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dlk
03-01-2013, 02:04 PM
Following Matt's posting on the maths for an approach to avoiding isobaric counter diffusion: http://www.thediveforum.com/technical-diving-faqs/789-diving-sums.html (there's another approach you could take, and I'll write it up later when I get the chance), does anyone feel like taking a stab at explaining ICD?

Warning: this might be a trap:)!

DLK

Scuttler
03-01-2013, 02:12 PM
It's the movement or diffusion of gasses in different directions which can result in bubbles forming without there necessarily being a change in depth or pressure.

Does that work for you?

dlk
03-01-2013, 02:19 PM
It's the movement or diffusion of gasses in different directions which can result in bubbles forming without there necessarily being a change in depth or pressure.

Does that work for you?

So far, yes. It's the how that gets confusing.

Hot Totty
03-01-2013, 02:23 PM
On gassing/off gassing of different gases due to a change in their partial pressure ie at gas switch

BTS
03-01-2013, 02:26 PM
Pesonally I don't care how, all I need to know is what it is and how to avoid any associated issues, no need to make it all scientificky...

Graham Smith
03-01-2013, 02:28 PM
I thought the basic understanding was that there is potentially three different causes / types of ICD??

Or there is always GI3's response
"Isobaric counter diffusion" is what happens when Santa Clause sucks the. Easter Bunny's dick - it's BULLSHIT.

G

Scuttler
03-01-2013, 02:36 PM
My schoolboy understanding of this is that it's governed by the permeability of the gasses. If the gas diffusing into the bubble has a greater permeability than the gas diffusing out then the bubble grows and vice versa.

jb2cool
03-01-2013, 02:41 PM
Graham, please tell me that's a real quote. That'd make my day.

My understanding (Might not be accurate) is that ICD can occur when you move from a mixture very low in nitrogen (trimix/heliox) to a rich nitrox mixture 50%. The nitrogen in the gas you are now breathing is a much higher partial pressure than the disolved gas in your body and therefore you start ongassing nitrogen faster than you are off-gassing helium and this pushes you over the limit and you start forming bubbles.

Wrap this up with words like m-value, diffusion, permeability and solubility and it'll sound more convincing.

Graham Smith
03-01-2013, 02:50 PM
Graham, please tell me that's a real quote. That'd make my day.



It's real....I'll dig out the original for you but it is also quoted in Mark Powells Deco for Divers.

He was total class..... ;)

G

dlk
03-01-2013, 03:00 PM
Best way to avoid it is not go diving.

I appreciate I'm being vague. Let me try to be a bit more precise.

According to one model, DCS is caused by bubbles forming, which can occur when the critical tension of gas in a tissue exceeds some maximum level (its M-value).
One way this might occur is when switching from a gas high in slow inert to one high in a fast inert (e.g., nitrox to trimix). I can explain this.
Another way it might occur is when switching from a fast inert (e.g., trimix) to a slow inert (e.g., nitrox), as we do regularly. I can't explain this (convincingly).

Critical tensions are total tissue tensions, i.e., the sum of the partial tensions. If I switch off a fast gas and onto a slow gas, I should be off-gassing the fast gas faster than on-gassing the slow gas, hence a reduction in total tissue tension not an increase.

So perhaps it's the solubility of the inerts (i.e., volume) as well as size (speed of diffusivity)? This is what Steve Burton says How to avoid a Isobaric Counter Diffusion hit (http://www.scubaengineer.com/isobaric_counter_diffusion.htm) and what (I believe) has led to the 1:5 rule of thumb used by several training agencies (increase N2 by 1% for every 5% reduction in He). But is this confusing total volume of dissolved inerts with total tissue tension?

Doolette & Mitchell present a model for IEDCS (inner ear decompression sickness)Biophysical basis for inner ear decompression sickness (http://jap.physiology.org/content/94/6/2145.long) seems more convincing. But it might mean the 1:5 rule is as arbitrary as the 0.5bar ppN2 increase.

BTS
03-01-2013, 03:23 PM
Ok but how does any of that fit with not putting trimix in your suit?

Scuttler
03-01-2013, 03:33 PM
Ok but how does any of that fit with not putting trimix in your suit?

I think dlk is focussing on deep tissue icd rather than superficial icd. The latter has been reproduced with subjects breathing nitrous oxide while in a helium "environment" at surface pressures.

Scuttler
03-01-2013, 03:36 PM
Best way to avoid it is not go diving.

I appreciate I'm being vague. Let me try to be a bit more precise.

According to one model, DCS is caused by bubbles forming, which can occur when the critical tension of gas in a tissue exceeds some maximum level (its M-value).
One way this might occur is when switching from a gas high in slow inert to one high in a fast inert (e.g., nitrox to trimix). I can explain this.
Another way it might occur is when switching from a fast inert (e.g., trimix) to a slow inert (e.g., nitrox), as we do regularly. I can't explain this (convincingly).

Critical tensions are total tissue tensions, i.e., the sum of the partial tensions. If I switch off a fast gas and onto a slow gas, I should be off-gassing the fast gas faster than on-gassing the slow gas, hence a reduction in total tissue tension not an increase.

So perhaps it's the solubility of the inerts (i.e., volume) as well as size (speed of diffusivity)? This is what Steve Burton says How to avoid a Isobaric Counter Diffusion hit (http://www.scubaengineer.com/isobaric_counter_diffusion.htm) and what (I believe) has led to the 1:5 rule of thumb used by several training agencies (increase N2 by 1% for every 5% reduction in He). But is this confusing total volume of dissolved inerts with total tissue tension?

Doolette & Mitchell present a model for IEDCS (inner ear decompression sickness)Biophysical basis for inner ear decompression sickness (http://jap.physiology.org/content/94/6/2145.long) seems more convincing. But it might mean the 1:5 rule is as arbitrary as the 0.5bar ppN2 increase.

