Gas limitations

[Mark Chase]

OK there are several threads I could start offering the DIR chaps a chance to educate me but folowing on from recent posts I would like to start with this.

When I plan a dive, I plan a 1.4 ish max PP02 at dive max depth. That being the sea bed i doubt if I will be there longer than it takes my ego to log it and the rest of the dive will be somthing like 1.3.

My travel gas has to provide a bail from the bottom option. This means that i will be within 10m of gas switch depth to travel from the top of the wreck

This invairably means that the piss gets taken out of me for taking two stages on a relitavly shallow 45m trimix dive but ridicule I can cope with death is harder for the kids to bear. On more serious stuff I will take a pony of somthing to get me to first stop depth.

One of the big problems I had with going DIR is they wont allow me to do this.

I was limited on back gas and limited on deco gas to the standard gas list provided by GUE. This lead to longer deco due to low pp02 on deeper dives and reduced bail out options.

Quote from JJ:

A diver will quickley see that a dive to 45m using 100% 02 and 50% Nitrox is more than suficient for efficient decompression, while adding 35% Nitrox to these has negligable impact on decompression but adds drag, weight and risk.

End quote

I would counter argue that a diver using 50% travel / deco has nothing to bail too at 45m


So my questions are:

1: Do DIR insist on the standard gas issue and if so what is the justification. I can see that standardising the mix across the teem is good but across ALL dives seems a little restrictive.

2: Why if standard mix is a must do, does JJ include a chapter in his training manual Beyound The Daylight Zone about air topping fills??


ATB

Mark Chase

[And]

<font color='#0000FF'>Wow, start with an easy one why don’t you. As I too have been contributing in the other post I’ll just let loose with how I interpret the DIR thing on gasses and the stuff I’ve learnt so far. If I’m wrong then I apologise and someone more knowledgeable may step in but I think I'm pretty close.

Originally Posted by [b

Quote[/b] ]1: Do DIR insist on the standard gas issue and if so what is the justification. I can see that standardising the mix across the teem is good but across ALL dives seems a little restrictive.

Why use standard gasses. So you get to know them, the mixing requirements and the deco curves, so you are comfortable using them, knowing they work for a range of depths. There is no custom deco table for each dive as you get to know the patterns and as such can rely less on computer programs. If you mix a special gas for a particular dive and then get blown out, then you may be tempted to use that gas on a dive where it is no longer optimal. Also, if you have specifically mixed a gas for a certain depth and that depth is wrong and you find yourself 5 metres above the wreck, do you drop down and go over 1.4 or call the dive?

JJ has this to say regarding standard mixes

Originally Posted by [b

Quote[/b] ]Many divers have been led to believe that great precision is necessary when calculating a “best mix” for a particular dive. In truth, such mixtures do not markedly improve a diver’s efficiency, but rather can place them at greater risk as they near individual limitations. In content, standard mixes allow divers to become familiar with a handful of gasses that provide them with very similar efficiency. This familiarity coupled with a more liberal safety margin make the following mixtures a wise choice for nearly any diving operation.

I can only guess the difference in deco you are experiencing is based on the helium content. The oxygen content in your depth ranges are 21, 18, and 15, there’s not much difference in the gaps. Buhlmann treats helium in a different manner than more current studies by NASA and by the research at WKPP shows. Whilst Buhlmann off gasses helium and nitrogen sequentially (ie adding to the deco), recent studies and the WKPP experience show that they are offgassed simultaneously, helium faster than nitrogen and so you only need to consider the nitrogen and the less of it the better.

JJ has this to say regarding 50% and deco

Originally Posted by [b

Quote[/b] ] All dive tables must include a table with the expected decompression, plus and minus at least 30% for depth and time. Divers should study these tables so they can evaluate the trend of the decompression should they need to extrapolate from it. Divers should also generate contingency tables to enable them to adjust their decompression schedule in the event of a lost or nonfunctional decompression bottle; they should be able to adjust their decompression with any combination of gasses. For example, a diver who loses their Oxygen bottle should be able to do the entire decompression on 50% Nitrox.

