C02 Retention / Decay times

Mark Chase · 2nd November 2007, 21:04

Quote: (Originally Posted by Depth-junkie)

Mark, if your plan makes you feel safe then fine. You go with what makes you comfortable. We can only come to our own conclusions as far as our diving is concerned of what we think is right and wrong.

My thinking is more along the lines of Heliox, Vestibular bend (IBCD) etc etc. For deep CCR dives i dont dive full heliox mixs, but hell they aint far off (very small N2 content)

If you feel happy to up your N2 count and blow of deep stops fine, thats upto you. But its not my way of thinking or of doing things, Dives which involve mandatory deco stops whilst still in deep water trimix depths create other varibles which come into play other than just gas volumes.

I am no doctor of science or deco master guru, but there is good reason why heliox divers deco on other heliox mixs (it's well documented by much cleverer people than me). When i do sub 70-80m dives my CCR dils are bordering on Heliox mixs, thats my thinking.

ATB
Gareth

Gareth

I know you do the dives and don't just talk about it and therefor i listen carefully to what you have to say. We will never agree on our ultimate strategies as i will not dive alpinist. ESPECIALLY on a KISS.

My plans are the best I can come up with with a balance of available training / experience and practical logic. I would like to have a personal team of safety divers each with a twinset of gas for me and I just carry two 80s of bottom mix but thats just not going to happen.

As it is,(bearing in mind my want to max the bottom time) I am most concerned about making the surface still breathing. Bent is acceptable, dead isn't. O2 tox deep is dead, running out of gas well before my 1st gas switch is close to dead if not dead.

The purpose of my question is to avoid the uncontrollable. I cant control an 02tox and I wanted to know if a C02 hit was manageable if i got off loop quick enough.

It would appear that the C02 question is answered (to my satisfaction at least)and I have useful information on the 02 issue.

I am enormously grateful to the contributors so far.

Many thanks

ATB

Mark

Mark Chase · 2nd November 2007, 21:07

Quote: (Originally Posted by sensor330)

What deco software is this. Looks interesting.

As the man said GUE decoplanner V3

It does VPM and GF deco. I put in the actual He i am using and then tweek the 02 to get 1.3 on the bottom then i input my bailout gas plan.

Sadly GUE dont like CCR so this software only works for OC but its excelent software which alows ery quick comparisons of options..

ATB

Mark Chase

Simon Mitchell · 2nd November 2007, 23:35

Quote: (Originally Posted by womble)

Common sense to me would dictate switching to a higher PPO2 would be better. . . " thinking along the same lines as higher PPO2s helping us to offgas quicker.

Hi Womble,

I think this understandable misconception is where your problem started. Higher PO2s help you get rid of inert gas. The physiology of CO2 transport in the body and its elimination is complex, but for our purposes here, suffice to say that CO2 elimination is not affected in any practically significant sense by the inspired fraction of oxygen. It is dependent on your minute volume (ie, the gas volume shifted in and out of the lungs over a minute). The higher the minute volume, the more CO2 you eliminate. Indeed, the whole process can be summarised by the simple equation:

PACO2 = VCO2 / VA where:

PACO2 is the alveolar CO2 (which is virtually the same as arterial CO2)
VCO2 is your body's production of CO2
VA is alveolar ventilation (or the minute volume I described above)

Put in plainer English, the PACO2 is directly proportional to CO2 production and inversely proportional to alveolar ventilation. Or even plainer still, at a constant CO2 production, the alveolar (and arterial) CO2 will go up if alveolar ventilation falls, or go down if alveolar ventilation goes up.

Hope this helps.

Warm regards,

Simon M

Abbo · 3rd November 2007, 00:34

Simon, slightly off thread, but what is it about hypercapnia, ultimately, that kills? Is it the narcosis causing drowning? Or is it narcosis depressing the CNS, shutting down the cardiovascular system? Or is it CO2 crowding out O2, causing death by hypoxia? Obviously in divers there is the additional effect that it raises the breathing rate, which raises WOB, causing increased CO2 production in the body, in a vicious circle, but in a dry chamber, hypercapnia would still kill wouldn't it, even if there were sufficient O2? How?

Thanks for your explanation: I have to admit I didn't know that PO2 has NO effect in reducing PaCO2.

rhelsley · 3rd November 2007, 01:14

Mark-

Regarding having adequate volume of bailout gas to reach the surface...

