Advanced Gas Switching
- Thread starter stuartv
- Start date
Please register or login
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
- Ability to post and comment on topics and discussions.
- A Free photo gallery to share your dive photos with the world.
- You can make this box go away
J-Vo
Contributor
PfcAJ
Contributor
Switch to back gas between stages/ deco bottles. Keep it clean.
OP
I think my analyzer is accurate to +/- 1% on FO2. So, a 10% mix could really be 9%.
If I switch to that at 20', I'm now breathing a ppO2 of 0.14.
You guys are okay with breathing 0.14 at 20'? Or you have analyzers that are a lot more precise and accurate than mine?
If I switch to that at 20', I'm now breathing a ppO2 of 0.14.
You guys are okay with breathing 0.14 at 20'? Or you have analyzers that are a lot more precise and accurate than mine?
PfcAJ
Contributor
Reality is that a .14-.16ish ppo2 for a few mins while you’re at rest between gases isn’t a problem.
If you’re doing big boy dives with big boy gases and you’ve legit got something that’s not breathable at 20’, break to a deep deco gas (18% perhaps).
For the extreme majority of deco dives, switching to back gas between deco gases makes sense. For the dives where it doesn’t, you’ll know and make proper adjustments based on experience and the situation.
If you’re doing big boy dives with big boy gases and you’ve legit got something that’s not breathable at 20’, break to a deep deco gas (18% perhaps).
For the extreme majority of deco dives, switching to back gas between deco gases makes sense. For the dives where it doesn’t, you’ll know and make proper adjustments based on experience and the situation.
If your instructor had chosen some reasonable 300ft gases then this wouldn't be an issue.
10/50 is diluent for (max) ~220ft dives, its especially useful with a mCCR and considered a standard dil by many. Its a terrible OC choice.
10/70 is the proper OC choice when you need 10% O2 but used for ~380-400ft dives.
If you went with 15/55 for 250ft workup dives, then 12/65 for your 300ft dive you wouldn't have such a crazy jump in your experience base and all this handwringing that goes with it. Going from 18-21% O2 normoxic dive to basically half that O2 is a huge jump.
1% low is a big error, fix that mix. You can't be sloppy about mixes on 350+ft dives.
OP
Epilogue:
I did a 250' dive on Sunday and hit 305' yesterday to finish out my class. I did the gas switches the way I had been thinking. I.e.:
- Carried intermediate deco gas slung on the left, on the bottom, deep deco gas slung on the left, on top, and O2 on a leash behind me.
- On descent, swapped from deep deco gas to back gas. Unclipped the tail of the deoc gas and swung it out in front of me to stow the hose/2nd stage, then clipped it back in place.
- On ascent, switched directly from back gas to deep deco gas at the appropriate depth.
- At 70', deployed intermediate gas reg (with normal gas switch protocol for verification), and switched directly from deep gas to intermediate gas. Repeat process to clean up deep gas cylinder.
- At 30', had plenty of time to do a bottle rotation to move deep gas to leash and O2 to slung on left, on top. As expected, the deep gas bottle was pretty floaty and rode in back just fine from there.
- At 20', deploy O2 reg (with normal protocol), then switch directly from intermediate gas to O2. Then plenty of time to clean up the intermediate cylinder.
Out of all the times I switched from one deco gas directly to another, I had one time where I swapped regs the wrong way and the hoses ended up with a half twist between them. When that happened, it was easy to see and untwist them 1/2 a twist to straighten them out.
My classmate/dive buddy was someone I had never met before. He was awesome! He is a GUE Tech 1/Cave 2 diver with many years of FL cave diving experience. Our instructor's approach was "y'all have different prior training and experience. I'm not going to try and change what you already know how to do. I'm just going to make sure what you're doing is safe." So, my buddy's manner of gas switches was not identical to mine. But, we talked it through to agree on how we would do things. Our first dive was a bit rough, but we learned more about how we were each doing things and how to communicate underwater. It was not bad at all. Just not smooth like it would be if we had been diving together previously. Our second dive was much better and smoother.
