Setpoint during fast decent, what do you use?

[berndo]

Nonsense, I got serious CCR training on 2 different rebreathers. I ve been diving CCR for 25 years.

Does your instructor dive this way too? High O2, high gas density and high END?

 

[Valyngar]

Haven't seen "my" way of doing this in posts. Autoset 0.7->1.3 10 meters before target dept. Diluent 1.0-1.1 at target depth. Autoset kicks in solenoid just before reaching bottom and get you nicely to target p02. Usually good buoyancy at target depth. Autoset 2nd computer to switch at dept to high set point. JJ as rebreather.

 

[GF99/99]

As @nickbutcher said - dil flushing becomes more efficient.

Operations is another reason. Suppose you dove with 15/55 to 75m; dil PPO was 1.27 on the bottom and your high set point was 1.3.

You still some of that gas left, but your next dive is 60m. What do you do? Well, if you don't have extra CCR tanks, you probably want to use 15/55 again. However, now your bottom dil PPO2 is 1.05.

OK so your saying that you want to have a dil with a lower PPO2 than your SP so that you can have more effective dil flushes, fair statement it has a strong argument but your saying that diving a 15/55 at 60m gives you a dil PPO2 of 1.05 and your complaining about this? Wouldn't a 15/55 be a nice 60m mix?

If not in the exact scenario you descried above what 1 mix would you propose to use for those 2 dives? I would have no issue diving a 15/55 at 75m and then at 60m for the next dive. I have no issues diving a 1.0 SP, but also for that scenario I would just dive 1.2 SP and just switch to high SP on the fly at about 50m I have never had issue with my solenoid screwing up my buoyancy when I switch over.

Majority of my diving is sub 100m and after 120m I always like to dive a 1.0 SP for sub 180m I might even consider 0.9 SP. But I do this for two reason and yes one of them is to make dil flushes more effective and to have more buffer if my PPO2 spikes at depth (its amazing how little O2 it takes to make your PPo2 spike at those depths) the other reason is I calibrate at 1.0 so I want to dive as close to this as possible. Also on a side note when you start getting super deep HPNS is of concern and I am trying to keep as much nitrogen in the mix as possible while still having a half decent END and gas density.

 

[GF99/99]

Haven't seen "my" way of doing this in posts. Autoset 0.7->1.3 10 meters before target dept. Diluent 1.0-1.1 at target depth. Autoset kicks in solenoid just before reaching bottom and get you nicely to target p02. Usually good buoyancy at target depth. Autoset 2nd computer to switch at dept to high set point. JJ as rebreather.

This is how I dive also. I'm just more old school and use the manual SP switch, but yes 5-10m be for target depth switch to high SP on the fly and when you reach target depth your pretty much good to go.

 

[mr_v]

OK so your saying that you want to have a dil with a lower PPO2 than your SP so that you can have more effective dil flushes, fair statement it has a strong argument but your saying that diving a 15/55 at 60m gives you a dil PPO2 of 1.05 and your complaining about this? Wouldn't a 15/55 be a nice 60m mix?

If not in the exact scenario you descried above what 1 mix would you propose to use for those 2 dives? I would have no issue diving a 15/55 at 75m and then at 60m for the next dive. I have no issues diving a 1.0 SP, but also for that scenario I would just dive 1.2 SP and just switch to high SP on the fly at about 50m I have never had issue with my solenoid screwing up my buoyancy when I switch over.

Majority of my diving is sub 100m and after 120m I always like to dive a 1.0 SP for sub 180m I might even consider 0.9 SP. But I do this for two reason and yes one of them is to make dil flushes more effective and to have more buffer if my PPO2 spikes at depth (its amazing how little O2 it takes to make your PPo2 spike at those depths) the other reason is I calibrate at 1.0 so I want to dive as close to this as possible. Also on a side note when you start getting super deep HPNS is of concern and I am trying to keep as much nitrogen in the mix as possible while still having a half decent END and gas density.

This thread is so off the original point that I don't know how to start replying to your post.

The mix, the set points, and gas loop addition procedures are individual choices.

It is possible to have PPO2 at max depth that is noticeably lower than the high set point. In that case, automatic rapid O2 addition to the loop _may_ affect buoyancy adversely. That is why some divers prefer managing loop, PPO2 manually while descending to the target depth.

