Switching diluents on ascent

Dirkadiver · 21st November 2008, 15:55

Quote: (Originally Posted by teomannaskali)

I am only speculating but I think the chap did not want to have a hypoxic dil plugged in to his adv . So he swapped to air, so in case of emergency he had something breathable (never need to use it if you are ascending, and maby he did not do a loop flush, just let out some bubbles because of the expanding gas during ascent).Teoman

Getting off the hypoxic mix is the only reason that makes any sense to me at all. Where he performed the switch doesn't. I could understand making this switch @ 20' or 30'

Dirkadiver · 21st November 2008, 15:57

Quote: (Originally Posted by mortenkjerulff)

Well, if you use a VR3 with Bühlmann for decompression, diluent switches will save you some decompression (sometimes a lot). So maybe that is why he did it...

Cheers
Morten

FWIW, the diver was using HH electronics. Maybe that will shed some light on the matter.

**Mark Chase** · 21st November 2008, 15:58

Some well known divers still get of helium deep. I spoke to Kevin Gurr at the LDS last year and he said he was still switching to air on ascent.

ATB

Mark

Dirkadiver · 21st November 2008, 16:10

Quote: (Originally Posted by Mark Chase)

if were planning a 120m five hour dive ill flush with reduced helium on ascent. Ill go from 8/70 to 18/45 or 21/35 on the ascent in order to reduce deco.

Mark

This sheds some light on question #2 in my original post & makes perfect sense. Very similar to us OC guys switching gases, with easing off the He & trying to limit uptake of N2.

Keep in mind, that the dive I've referenced wasn't very aggressive.

**Mark Chase** · 21st November 2008, 16:24

Quote: (Originally Posted by Dirkadiver)

FWIW, the diver was using HH electronics. Maybe that will shed some light on the matter.

HH uses GF deco so Buhlman based and that is going to punish helium more than VPMB will. End result is his deco will be noticeably shorter on an air flush than it would on helium all the way out.

Again i stress I personally wouldn't bother on dives under three hours i am merely pointing out it has a benefit in terms of deco and that some divers believe in it as a way of accelerating off gassing rather than just running more aggressive deco.

I am also pointing out we used to do this on OC al the time.

We didn't suffer inner ear bends and counterdefusion issues because we wernt doing significant bottom times. The recorded cases I am aware of where inner ear bends and counterdefusion were an actual manifestation were much deeper and for much longer duration than anything most average tec divers would ever envisage.

ATB

Mark

jkaterenchuk · 21st November 2008, 16:34

Quote: (Originally Posted by Mark Chase)

Yet in the good old OC days we used to switch from 13/60 or 18/45 to 32% or 50% without a second thought.

Just before i switched to CCR there was a move toward running 32/30 or 21/35 for deep deco but that was considered a new gimmick by many of my fellow divers.

On "normal" dives (60-80m 3hour run) I never bother swapping diluent but if were planning a 120m five hour dive ill flush with reduced helium on ascent. Ill go from 8/70 to 18/45 or 21/35 on the ascent in order to reduce deco.

8/70 all the way out = 295min deco

8/70 21/35 and 50% diluient switching = 260mins deco

VPMBE 3

ATB

Mark

I would be interested to know how the deco software really models the fact that you are off gassing into the loop which will rise the % of He to a higher level than if you were using 21/35 which you then rebreath? Does it assume that immediately upon connection of 21/35 and completing your DIL flush that your breathing that gas in the deco calculation or is there some kind of bufferring that takes into account the off gassing and rebreathing of the old higher He gas?

John

jkaterenchuk · 21st November 2008, 16:54

Quote: (Originally Posted by rossh)

It looks for big swings in ppInert between any of the selected mixes in the ascent phase.
The diver can adjust the threshold from the default 0.5 ATA.
No literature that I can point at right now. There are various reports on type 2 DCS that are thought to be as a result of IBCD problems with swaps to Air after helium. Type 3 DCS is thought to be an IBCD problem on deep dives too..

