Helium in your (last) deco gas?

Janos · July 31st, 2006, 11:04 PM

Now I know that DIR is very much pro-Helium. However I read somewhere that some DIR people use 50/25 instead of 50% Nitrox. I was curious as to the reasoning behind this - is it for a deco advantage or because it makes a heavy tank easier to carry?

Also, with these dives are you carrying an Oxygen bottle as well? Would you consider using Helium in your last deco gas?

Apologies if this is not approved DIR practice (I genuinely don't know one way or the other) if it's not, I'll ask elsewhere.

Janos

Ahmed Adly, Marlin Inn DC · July 31st, 2006, 11:19 PM

Quote: (Originally Posted by Janos)

Now I know that DIR is very much pro-Helium. However I read somewhere that some DIR people use 50/25 instead of 50% Nitrox. I was curious as to the reasoning behind this - is it for a deco advantage or because it makes a heavy tank easier to carry?

Also, with these dives are you carrying an Oxygen bottle as well? Would you consider using Helium in your last deco gas?

Apologies if this is not approved DIR practice (I genuinely don't know one way or the other) if it's not, I'll ask elsewhere.

Janos

Easier to carry. While deco 50 sinks, 50/25 will ride pretty well even when full. Also, where you would use 50/25 you would probably be also using O2 because you only need 50/25 for when it is nose clipped (I think).
So then No, you would have only pure O2 for deco. If you want it (O2) to ride better, I would use an Al80 to 100-120 bars.

Ahmed

**Alastair** · July 31st, 2006, 11:35 PM

Quote: (Originally Posted by Janos)

Now I know that DIR is very much pro-Helium. However I read somewhere that some DIR people use 50/25 instead of 50% Nitrox. I was curious as to the reasoning behind this - is it for a deco advantage or because it makes a heavy tank easier to carry?

Also, with these dives are you carrying an Oxygen bottle as well? Would you consider using Helium in your last deco gas?

I've pretty sure I've heard Jarrod saying he feels better with the added helium in the 50% mix on the DIR 2004 DVD. I've also read similiar from several other people. There was also a section on the V-planner website which used to detail how it theoretically led to a lower gas load:

http://www.hhssoftware.com/v-planner...tml#heliumdeco

Oxygen is the last deco gas - I can't see why you'd want to dilute it with anything as you want to make use of the oxygen window.

Cheers
Al

Janos · July 31st, 2006, 11:41 PM

Quote: (Originally Posted by Alastair)

Oxygen is the last deco gas - I can't see why you'd want to dilute it with anything as you want to make use of the oxygen window.

I'm going to duck out from debating the O2 window

Sorry (once again) I've not made myself clear. What I should have said was:

If you were doing a single stage dive, would you consider using 50/25 instead of 50% (no other deco bottles carried) or would you be unhappy about surfacing on a Helium mix.

I realise that sometimes the DIR answer is "we'd never be in that situation" so feel free to use that answer if it's appropriate.

Janos

**Alastair** · July 31st, 2006, 11:53 PM

I can't see why you'd bother as if the dive was short enough to only be using a 50% bottle i.e. no oxygen then you can't have incurred enough deco to warrant needing 50/25.

It's never come up in discussion though so I could be wrong.
Cheers
Al

lizardland · August 1st, 2006, 12:19 AM

Janos,

you know fine well that unless you go to your offboard gas then you will have helium in your loop at 21m

Personally, I like helium all the way up. I did about an hour long cave dive the other night on Tx40/20 with an average depth of about 18m with a fairly strenuous climb to get all the kit out and get to the chippy before it shut (I missed it). I was very surprised at how I felt at the end, no fatigue at all -- normally I'd be knackered even on nitrox. I've noticed this a lot on shallow dives in the last 12 months.

Open circuit I can't see any advantage in allowing any helium into your 6m bottle, closed circuit then you have to accept that unless you are flushing a lot then there will inevitably be a fair proportion of inerts in what you are breathing as you offgas.

I don't like the idea of coming off a high helium mix on to a nitrox or O2. I don't know if it is a real risk or not, but I like to use helium all the time. <40m dives will be fairly long soaks on trimix (by comparison to a deep dive), I'm not that happy with a sudden switch to a gas with no He content. The next time I need to fill my deco stages (in around 2.5yrs when they next need tested

) I will probably do 50/20 rather than EAN50 and maybe a 32/something.

*Hassan Adly* · August 1st, 2006, 02:12 AM

Hello,

My opinion:

50/25 in an aluminium 80 has neutral or slightly positive buoyancy when full in saltwater. This means that if you have it on a leash or nose-clipped to the hip d-ring, it won't hang down and cause drag like a full cylinder of nitrox 50% can, but rather drift smoothly next to your butt.

I haven't studied the deco benefits, though I daresay that the results should be better, especially on long dives with long deco.

I think that on any dive where I would consider putting helium into the 21m cylinder, I would most probably also be using O2 at 6m.

