Off Gassing of Helium

***schford*** · 23rd January 2005, 16:06

I have heard discussed that there is a theory that although helium is a lighter gas than nitrogen and on gasses and off gases quicker than it, that fatty tissues off gass it slower than nitrogen - this being the case then some dive profiles would result in a deco schedule where off gassing of helium from fatty tissue would be the controlling factor as the M value and half time for that theoretical compartment would control the dive.

Present-day dissolved gas models employ a concept for multiple inert gases (in a mix such as tri mix) which states that the total inert gas pressure in a hypothetical "tissue" compartment (eg the fatty tissue compartment) is the sum of the partial pressures of the inert gases present in the compartment, even though the various inert gases each have a different half-time for that compartment.

With deco models either having the same M value for Nitrogen and Helium or a higher (this is better) M value for Helium than Nitrogen and then calculating an intermediate M-value(which is an adjustment between the separate M-values for nitrogen and helium based on the proportion of these inert gases present in the compartment) and with Helium having a much shorter half time this should mean that on a typical deep trimix dive Helium either does not affect the deco schedule or shortens it.

I wondered if any one had any comments on this, could explain it to me or had a link to some web based research I could gander at - as obviously if this theory is correct any thing that off gasses slowly from fatty tissue would affect little *coughs* old me....

RonMicjan · 19th February 2005, 04:57

Well, the current theory goes that its the tissues with the most perfusion (blood supply) that have the most issues, fatty tissue have very low perfusion, so is not an issue, I would personally welcome a bends hit in my fatty tissue, whoops, that might include my brain tissue, maybe not.
but seriously, Most models lengthen deco time when there is helium in the mix, one reason might be because it hurts like hell if it comes out sideways, so conservatism is the watchword, the other reason is you are coming up to an atmosphere with zero HE in it, so the gradient between your tissue and ambient is quite large, especially compared to Nitrogen, which has a PP of .79 in ambient air. HE enters and exits 2.78 times faster than Nitrogen, so one might assume a dive with HE would get you out of the water sooner, but the models do not reflect that thinking. Bulhman kicks the crap out of you for even thinking about Helium in your mix. RGBM gets one out of the water much quicker (and so far without any insults from my experience) than the other models. Maybe Bruce Winke has something there... prevent the bubbles, then you dont have to hang and wait for them to be crushed again. my 2 coppers..

***schford*** · 19th February 2005, 11:34

Thanks Ron,

That all makes sence to me...

I wonder if Leon would be interested in upgrading the deco electronics for the Megs from GFB to RGBM - as all I have heard about RGBM is good stuff - in fact I was thinking of getting a HS explorer as backup computer and it would be nice if I could run it in RGBM mode along with the Meg...

Stuart

Mark Chase · 23rd February 2005, 17:12

I have run trimix in the unit from day one and stayed on mix throughout the dive including deco and I was pretty screwed up after even short 30min deco dives. My buddy was the same. We then started doing pure 02 rebreather deco for the last 15 -20mins of the deco profile and we felt great after the dives.

He is a fast gas and I understand it puts less strain on the body bur even with our deep stops and slow ascent rates it was screwing us up. OC Our profiles used 32%, 80% and occasionally 21/35 for deco and we got out feeling good after dives ranging from 90 mins to 160mins in depths ranging from 40 to 100m. Doing the same profiles and keeping HE in the mix all the way out just didn’t work for us on CCR. So whilst I am still open to its fast off gassing ability I am now convinced it is critical to use pure 02 to get rid of He during the shallow stops.