I don't know. I know that icd was one of the subjects discussed in detail at the 38th Undersea and Hyperbaric Medical Society Workshop. The minutes are available on rubicon if that helps.

BTS
03-01-2013, 03:37 PM
I think dlk is focussing on deep tissue icd rather than superficial icd.

Nowt superficial about skin bends...

dlk
03-01-2013, 03:42 PM
Ok but how does any of that fit with not putting trimix in your suit?

Transdermal ICD. Helium diffuses quickly into the body via the skin. Nitrogen diffuses out slowly.
More apparently, you freeze your nuts off.

dlk
03-01-2013, 04:27 PM
I don't know. I know that icd was one of the subjects discussed in detail at the 38th Undersea and Hyperbaric Medical Society Workshop. The minutes are available on rubicon if that helps.

The most relevant paper at that workshop seems to be Lambersen's. Paraphrasing, when it comes to trimix-nitrox switch ICD, he aligns with GI3. Kinda.
The search for Truth continues!:)

Tbh, I'd always suggest planning to avoid ICD issues on switching by using either the 0.5 bar or 5:1 guidelines... But it can make the logistics a challenge, and I've thoughtlessly moved from one guideline to the other when I couldn't make the first work. That got me wondering if that was really, really stupid or just a bit stupid, so I've been actively trying to understand where they come from and how definitive they are.

Mark Chase
03-01-2013, 06:42 PM
ICD is a real problem if you often dive below 150m

If you dont? it isnt.

ATB

Mark

matt
03-01-2013, 07:31 PM
Doolette & Mitchell present a model for IEDCS (inner ear decompression sickness)Biophysical basis for inner ear decompression sickness (http://jap.physiology.org/content/94/6/2145.long) seems more convincing. But it might mean the 1:5 rule is as arbitrary as the 0.5bar ppN2 increase.

Do you know where the 0.5 comes from?

Matt.

matt
03-01-2013, 07:38 PM
I broadly agree that the theory behind ICD is somewhat flaky and totally agree that it's kinda irrelevant for scuba divers as we don't dive deep enough or long enough.

http://www.funteqdiving.nl/website/Downloads/isobaric%20counter%20diffusion/ICD%20by%20Mark%20Ellyatt.pdf

Matt.

Janos
03-01-2013, 07:46 PM
Three types:

1) You dive a very rich trimix (in fact, let's make it Heliox - say 10/90). You spend a while on the bottom. Because you're diving Heliox, you won't on-gas any Nitrogen, but you will on-gas Helium. You do your stops on 10/90 until you hit (say) 21m. At this point (because you've got to stop) you're right on the saturation limit. If you switch to Nitrox 50 then the Nitrogen will rush in very quickly, far quicker than the Helium will leave, giving you a ICDB bend.

In practice you're going to not be diving Heliox and will stop deeper, but the principle holds true - don't spike the ppN2 when you're starting a stop (or indeed any other gas)

2) Similar to (1), but instead of the spike happens because you flush your suit with a fancy gas, and the fancy gas diffuses through your skin and into your body. I don't worry about this as it mainly is of concern to astronauts.

3) Inner ear IBCD. Your body has air spaces - notably the ears and sinuses. You need to fill these with air when you descend. These can act as 'wells' of inert gas so on the way up your innermost ear still has (say) 15/55 in, but you're breathing Nitrox 50. The high ppHe in the innermost part of the ear diffuses into the middle ear, causing a temporary spike and a potential bend. I like to think of this as Poole harbour and it's strange double tide. The tide rushes into the main part of Poole harbour, then starts to come back out. But while it's turned deep inside the harbour and is flooding out, at the entrance it's a bit delayed, and possibly even still coming in, leading to a strange double tide. So it is with your ear.

There you go. Clear as mud. I need a diagram to explain it better.

Janos

Scuba steve
03-01-2013, 07:57 PM
Ok so bear with me on this 1 and paint a picture . if you imagine looking at a bar door . and nitrogen molicules as big burly rude rugby players and helium as little danty girls in their high hell shoes . The girls want to leave the bar and the rugby players want to enter . There is only a limited space they can travel trough so the very rude and bigger rugby players burst there way into the bar . While the girls wait for the rugby lads to come in so they can leave . Whilst this happens and the bigger bullies there way in the smaller get frustrated and blow their tops (reduction in ambient pressure helium comes out of solution and form a bubble)

The .5bar comes from the use of 50% as a deco gas . The real problem is when using high helium at depth and the switching to an air mix as the "first deco gas" , this has been linked to ICD or inner ear
Bends and the use of air on these type of dive has been disbariged for year

matt
03-01-2013, 08:06 PM
The .5bar comes from the use of 50% as a deco gas

Nope, that's not related to the 0.5bar "limit" for the ICD calculation...

Scuba steve
03-01-2013, 09:15 PM
That's where the suggestions starts . It's also reducing the nitrogen content by 30% but increases or opens the oxygen window aswell

JPTaylor
03-01-2013, 10:06 PM
Pesonally I don't care how, all I need to know is what it is and how to avoid any associated issues, no need to make it all scientificky...

Dive closed circuit & don't switch your diluent, the unit will smoothly reduce partial pressures of He/N2...... simples!!

Mark Chase
03-01-2013, 10:12 PM
That's where the suggestions starts . It's also reducing the nitrogen content by 30% but increases or opens the oxygen window aswell



Ka Ching

ICD and the 02 Window :D

Two myths for the price of one thread :D


As I said before ICD dosent affect normal divers doing normal dives. In fact I have never herd of any one getting ICD on any dive shalower than 150m. There are incidents on world record attempt dives with the typical nausia vomiting and vertigo symptoms, but for dives in the 0-100m range its not rely an issue.