    Experienced divers are able to extrapolate from a memorized table to any reasonable schedule they may encounter. Skilled divers learn to make minimal use of written decompression tables; instead they focus on understanding how the tables are generated and invest time in understanding the decompression curve and how it is reshaped by alterations in the plan (lost gas or increased time/depth).

I can say with some confidence that we may disagree here. With a twinset and using the rule of thirds and diving with a buddy your bailout is your buddies supply. The DIR view is that a full on manifold failure is such a rarity that it is not worth planning for other than using your buddy. Team diving is central to the whole DIR philosophy.

As no bailout is required, the gas breathable at all depths is the backgas (There are some exceptions in deeper cave diving due to the ability to leave gas at the shallower part of the cave for later). Even the 15/55 mix has the same ppo2 as air at 4 mtrs so the need for a travel gas is negated.

Originally Posted by [b

Quote[/b] ]2: Why if standard mix is a must do, does JJ include a chapter in his training manual Beyound The Daylight Zone about air topping fills??

This applies to cave diving more than wreck. You remember when I talked about using the backgas as your bailout? You will still be using it for wing inflation. I do know that it is common for that backgas to be topped up as you won’t use that much on each dive. You are breathing the main gas from stage bottles and they can obviously be filled fresh each time. Bob will know more about this because he has done it for real, in France. I’m just nosy and have put Andy Kerslake through a number of intense inquisitions about cave diving

Phew, think that answers the main points. DIRQuest is a good source to find out more about deco and standard gasses.

Hope that helps

Andy

[Bob Cooper]

Originally Posted by [b

Quote[/b] (Mark Chase @ Aug. 29 2003,21:10)]OK there are several threads I could start offering the DIR chaps a chance to educate me but folowing on from recent posts I would like to start with this.

When I plan a dive, I plan a 1.4 ish max PP02 at dive max depth. That being the sea bed i doubt if I will be there longer than it takes my ego to log it and the rest of the dive will be somthing like 1.3.

My travel gas has to provide a bail from the bottom option. This means that i will be within 10m of gas switch depth to travel from the top of the wreck

This invairably means that the piss gets taken out of me for taking two stages on a relitavly shallow 45m trimix dive but ridicule I can cope with death is harder for the kids to bear. On more serious stuff I will take a pony of somthing to get me to first stop depth.

One of the big problems I had with going DIR is they wont allow me to do this.

I was limited on back gas and limited on deco gas to the standard gas list provided by GUE. This lead to longer deco due to low pp02 on deeper dives and reduced bail out options.

Quote from JJ:

A diver will quickley see that a dive to 45m using 100% 02 and 50% Nitrox is more than suficient for efficient decompression, while adding 35% Nitrox to these has negligable impact on decompression but adds drag, weight and risk.

End quote

I would counter argue that a diver using 50% travel / deco has nothing to bail too at 45m


So my questions are:

1: Do DIR insist on the standard gas issue and if so what is the justification. I can see that standardising the mix across the teem is good but across ALL dives seems a little restrictive.

2: Why if standard mix is a must do, does JJ include a chapter in his training manual Beyound The Daylight Zone about air topping fills??


ATB

Mark Chase

Originally Posted by [b

Quote[/b] ]I would counter argue that a diver using 50% travel / deco has nothing to bail too at 45m

As with most things DIR, we allow for two failures (with the exception of lights).  We have two first stages, two second stages, two methods of deploying an SMB etc. The third &quot;backup&quot; is our buddy.  So, if you are unfortunate enough to have two regulators fail, you have a buddy close by with a third.  If you have a deco gas failure you have a buddy with sufficient deco gas for you to use, or you can finish the deco on your backgas etc.  To answer your question, your backup gas at 45m is with your buddy.

Originally Posted by [b

Quote[/b] ]1: Do DIR insist on the standard gas issue and if so what is the justification. I can see that standardising the mix across the teem is good but across ALL dives seems a little restrictive.

Standard mixes mean everybody in the team have a uniform approach so we all do our gas switches at the same depth, we all plan the same bottom time, same ppO2 etc.  more important we can all share the gas we are breathing at *all* depths and at any time during the dive.

Originally Posted by [b

Quote[/b] ]2: Why if standard mix is a must do, does JJ include a chapter in his training manual Beyound The Daylight Zone about air topping fills??