Regards

Is your method the same or differrent(in what way) than that indicated in Steve Burtons article...see Link

Isobaric Counter Diffusion

John

the diving tiler · 21st November 2008, 18:24

[quote=Dirkadiver;224496]

Quote:

My specific question is this: we did a dive to 250' for about 15min. 3 of us were on OC & 1 was on a Rebreather. The Rebreather diver was diving 10/50 as diluent. At our 70' stop, I noticed bubbles coming up from below me & I knew the only person below me was the Rebreather diver, so I quickly started dropping back down & was frantically trying to get his attention to see what was going on. He kept signaling ok & was fumbling around with one of his offboard tanks & his CL. Once everything settled out, and I was convinced everything was ok, we finished our deco. On the boat, I asked what had happened and he said that he switched from his onboad diluent(10/50) to his offboard air(drysuit bottle). On this specific dive, does this make sense?

Hi I have being diving 60 to 70 meters, run times about 2 hours, 16/50 mix switch at 40m to off board air dill. I get out of water about 13 min earlier than not switching. At the 40m switch I do 3 long flushes so lots of bubbles. Hope this helps.

rossh · 21st November 2008, 23:23

Quote:

Is your method the same or differrent(in what way) than that indicated in Steve Burtons article

V-Planner has its own test method. As I posted above, a shift greater that 0.5 ATA in ppInert will trip it. That simple test is enough to find the potential problem areas.

Quote:

I would be interested to know how the deco software really models the fact that you are off gassing into the loop which will rise the % of He to a higher level than if you were using 21/35 which you then rebreath? Does it assume that immediately upon connection of 21/35 and completing your DIL flush that your breathing that gas in the deco calculation or is there some kind of bufferring that takes into account the off gassing and rebreathing of the old higher He gas?

When the diver plans a dil swap or SP change, V-Planner takes that change as an instant change. There are no attempts to blend them together over time. You can plan that as a stepped transition if required.

The actual volume of inert gas that a human absorbs in a dive is quite small. Off gassing in ascent and the time delay to build up enough off gassed extra to upset the balance of inerts, is offset by the fact that the loop is reducing the total fraction of inerts in the inspired mix through ascent and a high SP. Any change to the balance of inspired inerts in deco and ascent, is just not significant to the deco calculation. eg. Using a worst case sample test dive, a 1 hour dive at 80m, with a test increase of 2.5% He per ATA in deco, makes about 1% change in deco time. Not much change really. Thats usually less a than a single level change in conservatism.

Therefore exhaled excess He in the loop and the upset to the balance of Inert is not considered. If this issue concerns you still, then plan the changes into the dive ascent (add zero time legs after the bottom segment).

Regards

Lancer4545 · 22nd November 2008, 01:44

I never bother changing the dil during ascent for the simple reason that as you ascend with a constant PO2, the percentage of inerts in the loop is actually decreasing as the percentage of O2 is increasing. This is a steady and gradual change if you are doing a few planned stops along the duration of the ascent and will help avoid any potential of an ICD hit.

Then when you are at your final stop I do some quick mental math and usually run the loop at around 85-90% O2 for increasing offgassing. It's at this time you really notice the inerts building up in the loop as you get to a point where you can no longer maintain the setpoint and you need to flush the loop with O2 to get it stabilised again. Near the end of the deco you'll see you need to flush the loop less as there are less inerts being expired......

Bailing out is a different situation requiring much more gas planning......

Regards,

Lance

21st November 2008, 15:55	#21 (permalink)
Dirkadiver RBW Member Current Rebreather/s: Not Bought Yet Other Rebreather/s: Not Bought Yet Join Date: Jun 2008 Location: Charlotte NC Posts: 26	Re: Switching diluents on ascent Quote: (Originally Posted by teomannaskali) I am only speculating but I think the chap did not want to have a hypoxic dil plugged in to his adv . So he swapped to air, so in case of emergency he had something breathable (never need to use it if you are ascending, and maby he did not do a loop flush, just let out some bubbles because of the expanding gas during ascent).Teoman Getting off the hypoxic mix is the only reason that makes any sense to me at all. Where he performed the switch doesn't. I could understand making this switch @ 20' or 30'
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21st November 2008, 15:57	#22 (permalink)
Dirkadiver RBW Member Current Rebreather/s: Not Bought Yet Other Rebreather/s: Not Bought Yet Join Date: Jun 2008 Location: Charlotte NC Posts: 26	Re: Switching diluents on ascent Quote: (Originally Posted by mortenkjerulff) Well, if you use a VR3 with Bühlmann for decompression, diluent switches will save you some decompression (sometimes a lot). So maybe that is why he did it... Cheers Morten FWIW, the diver was using HH electronics. Maybe that will shed some light on the matter.
(Offline)