Theres the thing about switching to air deep from trimix, but in truth 10,s of 1000s of divers used to do this regular as clockwork without ill effect. Have a chat to Kevin Gurr next time you see him at a dive show. He was running Air as inboards diluient on his Sentianl and Borris and flying the unit on off board gas but flushing with Air on the ascent to reduce deco. Very old school diving I admit, but it seems some people still like the idea of getting off the HE as soon as possable.

50% is a comon gas choice for deco but no more so than 32%. In my early years on OC by far the most comon combination of deco mix was 32% and 80% and we would switch to 32% at 40m streight from back gas. QED I could have been doing a 80m dive on 14/65 and then gas switch directly on to 32% @ 40m for deco.

We all did this. This was the norm. No one got ICD

On deep dives down arround 100m we would add 21/35 as a travel gas and deco mix. Wed decend on 21/35 to 60m then switch to back gas (say 10/70) and on ascent wed switch back to 21/35 for deco at 66m. The 35% He wasn't to avoid ICD, it was to avoid the monster narcosis hit of switching to air.


As for the 02 Window?

That was investigated and found to be total myth with zero suporting evidance.

There are however two points often asociated with the myth that i do agree with. 1: it takes two mins for any gas switch to take full effect. So staying an extra 2mins at 21m on 50% does help get the gass fulley into the system. 2: Replacing as much of the nitrogen with 02 as possable, will speed up deco. Prety obvious when you think about it.

ATB

Mark

dlk
04-01-2013, 09:08 AM
Ka Ching

ICD and the 02 Window :D

Two myths for the price of one thread :D

...
Mark

Ha ha... Think of it this way then, I'm after the "maths behind the myths".

(Oh, and in the context it's used, there's nothing mythical about Scuba Steve's comment on the O2 window. The type he's referring to is simply increasing the off-gassing gradient.)

DLK

Janos
04-01-2013, 09:11 AM
As I said before ICD dosent affect normal divers doing normal dives. In fact I have never herd of any one getting ICD on any dive shalower than 150m.

Not even switching diluent to air? Quite a few examples of that.

Janos

dlk
04-01-2013, 09:20 AM
Ok so bear with me on this 1 and paint a picture . if you imagine looking at a bar door . and nitrogen molicules as big burly rude rugby players and helium as little danty girls in their high hell shoes . The girls want to leave the bar and the rugby players want to enter . There is only a limited space they can travel trough so the very rude and bigger rugby players burst there way into the bar . While the girls wait for the rugby lads to come in so they can leave . Whilst this happens and the bigger bullies there way in the smaller get frustrated and blow their tops (reduction in ambient pressure helium comes out of solution and form a bubble)
...


ICD is a diffusion process. Diffusion is governed by molecular weight (rate) and pressure gradient (amount). See Dalton's law & Graham's law.

The girls daintily skip past the rugby players without even noticing them.

DLK

dlk
04-01-2013, 09:35 AM
1) You dive a very rich trimix (in fact, let's make it Heliox - say 10/90). You spend a while on the bottom. Because you're diving Heliox, you won't on-gas any Nitrogen, but you will on-gas Helium. You do your stops on 10/90 until you hit (say) 21m. At this point (because you've got to stop) you're right on the saturation limit. If you switch to Nitrox 50 then the Nitrogen will rush in very quickly, far quicker than the Helium will leave, giving you a ICDB bend.
...

Janos

Nice, concise explanation.
Now the $64,000 question: Why will the nitrogen rush in much quicker than the helium leaves? Dalton and Graham seem to think otherwise.

When I looked at the numbers, I couldn't see how to make this happen. I either made an error or I'm missing something.
Steve Burton gives a rationalization (in the link I posted earlier) but I think he's confusing quantity with partial pressure.

DLK

Janos
04-01-2013, 09:50 AM
Nice, concise explanation.
Now the $64,000 question: Why will the nitrogen rush in much quicker than the helium leaves? Dalton and Graham seem to think otherwise.

When I looked at the numbers, I couldn't see how to make this happen. I either made an error or I'm missing something.
Steve Burton gives a rationalization (in the link I posted earlier) but I think he's confusing quantity with partial pressure.

DLK

Because the Nitrogen pp gradient is huge - 1.6 bar (ambient) to zero (tissues). The Helium coming out isn't a straightforward simple diffusion across a membrane - but a complex thing involving perfusion from slow tissues to blood to lungs etc...

Mark Chase
04-01-2013, 10:02 AM
Not even switching diluent to air? Quite a few examples of that.

Janos



Link me to them?

I have asked the question many times and no one has come up with any hard evidance. There was one incident of a inner ear bend but that dosen't necerrily mean it was ICD.


The problem with gas switching to air is narcosis not ICD. As i said i have gas switched from 60+% He to 32% nitrox many times. As has most decompresion divers doing dives in the last 20 years who wernt GUE trained.

ATB

Mark

Scuba steve
04-01-2013, 10:21 AM
The oxygen window is a realitive term for increasing the rate in off gassingnor excelerated deco . The futher the window is opened the "quicker" the off gassing process . It is a great theory for explaing off gasing and excelerated deco

Im only doing He diving since 07 and not sure on the good auld days were divers were divers . What i do know is that i have only ever seen 50% ,80% and 100% mixes on the deeper diver boats . I have seen diver doing deep air dives using 32% but the are only racking up shorter decos and cutting abit off . Any of the leading deep divers iv sat with and talked to all sing it from the towers that the first switch off helium should be to Ean50 .

dlk
04-01-2013, 10:39 AM
Because the Nitrogen pp gradient is huge - 1.6 bar (ambient) to zero (tissues). The Helium coming out isn't a straightforward simple diffusion across a membrane - but a complex thing involving perfusion from slow tissues to blood to lungs etc...

Here are some numbers.
Tmx 20/25 to Nx 32 @ 40m.