Mark Powell · August 1st, 2006, 10:08 AM

With a 50% deco gas on a single stage Trimix 21/35 dive then the switch from back gas to deco gas causes the helium to drop to zero, resulting in a big helium gradient, and accelerated helium offgassing. The nitrogen on the other hand goes from 44% in back gas to 50% in deco. So you are not increasing your nitrogen gradient, in fact it is decreasing slightly.

If we use 50/25 then we are reducing the helium content (although not by as much) and also reducing the nitrogen content. So we are accelerating offgassing for helium and nitrogen.

If you are using multiple stages then you can remove helium from the last deco gas and still reduce the nitrogen percentage. Especially if you are using 100% as then you have no inert gas at all.

*Rich Walker* · August 1st, 2006, 10:36 AM

It doesn't have any discernable effect on short ocean-type exposures. However for the longer cave dives, I believe it has benefit for a number of reasons.

It is easier to breathe due to the helium content leading to a "physically" easier decompression, less CO2 production etc.
The gradients Mark highlighted will also have an effect, particularly for the long exposures.
The cylinders do sit better, so whether this was important would depend very much on the cave profile.

Rich

davy · August 1st, 2006, 11:23 AM

I like helium in my decompression mixes...
It isn't "necessary" per see for short OW dives, but it makes the bottle ride better and I feel noticeably better after the dive as well...
I have an old injury that sometimes aches for a short while after a decodive, but I have never noticed it when using helium deco... so for me it is better.
In the 50% bottle I always like to have some helium so there is no ppN2 increase after switching...

I also found that I feel much much better after a long shallow dive with lots of ups and downs in the 10m range when using 30/30 or something like that...

July 31st, 2006, 11:04 PM	#1 (permalink)
Janos(Offline) "Two Sheds" Join Date: Nov 2005 Location: Surrey Posts: 400	Helium in your (last) deco gas? Now I know that DIR is very much pro-Helium. However I read somewhere that some DIR people use 50/25 instead of 50% Nitrox. I was curious as to the reasoning behind this - is it for a deco advantage or because it makes a heavy tank easier to carry? Also, with these dives are you carrying an Oxygen bottle as well? Would you consider using Helium in your last deco gas? Apologies if this is not approved DIR practice (I genuinely don't know one way or the other) if it's not, I'll ask elsewhere. Janos __________________ You can lead a horse to water but you can't climb a ladder with a large bell in both hands - Vic Reeves www.hellfins.com/shed

July 31st, 2006, 11:19 PM	#2 (permalink)
Ahmed Adly, Marlin Inn DC(Offline) New Member Join Date: Feb 2006 Location: hurghada Posts: 849	Quote: (Originally Posted by Janos) Now I know that DIR is very much pro-Helium. However I read somewhere that some DIR people use 50/25 instead of 50% Nitrox. I was curious as to the reasoning behind this - is it for a deco advantage or because it makes a heavy tank easier to carry? Also, with these dives are you carrying an Oxygen bottle as well? Would you consider using Helium in your last deco gas? Apologies if this is not approved DIR practice (I genuinely don't know one way or the other) if it's not, I'll ask elsewhere. Janos Easier to carry. While deco 50 sinks, 50/25 will ride pretty well even when full. Also, where you would use 50/25 you would probably be also using O2 because you only need 50/25 for when it is nose clipped (I think). So then No, you would have only pure O2 for deco. If you want it (O2) to ride better, I would use an Al80 to 100-120 bars. Ahmed Last edited by Ahmed Adly, Marlin Inn DC; July 31st, 2006 at 11:26 PM.

July 31st, 2006, 11:35 PM	#3 (permalink)
Alastair(Offline) DIRX Supporter Join Date: Nov 2005 Location: UK Posts: 693	Quote: (Originally Posted by Janos) Now I know that DIR is very much pro-Helium. However I read somewhere that some DIR people use 50/25 instead of 50% Nitrox. I was curious as to the reasoning behind this - is it for a deco advantage or because it makes a heavy tank easier to carry? Also, with these dives are you carrying an Oxygen bottle as well? Would you consider using Helium in your last deco gas? I've pretty sure I've heard Jarrod saying he feels better with the added helium in the 50% mix on the DIR 2004 DVD. I've also read similiar from several other people. There was also a section on the V-planner website which used to detail how it theoretically led to a lower gas load: http://www.hhssoftware.com/v-planner...tml#heliumdeco Oxygen is the last deco gas - I can't see why you'd want to dilute it with anything as you want to make use of the oxygen window. Cheers Al __________________ Helium is our friend - GI3

July 31st, 2006, 11:41 PM	#4 (permalink)
Janos(Offline) "Two Sheds" Join Date: Nov 2005 Location: Surrey Posts: 400	Quote: (Originally Posted by Alastair) Oxygen is the last deco gas - I can't see why you'd want to dilute it with anything as you want to make use of the oxygen window. I'm going to duck out from debating the O2 window Sorry (once again) I've not made myself clear. What I should have said was: If you were doing a single stage dive, would you consider using 50/25 instead of 50% (no other deco bottles carried) or would you be unhappy about surfacing on a Helium mix. I realise that sometimes the DIR answer is "we'd never be in that situation" so feel free to use that answer if it's appropriate. Janos __________________ You can lead a horse to water but you can't climb a ladder with a large bell in both hands - Vic Reeves www.hellfins.com/shed