You can successfully air top a fill and still get a useful mix.  The important thing about being in a team situation is that your buddy's gas must be breathable by you, so it figures that the gas must be the same (within reason)

I don't really understand why you are needing a travel gas and a bail out gas?  Although I suspect this might have something to do with solo diving?  But of course, we don't do that ;-)

Bob

[Mark Chase]

Excelent and well argued replies both, and I can now see the logic of the aproach.

Bob, I admit openly that I like to be a self contained unit on a dive. Whilst I might have a buddy with me and he or she may be totaly compitent, I like to have MY own bail out options. Call me paranoid. The up side to this is I am able to go solo at short notice as I did on my last Tmix trip, and not be taking a huge risk. The only aspect of my dive plan that has to change for a solo dive is limitations on depth and type of wreck penitration.

Whilst I could go on about the deco comitment of the standard gas mix at the shalow end if its depth range, I wont as I can see the off set benifit of standard deco profiles to go with the depths and pre set gas mix.

ATB

Mark Chase

[Lanny]

Hi Mark,

Is losing your buddy, plus having both regulators or tank valves fail simulateously, a realistic scenario? I would suggest not. Even if you lose a first stage/hose/tank valve, you will still have enough backgas to get to your first switch depth, unless you are seriously slow in conducting a shutdown.

Incidentally, if you do switch to your (frankly unneccessary) pony of air at depth, the effect will be a sudden and immediate onset of narcosis in an already high stress and high task loading situation - not a situation I would want to be in.

I really can't see any reasonable justification or any advantages whatsoever for an additional pony as bail out. &nbsp;It is also a massive line trap on the side of your twins. &nbsp;I don't wish to be overly critical but am trying to understand why you are doing what you are doing.

Regards,

[Tibbs]

It's the hit of air at 65m that quite frankly concerns me.

If you must carry a pony of gas, make it 10/70. At least then at any depth you will have a completely clear head to deal with any problems.

When I was going through my 'deep air' phase I had an arguement with a guy who dived mix. I didn't think air impaired me at 50m and he did. He took me down to 45m breathing backgas of 21/35 and carrying a stage of air. I breathed the mix on the way down and switched to the stage at 45m and it was like being hit by a train. Or like drinking half a bottle of tequila on an empty stomach and being asked to drive home. It really, REALLY scared me!

In a situation where you are likely to want to switch to your pony (ie a highly stressful one) the very last thing you want is to suffer a massive hit of narcosis. So please if you do have to carry it, and Lanny has made a good case for not, fill it with a non narcotic mix. There have been far too many deep air deaths already - don't be another one...

Chris

[Mark Chase]

Originally Posted by [b

Quote[/b] (Lanny @ Sep. 01 2003,00:15)]Hi Mark,

Is losing your buddy, plus having both regulators or tank valves fail simulateously, a realistic scenario? I would suggest not. Even if you lose a first stage/hose/tank valve, you will still have enough backgas to get to your first switch depth, unless you are seriously slow in conducting a shutdown.

Incidentally, if you do switch to your (frankly unneccessary) pony of air at depth, the effect will be a sudden and immediate onset of narcosis in an already high stress and high task loading situation - not a situation I would want to be in.

I really can't see any reasonable justification or any advantages whatsoever for an additional pony as bail out.  It is also a massive line trap on the side of your twins.  I don't wish to be overly critical but am trying to understand why you are doing what you are doing.

Regards,

The scinario would be more like big failure, look up and cant find buddy in the 2m viz coz he / she has finned off oblivious to the problem and is now a masive 5m away.

The failurs that you have outlined are not the ones I was covering my ass on to be honist. Total failure of the twin set is very unlikley and I can do very quick shut downs thanks to my slob knob

The situation would be more along the lines off: la de dar de dar isnt this a fantastic wreck. S#IT I have run out of gas. Or a free flow right at the end of the dive which dispite the shut down has not left me enough gas to get to first stop depth.

Remember that we plan to run down well below 100bar before leaving the bottom on 60+m dives. 100 bar is not a lot of gas at that depth.

Line trap on the pony &nbsp; What? worse than the two stages, manifolded twin set, reels canister tourch etc, etc. Dont make me laugh. Trimix rig is one BIG line trap. Never had a snag on the invereted pony yet but it would be an easy cut free if I did. If your that worried side sling it.

