Difference between an N2 and He Bend

Mike · 9th August 2005, 10:43

Quote: (Originally Posted by Simon Mitchell)

the pressure of He in the bubble will now be approximately 2.8 ATA. The pressure of He in lungs will still be virtually zero, and now the gradient for outgassing from bubble is 2.8 ATA instead of 1

This is interesting, a couple of us were briefly discussing this on the weekend after a bouncy deco. I suggested it was worth doing a 3m stop, vs using 6m as the last stop.

In the light of Simon's quote, it means that you will decompress quickest at the deepest stop where you use straight O2. In particular the 6m stop will be 1.6/1.3 times (ie over 20%) more efficient for decompresion than the 3m stop. That's significant! So you are better off doing all the time at 6m and skipping the 3m stop completely.

Wiz - my apologies, you were competely correct.

Although I guess it get's complicated by the fact that there are some bubbles present as well as dissolved gas, so you don't want these bubbles growing too much by going from 6m to the surface too quickly...

Bugger it, I don't know

Mike

Scuba_Vixen · 9th August 2005, 19:03

Quote: (Originally Posted by Mike)

This is interesting, a couple of us were briefly discussing this on the weekend after a bouncy deco. I suggested it was worth doing a 3m stop, vs using 6m as the last stop.

In the light of Simon's quote, it means that you will decompress quickest at the deepest stop where you use straight O2. In particular the 6m stop will be 1.6/1.3 times (ie over 20%) more efficient for decompresion than the 3m stop. That's significant! So you are better off doing all the time at 6m and skipping the 3m stop completely.

Wiz - my apologies, you were competely correct.

Although I guess it get's complicated by the fact that there are some bubbles present as well as dissolved gas, so you don't want these bubbles growing too much by going from 6m to the surface too quickly...

Bugger it, I don't know

Mike

Mike, Not exactly the case. If that were true, then selecting the "last stop at 6m" would give a shorter run time than using "last stop at 3m" when doing a dive plan.

Once you're on O2, the inert gas gradient between inspired and dissolved is at it's max, and that will stay constant for any depth, once on O2.

There's been ongoing debate in the tech forums for ages regarding the last stop: all the time done at 6m or some at 6 and some at 3.

Decompression is driven by the difference in the gas pressures within us, and the ambient pressure. Inspired gas is of course at ambient.

The real workings are better thought of as a partial pressure deficit. Arterial total pressure is greater than venous total pressure, hence the deficit that makes room for the dissolved gas to be absorbed into the venous blood flow. The deficit comes from the fact that the CO2 that is released by the body as it metabolizes O2 is less by volume than the O2.

"Qxygen window" theory proponents suggest making the whole O2 stop time at max PO2 (6m) gives the best offgassing, If you run the different deco planners and compare times with all the time at 6m and then the same dive with a 6m and 3m stop, the times are exactly the same. The mathematical models don't seem to favor all the O2 time at 6m over some at 6 and some at 3

The down side to all the O2 time at 6m is a greater CNS loading, which if you're doing multiple or repetitive dives, is usually better avoided.

I always plan my dives with the last stop at 3m, that way I have a minimum time I have to stay at 6m, but I can do the 3 m stop anywhere between 6m and 3 m. If there's a surface current, or conditions go to squat, I can make the last stop depth decision when I get there and can assess the situation.

For those that are ready for it, here's a link to the "Thread From Hell" about last stop depths.

http://thedecostop.com/forums/showth...essure+deficit

And if that's not enough to choke on, here's yet another (with links to still more): (Be sure to follow the links to some of the professional articles sited in the posts)

http://thedecostop.com/forums/showth...essure+deficit

Makes me want to go back to "Tic Tac Toe Ratio Turtle Bone Divination Deco on the fly"

Darlene

wizbang · 9th August 2005, 22:06

Hi Darlene, not trying to be picky but V-planner for the deeper/longer dives gives a difference/benefit for the 6m stop if using 100% (PPO2 >1.55).
For this example it is only 4 minutes but more often than not it is also easier to do the stop at 6m than 3m. Especially on the weekend Mike was talking about.
Eg 115m for 21min 8/60 1.3 with a....
6m stop PPO2 1.3 215 min CNS 118.1%
3m stop PPO2 1.3 205 min CNS 111.8%
6m stop PPO2 1.55 201 min CNS 148.6%
6m stop PPO2 1.55 205 min CNS 111.8%

Some might say the CNS difference alone is reason enough to do it at 3m.