 

[GF99/99]

Normaly I switch from 0.7 to 1.3 during descent at about 20m. I had a discussion: Dil 21/35, 20m, switch to 1.3, stay for 1 min, then very fast descent to 60m. A lot of dil is added, which brings the mix close to 21/35. But you still have the mix which was in the loop at 20m, this is a hotter mix. And during fast descent you consumed very little O2, so your mix at 60m might be to hot, pO2 for ex. 1.8.

I wonder if this is a problem in reality. I think the amount of dil you add during descent is much more then what you have inside the loop at 20m so I expect pO2 not to be very high. PO2 of 21/35 at 60m is 1.47. A bit higher because of the gas you had at 20, a bit lower because of consumption. These "a bit" are not precise at all, can they be calculated?

Or even more simpel: is this a problem in real life?

This thread is so off the original point that I don't know how to start replying to your post.

I think its pretty relevant what the OP is discussing. The title of the OP's post is "Setpoint during fast decent, what do you use?" The OP is discussing low high SP switches, depth to switch, dil mix, hot dil mix, diving at 60m. You can start replying to my post but just answer the questions I was curious about. As this is very relevant to SP during decent. Your dil choice and SP choice and switching plays a big role on your SP. To high and you spike it, to low and you have a huge O2 injection when you switch to high SP

I am not trying to be rude, I am trying to understand the logic behind your previous post. I am always opening to learning new things and hearing the logic behind them. Also just good discussion about how to different mixes and SP's. That's why I asked the question:

"OK so your saying that you want to have a dil with a lower PPO2 than your SP so that you can have more effective dil flushes, fair statement it has a strong argument but your saying that diving a 15/55 at 60m gives you a dil PPO2 of 1.05 and your complaining about this? Wouldn't a 15/55 be a nice 60m mix?

If not in the exact scenario you descried above what 1 mix would you propose to use for those 2 dives?"

Why not just match your dil with your bottom SP.

You arrive at depth and your PO2 will be really close to your SP. Flip over to high SP and have a nice dive

Keep it simple.

I simply mad a suggestion for the OP's question.

As @nickbutcher said - dil flushing becomes more efficient.

Operations is another reason. Suppose you dove with 15/55 to 75m; dil PPO was 1.27 on the bottom and your high set point was 1.3.

You still some of that gas left, but your next dive is 60m. What do you do? Well, if you don't have extra CCR tanks, you probably want to use 15/55 again. However, now your bottom dil PPO2 is 1.05.

You then made a very valid point but also you kind of contradicted your self. If there was a second 60m dive what I would do is dive the 15/55 why is having a PP02 of 1.05 a problem? This actually makes dil flushes more efficient as you suggested.
So what mix would you pick for a 75m and then 60m dive? Because I think the mix you suggested you disprove my original post above actually works out as a fairly good mix for both those dives.

I'm open to other to jump in too. What you would you dive for @mr_v scenario to not spike your PPO2 during a rapid decent?

 

[mr_v]

You then made a very valid point but also you kind of contradicted your self. If there was a second 60m dive what I would do is dive the 15/55 why is having a PP02 of 1.05 a problem? This actually makes dil flushes more efficient as you suggested.

It was just an example of the case where the in the first dive your PPO2 at the bottom is close to the set point. However, on the subsequent dive the delta between PPO2 and the set point was going to be greater, i.e., the letting the solenoid fill the gap automatically could lead to buoyancy issues. There was nothing wrong with gas choices.

Speaking of the thread... did you not cover various techniques for dealing with this stuff in your coursework?

 

[GF99/99]

Speaking of the thread... did you not cover various techniques for dealing with this stuff in your coursework?

Speaking of the thread and getting off topic. I don't think questioning my CCR course work/knowledge of a CCR is relevant, but yeah I have taken a CCR course or two...

 

[mr_v]

Speaking the thread and getting off topic. I don't think questioning my CCR course work/knowledge of a CCR is relevant, but yeah I have taken a CCR course or two...

Not trying to be a prick - simply asking

 

[grantctobin]

Not trying to be a prick - simply asking

lol that’s like asking AJ or Beckner etc if they have any cave experience