IBCD on bailout?

Padowan · 22nd June 2006, 13:46

All this talk about bailout gasses (number and choice) got me thinking....

I put together a little spreadsheet for me to look at gas switches for various gas choices to give me an idea of the switch depth (for a set PPO2) and the PPN2 to look for N2 spikes and IBCD (Isobaric Counter Diffusion) risks. This got me thinking, as I was only looking at the switches from one OC gas to another, so I included a row that showed the loop PPo2 and PPN2 so I get a feeling for the spikes bailing out on the bottom.

So, to cut a long story short. Is IBCD a significant risk doing a switch on the bottom before you start your ascent (and thefore before you start offgassing) or is it only a big problem once you've started things fizzing on the way up, and switch shallower?

***diverklondike*** · 22nd June 2006, 14:19

For that exact reason I keep He in all of my bailouts....

Depending on the exact depth/duration of the dive down to 275fsw they are a selection of the below...

16/40
20/35
40/30
100%

"He" is your friend...

Dive Safe...

Mark

Padowan · 22nd June 2006, 14:30

So what is considered an "acceptable" N2 spike? Is it an absolute figure (eg 0.5bar = 3.5bar-4bar), or perhaps a percentatge of the ambient N2 pressure (eg 20% = 3.5bar-4.2bar).

Mark Chase · 22nd June 2006, 15:23

Quote: (Originally Posted by Padowan)

So what is considered an "acceptable" N2 spike? Is it an absolute figure (eg 0.5bar = 3.5bar-4bar), or perhaps a percentatge of the ambient N2 pressure (eg 20% = 3.5bar-4.2bar).

Depends totaly on depth and bottom time.

I have gas switched on OC from 10/70 to 32% on several ocasions and on 16/55 to 32% loads more times and from 18/45 to 32% more times than I care to remember.

I havent suffered IBCD yet.

However i understand things would have been totaly diferent had i spent 45mins at 100 instead of 10.

A very good link

http://www.ncbi.nlm.nih.gov/entrez/q...=pubmed_docsum

ATB

Mark Chase

Gibbon · 22nd June 2006, 23:20

I think that if you look at the individual component gases and then look at the effective change in depth that you would need on the original mix to create the same pressure change (per gas) then ask yourself is it below the deco ceiling on the original mix again per gas.

So current deco ceiling = c.ceiling1
current mix = c.pg1, c.pg2, c.pg3
new mix =nm.pg1, nm.pg2, nm.pg3

Is (nm.pg1 < c.ceiling) and (nm.pg2 < c.ceiling) and (nm.pg3 < c.ceiling)??

If you dont agree with this then please say so as I would very much like to understand this better!

pchanning · 23rd June 2006, 12:39

Alright Si,

The new version of V-Planner has an added facility - a pp inert gas map. It plots (excel stylee) the ppo2 and depths of all gases and shows you whats going on.

Visually, it conveys an excellent means for getting a quick feel for different options. Well done to Ross H. Great stuff mate.

The CCR graphs are lovely and smooth, the OC ones look like a 3yr old drawing of errr.... whatever it is 3yr old's draw.

HTH
Paul

davy · 23rd June 2006, 14:02

Quote: (Originally Posted by Padowan)

So what is considered an "acceptable" N2 spike? Is it an absolute figure (eg 0.5bar = 3.5bar-4bar), or perhaps a percentatge of the ambient N2 pressure (eg 20% = 3.5bar-4.2bar).

Hey Padowan,

have a look at this
http://www.scubaengineer.com/isobari..._diffusion.htm

It's an interesting article viewed from a solubility point of view.

Padowan · 23rd June 2006, 14:25

Quote: (Originally Posted by davy)

Hey Padowan,

have a look at this
http://www.scubaengineer.com/isobari..._diffusion.htm

It's an interesting article viewed from a solubility point of view.

Interesting link, thanks!

22nd June 2006, 13:46	#1 (permalink)
Padowan Proper Boffin Current Rebreather/s: Inspiration Classic Other Rebreather/s: Join Date: Feb 2005 Location: Kent, UK. Posts: 518	IBCD on bailout? All this talk about bailout gasses (number and choice) got me thinking.... I put together a little spreadsheet for me to look at gas switches for various gas choices to give me an idea of the switch depth (for a set PPO2) and the PPN2 to look for N2 spikes and IBCD (Isobaric Counter Diffusion) risks. This got me thinking, as I was only looking at the switches from one OC gas to another, so I included a row that showed the loop PPo2 and PPN2 so I get a feeling for the spikes bailing out on the bottom. So, to cut a long story short. Is IBCD a significant risk doing a switch on the bottom before you start your ascent (and thefore before you start offgassing) or is it only a big problem once you've started things fizzing on the way up, and switch shallower? __________________ Padowan The future's bright, the future's Yellow .....for now padowan.net - Latest Video: Scapa Flow Rebreather Diving
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22nd June 2006, 14:19	#2 (permalink)
*diverklondike* Moderator Current Rebreather/s: Inspiration Classic Other CCR Other Rebreather/s: Inspiration Classic Other CCR Join Date: Oct 2005 Location: Delaware, USA Posts: 1,552	Re: IBCD on bailout? For that exact reason I keep He in all of my bailouts.... Depending on the exact depth/duration of the dive down to 275fsw they are a selection of the below... 16/40 20/35 40/30 100% "He" is your friend... Dive Safe... Mark
(Online)