ATB

Mark Chase

23rd January 2005, 16:06	#1 (permalink)
*schford* Despotic Overlord Current Rebreather/s: Megalodon Sport Kiss Other Rebreather/s: Join Date: Jan 2005 Location: London, UK Posts: 3,562	Off Gassing of Helium I have heard discussed that there is a theory that although helium is a lighter gas than nitrogen and on gasses and off gases quicker than it, that fatty tissues off gass it slower than nitrogen - this being the case then some dive profiles would result in a deco schedule where off gassing of helium from fatty tissue would be the controlling factor as the M value and half time for that theoretical compartment would control the dive. Present-day dissolved gas models employ a concept for multiple inert gases (in a mix such as tri mix) which states that the total inert gas pressure in a hypothetical "tissue" compartment (eg the fatty tissue compartment) is the sum of the partial pressures of the inert gases present in the compartment, even though the various inert gases each have a different half-time for that compartment. With deco models either having the same M value for Nitrogen and Helium or a higher (this is better) M value for Helium than Nitrogen and then calculating an intermediate M-value(which is an adjustment between the separate M-values for nitrogen and helium based on the proportion of these inert gases present in the compartment) and with Helium having a much shorter half time this should mean that on a typical deep trimix dive Helium either does not affect the deco schedule or shortens it. I wondered if any one had any comments on this, could explain it to me or had a link to some web based research I could gander at - as obviously if this theory is correct any thing that off gasses slowly from fatty tissue would affect little coughs old me....
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19th February 2005, 04:57	#2 (permalink)
RonMicjan Cap Ron scourge of the NW Current Rebreather/s: Megalodon Dolphin Home Build Other Rebreather/s: Sport Kiss Classic Kiss Dolphin Home Build Join Date: Feb 2005 Location: Cape Disapointment, The Graveyard of the Pacific Posts: 1,100	Well, the current theory goes that its the tissues with the most perfusion (blood supply) that have the most issues, fatty tissue have very low perfusion, so is not an issue, I would personally welcome a bends hit in my fatty tissue, whoops, that might include my brain tissue, maybe not. but seriously, Most models lengthen deco time when there is helium in the mix, one reason might be because it hurts like hell if it comes out sideways, so conservatism is the watchword, the other reason is you are coming up to an atmosphere with zero HE in it, so the gradient between your tissue and ambient is quite large, especially compared to Nitrogen, which has a PP of .79 in ambient air. HE enters and exits 2.78 times faster than Nitrogen, so one might assume a dive with HE would get you out of the water sooner, but the models do not reflect that thinking. Bulhman kicks the crap out of you for even thinking about Helium in your mix. RGBM gets one out of the water much quicker (and so far without any insults from my experience) than the other models. Maybe Bruce Winke has something there... prevent the bubbles, then you dont have to hang and wait for them to be crushed again. my 2 coppers.. __________________ Marine rescue, towing and salvage. Interfering with natural selection since 1983. www.tmishop.com Diving bits etc. www.seatowpdx.com The Summer Job
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19th February 2005, 11:34	#3 (permalink)
*schford* Despotic Overlord Current Rebreather/s: Megalodon Sport Kiss Other Rebreather/s: Join Date: Jan 2005 Location: London, UK Posts: 3,562	Thanks Ron, That all makes sence to me... I wonder if Leon would be interested in upgrading the deco electronics for the Megs from GFB to RGBM - as all I have heard about RGBM is good stuff - in fact I was thinking of getting a HS explorer as backup computer and it would be nice if I could run it in RGBM mode along with the Meg... Stuart
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23rd February 2005, 17:12	#4 (permalink)
Mark Chase I go down for ages Current Rebreather/s: Classic Kiss Other Rebreather/s: Inspiration Classic Join Date: Jan 2005 Location: Kent Posts: 2,700	I have run trimix in the unit from day one and stayed on mix throughout the dive including deco and I was pretty screwed up after even short 30min deco dives. My buddy was the same. We then started doing pure 02 rebreather deco for the last 15 -20mins of the deco profile and we felt great after the dives. He is a fast gas and I understand it puts less strain on the body bur even with our deep stops and slow ascent rates it was screwing us up. OC Our profiles used 32%, 80% and occasionally 21/35 for deco and we got out feeling good after dives ranging from 90 mins to 160mins in depths ranging from 40 to 100m. Doing the same profiles and keeping HE in the mix all the way out just didn’t work for us on CCR. So whilst I am still open to its fast off gassing ability I am now convinced it is critical to use pure 02 to get rid of He during the shallow stops. 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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