I use decoweenie (thanks, Phi) and plan my CCR deco dives with a bulhman GF of 35/80 (or so) for my dives for the obvious deco benefits.

For my bailout plan, if SHTF, I use a plan cut from decoweenie but calculated with straight bulhman (no GF), which yields less time at deeper stops and more at the shallower end, thereby saving bailout gas. You'll find the difference in volumes required substantial between the two. It may not be as clean a deco, but bulhman is tried and true, and will get you to the surface alive, if not mildly bent. Good trade off if you ask me.

In fact I carry a range of bailout plans written on a slate that covers various depths & times based on my bailout mix and ccr bottom profile.

A question for the group, as I'm a simpleton when it comes to rebreathers, do modern CCR deco computers allow you to switch algorithms mid-dive to facilitate a plan such as this?

Great thread btw!!

russell helsley

Gene_Hobbs · 3rd November 2007, 03:05

Quote: (Originally Posted by UWSojourner)

Another interesting bit in that description is that high PiO2 was also listed as other factors that contributed to CO2 retention.

To take this one step further... Lanphier and Camporesi describe Lanphier's (1-4) work at NEDU to answer the question 'why don't divers breathe enough?'

Quote: (Originally Posted by Lanphier and Camporesi. Chapter 5. Respiration and Exertion, In: The Physiology and Medicine of Diving)

1. Higher Inspired Oxygen (PiO2) at 4 ata (404 kPa) accounted for not more than 25% of the elevation in End Tidal CO2 (etCO2) above values found at the same work rate when breathing air just below the surface.
2. Increased Work of Breathing accounted for most of the elevation of PACO2 (alveolar gas equation) in exposures above 1 ata (101 kPa), as indicated by the results when helium was substituted for nitrogen at 4 ata (404 kPa).
3. Inadequate ventilatory response to exertion was indicated by the fact that, despite resting values in the normal range, PetCO2 rose markedly with exertion even when the divers breathed air at a depth of only a few feet.

As for the question of CO2 and CNS O2 toxicity, Mike Natoli's MS thesis is a great review and answer to many questions that come up in relation to the topic (5).

Abbo, you might find the UHMS workshop on the 'Unconscious Diver' (6) a good starting point as well as some of the other work on hypercapnia.

References:
1. Lanphier. Nitrogen-Oxygen Mixture Physiology, Phases 1 and 2. NEDU Report 1955-07. RRR ID: 3326
2. Lanphier, Lambertsen, and Funderburk. Nitrogen-Oxygen Mixture Physiology - Phase 3. End-Tidal Gas Sampling System. Carbon Dioxide Regulation in Divers. Carbon Dioxide Sensitivity Tests. NEDU Report 1956-02. RRR ID: 3327
3. Lanphier. NITROGEN-OXYGEN MIXTURE PHYSIOLOGY. PHASE 4. CARBON DIOXIDE SENSITIVITY AS A POTENTIAL MEANS OF PERSONNEL SELECTION. PHASE 6: CARBON DIOXIDE REGULATION UNDER DIVING CONDITIONS NEDU Report 1958-07. RRR ID: 3362
4. Lanphier. Nitrogen-Oxygen Mixture Physiology. Phase 5. Added Respiratory Dead Space (Value in Personnel Selection tests) (Physiological Effects Under Diving Conditions). NEDU Report 1956-05. RRR ID: 3809
5. Natoli and Vann. Factors Affecting CNS Oxygen Toxicity in Humans. MS Thesis (Duke) 1996. RRR ID: 26
6. Lanphier EH (ed). Unconscious Diver: Respiratory Control and Other Contributing Factors. 25th Undersea and Hyperbaric Medical Society Workshop. UHMS Publication Number 52WS(RC)1-25-82. Bethesda: Undersea and Hyperbaric Medical Society; 1982; 160 pages. RRR ID: 4278

Drmike · 3rd November 2007, 08:06

Quote: (Originally Posted by Mark Chase)

Shut up moaning and dive a third large tin

There yah go

I dont think theres a magic shortcut. You do the dive - you need the gas volumes.