July 31st, 2006, 11:53 PM	#5 (permalink)
Alastair(Offline) DIRX Supporter Join Date: Nov 2005 Location: UK Posts: 693	I can't see why you'd bother as if the dive was short enough to only be using a 50% bottle i.e. no oxygen then you can't have incurred enough deco to warrant needing 50/25. It's never come up in discussion though so I could be wrong. Cheers Al __________________ Helium is our friend - GI3

August 1st, 2006, 12:19 AM	#6 (permalink)
lizardland(Offline) LCS Join Date: Nov 2005 Location: Evidently Chickentown Posts: 591	Janos, you know fine well that unless you go to your offboard gas then you will have helium in your loop at 21m Personally, I like helium all the way up. I did about an hour long cave dive the other night on Tx40/20 with an average depth of about 18m with a fairly strenuous climb to get all the kit out and get to the chippy before it shut (I missed it). I was very surprised at how I felt at the end, no fatigue at all -- normally I'd be knackered even on nitrox. I've noticed this a lot on shallow dives in the last 12 months. Open circuit I can't see any advantage in allowing any helium into your 6m bottle, closed circuit then you have to accept that unless you are flushing a lot then there will inevitably be a fair proportion of inerts in what you are breathing as you offgas. I don't like the idea of coming off a high helium mix on to a nitrox or O2. I don't know if it is a real risk or not, but I like to use helium all the time. <40m dives will be fairly long soaks on trimix (by comparison to a deep dive), I'm not that happy with a sudden switch to a gas with no He content. The next time I need to fill my deco stages (in around 2.5yrs when they next need tested) I will probably do 50/20 rather than EAN50 and maybe a 32/something. __________________ Can you imagine drifting along in the sea with your mouth open and a load of f***ing plankton going in? You'd like it, would you? www.westons-cider.co.uk The Lot isn't the only place to dive: http://www.lulu.com/content/613554

August 1st, 2006, 02:12 AM	#7 (permalink)
Hassan Adly (Online) GUE Instructor Join Date: Feb 2006 Location: Red Sea Posts: 286	Hello, My opinion: 50/25 in an aluminium 80 has neutral or slightly positive buoyancy when full in saltwater. This means that if you have it on a leash or nose-clipped to the hip d-ring, it won't hang down and cause drag like a full cylinder of nitrox 50% can, but rather drift smoothly next to your butt. I haven't studied the deco benefits, though I daresay that the results should be better, especially on long dives with long deco. I think that on any dive where I would consider putting helium into the 21m cylinder, I would most probably also be using O2 at 6m. __________________ Hassan Adly Red Sea Discovery "Today I will finally see the sea again, which will smell of salt, wind, sand - and the cold of winter. Finally I will not only travel on it but dive in it, again I will become water, a bird - and I will remember the feeling of gliding above the abyss" Elena Sundukova, tragically lost at sea, January 2007

August 1st, 2006, 10:08 AM	#8 (permalink)
Mark Powell (Online) New Member Join Date: Jan 2006 Location: Guildford Posts: 49	With a 50% deco gas on a single stage Trimix 21/35 dive then the switch from back gas to deco gas causes the helium to drop to zero, resulting in a big helium gradient, and accelerated helium offgassing. The nitrogen on the other hand goes from 44% in back gas to 50% in deco. So you are not increasing your nitrogen gradient, in fact it is decreasing slightly. If we use 50/25 then we are reducing the helium content (although not by as much) and also reducing the nitrogen content. So we are accelerating offgassing for helium and nitrogen. If you are using multiple stages then you can remove helium from the last deco gas and still reduce the nitrogen percentage. Especially if you are using 100% as then you have no inert gas at all. __________________ Mark Powell Dive-Tech: Technical Diver Training http://www.dive-tech.co.uk Visit the online technical diving shop: Analox, Fourth Element, Halcyon and Apeks

August 1st, 2006, 10:36 AM	#9 (permalink)
Rich Walker (Online) GUE Instructor Join Date: Nov 2005 Location: Sheffield Posts: 299	It doesn't have any discernable effect on short ocean-type exposures. However for the longer cave dives, I believe it has benefit for a number of reasons. It is easier to breathe due to the helium content leading to a "physically" easier decompression, less CO2 production etc. The gradients Mark highlighted will also have an effect, particularly for the long exposures. The cylinders do sit better, so whether this was important would depend very much on the cave profile. Rich __________________ Education, Equipment, Experience GUE training with Rich Walker

August 1st, 2006, 11:23 AM	#10 (permalink)
davy (Online) DIveR Join Date: Nov 2005 Location: Belgium Posts: 274	I like helium in my decompression mixes... It isn't "necessary" per see for short OW dives, but it makes the bottle ride better and I feel noticeably better after the dive as well... I have an old injury that sometimes aches for a short while after a decodive, but I have never noticed it when using helium deco... so for me it is better. In the 50% bottle I always like to have some helium so there is no ppN2 increase after switching... I also found that I feel much much better after a long shallow dive with lots of ups and downs in the 10m range when using 30/30 or something like that...