Question Bailout gas configs for tech/deep chestmount rebreathers

[LFMarm]

Am curious about acceptable CNS and POT (pulmonary oxygen toxicity) limits for MOD3 dives.

What limits would you use for planning 100m/330ft, 125m/410ft and 150m/500ft dives?

I don't dive deeper than 100m and in all dives I stay with CNS<100% (never longer than 3.5h).

 

[inquis]

So let's say you've got 21/35 and you're doing a 150' helitrox dive. If you lose that bottle of 21/35 you'd plug in a 50% bottle?

Assuming OW, yes that seems to lessen risk compared to not doing so. Consider, you're on the way up and ideally wouldn't touch the now extremely hot dil. Although, isn't the wing inflation split off the dil feed in some units? If so, then it'd be nice to be able to correct from overventing the wing on ascent.

I suspect what you're getting at is what do you do if you erroneously collapse the loop leaving yourself with nothing to inhale? Let's further assume your teammate let you down and cannot donate, so you absolutely must add *something*. Certainly a touch of 50% is better than O2, but what is the impact? Odds are that you are well above 150 ft when you screwed up, but assuming not... For an ideal loop volume of 6L that is erroneously collapsed to 4L, adding 1 L of 50% to a 1.2 bar PO2 loop at 150 ft takes the PO2 to 1.5 bar. Seems far better than the alternative.

 

[DiveTucson]

So your drive gas and deep bailout are on a single reg?

For open water deeper dives that is correct.

I am hesitant to get into specific gas discussions here but the concept I prefer is the Deep BO / Drive Gas is set to a 1.0 or 1.1 then the intermediate BO gas is set to a 1.6 somewhere not very far off the max depth possibly.

There is some risk here but we are not talking a massive difference of when you can switch to the intermediate BO gas much deeper than your example of the 150' with 21/35 to 70' for 50%.

Looking at a standard BM unit with onboard dil in the event of a unit failure are you not bailing out to a single deep BO cylinder with a single regulator? With the setup above a catastrophic loss of the drive gas / deep bailiout won't force you off the unit assuming no other failures you can ascend with your current loop volume until you reach a point where you could safely plug in your intermediate BO back into the unit and now that becomes your drive gas and continue your ascent on the loop.

I don't think this strategy works well at all on a dive to 150' and I agree in that case the 21/35 should be in the doubles and then just carry an AL80 of 50%. More applicable to stuff in the 80-130M range.

Just my thoughts.

 

[Rayk]

I wouldn’t dive with so called “dilout” setup where deep bailout as well as diluent both comes from same tank/1st stage. That beyond level of risk I am willing to take. Is that a common practice on the that side of the pond?

 

[rjack321]

So your drive gas and deep bailout are on a single reg?

This is why the sidewinder (not a CM I know but some commonalities) is "errrrr" for deep ocean diving. Using L/R SM tanks for dil/Bout on the right and BOV/Bout on the left leaves some room for 2x 40s underslung or maybeeee an 80 and a 40 underslung. But it's like swimming a brick though the ocean.

Being able to drop deco gases is why this unit is "cave only" once past a 1 deco bottle dive.

 

[rjack321]

Looking at a standard BM unit with onboard dil in the event of a unit failure are you not bailing out to a single deep BO cylinder with a single regulator? With the setup above a catastrophic loss of the drive gas / deep bailiout won't force you off the unit assuming no other failures you can ascend with your current loop volume until you reach a point where you could safely plug in your intermediate BO back into the unit and now that becomes your drive gas and continue your ascent on the loop.

In the event of a dil side failure on a conventional unit... It's super easy with a BOV to take a gulp of BO gas and use it as dil. Also not that hard to do if you are DSV person. Many CLs also have an offboard port too. So no, you aren't "dil-less" with one failure

 

[kensuf]

For open water deeper dives that is correct.

I am hesitant to get into specific gas discussions here but the concept I prefer is the Deep BO / Drive Gas is set to a 1.0 or 1.1 then the intermediate BO gas is set to a 1.6 somewhere not very far off the max depth possibly.

There is some risk here but we are not talking a massive difference of when you can switch to the intermediate BO gas much deeper than your example of the 150' with 21/35 to 70' for 50%.

Looking at a standard BM unit with onboard dil in the event of a unit failure are you not bailing out to a single deep BO cylinder with a single regulator? With the setup above a catastrophic loss of the drive gas / deep bailiout won't force you off the unit assuming no other failures you can ascend with your current loop volume until you reach a point where you could safely plug in your intermediate BO back into the unit and now that becomes your drive gas and continue your ascent on the loop.

I don't think this strategy works well at all on a dive to 150' and I agree in that case the 21/35 should be in the doubles and then just carry an AL80 of 50%. More applicable to stuff in the 80-130M range.

Just my thoughts.

Ben, you're a friend of mine, but I disagree with some of your logic here.

On a standard BM unit...

Scenario 1: If I lose DIL I can plumb in my deep bailout to function as DIL as I am working my way to the surface. This assumes the loop is intact and you can stay on it. FWIW, my deep bailout is usually a PO2 of 1.3 with a MOD of 1.4.

Scenario 2: If I lose my deep bailout, I still have a fully functional DIL. This assume the loop is intact and you can stay on it.

Scenario 3: If I lose my rebreather, I still have a fully functional bailout that is completely isolated from the rebreather. All I have to do is jump on the necklace reg and go up from there then switch to deco bottles as necessary.

And when I configure my Fathom for the "Tech Config" (two LP50s and a LOLA manifold like the GUE JJ config), the LP50s have the deep gas in it (with full redundancy) and only deco gases are in the stage bottles. I suspect others diving the unit this way do the same thing.

At the end of the day everyone has to find their level of risk tolerance. For me, having all of my eggs in the same basket (DIL and deep bailout) is beyond the level of risk that I am willing to accept.

I'm happy to go get a beer with you and debate this further.

 

[DiveTucson]

I'm happy to go get a beer with you and debate this further.

Sounds like a plan!

 

[kensuf]

Was referring to deeper dives where I have 10/70 and 21/35.

If I am doing a 50m dive on 21/35 and 50% and I lose 21/35, 2 options (a) if rebreather is still working, I go up and start the deco, (b) if I am on BO, I swim up to 21m and go on 50% OC (first stop would be a 18m). Definitely not ideal but also unlikely to have 2 major failures concurrenlty.

So at 100m you'd plumb in 21/35 if you lost the 10/70 bottle?

I'm just trying to make sure I understand the plan here.

 

[LFMarm]

So at 100m you'd plumb in 21/35 if you lost the 10/70 bottle?

I'm just trying to make sure I understand the plan here.

Correct, I would connect the 21/35 on my quick way up (not at 100m) and normally not needing any diligent during deco. This doesn’t work as well if you lose both rebreather and 10/70