Anyway, my buddy's process for the gas switches and bottle management was similar to what I was doing, but also had some significant differences. He also slung everything on the left or on a leash. But, he did things like put 2 cylinders on the leash at one point, and he also did some things where he just (temporarily) had 1 or 2 cylinders just nose clipped to his left chest D-ring. He evolved his process some during the course of the class. He started with switching to back gas between each deco gas, but by the last dive, he changed to switching directly from one deco gas to the next. He was still doing "more" with moving cylinders around and ending up with only 1 slung on the left by the end. But, it always looked too complicated, to me, compared to the way I was doing it.
The hardest part of the process I used was cleaning up the intermediate gas cylinder after I finished with it. That cylinder was always clipped on my left, on the bottom. To clean it up meant reaching down between my body and the cylinders with my left hand to unclip the tail of the bottom cylinder, so I could swing it around and clean it up, then again to clip it back on. Not that hard, really - and that was the hardest thing, I think.
So, thank you to you all who contributed to this thread. It has been a good learning experience for me. It was definitely helpful to go into this class with a lot of different perspectives to put forward and talk about.
I did a 250' dive on Sunday and hit 305' yesterday to finish out my class. I did the gas switches the way I had been thinking. I.e.:
- Carried intermediate deco gas slung on the left, on the bottom, deep deco gas slung on the left, on top, and O2 on a leash behind me.
- On descent, swapped from deep deco gas to back gas. Unclipped the tail of the deoc gas and swung it out in front of me to stow the hose/2nd stage, then clipped it back in place.
- On ascent, switched directly from back gas to deep deco gas at the appropriate depth.
- At 70', deployed intermediate gas reg (with normal gas switch protocol for verification), and switched directly from deep gas to intermediate gas. Repeat process to clean up deep gas cylinder.
- At 30', had plenty of time to do a bottle rotation to move deep gas to leash and O2 to slung on left, on top. As expected, the deep gas bottle was pretty floaty and rode in back just fine from there.
- At 20', deploy O2 reg (with normal protocol), then switch directly from intermediate gas to O2. Then plenty of time to clean up the intermediate cylinder.
Out of all the times I switched from one deco gas directly to another, I had one time where I swapped regs the wrong way and the hoses ended up with a half twist between them. When that happened, it was easy to see and untwist them 1/2 a twist to straighten them out.
My classmate/dive buddy was someone I had never met before. He was awesome! He is a GUE Tech 1/Cave 2 diver with many years of FL cave diving experience. Our instructor's approach was "y'all have different prior training and experience. I'm not going to try and change what you already know how to do. I'm just going to make sure what you're doing is safe." So, my buddy's manner of gas switches was not identical to mine. But, we talked it through to agree on how we would do things. Our first dive was a bit rough, but we learned more about how we were each doing things and how to communicate underwater. It was not bad at all. Just not smooth like it would be if we had been diving together previously. Our second dive was much better and smoother.
Anyway, my buddy's process for the gas switches and bottle management was similar to what I was doing, but also had some significant differences. He also slung everything on the left or on a leash. But, he did things like put 2 cylinders on the leash at one point, and he also did some things where he just (temporarily) had 1 or 2 cylinders just nose clipped to his left chest D-ring. He evolved his process some during the course of the class. He started with switching to back gas between each deco gas, but by the last dive, he changed to switching directly from one deco gas to the next. He was still doing "more" with moving cylinders around and ending up with only 1 slung on the left by the end. But, it always looked too complicated, to me, compared to the way I was doing it.
The hardest part of the process I used was cleaning up the intermediate gas cylinder after I finished with it. That cylinder was always clipped on my left, on the bottom. To clean it up meant reaching down between my body and the cylinders with my left hand to unclip the tail of the bottom cylinder, so I could swing it around and clean it up, then again to clip it back on. Not that hard, really - and that was the hardest thing, I think.
So, thank you to you all who contributed to this thread. It has been a good learning experience for me. It was definitely helpful to go into this class with a lot of different perspectives to put forward and talk about.
Similar threads
