Diluent Switching why not?

Gilles · 2nd December 2006, 22:51

"Fools walk in where angels

fear to tread" and I ain't no angel. So there! no one can insult as I've done it already.

There's a very strong consensus of not switching dils on ascent (see dil switching poll). This is also consistent with what I've been instructed.

Assuming the following

1) 40 L/m RMV on bottom, and 20 L/m deco bailout
2) 1st bail to dil equivalent offboard
3) Keeping dPN2 to +0.5 ATA on bailout switches (as per Vplanner default)
4) VPM-B/E +2
5) Dives deeper than 60m
6) 5 min on bottom on OC bail-out before ascent can begin (to swim to line, cave entrance, etc)
7) Surface supplied O2

One can surface significantly sooner (some 10-20% total dive time) bailing out than if staying on loop with same dil. Something is not right with this.

In the case where all is hunky-dory, and you're ascending with unused bail-out gas...

Could/should one utilize their bail-out to flush their loop at appropriate depths (and with appropriate bailout mix, eg EAN50 @ 20m) to reduce the He concentration (as per a dPN2 limit), in order to reduce deco to being a few minutes less than OC

bail-out?

Is this much more complicated and risky in practise? I missed the opportunity to query this further and thus seek your opinions.

May be a useful discussion for you Heliox'ies.

Gilles · 17th December 2006, 05:24

Obviously I haven't provided enough to chew on to get response on this thread. As I'm now confined to Internet diving for the time being, here goes. I have never been to 100m!

Attached are plotted profiles for a 100m dive for 20 minute bottom times. Bail-out is assumed at minute 15 and another 5 minutes on bottom are required to reach line, exit, etc.

If one is not bailing out, and all is fine, why not use your available bail-out gas to flush your loop and reduce the deco/CNS/OTU/etc?
Staying on the same diluent (especially He rich ones) increases your time, your CNS, and your OTU. Does it not thus increase your risk?

I also provide a comparison using Heliox. Heliox keeps you under longer than Tmx, but the effect of switching diluent on ascent remains.

More details provided upon request.

dantheman · 17th December 2006, 16:50

There are 2 major observations I'll make about your question:

Shorter deco isn't necessarily better deco. The totality of your environment should be considered, and there's no pithy rule-of-thumb for it. That's why we talk about acceptable risk...
Bailout is an exceptional situation. Given that there is no such thing as "right" deco, only acceptable risks, folks should decide what the risks are and plan for them. In other words, does a bailout situation mean getting to the surface sooner (water temp, seas, etc) and therefore an expedited (if sub-optimal otherwise) schedule is tolerable. Or do you now expect to sit for a while, and do extra deco to allow for possibly altered physiological responses to extreme pressure and partial pressures...

A couple more observations:

are you assuming no change in SP on deco?
have you enabled the special skip-stops on TMx/O2 feature

I don't have any experience w/HeOx, but my instinct tells me

except at the extremes, it's probably not the right tool
introducing some N2 late in the deco makes sense as a off-gassing strategy: knowing how much and when is the problem

--dan

Mark Chase · 17th December 2006, 18:01

My view is on the average dive its not particularly relevant. 45mins at 65m being and average sort of dive. 148mins on 16/50 dill 138mins if i add a 21/35 flush at 55m 125min if i air flushed at 55m

So even flushing with the dreaded air I only gain 23mins. For all the hassle of nicking the tops off what is bound to be my bailout bottles I cant see the point. Id rather do the extra 23mins and keep my bailouts in tact.

However on a big dive 100m+ I would definitely be doing dill switches on the way up and probably at least two of them. 30mins at 100 using dill 10/70 and switching to my 18/45 and my 50% bailout gases i could get the time down from 214min to 175. 40mins is worth it.

ATB

Mark Chase

CharlieT · 17th December 2006, 19:46

The mystery is explained

Now I know where I was going wrong. I kept on running out of air when I was trying to do 45 mins at 65m on my original 12 litre cylinder.

Someone said it was only "an average sort of dive" so I thought I'm up for that then

Only teasing, Mark, promise

Charlie

diverklondike · 17th December 2006, 20:25

Quote: (Originally Posted by sabgia)

Obviously I haven't provided enough to chew on to get response on this thread. As I'm now confined to Internet diving for the time being, here goes. I have never been to 100m!