Is a link to V-planners (Ross') reasoning for why the 6m stop for 100% is worthwhile.
http://www.hhssoftware.com/v-planner/decomyths.html

Just some more food for thought.
And perhaps a touch off topic. DOH

dteubner · 9th August 2005, 22:06

Quote: (Originally Posted by Mike)

In the light of Simon's quote, it means that you will decompress quickest at the deepest stop where you use straight O2. In particular the 6m stop will be 1.6/1.3 times (ie over 20%) more efficient for decompresion than the 3m stop. That's significant! So you are better off doing all the time at 6m and skipping the 3m stop completely.

Mike

This is only true for gas present as bubbles. Dissolved gas doesn't care what the ambient pressure is (except that too low an ambient pressure causes bubbles).

For ordinary deco (hopefully) the vast majority of the gas present is dissolved and there should be no real difference in offgassing on O2 at 3 and 6m, and, like Darlene says, there is increased O2 exposure at 6m

At the end of the day there is a trade off between increased O2 exposure and increased off gassing from any bubbles present.

This is a thing where there can only be opinion. There is no real evidence at all. Models do not measure what is happening in the body at all.

Dave T

Simon Mitchell · 9th August 2005, 23:52

Mike,

Dave is right. My earlier comments pertained to the diver being recompressed for DCS after completion of a dive (usually a significant time after completion), and cannot really be extrapolated to the decompression phase of a dive. During decompression, the bubble is in an evolutionary state and its growth is being driven by supersaturation of gas in the surrounding tissues. At that time its behaviour is dictated largely by tissue to bubble partial pressure gradients rather than bubble to lung gradients. It is a very complicated dynamic process that we are trying to describe in simple terms but I hope you can see what I mean.

Regards,

Simon M

Mike · 9th August 2005, 23:58

Quote: (Originally Posted by dteubner)

This is only true for gas present as bubbles. Dissolved gas doesn't care what the ambient pressure is (except that too low an ambient pressure causes bubbles).

Doesn't it?
Surely even for dissolved gas it is the concentration gradient for the gas that is driving the diffusion out of the tissue into the blood, and from the blood to the lungs? This concentration can be considered to be relative to pressure

Really simplisticually I had imagined it (at 6m, on O2) being something like;
Tissue N2 concentration= 1.6
Blood N2 concentration= 0.8
Lung (free gas) N2 = 0.0

VS at 3m
Tissue PPN2 = 1.3
Blood PPN2 = 0.65
Lung PPN2 = 0

(I know the figures I have used are not an accurate representation of the concentration, but I'm assuming the concentration is relative what the partial pressure would be, and it is only the ratio between them I'm interested in.
Also I'm just assuming blood is a transport medium between tissue and free gas in the lungs, and therefore it will be somewhere between the concentrations of each)

BUGGER!
Ignore the stuff in italics - it's crap. For dissolved gas it's concentration that drives the diffusion - ie number of molecules per volume - and it is completely independant of pressure. Therefore the concentration gradient and hence the rate of diffusion doesn't change by moving from 6m to 3m.

So:
-For dissolved gas, it's going to diffuse out at the same rate reguardless of depth. (If you are breathing O2 only)
-As you come shallower some gas is going to come out of solution into bubbles. If these bubbles are kept small, it aids off gassing, if they get big, it both hinders offgassing and hurts. And we don't know exactly how to calculate the size of the bubbles, but VPM and RGBM are guesses at how to keep bubble size at the optimum for off gassing.