Saturation targets (end points):
Before switch: ppN2=2.75, ppHe=1.25, total p=4.0
After switch: ppN2=3.4, ppHe=0, total p=3.4

Gradients:
N2 2.75->3.4 = +0.65
He 1.25->0 = -1.25

Relative speed of diffusivity: He=2.65xN2

Diffusivity modified gradients:
N2: +0.65
He: -1.25x2.65 = -3.31

You seem to be off-gassing He at 5 times the rate you're on-gassing N2. If we can formulate if/where where this is reversed then we have a guideline for ICD control. I am presuming this has already been done (and is maybe the basis of the 0.5 bar rule)?

EDIT: I cannot find any switches where this is reversed, i.e., I cannot find any circumstances where you would on-gas N2 faster than off-gassing He.
Is it a transient spike in a supersaturation window??

Oh, and re perfusion limited models, that's sorta what Doolette & Mitchell propose in their IEDCS model (serial compartments), also referenced earlier.

DLK

dlk
04-01-2013, 10:55 AM
The oxygen window is a realitive term for increasing the rate in off gassingnor excelerated deco . The futher the window is opened the "quicker" the off gassing process . It is a great theory for explaing off gasing and excelerated deco
...

There are at least 2, and possibly 3 (depending on your thinking) interpretations for the term Oxygen Window.
Increasing the off-gassing gradient (as you're proposing) is entirely valid and beyond contention.

Mark is referring to the O2 window partial pressure vacancy theory, or inherent unsaturation. This is an interesting model, but very contentious.

DLK

Major Clanger
04-01-2013, 11:17 AM
ICD is when v-planner says you shouldn't use that mix as a deco gas. I've little interest in the science behind it other than a basic working understanding. Interestingly (to me as I use them), neither the ostc planner or ideco raise an eyebrow at it. Why you all splitting airs over what's clearly explained in deco for divers and simlar?

dlk
04-01-2013, 11:27 AM
ICD is when v-planner says you shouldn't use that mix as a deco gas. I've little interest in the science behind it other than a basic working understanding.

Nothing wrong with that.


Interestingly (to me as I use them), neither the ostc planner or ideco raise an eyebrow at it. Why you all splitting airs over what's clearly explained in deco for divers and simlar?

Because it isn't.

Major Clanger
04-01-2013, 11:32 AM
Explained well enough for most to know they font have to bother about it.

JPTaylor
04-01-2013, 11:32 AM
Link me to them?

Not everything is on the internet!! ;)

dlk
04-01-2013, 11:52 AM
Explained well enough for most to know they font have to bother about it.

Agreed.
That's why I put this in Physics & Physiology, and not Technical Diving or Decompression.
I want answers, and I want them now, dammit!!:)

matt
04-01-2013, 12:02 PM
Here are some numbers.
Tmx 20/25 to Nx 32 @ 40m.

Saturation targets (end points):
Before switch: ppN2=2.75, ppHe=1.25, total p=4.0
After switch: ppN2=3.4, ppHe=0, total p=3.4

Gradients:
N2 2.75->3.4 = +0.65
He 1.25->0 = -1.25


One question on this - just looking at N2 component. FN2=100-(20+25)=55 (0.55). 40m -> 5 bar * 0.55 = 2.75.

OK - I figured that bit out. Is 2.75 right or do you need to take into account supersaturation as suggested in Bruce's diagram below:

http://www.advanceddivermagazine.com/articles/icd/fig%201%20tension.jpg

Saying:

Bowenworld Article (http://www.advanceddivermagazine.com/articles/icd/icd.html)



Depicted in Fig 1 is a comparative representation of the time courses of changes in helium, nitrogen, and sum of the two, tissue tensions for 480 min nitrogen tissue compartments and 240 min helium tissue compartments. The depth is 200 fsw with abrupt change from normoxic nitrox to normoxic heliox. Note the buildup in time of total inert gas tension, with a maxima after some 400 min. With faster tissue compartments, this maxima builds more quickly, on time scales of the slowest tissues involved. Actually the curves remain the same as shown, but axis time scales are shortened by the ratio of the fast tissue halftime, t, divided by 240 for the helium compartment, t/240, and similarly for the nitrox compartment, tissue halftime, t, divided by 480, that is, t/480. This is quite obviously not a good scenario for the mixed gas diver. If the gases were flip flopped, a minima would develop, identical in shape to inverted

Matt.

matt
04-01-2013, 12:05 PM
Explained well enough for most to know they font have to bother about it.

What type of Font?
https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcTZgnxGmgiCW1JprbjrOOBeQW29ACmDy dQd6ezmQK4QlaqITCR32w

JPTaylor
04-01-2013, 12:20 PM
The only *practical* guideline I'm aware of is that the percentage increase in N2 on a gas switch should be no more than 1/5th of the decrease in the He percentage.

(Covered in Mark Powell's book "Deco for Divers")

Major Clanger
04-01-2013, 12:41 PM
Agreed.
That's why I put this in Physics & Physiology, and not Technical Diving or Decompression.
I want answers, and I want them now, dammit!!:)

Lol, I could tell you the answer but it would involve buying me lots of beer and curry. The answer may be wrong as well.

Major Clanger
04-01-2013, 12:42 PM
What type of Font?
https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcTZgnxGmgiCW1JprbjrOOBeQW29ACmDy dQd6ezmQK4QlaqITCR32w

That's it, the font for fat fingers :p

dlk
04-01-2013, 12:48 PM
One question on this - just looking at N2 component. FN2=100-(20+25)=55 (0.55). 40m -> 5 bar * 0.55 = 2.75.

OK - I figured that bit out. Is 2.75 right or do you need to take into account supersaturation as suggested in Bruce's diagram below:
...


Good link, thanks.

