Isobaric Counter Diffusion (ICD)

[Searcaigh]

Following reading a recent post of diving the Arch in the Blue Hole at Dahab and watching the video in the post I was a bit confused as to why EAN32 was being used as the first deco gas.

I decided to establish a new thread on this subject rather than hijack the original tread.

From what I understand about ICD a change from the bottom gas 18/30 to EAN32 could result in DCS as the N2 difference is 16% which is more than the ideal.

Or am I incorrect?

I've always used EAN50 when changing from 18/40

 

[Wibble]

MultiDeco really whinges at you if you change the nitrogen levels too much.

I look at the ICD logic like this:
Going from 18/40 (= 42% nitrogen) to 32% (=78% nitrogen) seems that you're going to get a lot of helium replaced with nitrogen.
Doing the same with 50% (=50% N2) will be more effective as the nitrogen content's not changing much, but there's a big change in O2.

 

[sea_ledford]

My understanding is ICD isn't really an issue until you are deeper than about 100M. I'm sure there is a shallower and longer threshold as well (like 250 feet for a two hour bottom time or something). But if you are still in the normoxic range, it is probably mostly academic.

 

[100days-a-year]

Does anyone have any knowledge of recreational teck divers getting an ICD hit?
What peer reviewed evidence is there to suggest it is anything but academic?

 

[Tracy]

Following reading a recent post of diving the Arch in the Blue Hole at Dahab and watching the video in the post I was a bit confused as to why EAN32 was being used as the first deco gas.

I decided to establish a new thread on this subject rather than hijack the original tread.

From what I understand about ICD a change from the bottom gas 18/30 to EAN32 could result in DCS as the N2 difference is 16% which is more than the ideal.

Or am I incorrect?

I've always used EAN50 when changing from 18/40

While you are correct, that change is outside the parameters and wouldn't be an ideal choice, I was taught and have always learned that IBCD is of little concern with normoxic dives of nominal length.

 

[NothingClever]

My understanding is ICD isn't really an issue until you are deeper than about 100M.

The depth threshold to start thinking about ICD is 80m for TDI.

 

[J-Vo]

I witnessed a hypoxic class using 36% for travel/deco to "avoid carrying the extra bottle" on the final dive they were switching from 10/70 to 36% at 120' (fresh water at altitude). That caught my eye as a little aggressive, but they didn't have any problems with ICD. I have no idea why they where using 10/70 for a dive to 280' in open circuit

 

[Wibble]

I witnessed a hypoxic class using 36% for travel/deco to "avoid carrying the extra bottle" on the final dive they were switching from 10/70 to 36% at 120' (fresh water at altitude). That caught my eye as a little aggressive, but they didn't have any problems with ICD. I have no idea why they where using 10/70 for a dive to 280' in open circuit

Was that a bounce dive to 95m/280' where they just used 32% and 80%? i.e. only two deco cylinders.

 

[J-Vo]

Was that a bounce dive to 95m/280' where they just used 32% and 80%? i.e. only two deco cylinders.

10/70 on the bottom, 36% and 100% for deco. If memory serves me well, 15 minute bottom time. but I'm not sure. I only no the mixes as I was curious about not taking 3 bottles. Personally, one of the big learning curves of Hypoxic was managing 3 and 4 deco bottles. I thought the instructor was doing them a disservice by avoiding it..


Edit: typo

 

[PfcAJ]

10/70 on the bottom, 36% and 100% for deco. If memory serves me well, 15 minute bottom time. bit I'm not sure. I only no the mixes as I was curious about not taking 3 bottles. Personally, one of the big learning curves of Hypoxic was managing 3 and 4 deco bottles. I thought the instructor was doing them a disservice by avoiding it.

Looney toons.

Delete either of those two bottles and you’re REALLY in a pickle. Too much space between the 36% bottle and oxygen.