Right.
Thank you all for helping to beat this into my head, and please return to your scheduled program.

Mike
<edit> and if I had waiting rather than typing, Simon would have explained it for me anyway...

Barrie Law · 10th August 2005, 00:56

I think the point should not be missed when considering a 6mtr or a 3 mtr last stop depth that other factors can be considered;

CNS loading up to the last stop.

The number of inert gasses to be decompressed helium, nitrogen?

The difference in the inert gas properties and their decompression requirements.

The CCR's ability to maintain a setpoint eg; 1.3 PP02

Our ability on CCR to 02 flush at 6 mtrs and reflush during last stops.

Our ability on CCR to go from last stop to the surface in as long as we choose.

The increased body temperature received from breathing the 100% 02 in the loop.

The heat from the sun at 3 mtrs is quite pleasant, even in winter.

So I elect to do my final stops at 3.5 mtrs on a 1.3 setpoint and I choose to use Heliox mixes for my dil and bailout gasses, I often monitor the He offgassing using the counterlungs and their expansion when at stop depths and it becomes more noticeable running quite aggressive deco schedules.Some cramps in the lower legs caused concern for a while but this ceased when I got a better fitting drysuit and warmth is critical on the last stops as they are by far the longest.
I ascend slowly from 3.5 mtrs to the surface and on an smb do not find conditions up top effect my position in the water even when it is quite ruff as the Irish coast can be!!
This works for me and I enjoy my diving and feel comfortable with heliox but I have not been able to draw a line in the sand on limits despite getting out of the water and talking like donald duck for several minutes without a twinge so I have come to the conclusion that all bends are earned either before, during or after a dive and the common ingredients are Nitrogen, Ascent rates and body temperature.
I am not saying you can not get bent on Heliox but I do believe it is more forgiving than Trimix or Nitrox and a single inert gas for decompression seems a better choice for me to depths before HPNS becomes a consideration.

Cheers

rossh · 10th August 2005, 02:04

Quote:

..... but He dosent actualy benifit from gradient of 02 to off gas the HE as there is no HE in the air we breath so even air would off gas it.

Hi Mark, In addition to many fine comments above,

On your typical 10/50 CCR dive, the inspired ppN2 component is equal to surface values at around 15m depth. From here up the diver is off gassing N2 to tissue values less than surface amounts. By the time the dive is over and the diver breaths air again, tissue pressures of N2 are below normal surface values. Breathing air means he will be on gassing N2.

The use of surface O2 permits more complete off gassing of all inerts, vs when air breathing the He is leaving and N2 is arriving.

Regards

PIKEY · 14th August 2005, 05:07

Quote: (Originally Posted by Barrie Law)

I think the point should not be missed when considering a 6mtr or a 3 mtr last stop depth that other factors can be considered;

CNS loading up to the last stop.

The number of inert gasses to be decompressed helium, nitrogen?

The difference in the inert gas properties and their decompression requirements.

The CCR's ability to maintain a setpoint eg; 1.3 PP02

Our ability on CCR to 02 flush at 6 mtrs and reflush during last stops.

Our ability on CCR to go from last stop to the surface in as long as we choose.

The increased body temperature received from breathing the 100% 02 in the loop.

The heat from the sun at 3 mtrs is quite pleasant, even in winter.

So I elect to do my final stops at 3.5 mtrs on a 1.3 setpoint and I choose to use Heliox mixes for my dil and bailout gasses, I often monitor the He offgassing using the counterlungs and their expansion when at stop depths and it becomes more noticeable running quite aggressive deco schedules.Some cramps in the lower legs caused concern for a while but this ceased when I got a better fitting drysuit and warmth is critical on the last stops as they are by far the longest.
I ascend slowly from 3.5 mtrs to the surface and on an smb do not find conditions up top effect my position in the water even when it is quite ruff as the Irish coast can be!!
This works for me and I enjoy my diving and feel comfortable with heliox but I have not been able to draw a line in the sand on limits despite getting out of the water and talking like donald duck for several minutes without a twinge so I have come to the conclusion that all bends are earned either before, during or after a dive and the common ingredients are Nitrogen, Ascent rates and body temperature.
I am not saying you can not get bent on Heliox but I do believe it is more forgiving than Trimix or Nitrox and a single inert gas for decompression seems a better choice for me to depths before HPNS becomes a consideration.