The supersaturation is what I'm wondering about (transient spike), but not I think as outlined here. Bruce's diagram & explanation cover slow-fast switches, which I get. I need to do some more sums to see if they also cover fast-slow. (He says not... that the maxima inverts to a minima, as per my calcs [it's an easy spreadsheet].)

dlk
04-01-2013, 12:58 PM
The only *practical* guideline I'm aware of is that the percentage increase in N2 on a gas switch should be no more than 1/5th of the decrease in the He percentage.
...
(Covered in Mark Powell's book "Deco for Divers")

The 1/5th comes, I believe, from Steve Burton's approach, which notes that N2 is 4.65 x more soluble than He. Linked earlier.
I am unsure how solubility factors into it, unless it's a dual-phase model, which would confound things to the extent that 1/5th is entirely arbitrary.

Mark's book is great, and I'd recommend it for all. It's a well presented collection and analysis of a wide range of primary materials.
It's the primary material itself I'm concerned with.

matt
04-01-2013, 01:43 PM
Good link, thanks.

Same picture, same author, different article: http://www.tecdivingdahab.com/web_documents/wienkeicd-2004-08sources.pdf

And one from the CaveDivingGroup: CDG - Isobaric Counter Gas Transport (http://www.cavedivinggroup.org.uk/Articles/ICGT.html)

Matt.

dlk
04-01-2013, 03:51 PM
Thank you all, question answered.
You'll be pleased(!) to hear (pending data on flux constants and lipid-aqueous partition co-efficients*) the answer is... "no one really knows". Ha ha ha...

I'll post an overview summary a bit later.

*I'm not making that up.

DLK

Mark Chase
04-01-2013, 03:56 PM
The oxygen window is a realitive term for increasing the rate in off gassingnor excelerated deco . The futher the window is opened the "quicker" the off gassing process . It is a great theory for explaing off gasing and excelerated deco

Im only doing He diving since 07 and not sure on the good auld days were divers were divers . What i do know is that i have only ever seen 50% ,80% and 100% mixes on the deeper diver boats . I have seen diver doing deep air dives using 32% but the are only racking up shorter decos and cutting abit off . Any of the leading deep divers iv sat with and talked to all sing it from the towers that the first switch off helium should be to Ean50 .



I on the otherhand would say 90% + of the tec divers i have dived with in the last 15 years used 32% and 80%.

50% and 100% didnt make an apearance till arround 2005 when GUE became popular.

Getting 100% fills is a PITA and running 50 & 80 dosent work logisticly.

ATB

Mark

Mark Powell
04-01-2013, 04:04 PM
EDIT: I cannot find any switches where this is reversed, i.e., I cannot find any circumstances where you would on-gas N2 faster than off-gassing He.
I don't think there are any.


Is it a transient spike in a supersaturation window??

That's my reading of the Doolette and Mitchell paper. The way I like to think about it is that it's a combination of physics and physiology.

If you look at the physics on their own then ICD can't happen as the Helium is off-gassing faster than the Nitrogen will be on-gassing. Unless there is a physiological oddity in the inner ear that causes a lag in the helium off-gassing to allow a transient spike in the supersaturation window.

Mark Powell
04-01-2013, 04:07 PM
I on the otherhand would say 90% + of the tec divers i have dived with in the last 15 years used 32% and 80%.

50% and 100% didnt make an apearance till arround 2005 when GUE became popular.

Getting 100% fills is a PITA and running 50 & 80 dosent work logisticly.

ATB

Mark
There was a quite a lot of people using 50% and 80% even before GUE started advocating 50%. I think the two reasons for this were that IANTD Adv Nitrox was a 50% ticket until quite recently and if you have half a stage of 80% and top it up with air it is usually pretty close to 50% and so people used to cycle their stages.

Mark Chase
04-01-2013, 04:13 PM
There was a quite a lot of people using 50% and 80% even before GUE started advocating 50%. I think the two reasons for this were that IANTD Adv Nitrox was a 50% ticket until quite recently and if you have half a stage of 80% and top it up with air it is usually pretty close to 50% and so people used to cycle their stages.


But Mark i am talking about doing deep dives with trimix back gas and multiple deco gas.

I cant honestly say i dived with anyone between 2000 and 2005 who used 50 & 100. We were all on 32 & 80 or something between 32 and 40% & 80.

Single gas deco is a whole diferent subject. Again in the day all sorts were used 40/50/60/70/80% I have seen them all used for single deco gas dives. Today its rare to see anything except 50% but not so prior to 05.

ATB

Scuba steve
04-01-2013, 04:24 PM
On the deep dive boats (i would consider anything deeper than 80m deep ) that I'm on everyone dive 50% and either 80% or 100% , and they are all different dive groups diving different set ups and a mixture of OC and CCR ,

JPTaylor
04-01-2013, 04:28 PM
On the deep dive boats (i would consider anything deeper than 80m deep ) that I'm on everyone dive 50% and either 80% or 100% , and they are all different dive groups diving different set ups and a mixture of OC and CCR ,

On most deep stuff (as you define it), I don't carry any nitrox/deco gases....

matt
04-01-2013, 04:29 PM
I on the otherhand would say 90% + of the tec divers i have dived with in the last 15 years used 32% and 80%.

50% and 100% didnt make an apearance till arround 2005 when GUE became popular.

Getting 100% fills is a PITA and running 50 & 80 dosent work logisticly.

ATB

Mark

I dived 50%/100% since the late 1990's...does that make me trendy? Learnt in the Philippines with John Bennett (RIP).

Matt.

matt
04-01-2013, 04:33 PM
Thank you all, question answered.
You'll be pleased(!) to hear (pending data on flux constants and lipid-aqueous partition co-efficients*) the answer is... "no one really knows". Ha ha ha...

I'll post an overview summary a bit later.

*I'm not making that up.

DLK

No different to deco in general then, or CNS, or OTU or all the rest...best guess, throw of the dice.

Matt.

Scuba steve
04-01-2013, 04:50 PM
So if the box goes tits up what do you do ?? When do you consider the use of extra stages for deco /bailout ?