22nd June 2006, 14:30	#3 (permalink)
Padowan Proper Boffin Current Rebreather/s: Inspiration Classic Other Rebreather/s: Join Date: Feb 2005 Location: Kent, UK. Posts: 518	Re: IBCD on bailout? So what is considered an "acceptable" N2 spike? Is it an absolute figure (eg 0.5bar = 3.5bar-4bar), or perhaps a percentatge of the ambient N2 pressure (eg 20% = 3.5bar-4.2bar). __________________ Padowan The future's bright, the future's Yellow .....for now padowan.net - Latest Video: Scapa Flow Rebreather Diving
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22nd June 2006, 15:23	#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: 2,539	Re: IBCD on bailout? Quote: (Originally Posted by Padowan) So what is considered an "acceptable" N2 spike? Is it an absolute figure (eg 0.5bar = 3.5bar-4bar), or perhaps a percentatge of the ambient N2 pressure (eg 20% = 3.5bar-4.2bar). Depends totaly on depth and bottom time. I have gas switched on OC from 10/70 to 32% on several ocasions and on 16/55 to 32% loads more times and from 18/45 to 32% more times than I care to remember. I havent suffered IBCD yet. However i understand things would have been totaly diferent had i spent 45mins at 100 instead of 10. A very good link http://www.ncbi.nlm.nih.gov/entrez/q...=pubmed_docsum 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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22nd June 2006, 23:20	#5 (permalink)
Gibbon Southern Monkey Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Aug 2005 Location: East Devon Posts: 226	Re: IBCD on bailout? I think that if you look at the individual component gases and then look at the effective change in depth that you would need on the original mix to create the same pressure change (per gas) then ask yourself is it below the deco ceiling on the original mix again per gas. So current deco ceiling = c.ceiling1 current mix = c.pg1, c.pg2, c.pg3 new mix =nm.pg1, nm.pg2, nm.pg3 Is (nm.pg1 < c.ceiling) and (nm.pg2 < c.ceiling) and (nm.pg3 < c.ceiling)?? If you dont agree with this then please say so as I would very much like to understand this better!
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23rd June 2006, 12:39	#6 (permalink)
pchanning PFO free :) Current Rebreather/s: Inspiration Classic Other Rebreather/s: Join Date: Feb 2005 Location: fixed! Posts: 454	Re: IBCD on bailout? Alright Si, The new version of V-Planner has an added facility - a pp inert gas map. It plots (excel stylee) the ppo2 and depths of all gases and shows you whats going on. Visually, it conveys an excellent means for getting a quick feel for different options. Well done to Ross H. Great stuff mate. The CCR graphs are lovely and smooth, the OC ones look like a 3yr old drawing of errr.... whatever it is 3yr old's draw. HTH Paul __________________ Cheers Paul The key to enlightenment... is survival. Last edited by pchanning : 23rd June 2006 at 12:42.
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23rd June 2006, 14:02	#7 (permalink)
davy New Member Current Rebreather/s: RB80 / Clone Home Build Other Rebreather/s: RB80 / Clone Home Build Join Date: Aug 2005 Location: Belgium Posts: 21	Re: IBCD on bailout? Quote: (Originally Posted by Padowan) So what is considered an "acceptable" N2 spike? Is it an absolute figure (eg 0.5bar = 3.5bar-4bar), or perhaps a percentatge of the ambient N2 pressure (eg 20% = 3.5bar-4.2bar). Hey Padowan, have a look at this http://www.scubaengineer.com/isobari..._diffusion.htm It's an interesting article viewed from a solubility point of view.
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23rd June 2006, 14:25	#8 (permalink)
Padowan Proper Boffin Current Rebreather/s: Inspiration Classic Other Rebreather/s: Join Date: Feb 2005 Location: Kent, UK. Posts: 518	Re: IBCD on bailout? Quote: (Originally Posted by davy) Hey Padowan, have a look at this http://www.scubaengineer.com/isobari..._diffusion.htm It's an interesting article viewed from a solubility point of view. Interesting link, thanks! __________________ Padowan The future's bright, the future's Yellow .....for now padowan.net - Latest Video: Scapa Flow Rebreather Diving
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