Cheers

Keep on believing

wrecked · 14th August 2005, 06:19

Quote: (Originally Posted by Mark Chase)

Something that cropped up last year but has since vanished was the discussion on the Hyperbaric units ditching pure 02 for the treatment of bends and moving over to 50/50 Heliox for treatment?? It would be nice to get some more info on that.

Yes 50/50 is used on more serious bends particulary He Bends

9th August 2005, 10:43	#21 (permalink)
Mike Prism 'prentice Current Rebreather/s: Prism Topaz Other Rebreather/s: Inspiration Classic Evolution Join Date: Feb 2005 Location: Melbourne Posts: 329	Quote: (Originally Posted by Simon Mitchell) the pressure of He in the bubble will now be approximately 2.8 ATA. The pressure of He in lungs will still be virtually zero, and now the gradient for outgassing from bubble is 2.8 ATA instead of 1 This is interesting, a couple of us were briefly discussing this on the weekend after a bouncy deco. I suggested it was worth doing a 3m stop, vs using 6m as the last stop. In the light of Simon's quote, it means that you will decompress quickest at the deepest stop where you use straight O2. In particular the 6m stop will be 1.6/1.3 times (ie over 20%) more efficient for decompresion than the 3m stop. That's significant! So you are better off doing all the time at 6m and skipping the 3m stop completely. Wiz - my apologies, you were competely correct. Although I guess it get's complicated by the fact that there are some bubbles present as well as dissolved gas, so you don't want these bubbles growing too much by going from 6m to the surface too quickly... Bugger it, I don't know Mike
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9th August 2005, 19:03	#22 (permalink)
Scuba_Vixen e/mCCR Dolphin Pilot Current Rebreather/s: Other CCR Home Build Other Rebreather/s: Other CCR Home Build Join Date: May 2005 Location: St. Croix USVI Posts: 557	Quote: (Originally Posted by Mike) This is interesting, a couple of us were briefly discussing this on the weekend after a bouncy deco. I suggested it was worth doing a 3m stop, vs using 6m as the last stop. In the light of Simon's quote, it means that you will decompress quickest at the deepest stop where you use straight O2. In particular the 6m stop will be 1.6/1.3 times (ie over 20%) more efficient for decompresion than the 3m stop. That's significant! So you are better off doing all the time at 6m and skipping the 3m stop completely. Wiz - my apologies, you were competely correct. Although I guess it get's complicated by the fact that there are some bubbles present as well as dissolved gas, so you don't want these bubbles growing too much by going from 6m to the surface too quickly... Bugger it, I don't know Mike Mike, Not exactly the case. If that were true, then selecting the "last stop at 6m" would give a shorter run time than using "last stop at 3m" when doing a dive plan. Once you're on O2, the inert gas gradient between inspired and dissolved is at it's max, and that will stay constant for any depth, once on O2. There's been ongoing debate in the tech forums for ages regarding the last stop: all the time done at 6m or some at 6 and some at 3. Decompression is driven by the difference in the gas pressures within us, and the ambient pressure. Inspired gas is of course at ambient. The real workings are better thought of as a partial pressure deficit. Arterial total pressure is greater than venous total pressure, hence the deficit that makes room for the dissolved gas to be absorbed into the venous blood flow. The deficit comes from the fact that the CO2 that is released by the body as it metabolizes O2 is less by volume than the O2. "Qxygen window" theory proponents suggest making the whole O2 stop time at max PO2 (6m) gives the best offgassing, If you run the different deco planners and compare times with all the time at 6m and then the same dive with a 6m and 3m stop, the times are exactly the same. The mathematical models don't seem to favor all the O2 time at 6m over some at 6 and some at 3 The down side to all the O2 time at 6m is a