Setpoint during fast decent, what do you use?

[Agro]

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?

 

[beldridg]

I keep my Low SP (which, for me, is 0.5) until I get to the bottom and manually add Dil & O2 during the descent (O2 until 5-6m and then generally just Dil). I then switch it to High SP at the bottom of the descent.

I also switch back to my Low SP when I start my ascent and manually manage my PPO2 during ascent & deco.

Basically, the only time my solenoid is firing is on the bottom portion of the dive.

- brett

 

[tbone1004]

I still like to have a high ppO2 cell validation check and start with a 1.0 in the loop. I also dive with a fairly rich mix using the same gases that I would dive on OC, and I have not had any real issues with high ppO2's once you reach depth.

Now, on my Meg, I just leave it on low setpoint the whole time and run it in parachute mode, but when I am doing working dives and need it to run itself, I will leave it on low setpoint until I stabilize at depth and then switch it over to high. Not an issue in real world in my experience.

 

[beldridg]

Just to add to my previous response and concur with @tbone1004, what I have generally found is that your loop volume will bottom out before your PO2 spikes too quickly and you have to add Dil if you want to breathe so it is somewhat self-correcting (as long as you add Dil and not O2).

- brett

 

[Agro]

I still like to have a high ppO2 cell validation check and start with a 1.0 in the loop. I also dive with a fairly rich mix using the same gases that I would dive on OC, and I have not had any real issues with high ppO2's once you reach depth.

Even if you make very fast descent? Until now I never ever did so, only "normal" descent or "a bit fast" descent.

I do not run the unit manualy (except on deco), therefor I have an ECCR. My unit even has an automatic switch option: at depth X (for ex. 20m) it switches to high SP. I normaly use this and I like it. But if a fast descent should cause a problem I can and will use the low SP until target depth. If there is no problem I will keep on switching to high SP at 20m.

 

[macado]

Less complicated than you think. I do plenty of very fast hot drops to 200-250ft (~60-76m). Negative entries or literally pointing a DPV to the bottom. I leave my low setpoint at .7 and hammer on my dil MAV as I descend.

I'm typically diving something like 15/55 or 12/60 for diluent. I am usually dil matched to ~1.0 on bottom. I agree with @beldridg you'll typically bottom out your counterlung and have to add diluent before the ppO2 spikes too high.

Once I get to the bottom I manually add more O2 and then change my high setpoint to 1.2.

 

[grantctobin]

Less complicated than you think. I do plenty of very fast hot drops to 200-250ft (~60-76m). Negative entries or literally pointing a DPV to the bottom. I leave my low setpoint at .7 and hammer on my dil MAV as I descend.

I'm typically diving something like 15/55 or 12/60 for diluent. I am usually dil matched to ~1.0 on bottom. I agree with @beldridg you'll typically bottom out your counterlung and have to add diluent before the ppO2 spikes too high.

Once I get to the bottom I manually add more O2 and then change my high setpoint to 1.2.

This is the way

 

[tbone1004]

Even if you make very fast descent? Until now I never ever did so, only "normal" descent or "a bit fast" descent.

I do not run the unit manualy (except on deco), therefor I have an ECCR. My unit even has an automatic switch option: at depth X (for ex. 20m) it switches to high SP. I normaly use this and I like it. But if a fast descent should cause a problem I can and will use the low SP until target depth. If there is no problem I will keep on switching to high SP at 20m.

works fine for me on hot drops over here which we do all the time on the east coast because of currents. I would either turn automatic switching off, or set it to just above your intended depth, so in your 60m example set it to 50m and you'll probably be fine. Personally I just leave it off and switch manually. When you do a true hot drop like @macado said you typically have to shake everything out once you get to the bottom and get your life back together so adding a setpoint switch to that process is not a big deal.

The last thing I would recommend doing is stopping at some mid-water depth and stabilizing. If you're hot dropping you're typically doing it for a reason, usually current, so you need to get to your destination ASAP and keeping a stable ppO2 is on the bottom of my priority list. You may see ppO2's slightly higher than your setpoint depending on your mix and how deep you're going *the deeper you go the more dil going in so the lower the ultimate ppO2 will be*, but at any trimix depth I've never seen it get above 1.5 and while that's high, it falls down very quickly even without a dilflush. I've seen a 1.7 on an EAN32 hot drop to 30m before but that was on an mCCR and that's a short descent with a constant flow of O2 coming into it so not a lot of dil to knock it back. It came down pretty quick though.

 

[Agro]

Sounds very good. Thanks a lot. I will probably keep automatic switching but put it close to target depth (50/60m). 1.6/1.7 for a short time is totaly OK for me.

If for any reason I do not reach target depth (45m instead of 60) I still can do a manual switch.

 

[lostsheep]

Once I get to the bottom I manually add more O2 and then change my high setpoint to 1.2.

Im curious, what is the thinking for manually bumping up to 1.2 vs just letting the solenoid do it?

That never occurred to me but I’m going to have to give it a shot. I switch when I hit the bottom and I always do a brief pause after the switch to make sure the solenoid is behaving.