People often moan about carrying one or two tanks saying what a pain it is - but I suspect if they spent some time organising their tanks better (the way they carry them) they wouldnt mind carrying them so much and their dives would be overall safer (from bailout perspective)

My advice would be get biggest tanks you can and sidemount them*, forget about them. Thats the min baseline. I add to this baseline depending on the dive. Im so used to diving that way I just dont feel the tanks are there at all.

Any additional tanks added to this baseline are clipped off high and tight - no danglies. These are the only tanks I 'feel' and its important to rig them right (high and tight) I feel them because you loose easy access to your chest D rings and feel cluttered. I also preffer if I can to keep one side clear so I can easily clip light, secondary etc off on one clear chest d ring - but thats not always possible.

If diving with more than 3 tanks (so lose easy access to both chest d rings) I often clip my secondary to my wrist so I dont have to dig it out from under the front two tanks and I dont clip off my light to chest d rings

*biggest I have sidemounted so far was 18L steel tanks sidemounted with polystyrene floats attached (as shown to me pre by by Jerome) Nice having 36L of tank vol as a baseline, but usually I use AL80s as min baseline.

Id rather take 10% extra bottome bail gas than 100% extra shallow gas because you can have a long time to alert the surface sometimes before you need the shallow gas and depth is where things go wrong and go wrong fast (additional stress from paranoia about not having enough bottom bail gas to swim back to line increases RMV and/or makes you make bad decisions (such as an imediate ascent when it would be possible to make it back to upline) I think your better off having bags of bottom bail gas so you have time to find your buddies (if diving same wreck same time) get their gas and avoid a solo free floating bailout. If your on the upline you have people around that can help easy for surface support to drop tanks. much safer and again importantly you will be less stressed (lower rmv)

One final point I learnt the hard way is to have spgs on bottom bail tank(s) the reason is simple. when you bail to OC you will (if your anything like me) imediately start to worry you will run out of deep bail gas before you reach the depth you can switch. That stress can encourage you to do silly things like an imediate ascent instead of swimming back to the upline (where that possible), or ascending, or turning dive (cave) before alerting your buddy(s)

Lets say youve just bailed and signalled your buddy (who is leading cave dive) he hasnt noticed, hes still swimming away from you quite a distance now- do you 'waste gas' catching up with him to alert him youve bailed? One part of you is screamming you should the other is screaming GET OUT GET OUT!

Stress/paranoia about running out of bottom bail gas can encourage you to do the wrong thing and not catch up with him. (I was quite surprised when I turned around and found no buddy

) Of course if you do 'waste' some of that gas to catch up with him you benefit far more because now you have access to his gas too! Not having an spg adds stress as you will get paranoid (i did) the stress can also increase your rmv!! If you have an spg you can see how much volume you have how much your using and that will help you I believe keep a little calmer. Anything that will minimise your stress and hence rmv is critical if you want to survive a bail

Drmike · 3rd November 2007, 08:10

Quote: (Originally Posted by rhelsley)

Mark-

Regarding having adequate volume of bailout gas to reach the surface...

I use decoweenie (thanks, Phi) and plan my CCR deco dives with a bulhman GF of 35/80 (or so) for my dives for the obvious deco benefits.

For my bailout plan, if SHTF, I use a plan cut from decoweenie but calculated with straight bulhman (no GF), which yields less time at deeper stops and more at the shallower end, thereby saving bailout gas. You'll find the difference in volumes required substantial between the two. It may not be as clean a deco, but bulhman is tried and true, and will get you to the surface alive, if not mildly bent. Good trade off if you ask me.

In fact I carry a range of bailout plans written on a slate that covers various depths & times based on my bailout mix and ccr bottom profile.

A question for the group, as I'm a simpleton when it comes to rebreathers, do modern CCR deco computers allow you to switch algorithms mid-dive to facilitate a plan such as this?

Great thread btw!!

russell helsley

VR3 is conservative - yes if there was any doubt about having enough gas Id bend the VR3 if I bailed by blowing the deep stops. It would still calc deco so id follow rest of profile

Drmike · 3rd November 2007, 08:25

Quote: (Originally Posted by Mark Chase)

The purpose of my question is to avoid the uncontrollable.

This is the right approach I think.

On deep dives where is the risk? Running out of OC bail gas is a risk - but its a very easily managed risk simply by staging or carrying more tanks of gas.

