10% O2, 60% HE, & 30% N2 in the diluent (dil) cylinder. You can add both dil & pure O2 into the loop of the CCR so you can add O2 to bring the mix up to 30% O2 which will drop the percentage of both HE & N2 in the loop. Just doing the math in my head this would result in a mix around 30/50. So no you can't custom blend any mix you want however your mix will change in relation to the one gas you do control which is the O2.

All of this is being discussed in percentage of gas however when diving CCR we think of it in partial pressure. We maintain a constant partial pressure of O2, lets say 1.0. As we change depth we add either O2 or diluent to maintain 1.0 of O2. During ascent, due to expanding gas and dropping PO2 we vent gas from the loop while adding O2 to again maintain the desired partial pressure.

 

[DaleC]

I understood the OP to be asking: Why not have separate tanks for OC diving that mix the gas you are breathing. Taking Helium out of the mix this would be air and O2. So you could optimize at 60' (for example) by running 40% and at depth (say 100') the computer would deliver 32%. Upon ascent, at 70 feet if deco were required, the computer would deliver 50/50 and at 20' 100% (or 80% if you prefer). Basically a continuous nitrox blender with automated computer control that would deliver optimized mix for conditions without the scrubbing (CO2) issues of RB's.

The biggest problem would be the lack of redundancy. If you had a failure in either tank you would be forced to breath 02 at depth or do a prolonged air deco schedule, which you may not have reserves for. Current OC diving accounts for failures like that.

Related to that is the fact that OC diving is self limiting in regards to behavior due to the fact that one has to pre-plan re. gas. That need forces one to think about the dive. The device the OP suggests (which could be built) would allow the diver to negate the planning and get into very committed scenarios without realizing it.

 

[tamas970]

Recreational and trimix doesn't go together, that's strictly tec area. One needs to learn trimix deco theory+practice beforehand
and as mentioned before, shell out quite a nice sum on equipment (that rebreater of course...) and gases.

Anyway, I don't think that there is a huge advantage of having continuous ratio adjustment vs swapping stages if we look
at the decompression procedure. Maybe less reserve requirement could be one reason (one could use the same tanks for
bottom or deco reserve, just different ratios).

Folks I will start this post by saying I'm sorry if its old news or I'm asking something that's odviously impossible.
Out at the Poor Knights dive site on a surface interval I watched a couple of deep divers prepping for a dive.
Given the technology available in this the 21st century why don't deep recreational divers have a rig with bottles of the gasses they use attached to it and a computer doing the gas mixing for the various depths and deco requirements?
again I apologise but I can't see the answer.

 

[tbone1004]

oh there is a massive benefit of constant PO2 from a decompression standpoint if you're not doing square profiles on the dives, or doing dives to different depths and don't want multiple nitrox mixes. The 7even is actually pretty nifty for a recreational ccr

 

[Dr. Lecter]

Recreational and trimix doesn't go together, that's strictly tec area.

Quick, you'd better go warn the GUE rec divers who dive helium in the deep end of the pool.

 

[victorzamora]

Recreational and trimix doesn't go together, that's strictly tec area. One needs to learn trimix deco theory+practice beforehand
and as mentioned before, shell out quite a nice sum on equipment (that rebreater of course...) and gases.

Anyway, I don't think that there is a huge advantage of having continuous ratio adjustment vs swapping stages if we look
at the decompression procedure. Maybe less reserve requirement could be one reason (one could use the same tanks for
bottom or deco reserve, just different ratios).

"Recreational" to me read as rec vs commercial, not rec vs tec. Deco or cave or wreck or not....you're still diving recreationally.

 

[Frosty]

I understood the OP to be asking: Why not have separate tanks for OC diving that mix the gas you are breathing. Taking Helium out of the mix this would be air and O2. So you could optimize at 60' (for example) by running 40% and at depth (say 100') the computer would deliver 32%. Upon ascent, at 70 feet if deco were required, the computer would deliver 50/50 and at 20' 100% (or 80% if you prefer). Basically a continuous nitrox blender with automated computer control that would deliver optimized mix for conditions without the scrubbing (CO2) issues of RB's.

The biggest problem would be the lack of redundancy. If you had a failure in either tank you would be forced to breath 02 at depth or do a prolonged air deco schedule, which you may not have reserves for. Current OC diving accounts for failures like that.

Related to that is the fact that OC diving is self limiting in regards to behavior due to the fact that one has to pre-plan re. gas. That need forces one to think about the dive. The device the OP suggests (which could be built) would allow the diver to negate the planning and get into very committed scenarios without realizing it.

Thanks dale--you say it so much better than I do.
I clearly get your point re redundancy thank you.
Again forgive my ignorance but aren't you just in the same situation you would be carrying the three "mixed' tanks/regulators etc for the varying steps of a dive?. --Im thinking one 'primary" rig and a "bailout" rig.

 

[wedivebc]

Thanks dale--you say it so much better than I do.
I clearly get your point re redundancy thank you.
Again forgive my ignorance but aren't you just in the same situation you would be carrying the three "mixed' tanks/regulators etc for the varying steps of a dive?. --Im thinking one 'primary" rig and a "bailout" rig.

Open circuit diving is hugely inefficient from a gas consumption point of view and automating the delivery system adds complexity without a clear advantage.

 

[RJP]

Quick, you'd better go warn the GUE rec divers who dive helium in the deep end of the pool.

Problem with that is it would require telling them that they ARE recreational divers.

:d