21st November 2008, 15:58	#23 (permalink)
Mark Chase So many CCR So little etc Current Rebreather/s: rEvo Other Rebreather/s: Inspiration Classic Classic Kiss rEvo Join Date: Jan 2005 Location: Kent Posts: 3,268	Re: Switching diluents on ascent Some well known divers still get of helium deep. I spoke to Kevin Gurr at the LDS last year and he said he was still switching to air on ascent. ATB Mark __________________ See my "Doing It Chasey" video where I'm locked into a padded room, naked, with two ball bearings and within an Hour, I manage to lose one and break the other!!! Kevin Juergensen 16/11/08 [/quote]
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21st November 2008, 16:10	#24 (permalink)
Dirkadiver RBW Member Current Rebreather/s: Not Bought Yet Other Rebreather/s: Not Bought Yet Join Date: Jun 2008 Location: Charlotte NC Posts: 26	Re: Switching diluents on ascent Quote: (Originally Posted by Mark Chase) if were planning a 120m five hour dive ill flush with reduced helium on ascent. Ill go from 8/70 to 18/45 or 21/35 on the ascent in order to reduce deco. Mark This sheds some light on question #2 in my original post & makes perfect sense. Very similar to us OC guys switching gases, with easing off the He & trying to limit uptake of N2. Keep in mind, that the dive I've referenced wasn't very aggressive.
(Offline)

21st November 2008, 16:24	#25 (permalink)
Mark Chase So many CCR So little etc Current Rebreather/s: rEvo Other Rebreather/s: Inspiration Classic Classic Kiss rEvo Join Date: Jan 2005 Location: Kent Posts: 3,268	Re: Switching diluents on ascent Quote: (Originally Posted by Dirkadiver) FWIW, the diver was using HH electronics. Maybe that will shed some light on the matter. HH uses GF deco so Buhlman based and that is going to punish helium more than VPMB will. End result is his deco will be noticeably shorter on an air flush than it would on helium all the way out. Again i stress I personally wouldn't bother on dives under three hours i am merely pointing out it has a benefit in terms of deco and that some divers believe in it as a way of accelerating off gassing rather than just running more aggressive deco. I am also pointing out we used to do this on OC al the time. We didn't suffer inner ear bends and counterdefusion issues because we wernt doing significant bottom times. The recorded cases I am aware of where inner ear bends and counterdefusion were an actual manifestation were much deeper and for much longer duration than anything most average tec divers would ever envisage. ATB Mark __________________ See my "Doing It Chasey" video where I'm locked into a padded room, naked, with two ball bearings and within an Hour, I manage to lose one and break the other!!! Kevin Juergensen 16/11/08 [/quote]
(Offline)

21st November 2008, 16:34	#26 (permalink)
jkaterenchuk . Current Rebreather/s: Megalodon Other Rebreather/s: Inspiration Classic Classic Kiss Join Date: May 2006 Location: PA Posts: 1,567	Re: Switching diluents on ascent Quote: (Originally Posted by Mark Chase) Yet in the good old OC days we used to switch from 13/60 or 18/45 to 32% or 50% without a second thought. Just before i switched to CCR there was a move toward running 32/30 or 21/35 for deep deco but that was considered a new gimmick by many of my fellow divers. On "normal" dives (60-80m 3hour run) I never bother swapping diluent but if were planning a 120m five hour dive ill flush with reduced helium on ascent. Ill go from 8/70 to 18/45 or 21/35 on the ascent in order to reduce deco. 8/70 all the way out = 295min deco 8/70 21/35 and 50% diluient switching = 260mins deco VPMBE 3 ATB Mark I would be interested to know how the deco software really models the fact that you are off gassing into the loop which will rise the % of He to a higher level than if you were using 21/35 which you then rebreath? Does it assume that immediately upon connection of 21/35 and completing your DIL flush that your breathing that gas in the deco calculation or is there some kind of bufferring that takes into account the off gassing and rebreathing of the old higher He gas? John
(Online)