But IBCD poorly understood deco these are risks we cant easily control because we just dont have enough data on them - these make these things much larger 'risks' in my view.

So Im very conservative when it comes to dealing with these unknowns.

Risk of IBCD (we dont know how much risk it is so must assume worse) can be minimised by no big spikes in PN2 = but that means more tanks - but thats easy enough to manage. Adding extra tanks is easy - result of IBCD can be leithal.

Risk of poorly understood deco - be conservative. Ive had no issues so far doing vpmb with Buhlman tail. Ive also had good experiences with Buhlman, But I would never use pure vpmb (maybe when its finished its metamorphisis to match non bubble models

).

I also agree with Gareth - loads of He!

On deep dives I set min deco stop time to 1secs - it can make a HUGE difference to gas volumes needed and total deco time needed. theres no point sitting there on gassing and using up gas during deco (as you will be on deep deco part of deep dives) when you dont need to be!!

womble · 3rd November 2007, 18:56

Quote: (Originally Posted by Simon Mitchell)

Hi Womble,

I think this understandable misconception is where your problem started. Higher PO2s help you get rid of inert gas. The physiology of CO2 transport in the body and its elimination is complex, but for our purposes here, suffice to say that CO2 elimination is not affected in any practically significant sense by the inspired fraction of oxygen. It is dependent on your minute volume (ie, the gas volume shifted in and out of the lungs over a minute). The higher the minute volume, the more CO2 you eliminate. Indeed, the whole process can be summarised by the simple equation:

PACO2 = VCO2 / VA where:

PACO2 is the alveolar CO2 (which is virtually the same as arterial CO2)
VCO2 is your body's production of CO2
VA is alveolar ventilation (or the minute volume I described above)

Put in plainer English, the PACO2 is directly proportional to CO2 production and inversely proportional to alveolar ventilation. Or even plainer still, at a constant CO2 production, the alveolar (and arterial) CO2 will go up if alveolar ventilation falls, or go down if alveolar ventilation goes up.

Hope this helps.

Warm regards,

Simon M

Simon,

yes, this makes sense. Thankyou.

Basically, If you stop and do nothing (lower VCO2) you will get a drop in arterial/alveolar CO2. This bit we all agree on. And as the loop is an extention of your lungs, will this result in a reduction of loop CO2 (if you dont bailout and dil flush instead)??

Secondly, alveolar ventilation (VA) changes you explain above also make sense. I also must apologise, I misread one of your earlier posts. You said Hypoventilation, I misread that as hyperventilation. So yes, If you are hypoventilating with regards to CO2 production (PACO2), you are just gonna get worse and worse. This would include, If I'm on the right track, shallow rapid breathing, as you may be experiencing with a CO2 hit.

High WOB on the unit (if still onloop) or high WOB on bailout (eg poor performance reg, dense gas etc) would affect both VCO2, as you are working harder to get the gas, AND it would affect VA. Is this assumption correct??

This thread has certainly given me a lot of food for thought.