greater CNS loading, which if you're doing multiple or repetitive dives, is usually better avoided. I always plan my dives with the last stop at 3m, that way I have a minimum time I have to stay at 6m, but I can do the 3 m stop anywhere between 6m and 3 m. If there's a surface current, or conditions go to squat, I can make the last stop depth decision when I get there and can assess the situation. For those that are ready for it, here's a link to the "Thread From Hell" about last stop depths. http://thedecostop.com/forums/showth...essure+deficit And if that's not enough to choke on, here's yet another (with links to still more): (Be sure to follow the links to some of the professional articles sited in the posts) http://thedecostop.com/forums/showth...essure+deficit Makes me want to go back to "Tic Tac Toe Ratio Turtle Bone Divination Deco on the fly" Darlene
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9th August 2005, 22:06	#23 (permalink)
wizbang WEB MERMEN Current Rebreather/s: Inspiration Classic Dolphin Home Build Other Rebreather/s: Not Bought Yet Inspiration Vision Evolution Prism Topaz Sport Kiss Classic Kiss Dolphin Home Build Join Date: Feb 2005 Location: Perth Australia Posts: 386	Hi Darlene, not trying to be picky but V-planner for the deeper/longer dives gives a difference/benefit for the 6m stop if using 100% (PPO2 >1.55). For this example it is only 4 minutes but more often than not it is also easier to do the stop at 6m than 3m. Especially on the weekend Mike was talking about. Eg 115m for 21min 8/60 1.3 with a.... 6m stop PPO2 1.3 215 min CNS 118.1% 3m stop PPO2 1.3 205 min CNS 111.8% 6m stop PPO2 1.55 201 min CNS 148.6% 6m stop PPO2 1.55 205 min CNS 111.8% Some might say the CNS difference alone is reason enough to do it at 3m. Is a link to V-planners (Ross') reasoning for why the 6m stop for 100% is worthwhile. http://www.hhssoftware.com/v-planner/decomyths.html Just some more food for thought. And perhaps a touch off topic. DOH
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9th August 2005, 22:06	#24 (permalink)
dteubner Pedant Current Rebreather/s: Sport Kiss Classic Kiss Other Rebreather/s: Join Date: Mar 2005 Location: Adelaide, Australia Posts: 217	Quote: (Originally Posted by Mike) In the light of Simon's quote, it means that you will decompress quickest at the deepest stop where you use straight O2. In particular the 6m stop will be 1.6/1.3 times (ie over 20%) more efficient for decompresion than the 3m stop. That's significant! So you are better off doing all the time at 6m and skipping the 3m stop completely. Mike This is only true for gas present as bubbles. Dissolved gas doesn't care what the ambient pressure is (except that too low an ambient pressure causes bubbles). For ordinary deco (hopefully) the vast majority of the gas present is dissolved and there should be no real difference in offgassing on O2 at 3 and 6m, and, like Darlene says, there is increased O2 exposure at 6m At the end of the day there is a trade off between increased O2 exposure and increased off gassing from any bubbles present. This is a thing where there can only be opinion. There is no real evidence at all. Models do not measure what is happening in the body at all. Dave T
(Online)

9th August 2005, 23:52	#25 (permalink)
Simon Mitchell New Member Current Rebreather/s: MK 15.X Other Rebreather/s: Join Date: May 2005 Location: New Zealand Posts: 177	Mike, Dave is right. My earlier comments pertained to the diver being recompressed for DCS after completion of a dive (usually a significant time after completion), and cannot really be extrapolated to the decompression phase of a dive. During decompression, the bubble is in an evolutionary state and its growth is being driven by supersaturation of gas in the surrounding tissues. At that time its behaviour is dictated largely by tissue to bubble partial pressure gradients rather than bubble to lung gradients. It is a very complicated dynamic process that we are trying to describe in simple terms but I hope you can see what I mean. Regards, Simon M
(Offline)