Attached are plotted profiles for a 100m dive for 20 minute bottom times. Bail-out is assumed at minute 15 and another 5 minutes on bottom are required to reach line, exit, etc.

If one is not bailing out, and all is fine, why not use your available bail-out gas to flush your loop and reduce the deco/CNS/OTU/etc?
Staying on the same diluent (especially He rich ones) increases your time, your CNS, and your OTU. Does it not thus increase your risk?

I also provide a comparison using Heliox. Heliox keeps you under longer than Tmx, but the effect of switching diluent on ascent remains.

More details provided upon request.

Gilles,

The reason that you may not be getting lots of people biting is that there have been quite a few recent threads and posts on the same topic...

Dive Safe...

Mark

Gilles · 17th December 2006, 21:48

Quote: (Originally Posted by diverklondike)

Gilles,

The reason that you may not be getting lots of people biting is that there have been quite a few recent threads and posts on the same topic...

Dive Safe...

Mark

Hi Mark;

Perhaps my forum searching has been crap, but the only relevant topic I found was the dil switching poll, with a strong consensus against. Little reason against has been presented. Many say they just do it

Very little has been discussed about ICD mitigation (probably because little is known). I missed the opportunity to scrutinize my instruction against dil switching.

I don't like serving process without understanding the purpose it serves

. I'm sure all others are likewise

.

Other assumption(s) not mentioned. Bottom SP 1.1, switched to 1.2 on begining of ascent, and O2 flush @ 4m. All gases designed to keep within a dPN2 of 0.5 ATA at switch.

All replies/discussion (and links to earlier threads on topic) appreciated!

Gilles · 17th December 2006, 21:57

Quote: (Originally Posted by dantheman)

There are 2 major observations I'll make about your question:

Shorter deco isn't necessarily better deco. The totality of your environment should be considered, and there's no pithy rule-of-thumb for it. That's why we talk about acceptable risk...
Bailout is an exceptional situation. Given that there is no such thing as "right" deco, only acceptable risks, folks should decide what the risks are and plan for them. In other words, does a bailout situation mean getting to the surface sooner (water temp, seas, etc) and therefore an expedited (if sub-optimal otherwise) schedule is tolerable. Or do you now expect to sit for a while, and do extra deco to allow for possibly altered physiological responses to extreme pressure and partial pressures...

A couple more observations:

are you assuming no change in SP on deco?
have you enabled the special skip-stops on TMx/O2 feature

I don't have any experience w/HeOx, but my instinct tells me

except at the extremes, it's probably not the right tool
introducing some N2 late in the deco makes sense as a off-gassing strategy: knowing how much and when is the problem

--dan

Thanks Dan;

Is risk not made more acceptable when it is reduced?

I'm exploring simply making use of your bail-out gas (and MGP if you have one) when surfacing normally.

I like to switch from 1.1 to 1.2 at the beginning of the ascent. Accelerated stops dis-abled.

dantheman · 18th December 2006, 00:33

Quote: (Originally Posted by sabgia)

I found was the dil switching poll, with a strong consensus against.

Very little has been discussed about ICD mitigation (probably because little is known). I missed the opportunity to scrutinize my instruction against dil switching.

Other assumption(s) not mentioned. Bottom SP 1.1, switched to 1.2 on begining of ascent, and O2 flush @ 4m.
...
Is risk not made more acceptable when it is reduced?

There are multiple dimensions to the topic.

Like any computer scientist, I was all about the schedule, until I actually implemented deco software. Now I realize, schedule risk is almost nil, since radically different schedules "work".

Risk isn't just about schedule. in my humble opinion it's far more about logistics. This is especially true for gas-switching. When I first came to the dark side, I wanted to pimp my ride, but I noticed more than a few more experienced folks talk about rolling-back their mods. So I decided to wait and see.

They were right. Most mods are not needed, IMVHO. At least, I haven't seen a sufficient argument in most cases to outweight the increased risk.

Where I am (slowly) going with this is: gas-switching is a mod. It complicates your logistics, and probably doesn't serve a worthy-enough goal (schedule reduction). (Obviously, we are not speaking of extreme exposures, and the like ...)