2nd November 2007, 21:04	#41 (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: C02 Retention / Decay times Quote: (Originally Posted by Depth-junkie) Mark, if your plan makes you feel safe then fine. You go with what makes you comfortable. We can only come to our own conclusions as far as our diving is concerned of what we think is right and wrong. My thinking is more along the lines of Heliox, Vestibular bend (IBCD) etc etc. For deep CCR dives i dont dive full heliox mixs, but hell they aint far off (very small N2 content) If you feel happy to up your N2 count and blow of deep stops fine, thats upto you. But its not my way of thinking or of doing things, Dives which involve mandatory deco stops whilst still in deep water trimix depths create other varibles which come into play other than just gas volumes. I am no doctor of science or deco master guru, but there is good reason why heliox divers deco on other heliox mixs (it's well documented by much cleverer people than me). When i do sub 70-80m dives my CCR dils are bordering on Heliox mixs, thats my thinking. ATB Gareth Gareth I know you do the dives and don't just talk about it and therefor i listen carefully to what you have to say. We will never agree on our ultimate strategies as i will not dive alpinist. ESPECIALLY on a KISS. My plans are the best I can come up with with a balance of available training / experience and practical logic. I would like to have a personal team of safety divers each with a twinset of gas for me and I just carry two 80s of bottom mix but thats just not going to happen. As it is,(bearing in mind my want to max the bottom time) I am most concerned about making the surface still breathing. Bent is acceptable, dead isn't. O2 tox deep is dead, running out of gas well before my 1st gas switch is close to dead if not dead. The purpose of my question is to avoid the uncontrollable. I cant control an 02tox and I wanted to know if a C02 hit was manageable if i got off loop quick enough. It would appear that the C02 question is answered (to my satisfaction at least)and I have useful information on the 02 issue. I am enormously grateful to the contributors so far. Many thanks ATB Mark __________________ 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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2nd November 2007, 21:07	#42 (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: C02 Retention / Decay times Quote: (Originally Posted by sensor330) What deco software is this. Looks interesting. As the man said GUE decoplanner V3 It does VPM and GF deco. I put in the actual He i am using and then tweek the 02 to get 1.3 on the bottom then i input my bailout gas plan. Sadly GUE dont like CCR so this software only works for OC but its excelent software which alows ery quick comparisons of options.. 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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2nd November 2007, 23:35	#43 (permalink)
Simon Mitchell New Member Current Rebreather/s: MK 15.X Other Rebreather/s: Join Date: May 2005 Location: New Zealand Posts: 177	Re: C02 Retention / Decay times Quote: (Originally Posted by womble) Common sense to me would dictate switching to a higher PPO2 would be better. . . " thinking along the same lines as higher PPO2s helping us to offgas quicker. Hi Womble, I think this understandable misconception is where your problem started. Higher PO2s help you get rid of inert gas. The physiology of CO2 transport in the body and its elimination is complex, but for our purposes here, suffice to say that CO2 elimination is not affected in any practically significant sense by the inspired fraction of oxygen. It is dependent on your minute volume (ie, the gas volume shifted in and out of the lungs over a minute). The higher the minute volume, the more CO2 you eliminate. Indeed, the whole process can be summarised by the simple equation: PACO2 = VCO2 / VA where: PACO2 is the alveolar CO2 (which is virtually the same as arterial CO2) VCO2 is your body's production of CO2 VA is alveolar ventilation (or the minute volume I described above) Put in plainer English, the PACO2 is directly proportional to CO2 production and inversely proportional to alveolar ventilation. Or even plainer still, at a constant CO2 production, the alveolar (and arterial) CO2 will go up if alveolar ventilation falls, or go down if alveolar ventilation goes up. Hope this helps. Warm regards, Simon M Last edited by Simon Mitchell : 2nd November 2007 at 23:37. Reason: As usual, grammar and spelling!
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3rd November 2007, 00:34	#44 (permalink)
Abbo Nicholas Smith Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Aug 2006 Location: Tokyo Posts: 445	Re: C02 Retention / Decay times Simon, slightly off thread, but what is it about hypercapnia, ultimately, that kills? Is it the narcosis causing drowning? Or is it narcosis depressing the CNS, shutting down the cardiovascular system? Or is it CO2 crowding out O2, causing death by hypoxia? Obviously in divers there is the additional effect that it raises the breathing rate, which raises WOB, causing increased CO2 production in the body, in a vicious circle, but in a dry chamber, hypercapnia would still kill wouldn't it, even if there were sufficient O2? How? Thanks for your explanation: I have to admit I didn't know that PO2 has NO effect in reducing PaCO2. Last edited by Abbo : 3rd November 2007 at 00:48.