9th August 2005, 23:58	#26 (permalink)
Mike Prism 'prentice Current Rebreather/s: Prism Topaz Other Rebreather/s: Inspiration Classic Evolution Join Date: Feb 2005 Location: Melbourne Posts: 329	Quote: (Originally Posted by dteubner) This is only true for gas present as bubbles. Dissolved gas doesn't care what the ambient pressure is (except that too low an ambient pressure causes bubbles). Doesn't it? Surely even for dissolved gas it is the concentration gradient for the gas that is driving the diffusion out of the tissue into the blood, and from the blood to the lungs? This concentration can be considered to be relative to pressure Really simplisticually I had imagined it (at 6m, on O2) being something like; Tissue N2 concentration= 1.6 Blood N2 concentration= 0.8 Lung (free gas) N2 = 0.0 VS at 3m Tissue PPN2 = 1.3 Blood PPN2 = 0.65 Lung PPN2 = 0 (I know the figures I have used are not an accurate representation of the concentration, but I'm assuming the concentration is relative what the partial pressure would be, and it is only the ratio between them I'm interested in. Also I'm just assuming blood is a transport medium between tissue and free gas in the lungs, and therefore it will be somewhere between the concentrations of each) BUGGER! Ignore the stuff in italics - it's crap. For dissolved gas it's concentration that drives the diffusion - ie number of molecules per volume - and it is completely independant of pressure. Therefore the concentration gradient and hence the rate of diffusion doesn't change by moving from 6m to 3m. So: -For dissolved gas, it's going to diffuse out at the same rate reguardless of depth. (If you are breathing O2 only) -As you come shallower some gas is going to come out of solution into bubbles. If these bubbles are kept small, it aids off gassing, if they get big, it both hinders offgassing and hurts. And we don't know exactly how to calculate the size of the bubbles, but VPM and RGBM are guesses at how to keep bubble size at the optimum for off gassing. Right. Thank you all for helping to beat this into my head, and please return to your scheduled program. Mike <edit> and if I had waiting rather than typing, Simon would have explained it for me anyway...
(Offline)