All that said, IBCD avoidance (to the degree we know anything), is likely best done by avoiding radical gas-swtiches on deco. There is a great tool for doing that: your rebreather. I recommend the article by Weinke, et al in ADM sometime last year .. don't remember the month. He laid it out really well (almost sneakily

)

To address your original post, I tend to bump up my SP to 1.4 on deco, starting at 70ft. Given a nice low SP on the bottom, 1.0-1.1, you're unlikely to get anywhere near a CNS clock prob (if you believe in it), let alone OTU issue. (This would not be true for OC bailout in my experience: you almost always get more clock on OC). With this strategy, most of my bailout schedules deviate by at most 5 minutes.

--dan

Gilles · 18th December 2006, 21:13

To all/any who have a potential contribution to this post, I ask

1) Is reduced deco obligation not reduced risk? (less CNS, OTU, and sooner to bed with wife/girlfriend/boyfriend).

2) Given the tried and proven OC

procedures for switching gas, is it not operationally feasible to do likewise on Rebreather (those of us with MGB)?

Granted the logistical complexity of all the bail out gas, but I won't go without adequate amounts and quality and a plan on how to use it.

3) Assuming you organize adequate bail-out, is there not a significant synergy in using it to flush your loop at appropriate levels even when you are not bailing out (assuming the profile and gas induces a safe time advantage).