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3rd November 2007, 01:14	#45 (permalink)
rhelsley New Member Current Rebreather/s: Home Build Other Rebreather/s: Home Build Join Date: May 2005 Location: seattle, usa Posts: 32	Re: C02 Retention / Decay times Mark- Regarding having adequate volume of bailout gas to reach the surface... I use decoweenie (thanks, Phi) and plan my CCR deco dives with a bulhman GF of 35/80 (or so) for my dives for the obvious deco benefits. For my bailout plan, if SHTF, I use a plan cut from decoweenie but calculated with straight bulhman (no GF), which yields less time at deeper stops and more at the shallower end, thereby saving bailout gas. You'll find the difference in volumes required substantial between the two. It may not be as clean a deco, but bulhman is tried and true, and will get you to the surface alive, if not mildly bent. Good trade off if you ask me. In fact I carry a range of bailout plans written on a slate that covers various depths & times based on my bailout mix and ccr bottom profile. A question for the group, as I'm a simpleton when it comes to rebreathers, do modern CCR deco computers allow you to switch algorithms mid-dive to facilitate a plan such as this? Great thread btw!! russell helsley
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3rd November 2007, 03:05	#46 (permalink)
Gene_Hobbs Resident bibliophile Current Rebreather/s: Not Bought Yet Other Rebreather/s: Not Bought Yet Join Date: Sep 2006 Location: Durham, NC, USA Posts: 122	Re: C02 Retention / Decay times Quote: (Originally Posted by UWSojourner) Another interesting bit in that description is that high PiO2 was also listed as other factors that contributed to CO2 retention. To take this one step further... Lanphier and Camporesi describe Lanphier's (1-4) work at NEDU to answer the question 'why don't divers breathe enough?' Quote: (Originally Posted by Lanphier and Camporesi. Chapter 5. Respiration and Exertion, In: The Physiology and Medicine of Diving) 1. Higher Inspired Oxygen (PiO2) at 4 ata (404 kPa) accounted for not more than 25% of the elevation in End Tidal CO2 (etCO2) above values found at the same work rate when breathing air just below the surface. 2. Increased Work of Breathing accounted for most of the elevation of PACO2 (alveolar gas equation) in exposures above 1 ata (101 kPa), as indicated by the results when helium was substituted for nitrogen at 4 ata (404 kPa). 3. Inadequate ventilatory response to exertion was indicated by the fact that, despite resting values in the normal range, PetCO2 rose markedly with exertion even when the divers breathed air at a depth of only a few feet. As for the question of CO2 and CNS O2 toxicity, Mike Natoli's MS thesis is a great review and answer to many questions that come up in relation to the topic (5). Abbo, you might find the UHMS workshop on the 'Unconscious Diver' (6) a good starting point as well as some of the other work on hypercapnia. References: 1. Lanphier. Nitrogen-Oxygen Mixture Physiology, Phases 1 and 2. NEDU Report 1955-07. RRR ID: 3326 2. Lanphier, Lambertsen, and Funderburk. Nitrogen-Oxygen Mixture Physiology - Phase 3. End-Tidal Gas Sampling System. Carbon Dioxide Regulation in Divers. Carbon Dioxide Sensitivity Tests. NEDU Report 1956-02. RRR ID: 3327 3. Lanphier. NITROGEN-OXYGEN MIXTURE PHYSIOLOGY. PHASE 4. CARBON DIOXIDE SENSITIVITY AS A POTENTIAL MEANS OF PERSONNEL SELECTION. PHASE 6: CARBON DIOXIDE REGULATION UNDER DIVING CONDITIONS NEDU Report 1958-07. RRR ID: 3362 4. Lanphier. Nitrogen-Oxygen Mixture Physiology. Phase 5. Added Respiratory Dead Space (Value in Personnel Selection tests) (Physiological Effects Under Diving Conditions). NEDU Report 1956-05. RRR ID: 3809 5. Natoli and Vann. Factors Affecting CNS Oxygen Toxicity in Humans. MS Thesis (Duke) 1996. RRR ID: 26 6. Lanphier EH (ed). Unconscious Diver: Respiratory Control and Other Contributing Factors. 25th Undersea and Hyperbaric Medical Society Workshop. UHMS Publication Number 52WS(RC)1-25-82. Bethesda: Undersea and Hyperbaric Medical Society; 1982; 160 pages. RRR ID: 4278 __________________ http://rubicon-foundation.org/ Home of the Rubicon Research Repository. For help getting started with the Repository, please visit our FAQ page. PLEASE support our work. "Oxygen is addictive and deadly. Everyone who uses it will eventually die" --RW Hamilton, PhD 1991
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3rd November 2007, 08:06	#47 (permalink)
