Who is riding trimix dil all the way to the surface and who is flushing to a nitrox dil?

[bubblemonkey2]

you are off gassing He into the loop

Do we know how much actual volume/moles/molecules of gas is involved here?
How much He do we think enters and exits the body? I've heard it is less than a bar-liter (surface equivalent volume)

Might be a small fraction of overall loop volume. Less for helium than for nitrogen, due to lower solubility? Also gradually vented/replaced during ascent.

Found someone's work to quantify this here (for nitrogen)
https://wl.apsva.us/wp-content/uploads/sites/38/2017/03/IA-Sample-1.pdf

 

[broncobowsher]

Many thanks for all of the replies. My post is more about who is making a flush/switch to say 50% at 18 metres while staying on the deco plan for He dil in order to increase deco efficiency and therefore building in extra safety margin. In a way it follows on from this thread from three years ago - Dil switch off helium during deco

In our case we are looking at around +/- 2.5 hour run times, and frankly in the Red Sea being too warm before getting into the water is much more likely than being too cold towards the end of the dive so there is no push to shorten dives (baring the arrival of a grumpy OWT), but instead we want to do everything reasonably possible to reduce the chance of a DCS incident during the project.

Over thinking it, making things too complex for a fairly mild dive.

I know of the theory, never done it. For 2-3 hour dives up to 60M-200ft there is more chance of a mistake than there is from the benefit of switching DIL. Might save a few minutes of deco, which I can do with a slight bump in GF or PPO2. Not going to save any DIL either.

Now doing 100+M dives, maybe a benefit. Bonus if you can work that deco DIL being the same mix as a bailout.

 

[rjack321]

In our case we are looking at around +/- 2.5 hour run times, and frankly in the Red Sea being too warm before getting into the water is much more likely than being too cold towards the end of the dive so there is no push to shorten dives (baring the arrival of a grumpy OWT), but instead we want to do everything reasonably possible to reduce the chance of a DCS incident during the project.

2.5+ hours in the ocean? You might want to rethink that. Weather, currents, sharks, boat traffic, all sorts of unforeseen things can happen in that amount of time.

Dil switching is not risk free either. In your case I would just bump down your GF high if you wanted to be more conservative since its kinda impossible to know to what extent a switch from 18/45 dil to 50% dil does conservatism-wise. To increase conservatism... I wouldn't bother with a dil switch and instead:
1) skip alcohol on the whole trip,
2) use pee valves and drink like fishes,
3) shorten the planned TTS,
4) dive 50/70,
5) pad your SIs
6) include at least 1 safety diver per team of three, and
7) if conditions allow hang out at 3-6m for as much extra time as you want.

 

[Alex Ouroboros]

I for one don’t bother, I’d just ride it out, as others have said it’s flushing out anyway, I used to have mates that would change dil (with a flush) on the way up on deep deep stuff, my personal issue was that in reality the gases on ascent are coming out of our system so telling the computer that your now on mix X isn’t strictly true, put basically I’ve known a few folks get bendy when they exit the water and sometimes there has been this school of thought, another thing that I had drummed in to me by one of my (legendary) instructors was to work out the Po2 setting requirement prior to dive and apart from changing set point to high when nearly at target depth he disproved of altering it on ascent , ie “tweak it to 1.5 at 6m” to hurry up the deco, he alluded that there was a consensus among the elite that the Po2 being raised may have been responsible for some people being lost, as this was their habit (you may of heard of some of them but I’m not wanting to divulge as it’s not scientifically proven and just a theory) sorry to digress

 

[Alex Ouroboros]

2.5+ hours in the ocean? You might want to rethink that. Weather, currents, sharks, boat traffic, all sorts of unforeseen things can happen in that amount of time.