2nd December 2006, 22:51	#1 (permalink)
Gilles Worship the feminine Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Sep 2005 Location: Den Haag (Netherlands) Posts: 762	Diluent Switching why not? "Fools walk in where angels fear to tread" and I ain't no angel. So there! no one can insult as I've done it already. There's a very strong consensus of not switching dils on ascent (see dil switching poll). This is also consistent with what I've been instructed. Assuming the following 1) 40 L/m RMV on bottom, and 20 L/m deco bailout 2) 1st bail to dil equivalent offboard 3) Keeping dPN2 to +0.5 ATA on bailout switches (as per Vplanner default) 4) VPM-B/E +2 5) Dives deeper than 60m 6) 5 min on bottom on OC bail-out before ascent can begin (to swim to line, cave entrance, etc) 7) Surface supplied O2 One can surface significantly sooner (some 10-20% total dive time) bailing out than if staying on loop with same dil. Something is not right with this. In the case where all is hunky-dory, and you're ascending with unused bail-out gas... Could/should one utilize their bail-out to flush their loop at appropriate depths (and with appropriate bailout mix, eg EAN50 @ 20m) to reduce the He concentration (as per a dPN2 limit), in order to reduce deco to being a few minutes less than OC bail-out? Is this much more complicated and risky in practise? I missed the opportunity to query this further and thus seek your opinions. May be a useful discussion for you Heliox'ies. __________________ Gilles http://www.dirrebreather.com
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17th December 2006, 16:50	#3 (permalink)
dantheman Emoticonoclast Current Rebreather/s: Sport Kiss rEvo Other Rebreather/s: Classic Kiss rEvo Join Date: May 2005 Location: NorthEast USA Posts: 391	Re: Diluent Switching why not? There are 2 major observations I'll make about your question: Shorter deco isn't necessarily better deco. The totality of your environment should be considered, and there's no pithy rule-of-thumb for it. That's why we talk about acceptable risk... Bailout is an exceptional situation. Given that there is no such thing as "right" deco, only acceptable risks, folks should decide what the risks are and plan for them. In other words, does a bailout situation mean getting to the surface sooner (water temp, seas, etc) and therefore an expedited (if sub-optimal otherwise) schedule is tolerable. Or do you now expect to sit for a while, and do extra deco to allow for possibly altered physiological responses to extreme pressure and partial pressures... A couple more observations: are you assuming no change in SP on deco? have you enabled the special skip-stops on TMx/O2 feature I don't have any experience w/HeOx, but my instinct tells me except at the extremes, it's probably not the right tool introducing some N2 late in the deco makes sense as a off-gassing strategy: knowing how much and when is the problem --dan Last edited by dantheman : 17th December 2006 at 16:53. Reason: browser *** up*
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17th December 2006, 18:01	#4 (permalink)
Mark Chase I go down for ages Current Rebreather/s: Classic Kiss Other Rebreather/s: Inspiration Classic Join Date: Jan 2005 Location: Kent Posts: 2,700	Re: Diluent Switching why not? My view is on the average dive its not particularly relevant. 45mins at 65m being and average sort of dive. 148mins on 16/50 dill 138mins if i add a 21/35 flush at 55m 125min if i air flushed at 55m So even flushing with the dreaded air I only gain 23mins. For all the hassle of nicking the tops off what is bound to be my bailout bottles I cant see the point. Id rather do the extra 23mins and keep my bailouts in tact. However on a big dive 100m+ I would definitely be doing dill switches on the way up and probably at least two of them. 30mins at 100 using dill 10/70 and switching to my 18/45 and my 50% bailout gases i could get the time down from 214min to 175. 40mins is worth it. ATB Mark Chase __________________ Is it supposed to make that noise ? I took my unit to the dive shop and demanded they bolt on every thing that would fit. Join my elite diving teem and get a Tshirt "Doing It Chasey"Hammerhead Eccr Advanced Diving System
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17th December 2006, 19:46	#5 (permalink)
CharlieT Custom Title Allowed! Current Rebreather/s: Inspiration Vision Other Rebreather/s: Join Date: Nov 2005 Location: London Posts: 449	Re: Diluent Switching why not? The mystery is explained Now I know where I was going wrong. I kept on running out of air when I was trying to do 45 mins at 65m on my original 12 litre cylinder. Someone said it was only "an average sort of dive" so I thought I'm up for that then Only teasing, Mark, promise Charlie
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17th December 2006, 20:25	#6 (permalink)
diverklondike Just one of the Peasants Current Rebreather/s: Inspiration Classic Other CCR Other Rebreather/s: Inspiration Classic Other CCR Join Date: Oct 2005 Location: Delaware, USA Posts: 1,614	Re: Diluent Switching why not? Quote: (Originally Posted by sabgia) Obviously I haven't provided enough to chew on to get response on this thread. As I'm now confined to Internet diving for the time being, here goes. I have never been to 100m! Attached are plotted profiles for a 100m dive for 20 minute bottom times. Bail-out is assumed at minute 15 and another 5 minutes on bottom are required to reach line, exit, etc. If one is not bailing out, and all is fine, why not use your available bail-out gas to flush your loop and reduce the deco/CNS/OTU/etc? Staying on the same diluent (especially He rich ones) increases your time, your CNS, and your OTU. Does it not thus increase your risk? I also provide a comparison using Heliox. Heliox keeps you under longer than Tmx, but the effect of switching diluent on ascent remains. More details provided upon request. Gilles, The reason that you may not be getting lots of people biting is that there have been quite a few recent threads and posts on the same topic... Dive Safe... Mark
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17th December 2006, 21:48	#7 (permalink)
Gilles Worship the feminine Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Sep 2005 Location: Den Haag (Netherlands) Posts: 762	Re: Diluent Switching why not? Quote: (Originally Posted by diverklondike) Gilles, The reason that you may not be getting lots of people biting is that there have been quite a few recent threads and posts on the same topic... Dive Safe... Mark Hi Mark; Perhaps my forum searching has been crap, but the only relevant topic I found was the dil switching poll, with a strong consensus against. Little reason against has been presented. Many say they just do it Very little has been discussed about ICD mitigation (probably because little is known). I missed the opportunity to scrutinize my instruction against dil switching. I don't like serving process without understanding the purpose it serves . I'm sure all others are likewise . Other assumption(s) not mentioned. Bottom SP 1.1, switched to 1.2 on begining of ascent, and O2 flush @ 4m. All gases designed to keep within a dPN2 of 0.5 ATA at switch. All replies/discussion (and links to earlier threads on topic) appreciated! __________________ Gilles http://www.dirrebreather.com
(Offline)