Drmike Custom Title Allowed! Current Rebreather/s: MK 15.X Ouroboros Other CCR Home Build Other Rebreather/s: Inspiration Classic Other CCR Home Build Join Date: Feb 2005 Posts: 3,325	Re: C02 Retention / Decay times Quote: (Originally Posted by Mark Chase) Shut up moaning and dive a third large tin There yah go I dont think theres a magic shortcut. You do the dive - you need the gas volumes. People often moan about carrying one or two tanks saying what a pain it is - but I suspect if they spent some time organising their tanks better (the way they carry them) they wouldnt mind carrying them so much and their dives would be overall safer (from bailout perspective) My advice would be get biggest tanks you can and sidemount them, forget about them. Thats the min baseline. I add to this baseline depending on the dive. Im so used to diving that way I just dont feel the tanks are there at all. Any additional tanks added to this baseline are clipped off high and tight - no danglies. These are the only tanks I 'feel' and its important to rig them right (high and tight) I feel them because you loose easy access to your chest D rings and feel cluttered. I also preffer if I can to keep one side clear so I can easily clip light, secondary etc off on one clear chest d ring - but thats not always possible. If diving with more than 3 tanks (so lose easy access to both chest d rings) I often clip my secondary to my wrist so I dont have to dig it out from under the front two tanks and I dont clip off my light to chest d rings biggest I have sidemounted so far was 18L steel tanks sidemounted with polystyrene floats attached (as shown to me pre by by Jerome) Nice having 36L of tank vol as a baseline, but usually I use AL80s as min baseline. Id rather take 10% extra bottome bail gas than 100% extra shallow gas because you can have a long time to alert the surface sometimes before you need the shallow gas and depth is where things go wrong and go wrong fast (additional stress from paranoia about not having enough bottom bail gas to swim back to line increases RMV and/or makes you make bad decisions (such as an imediate ascent when it would be possible to make it back to upline) I think your better off having bags of bottom bail gas so you have time to find your buddies (if diving same wreck same time) get their gas and avoid a solo free floating bailout. If your on the upline you have people around that can help easy for surface support to drop tanks. much safer and again importantly you will be less stressed (lower rmv) One final point I learnt the hard way is to have spgs on bottom bail tank(s) the reason is simple. when you bail to OC you will (if your anything like me) imediately start to worry you will run out of deep bail gas before you reach the depth you can switch. That stress can encourage you to do silly things like an imediate ascent instead of swimming back to the upline (where that possible), or ascending, or turning dive (cave) before alerting your buddy(s) Lets say youve just bailed and signalled your buddy (who is leading cave dive) he hasnt noticed, hes still swimming away from you quite a distance now- do you 'waste gas' catching up with him to alert him youve bailed? One part of you is screamming you should the other is screaming GET OUT GET OUT! Stress/paranoia about running out of bottom bail gas can encourage you to do the wrong thing and not catch up with him. (I was quite surprised when I turned around and found no buddy ) Of course if you do 'waste' some of that gas to catch up with him you benefit far more because now you have access to his gas too! Not having an spg adds stress as you will get paranoid (i did) the stress can also increase your rmv!! If you have an spg you can see how much volume you have how much your using and that will help you I believe keep a little calmer. Anything that will minimise your stress and hence rmv is critical if you want to survive a bail __________________ Cave diving is a sport Wreck diving is a sport Diving in general is a sport 'Rebreather diving' is not a sport its the delusional obsession with a highly dangerous and often inappropriate piece of equipment
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3rd November 2007, 08:10	#48 (permalink)
Drmike Custom Title Allowed! Current Rebreather/s: MK 15.X Ouroboros Other CCR Home Build Other Rebreather/s: Inspiration Classic Other CCR Home Build Join Date: Feb 2005 Posts: 3,325	Re: C02 Retention / Decay times Quote: (Originally Posted by rhelsley) Mark- Regarding having adequate volume of bailout gas to reach the surface... I use decoweenie (thanks, Phi) and plan my CCR deco dives with a bulhman GF of 35/80 (or so) for my dives for the obvious deco benefits. For my bailout plan, if SHTF, I use a plan cut from decoweenie but calculated with straight bulhman (no GF), which yields less time at deeper stops and more at the shallower end, thereby saving bailout gas. You'll find the difference in volumes required substantial between the two. It may not be as clean a deco, but bulhman is tried and true, and will get you to the surface alive, if not mildly bent. Good trade off if you ask me. In fact I carry a range of bailout plans written on a slate that covers various depths & times based on my bailout mix and ccr bottom profile. A question for the group, as I'm a simpleton when it comes to rebreathers, do modern CCR deco computers allow you to switch algorithms mid-dive to facilitate a plan such as this? Great thread btw!! russell helsley VR3 is conservative - yes if there was any doubt about having enough gas Id bend the VR3 if I bailed by blowing the deep stops. It would still calc deco so id follow rest of profile __________________ Cave diving is a sport Wreck diving is a sport Diving in general is a sport 'Rebreather diving' is not a sport its the delusional obsession with a highly dangerous and often inappropriate piece of equipment