On most deep stuff (as you define it), I don't carry any nitrox/deco gases....

JPTaylor
04-01-2013, 05:00 PM
So if the box goes tits up what do you do ?? When do you consider the use of extra stages for deco /bailout ?

I carry some B/O stages, it's just that there both got Trimix in to get me back to shallows then it's hang tanks, drop tanks or on very few occasions support diver.

matt
04-01-2013, 05:02 PM
So if the box goes tits up what do you do ?? When do you consider the use of extra stages for deco /bailout ?

No point carrying a 50% and drowning (or tox'ing) at 100m. Deads dead, depth is irrelevant.

Scuba steve
04-01-2013, 05:12 PM
And if there's not enough time to get back to the shot or you can't find it ??? . Iv been on boats were the red bags have come up and only been by the grace of someone up there that we managed to see them in d distance .

50% or 100% is no good to you at 100 but it is there if the unit goes tits up and you decide to start your ascent from we're you are

Scuba steve
04-01-2013, 05:12 PM
And if there's not enough time to get back to the shot or you can't find it ??? . Iv been on boats were the red bags have come up and only been by the grace of someone up there that we managed to see them in d distance .

50% or 100% is no good to you at 100 but it is there if the unit goes tits up and you decide to start your ascent from we're you are

Janos
04-01-2013, 05:21 PM
If you look at the physics on their own then ICD can't happen as the Helium is off-gassing faster than the Nitrogen will be on-gassing. Unless there is a physiological oddity in the inner ear that causes a lag in the helium off-gassing to allow a transient spike in the supersaturation window.

Can you not have a situation where you're deco is controlled by the mid- or slow- tissues, when switching allows rapid on-gassing of N2 into blood (and fast tissues) while the slower tissues are still off-gassing into the blood. This would lead to a spike in the blood, and so fast tissues.

All a bit theoretical for me, as I don't dil switch.

Janos

matt
04-01-2013, 05:28 PM
And if there's not enough time to get back to the shot or you can't find it ??? . Iv been on boats were the red bags have come up and only been by the grace of someone up there that we managed to see them in d distance .

50% or 100% is no good to you at 100 but it is there if the unit goes tits up and you decide to start your ascent from we're you are

I would always return to shot myself - other on some forums disagree. I'm a shot and station diver when I can be.

If you're at 21m on the way down then the 50% is useful, but on the way up you'll have to skip a lot of time. In my opinion you won't get of the deck with this plan.

Matt.

Janos
04-01-2013, 05:56 PM
And if there's not enough time to get back to the shot or you can't find it ??? . Iv been on boats were the red bags have come up and only been by the grace of someone up there that we managed to see them in d distance .

50% or 100% is no good to you at 100 but it is there if the unit goes tits up and you decide to start your ascent from we're you are

For the deep dives SOP is to always come back to the shot. We alway reel off from the shot, and so always make it back. Bailout planning obviously take this into account. If for any reason you don't make it back to the shot, then unless you have a really, really good reason, you don't get invited back onboard.

Janos

dlk
04-01-2013, 06:07 PM
Can you not have a situation where you're deco is controlled by the mid- or slow- tissues, when switching allows rapid on-gassing of N2 into blood (and fast tissues) while the slower tissues are still off-gassing into the blood. This would lead to a spike in the blood, and so fast tissues.

No (in theory). The maths doesn't allow it.
Unless you use a serial compartment model like the old DCIEM tables or an asymmetric wash-in / wash-out. Neither of which, I believe, are used by recreational mix divers. I don't know about military. Nor if the 88s are based on a sequential model or if there are trimix versions (I assume not, but you'd know better). And not, I think, in VPM, although I need to re-examine to be sure. Theoretically, it might be possible in an RGBM, but who the hell would know other than Bruce Weinke (although his stance on ICD would indicate not)... if you want real brain ache, read his research papers (not internet diver abstracts) - redefines obfuscation.

The Doolette & Mitchell IEDCS theory is kind of a serial model, this and your earlier comment about diffusion not being quite so simple are, for my money, on the money.


All a bit theoretical for me, as I don't dil switch.

Bail out?

Scuba steve
04-01-2013, 06:44 PM
I personally don't like leaving my safety in anyone's else's hands when I can do what I have to or can carry what I have to . My safety is my primary responsibility . Iv been on a dive where we have had to bag off the wreck and 1.5 hrs into the deco the support diver appears . They didn't see the bag .

Alls well and good when things go to plan . But when they don't ???

I'm sure everyone has heard of the incident pit . So why let it happen to you ?

Mark Chase
04-01-2013, 08:27 PM
I dived 50%/100% since the late 1990's...does that make me trendy? Learnt in the Philippines with John Bennett (RIP).

Matt.

WTF did you get your fills?


All i had was Kent Diving in orpington and Dive Macheen in Tonbridge and getting as decent fill of 100% was about as easy as getting the wife to give me a blow job after id had a night out with the lads.

ATB


Mark

Scuba steve
04-01-2013, 08:59 PM
My plan would always to return to the shot and deco off the station . The best plan are made to be broken . I can count on 1 hand the number of times I haven't returned to the shot but still it happens . And I'd like to think that if this happened I had all I needed .

matt
04-01-2013, 09:15 PM
WTF did you get your fills?


All i had was Kent Diving in orpington and Dive Macheen in Tonbridge and getting as decent fill of 100% was about as easy as getting the wife to give me a blow job after id had a night out with the lads.

ATB


Mark

The club had O2 included in the members-fee along with a compressor. There's wasn't a pump - think they came along mid naughties. It was definitely the 1990's as I was with that club 1991-1999 and I didn't do the cert until 2000.

Matt.

simon mitchell
10-01-2013, 11:34 PM
Hello,

This is a confusing topic which is frequently misunderstood. I will try to explain aspects of it here.