21st November 2008, 16:54	#27 (permalink)
jkaterenchuk . Current Rebreather/s: Megalodon Other Rebreather/s: Inspiration Classic Classic Kiss Join Date: May 2006 Location: PA Posts: 1,567	Re: Switching diluents on ascent Quote: (Originally Posted by rossh) It looks for big swings in ppInert between any of the selected mixes in the ascent phase. The diver can adjust the threshold from the default 0.5 ATA. No literature that I can point at right now. There are various reports on type 2 DCS that are thought to be as a result of IBCD problems with swaps to Air after helium. Type 3 DCS is thought to be an IBCD problem on deep dives too.. Regards Is your method the same or differrent(in what way) than that indicated in Steve Burtons article...see Link Isobaric Counter Diffusion John
(Online)

21st November 2008, 18:24	#28 (permalink)
the diving tiler Deep Blue Current Rebreather/s: Inspiration Vision Other Rebreather/s: Join Date: Apr 2007 Location: Ireland Posts: 182	Re: Switching diluents on ascent [quote=Dirkadiver;224496] Quote: My specific question is this: we did a dive to 250' for about 15min. 3 of us were on OC & 1 was on a Rebreather. The Rebreather diver was diving 10/50 as diluent. At our 70' stop, I noticed bubbles coming up from below me & I knew the only person below me was the Rebreather diver, so I quickly started dropping back down & was frantically trying to get his attention to see what was going on. He kept signaling ok & was fumbling around with one of his offboard tanks & his CL. Once everything settled out, and I was convinced everything was ok, we finished our deco. On the boat, I asked what had happened and he said that he switched from his onboad diluent(10/50) to his offboard air(drysuit bottle). On this specific dive, does this make sense? Hi I have being diving 60 to 70 meters, run times about 2 hours, 16/50 mix switch at 40m to off board air dill. I get out of water about 13 min earlier than not switching. At the 40m switch I do 3 long flushes so lots of bubbles. Hope this helps.
(Offline)

21st November 2008, 23:23	#29 (permalink)
rossh RBW Member Current Rebreather/s: Not Bought Yet Other Rebreather/s: Not Bought Yet Join Date: Jul 2005 Location: Canada Posts: 157	Re: Switching diluents on ascent Quote: Is your method the same or differrent(in what way) than that indicated in Steve Burtons article V-Planner has its own test method. As I posted above, a shift greater that 0.5 ATA in ppInert will trip it. That simple test is enough to find the potential problem areas. Quote: I would be interested to know how the deco software really models the fact that you are off gassing into the loop which will rise the % of He to a higher level than if you were using 21/35 which you then rebreath? Does it assume that immediately upon connection of 21/35 and completing your DIL flush that your breathing that gas in the deco calculation or is there some kind of bufferring that takes into account the off gassing and rebreathing of the old higher He gas? When the diver plans a dil swap or SP change, V-Planner takes that change as an instant change. There are no attempts to blend them together over time. You can plan that as a stepped transition if required. The actual volume of inert gas that a human absorbs in a dive is quite small. Off gassing in ascent and the time delay to build up enough off gassed extra to upset the balance of inerts, is offset by the fact that the loop is reducing the total fraction of inerts in the inspired mix through ascent and a high SP. Any change to the balance of inspired inerts in deco and ascent, is just not significant to the deco calculation. eg. Using a worst case sample test dive, a 1 hour dive at 80m, with a test increase of 2.5% He per ATA in deco, makes about 1% change in deco time. Not much change really. Thats usually less a than a single level change in conservatism. Therefore exhaled excess He in the loop and the upset to the balance of Inert is not considered. If this issue concerns you still, then plan the changes into the dive ascent (add zero time legs after the bottom segment). Regards
(Offline)

22nd November 2008, 01:44	#30 (permalink)
Lancer4545 RBW Member Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Jun 2007 Location: Cairns QLD Australia Posts: 892	Re: Switching diluents on ascent I never bother changing the dil during ascent for the simple reason that as you ascend with a constant PO2, the percentage of inerts in the loop is actually decreasing as the percentage of O2 is increasing. This is a steady and gradual change if you are doing a few planned stops along the duration of the ascent and will help avoid any potential of an ICD hit. Then when you are at your final stop I do some quick mental math and usually run the loop at around 85-90% O2 for increasing offgassing. It's at this time you really notice the inerts building up in the loop as you get to a point where you can no longer maintain the setpoint and you need to flush the loop with O2 to get it stabilised again. Near the end of the deco you'll see you need to flush the loop less as there are less inerts being expired...... Bailing out is a different situation requiring much more gas planning...... Regards, Lance
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