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3rd November 2007, 08:25	#49 (permalink)
Drmike Custom Title Allowed! Current Rebreather/s: MK 15.X Ouroboros Other CCR Home Build Other Rebreather/s: Inspiration Classic Other CCR Home Build Join Date: Feb 2005 Posts: 3,325	Re: C02 Retention / Decay times Quote: (Originally Posted by Mark Chase) The purpose of my question is to avoid the uncontrollable. This is the right approach I think. On deep dives where is the risk? Running out of OC bail gas is a risk - but its a very easily managed risk simply by staging or carrying more tanks of gas. But IBCD poorly understood deco these are risks we cant easily control because we just dont have enough data on them - these make these things much larger 'risks' in my view. So Im very conservative when it comes to dealing with these unknowns. Risk of IBCD (we dont know how much risk it is so must assume worse) can be minimised by no big spikes in PN2 = but that means more tanks - but thats easy enough to manage. Adding extra tanks is easy - result of IBCD can be leithal. Risk of poorly understood deco - be conservative. Ive had no issues so far doing vpmb with Buhlman tail. Ive also had good experiences with Buhlman, But I would never use pure vpmb (maybe when its finished its metamorphisis to match non bubble models ). I also agree with Gareth - loads of He! On deep dives I set min deco stop time to 1secs - it can make a HUGE difference to gas volumes needed and total deco time needed. theres no point sitting there on gassing and using up gas during deco (as you will be on deep deco part of deep dives) when you dont need to be!! __________________ Cave diving is a sport Wreck diving is a sport Diving in general is a sport 'Rebreather diving' is not a sport its the delusional obsession with a highly dangerous and often inappropriate piece of equipment Last edited by Drmike : 3rd November 2007 at 08:39.
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3rd November 2007, 18:56	#50 (permalink)
womble Custom Title Allowed! Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Aug 2006 Location: UK Posts: 196	Re: C02 Retention / Decay times Quote: (Originally Posted by Simon Mitchell) Hi Womble, I think this understandable misconception is where your problem started. Higher PO2s help you get rid of inert gas. The physiology of CO2 transport in the body and its elimination is complex, but for our purposes here, suffice to say that CO2 elimination is not affected in any practically significant sense by the inspired fraction of oxygen. It is dependent on your minute volume (ie, the gas volume shifted in and out of the lungs over a minute). The higher the minute volume, the more CO2 you eliminate. Indeed, the whole process can be summarised by the simple equation: PACO2 = VCO2 / VA where: PACO2 is the alveolar CO2 (which is virtually the same as arterial CO2) VCO2 is your body's production of CO2 VA is alveolar ventilation (or the minute volume I described above) Put in plainer English, the PACO2 is directly proportional to CO2 production and inversely proportional to alveolar ventilation. Or even plainer still, at a constant CO2 production, the alveolar (and arterial) CO2 will go up if alveolar ventilation falls, or go down if alveolar ventilation goes up. Hope this helps. Warm regards, Simon M Simon, yes, this makes sense. Thankyou. Basically, If you stop and do nothing (lower VCO2) you will get a drop in arterial/alveolar CO2. This bit we all agree on. And as the loop is an extention of your lungs, will this result in a reduction of loop CO2 (if you dont bailout and dil flush instead)?? Secondly, alveolar ventilation (VA) changes you explain above also make sense. I also must apologise, I misread one of your earlier posts. You said Hypoventilation, I misread that as hyperventilation. So yes, If you are hypoventilating with regards to CO2 production (PACO2), you are just gonna get worse and worse. This would include, If I'm on the right track, shallow rapid breathing, as you may be experiencing with a CO2 hit. High WOB on the unit (if still onloop) or high WOB on bailout (eg poor performance reg, dense gas etc) would affect both VCO2, as you are working harder to get the gas, AND it would affect VA. Is this assumption correct?? This thread has certainly given me a lot of food for thought. __________________ "What could possibly go wrong?? . . "
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