10th August 2005, 00:56	#27 (permalink)
Barrie Law RebreatherWorld Sponsor Current Rebreather/s: Inspiration Classic Other Rebreather/s: Join Date: Jun 2005 Location: Ireland Posts: 459	Difference between an N2 and He Bend I think the point should not be missed when considering a 6mtr or a 3 mtr last stop depth that other factors can be considered; CNS loading up to the last stop. The number of inert gasses to be decompressed helium, nitrogen? The difference in the inert gas properties and their decompression requirements. The CCR's ability to maintain a setpoint eg; 1.3 PP02 Our ability on CCR to 02 flush at 6 mtrs and reflush during last stops. Our ability on CCR to go from last stop to the surface in as long as we choose. The increased body temperature received from breathing the 100% 02 in the loop. The heat from the sun at 3 mtrs is quite pleasant, even in winter. So I elect to do my final stops at 3.5 mtrs on a 1.3 setpoint and I choose to use Heliox mixes for my dil and bailout gasses, I often monitor the He offgassing using the counterlungs and their expansion when at stop depths and it becomes more noticeable running quite aggressive deco schedules.Some cramps in the lower legs caused concern for a while but this ceased when I got a better fitting drysuit and warmth is critical on the last stops as they are by far the longest. I ascend slowly from 3.5 mtrs to the surface and on an smb do not find conditions up top effect my position in the water even when it is quite ruff as the Irish coast can be!! This works for me and I enjoy my diving and feel comfortable with heliox but I have not been able to draw a line in the sand on limits despite getting out of the water and talking like donald duck for several minutes without a twinge so I have come to the conclusion that all bends are earned either before, during or after a dive and the common ingredients are Nitrogen, Ascent rates and body temperature. I am not saying you can not get bent on Heliox but I do believe it is more forgiving than Trimix or Nitrox and a single inert gas for decompression seems a better choice for me to depths before HPNS becomes a consideration. Cheers __________________ Barrie Law Rebreather World Store Supplying the Rebreather World Tel:US +19548403892 Fax:US+19543333792 Tel:EU+442071930496 Fax:EU+442077606344 Mobile+353876880628 Skype:barrielaw Email: barrie@rebreatherworldstore.com
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10th August 2005, 02:04	#28 (permalink)
rossh Custom Title Allowed! Current Rebreather/s: Not Bought Yet Other Rebreather/s: Not Bought Yet Join Date: Jul 2005 Location: Canada Posts: 111	Quote: ..... but He dosent actualy benifit from gradient of 02 to off gas the HE as there is no HE in the air we breath so even air would off gas it. Hi Mark, In addition to many fine comments above, On your typical 10/50 CCR dive, the inspired ppN2 component is equal to surface values at around 15m depth. From here up the diver is off gassing N2 to tissue values less than surface amounts. By the time the dive is over and the diver breaths air again, tissue pressures of N2 are below normal surface values. Breathing air means he will be on gassing N2. The use of surface O2 permits more complete off gassing of all inerts, vs when air breathing the He is leaving and N2 is arriving. Regards __________________ Regards, Ross Hemingway V-Planner Live for the X1 V-Planner VPM-B Nitrox Trimix CCR Deco planning for Technical Divers www.v-planner.com
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14th August 2005, 05:07	#29 (permalink)
PIKEY New Member Current Rebreather/s: Other Rebreather/s: Join Date: Jul 2005 Location: ireland Posts: 10	Quote: (Originally Posted by Barrie Law) I think the point should not be missed when considering a 6mtr or a 3 mtr last stop depth that other factors can be considered; CNS loading up to the last stop. The number of inert gasses to be decompressed helium, nitrogen? The difference in the inert gas properties and their decompression requirements. The CCR's ability to maintain a setpoint eg; 1.3 PP02 Our ability on CCR to 02 flush at 6 mtrs and reflush during last stops. Our ability on CCR to go from last stop to the surface in as long as we choose. The increased body temperature received from breathing the 100% 02 in the loop. The heat from the sun at 3 mtrs is quite pleasant, even in winter. So I elect to do my final stops at 3.5 mtrs on a 1.3 setpoint and I choose to use Heliox mixes for my dil and bailout gasses, I often monitor the He offgassing using the counterlungs and their expansion when at stop depths and it becomes more noticeable running quite aggressive deco schedules.Some cramps in the lower legs caused concern for a while but this ceased when I got a better fitting drysuit and warmth is critical on the last stops as they are by far the longest. I ascend slowly from 3.5 mtrs to the surface and on an smb do not find conditions up top effect my position in the water even when it is quite ruff as the Irish coast can be!! This works for me and I enjoy my diving and feel comfortable with heliox but I have not been able to draw a line in the sand on limits despite getting out of the water and talking like donald duck for several minutes without a twinge so I have come to the conclusion that all bends are earned either before, during or after a dive and the common ingredients are Nitrogen, Ascent rates and body temperature. I am not saying you can not get bent on Heliox but I do believe it is more forgiving than Trimix or Nitrox and a single inert gas for decompression seems a better choice for me to depths before HPNS becomes a consideration. Cheers Keep on believing
(Offline)

14th August 2005, 06:19	#30 (permalink)
wrecked Wrecked Current Rebreather/s: Sport Kiss Other Rebreather/s: Join Date: May 2005 Location: Lilydale, Vic Posts: 24	Quote: (Originally Posted by Mark Chase) Something that cropped up last year but has since vanished was the discussion on the Hyperbaric units ditching pure 02 for the treatment of bends and moving over to 50/50 Heliox for treatment?? It would be nice to get some more info on that. Yes 50/50 is used on more serious bends particulary He Bends
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