First of all, let's be clear that the gas switch we are interested in is from a high helium mix (eg heliox or trimix) to a high nitrogen mix (eg air or nitrox). Although switches in the other direction are interesting for theoretical reasons there is no reason to do them in diving, and discussing them (eg the graph posted earlier attributed to Bruce Wienke) is irrelevant, and just serves to confuse things. Similarly, discussing a situation in which you are breathing air and then put trimix in your drysuit is interesting for theoretical reasons, but it is of no practical relevance to technical diving and once again, it just confuses things. I reiterate that we are interested in is from a high helium mix (eg heliox or trimix) to a high nitrogen mix (eg air or nitrox).

The reasons a "helium to nitrogen" gas switch is made are twofold. First, in open circuit diving it saves helium which is becoming increasingly expensive. Second, it is perceived that it may accelerate decompression. This relies on the helium leaving the tissues faster than the nitrogen entering them. Since helium is small, it has higher diffusivity than nitrogen, though flux through tissue is also influenced by gas solubility in the tissue (which varies for any gas according to the tissue composition). This consideration makes the picture a little less clearer for helium and nitrogen, especially in tissues of a certain composition. But let's assume for now that helium does diffuse out of a tissue into the blood faster than nitrogen diffuses from the blood into the tissue. This process of the gases diffusing in the opposite direction (one outward and one inward) is called counter-diffusion, and the fact that it will occur after a gas switch even if the diver sits at the same depth (same ambient pressure) is where the term "isobaric" comes from. In reality, these switches are rarely actually isobaric in technical diving because we are usually in the process of ascending, but that is a largely irrelevant detail. A simple depiction of what we assume is happening is shown in the diagram below which shows a tissue and a blood vessel passing through it just after a gas switch from helium to nitrogen. Helium in the tissue moves into the blood faster than the nitrogen moves into the tissue.

https://dl.dropboxusercontent.com/u/22145620/Normal%20tissue.jpg

If our assumption holds true, the potential benefit of a helium to nitrogen gas switch during decompression is illustrated in the figure below which comes from a paper that David Doolette and I have written on decompression from technical dives for the journal Diving and Hyperbaric Medicine. This will be published in June as far as I know and once it comes out I will let you know. Anyway, assume the diver has been breathing heliox 10/90 at 90m (10 ATA) and switches to breathing nitrox 10 (10% oxygen 90% nitrogen) at time zero (on the horizontal axis on the graph) whilst remaining at 90m. I know no one would do this, but we are trying to illustrate the principle of a helium to nitrogen gas switch at this stage, rather than describe reality.

https://dl.dropbox.com/u/22145620/Technical%20diving%20part%202%20Figure%202.jpg

You can see that immediately after the switch the pressure of helium in the tissue decreases and the pressure of nitrogen increases, but the former process takes place more quickly so that the total pressure of dissolved inert gas in the tissue declines even though we are still breathing 90% inert gas. The tissue becomes undersaturated with respect to ambient pressure (the horizontal line at 10ATA) and this would allow a larger ascent (decrease in ambient pressure) before the total tissue inert gas pressure exceeded ambient pressure and the tissue therefore became supersaturated. In this way, the ascent would be accelerated.

In theory, this description of events following a helium to nitrogen gas switch should hold true for almost all tissues in the body, and such switches should not cause any problems, and should accelerate decompression to some extent. Whether this acceleration is real or significant in practical terms is highly debatable, and David and I cover it in the paper I mentioned above (so I can't pre-publish it on the internet here). I say “almost all tissues” because there is one notable exception: the inner ear. The inner er has a unique anatomy which makes it vulnerable to injury by counter-diffusing gases. It differs from the simple depiction in the first diagram above in that the sensitive functional inner ear tissue (which receives all the blood flow) has a relatively large reservoir of fluid immediately adjacent to it (called the perilymph). This does not have its own blood supply, and any movement of gas in and out of it must occur through the sensitive functional inner ear tissue which, as I said above, receives all the blood flow. During bottom time on a deep dive this reservoir of fluid absorbs helium, and becomes a helium reservoir. (Note: What follows is not strictly correct but is a conceptually adequate way of thinking about it). When we make a gas switch from helium to nitrogen, the sensitive inner ear tissue is already probably supersaturated with helium (remember, we are decompressing here). We make the switch, which in a normal tissue will result in faster loss of helium than nitrogen from the tissue (first diagram above), but in the inner ear there is also a big dump of helium from the fluid reservoir into the tissue (this helium has no other way of escaping except through the tissue) and the combination of the arrival of the nitrogen, and the dump of helium from the reservoir results in a transient INCREASE in supersaturation instead of a decrease. This could be enough to precipitate bubble formation. I have tried to illustrate this in the diagram below.

https://dl.dropbox.com/u/22145620/Inner%20ear%20after%20helium%20to%20nitrogen%20gas %20switch.jpg

The liklihood of this being problem is influenced to a large extent by the amount of supersaturation in the inner ear before the switch is made, which essentially means the quality of the decompression of the inner ear prior to the switch. These switches can be (and are) done with a very low incidence of problems.

Anyway, I hope that has helped with understanding of the problem, which frequently gets confused on internet forums. I must, in closing, acknowledge the fantastic work done by David Doolette on this problem. We have collaborated on the various papers, but he is the modelling genius.

Simon M

simon mitchell
10-01-2013, 11:36 PM
Hello,

This is a confusing topic which is frequently misunderstood. I will try to explain aspects of it here.