Dil switching is not risk free either. In your case I would just bump down your GF high if you wanted to be more conservative since its kinda impossible to know to what extent a switch from 18/45 dil to 50% dil does conservatism-wise. To increase conservatism... I wouldn't bother with a dil switch and instead:
1) skip alcohol on the whole trip,
2) use pee valves and drink like fishes,
3) shorten the planned TTS,
4) dive 50/70,
5) pad your SIs
6) include at least 1 safety diver per team of three, and
7) if conditions allow hang out at 3-6m for as much extra time as you want.

Don’t wish to appear a black cat, but 2.5hrs on a sea dive isn’t considered that long around the uk, near shipping lanes it’s max for sure but some guys are doing 7hrs or more.

 

[rjack321]

Don’t wish to appear a black cat, but 2.5hrs on a sea dive isn’t considered that long around the uk, near shipping lanes it’s max for sure but some guys are doing 7hrs or more.

Sure people do more, sometimes vastly more. But if you're trying to be "as safe as possible", dil switching (or not) is just hand waving around all those other risks. I'd wager 1000x more people get bent from booze and dehydration than dil switching or lack thereof. It just doesn't sound so cool and techie to go on a dry wreck diving holiday vs fiddling with dil switches on a 60m dive.

 

[Alex Ouroboros]

Sure people do more, sometimes vastly more. But if you're trying to be "as safe as possible", dil switching (or not) is just hand waving around all those other risks. I'd wager 1000x more people get bent from booze and dehydration than dil switching or lack thereof. It just doesn't sound so cool and techie to go on a dry wreck diving holiday vs fiddling with dil switches on a 60m dive.

Can’t argue with your logic there tbf.

 

[RedSeaDiver2]

Over thinking it, making things too complex for a fairly mild dive.

I know of the theory, never done it. For 2-3 hour dives up to 60M-200ft there is more chance of a mistake than there is from the benefit of switching DIL. Might save a few minutes of deco, which I can do with a slight bump in GF or PPO2. Not going to save any DIL either.

Now doing 100+M dives, maybe a benefit. Bonus if you can work that deco DIL being the same mix as a bailout.

The plan isn't to save time on deco - it is to run the same time, but use the N50 switch to build in an extra safety factor.

Risk assessment and mitigation is looked at differently when you have a dozen divers doing scientific diving doing the same profile day after day after day.

 

[RedSeaDiver2]

2.5+ hours in the ocean? You might want to rethink that. Weather, currents, sharks, boat traffic, all sorts of unforeseen things can happen in that amount of time.

Dil switching is not risk free either. In your case I would just bump down your GF high if you wanted to be more conservative since its kinda impossible to know to what extent a switch from 18/45 dil to 50% dil does conservatism-wise. To increase conservatism... I wouldn't bother with a dil switch and instead:
1) skip alcohol on the whole trip,
2) use pee valves and drink like fishes,
3) shorten the planned TTS,
4) dive 50/70,
5) pad your SIs
6) include at least 1 safety diver per team of three, and
7) if conditions allow hang out at 3-6m for as much extra time as you want.

Don't really have much option about the 2.5 hour run time in the ocean - unfortunately the wreck sank where it sank and we just have to work with the location.

There won't be any alcohol on the trip - this is a scientific diving expedition
We all will be using Pee valves and drinking plenty of water
There isn't a lot we can do about the TTS as we need working time on the bottom
I think everyone is diving 50/70 anyway
The dive plan is three teams of four divers each - Day 1 is T1 diving the morning, T2 diving the afternoon, T3 is surface support/dry day, Day 2 is T3 diving the morning, T1 diving the afternoon, T2 is surface support/dry day etc. Basically a minimum of a 24 hour surface interval with every third day a dry day
There will be safety divers on the deco line
Will be able to hang at 6m, unlikely at 3m

 

[grantctobin]

you have a dozen divers doing scientific diving doing the same profile day after day after day.

on that note, every fourth day totally off will definitely not be a bad idea, and probably be a huge benefit in your risk curve

Edit: posted before your second message