17th December 2006, 21:57	#8 (permalink)
Gilles Worship the feminine Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Sep 2005 Location: Den Haag (Netherlands) Posts: 762	Re: Diluent Switching why not? Quote: (Originally Posted by dantheman) There are 2 major observations I'll make about your question: Shorter deco isn't necessarily better deco. The totality of your environment should be considered, and there's no pithy rule-of-thumb for it. That's why we talk about acceptable risk... Bailout is an exceptional situation. Given that there is no such thing as "right" deco, only acceptable risks, folks should decide what the risks are and plan for them. In other words, does a bailout situation mean getting to the surface sooner (water temp, seas, etc) and therefore an expedited (if sub-optimal otherwise) schedule is tolerable. Or do you now expect to sit for a while, and do extra deco to allow for possibly altered physiological responses to extreme pressure and partial pressures... A couple more observations: are you assuming no change in SP on deco? have you enabled the special skip-stops on TMx/O2 feature I don't have any experience w/HeOx, but my instinct tells me except at the extremes, it's probably not the right tool introducing some N2 late in the deco makes sense as a off-gassing strategy: knowing how much and when is the problem --dan Thanks Dan; Is risk not made more acceptable when it is reduced? I'm exploring simply making use of your bail-out gas (and MGP if you have one) when surfacing normally. I like to switch from 1.1 to 1.2 at the beginning of the ascent. Accelerated stops dis-abled. __________________ Gilles http://www.dirrebreather.com
(Offline)

18th December 2006, 00:33	#9 (permalink)
dantheman Emoticonoclast Current Rebreather/s: Sport Kiss rEvo Other Rebreather/s: Classic Kiss rEvo Join Date: May 2005 Location: NorthEast USA Posts: 391	Re: Diluent Switching why not? Quote: (Originally Posted by sabgia) I found was the dil switching poll, with a strong consensus against. Very little has been discussed about ICD mitigation (probably because little is known). I missed the opportunity to scrutinize my instruction against dil switching. Other assumption(s) not mentioned. Bottom SP 1.1, switched to 1.2 on begining of ascent, and O2 flush @ 4m. ... Is risk not made more acceptable when it is reduced? There are multiple dimensions to the topic. Like any computer scientist, I was all about the schedule, until I actually implemented deco software. Now I realize, schedule risk is almost nil, since radically different schedules "work". Risk isn't just about schedule. in my humble opinion it's far more about logistics. This is especially true for gas-switching. When I first came to the dark side, I wanted to pimp my ride, but I noticed more than a few more experienced folks talk about rolling-back their mods. So I decided to wait and see. They were right. Most mods are not needed, IMVHO. At least, I haven't seen a sufficient argument in most cases to outweight the increased risk. Where I am (slowly) going with this is: gas-switching is a mod. It complicates your logistics, and probably doesn't serve a worthy-enough goal (schedule reduction). (Obviously, we are not speaking of extreme exposures, and the like ...) All that said, IBCD avoidance (to the degree we know anything), is likely best done by avoiding radical gas-swtiches on deco. There is a great tool for doing that: your rebreather. I recommend the article by Weinke, et al in ADM sometime last year .. don't remember the month. He laid it out really well (almost sneakily ) To address your original post, I tend to bump up my SP to 1.4 on deco, starting at 70ft. Given a nice low SP on the bottom, 1.0-1.1, you're unlikely to get anywhere near a CNS clock prob (if you believe in it), let alone OTU issue. (This would not be true for OC bailout in my experience: you almost always get more clock on OC). With this strategy, most of my bailout schedules deviate by at most 5 minutes. --dan
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18th December 2006, 21:13	#10 (permalink)
Gilles Worship the feminine Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Sep 2005 Location: Den Haag (Netherlands) Posts: 762	Re: Diluent Switching why not? To all/any who have a potential contribution to this post, I ask 1) Is reduced deco obligation not reduced risk? (less CNS, OTU, and sooner to bed with wife/girlfriend/boyfriend). 2) Given the tried and proven OC procedures for switching gas, is it not operationally feasible to do likewise on Rebreather (those of us with MGB)? Granted the logistical complexity of all the bail out gas, but I won't go without adequate amounts and quality and a plan on how to use it. 3) Assuming you organize adequate bail-out, is there not a significant synergy in using it to flush your loop at appropriate levels even when you are not bailing out (assuming the profile and gas induces a safe time advantage). __________________ Gilles http://www.dirrebreather.com
(Offline)

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