First of all, let's be clear that the gas switch we are interested in is from a high helium mix (eg heliox or trimix) to a high nitrogen mix (eg air or nitrox). Although switches in the other direction are interesting for theoretical reasons there is no reason to do them in diving, and discussing them (eg the graph posted earlier attributed to Bruce Wienke) is irrelevant, and just serves to confuse things. Similarly, discussing a situation in which you are breathing air and then put trimix in your drysuit is interesting for theoretical reasons, but it is of no practical relevance to technical diving and once again, it just confuses things. I reiterate that the switch we are interested in is from a high helium mix (eg heliox or trimix) to a high nitrogen mix (eg air or nitrox).

The reasons a "helium to nitrogen" gas switch is made are twofold. First, in open circuit diving it saves helium which is becoming increasingly expensive. Second, it is perceived that it may accelerate decompression. This relies on the helium leaving the tissues faster than the nitrogen entering them. Since helium is small, it has higher diffusivity than nitrogen, though flux through tissue is also influenced by gas solubility in the tissue (which varies for any gas according to the tissue composition). This consideration makes the picture a little less clearer for helium and nitrogen, especially in tissues of a certain composition. But let's assume for now that helium does diffuse out of a tissue into the blood faster than nitrogen diffuses from the blood into the tissue. This process of the gases diffusing in the opposite direction (one outward and one inward) is called counter-diffusion, and the fact that it will occur after a gas switch even if the diver sits at the same depth (same ambient pressure) is where the term "isobaric" comes from. In reality, these switches are rarely actually isobaric in technical diving because we are usually in the process of ascending, but that is a largely irrelevant detail. A simple depiction of what we assume is happening is shown in the diagram below which depicts a tissue and a blood vessel passing through it just after a gas switch from helium to nitrogen. Helium in the tissue moves into the blood faster than the nitrogen moves into the tissue.

https://dl.dropbox.com/u/22145620/Tissue%20gas%20exchange.jpg

If the assumption that this occurs holds true, the potential benefit of a helium to nitrogen gas switch during decompression is illustrated in the figure below which comes from a paper that David Doolette and I have written on decompression from technical dives for the journal Diving and Hyperbaric Medicine. This will be published in June as far as I know and once it comes out I will let you know. Anyway, assume the diver has been breathing heliox 10/90 at 90m (10 ATA) and switches to breathing nitrox 10 (10% oxygen 90% nitrogen) at time zero (on the horizontal axis on the graph) whilst remaining at 90m. I know no one would do this, but we are trying to illustrate the principle of a helium to nitrogen gas switch at this stage, rather than describe reality.

https://dl.dropbox.com/u/22145620/Technical%20diving%20part%202%20Figure%202.jpg

You can see that immediately after the switch the pressure of helium in the tissue decreases and the pressure of nitrogen increases, but the former process takes place more quickly so that the total pressure (the line labelled "sum") of dissolved inert gas in the tissue declines even though we are still breathing 90% inert gas. The tissue becomes undersaturated with respect to ambient pressure (the horizontal line at 10ATA) and this would allow a larger ascent (decrease in ambient pressure) before the total tissue inert gas pressure exceeded ambient pressure and the tissue therefore became supersaturated. In this way, the ascent would be accelerated.

In theory, this description of events following a helium to nitrogen gas switch should hold true for almost all tissues in the body, and such switches should not cause any problems, and should accelerate decompression to some extent. Whether this acceleration is real or significant in practical terms is highly debatable, and David and I cover it in the paper I mentioned above (so I can't pre-publish it on the internet here).

I say “almost all tissues” because there is one notable exception: the inner ear. The inner ear has a unique anatomy which makes it vulnerable to injury by counter-diffusing gases. It differs from the simple depiction in the first diagram above in that the sensitive functional inner ear tissue (which receives all the blood flow) has a relatively large reservoir of fluid immediately adjacent to it (called the perilymph). This does not have its own blood supply, and any movement of gas in and out of it must occur through the sensitive functional inner ear tissue which, as I said above, receives all the blood flow. During bottom time on a deep dive this reservoir of fluid absorbs helium, and becomes a helium reservoir. (Note: What follows is not strictly correct but is a conceptually adequate way of thinking about it). When we make a gas switch from helium to nitrogen, the sensitive inner ear tissue is already probably supersaturated with helium (remember, we are decompressing here). We make the switch, which in a normal tissue will result in faster loss of helium than nitrogen from the tissue as previously discussed, but in the inner ear there is also a "dump" of helium from the fluid reservoir into the tissue (this helium has no other way of escaping except through the tissue) and the combination of the arrival of the nitrogen, and the dump of helium from the reservoir results in a transient INCREASE in supersaturation instead of a decrease. This could be enough to precipitate bubble formation. I have tried to illustrate this in the diagram below.

https://dl.dropbox.com/u/22145620/Inner%20ear%20after%20helium%20to%20nitrogen%20gas %20switch.jpg

The liklihood of this being a problem is influenced to a large extent by the amount of supersaturation in the inner ear before the switch is made, which essentially means the quality of the decompression of the inner ear prior to the switch. These switches can be (and are) done with a very low incidence of problems if the decompression to that point has been adequate.

Anyway, I hope that has helped with understanding of the problem, which frequently gets confused on internet forums. I must acknowledge the fantastic work done by David Doolette on this problem. We have collaborated on the various papers, but he is the modelling genius.

Simon M

Scuttler
11-01-2013, 07:23 AM
I couldnt have explained it better myself :D

GLOC
09-06-2013, 08:31 PM
Bump because it is interesting and it came up somewhere else, so thought you might want to read some technical stuff ;)

Regards

matt
09-06-2013, 10:31 PM
Anyway, I hope that has helped with understanding of the problem, which frequently gets confused on internet forums. I must acknowledge the fantastic work done by David Doolette on this problem. We have collaborated on the various papers, but he is the modelling genius.

Simon M

I just re-read this after a helpful bump :-). Most of what I have read on ICD says that it is only a problem at-or-near-a-ceiling. Is this true and if so how near to a ceiling is near and can we ignore this (even for provocative dives) if we are not near a ceiling?

Thanks
Matt.