I have dived deep air to 70m so I am aware of the narcotic effect but it is not the kick in the face on the gas switch that is described and if you'r expecting the buzz its not too dificult to handle. Remember that you will be on an ever decreasing hit as ascent will be imediate. I have gas switched to 32% at 40m on all my 60+m dives without even noticing a narcosis hit. These dives would all have been in the 60m or less range at the point in the dive where gas loss might be an issue, so I chose air. Any deeper and I would chose mix.

The stress issue is another matter. If I sucked a tank dry I would not be stressed as I have a redundent supply and a well drilled bail out routeen. It would merrit a quick swear word but that is about it. &nbsp;

A diver on a 65m dive died having run out of gas. He found a buddy and got on the long hose but he was so stressed that he depleated the buddys supply of gas before the gas switch depth. Having sliped into self preservation mode the would be rescue diver bolted for the switch depth &nbsp;switched gas and looked round to find the other diver gone. The body has not been recovered.

This is a good example of human nature.

I like to think that the buddies I trust to dive with me at these depths are as 'with the program' as they appear but I am not going to rely on it.

ATB

Mark Chase

[Andy Phillips]

Hi,

Just want to add a bit to Marks comments above.

Mark and I have discussed this. We have dived together. The basic dive plan will be to dive the same gasses, time and indeed direction. We intend to be there for each other if the sh*t hits the fan, but, both of us agree whilst that is the case we are diving solo in the same place. No matter how well drilled or prepared you might be, it takes a rare individual who is prepared to risk his own life to save yours.
To carry one extra cylinder and allow yourself to be free of this dependance on another persons reaction seems like a stupid question to me. It is just simple logic. A safe option, and means I can add more nice warm argon to my suit at 6M.

Just an opinion,

Andrew

[Lanny]

Originally Posted by [b

Quote[/b] ]The situation would be more along the lines off: la de dar de dar isnt this a fantastic wreck. S#IT I have run out of gas.

What?  If you are incapable of monitoring your gas supply then you should not be diving and definitely should not be technical diving.  Looking at your depth gauge regularly is BSAC OD/PADI OW stuff.  If you are on deep air or on a too lean mix then the narcosis may impair your ability to remember to look at your gauges, so don't dive deep air and dive the right mix for the depth.

Originally Posted by [b

Quote[/b] ]Or a free flow right at the end of the dive which dispite the shut down has not left me enough gas to get to first stop depth.

Again, if you are diving thirds and your shutdowns are rapid enough, then you should have enough gas to get you to your first gas switch, without even counting your buddy's gas supply.  Even if you lose a lot of gas, I would not have a massive problem with busting some of my deep stops and extending my stops above 21m, in particular my 6m O2 stop.  This is where your ingrained knowledge of deco (or the stuff written in your wet notes!) pays dividends.

Originally Posted by [b

Quote[/b] ]
Line trap on the pony   What? worse than the two stages, manifolded twin set, reels canister tourch etc, etc. Dont make me laugh. Trimix rig is one BIG line trap.

The difference being that your twin set does not extend out beyond the width of your shoulders and your canister torch should be tucked away out of the slipstream under your right arm.  If I carry a reel it is on my butt D-ring and my spools are in my drysuit pockets - all out the way and not a line trap.  Deco or stage bottles are right in front of you and easily adjusted or disentangled, unlike a pony on the side of your twins, which is not.

Originally Posted by [b

Quote[/b] ]A diver on a 65m dive died having run out of gas. He found a buddy and got on the long hose but he was so stressed that he depleated the buddys supply of gas before the gas switch depth. Having sliped into self preservation mode the would be rescue diver bolted for the switch depth  switched gas and looked round to find the other diver gone. The body has not been recovered.

This is a good example of human nature.

This is a good example of piss-poor diving practices having tragic but easily avoidable consequences.

Safe diving,

[Mark Chase]

Originally Posted by [b

Quote[/b] (Lanny @ Sep. 02 2003,12:07)]

Originally Posted by [b

Quote[/b] ]If you are incapable of monitoring your gas supply then you should not be diving and definitely should not be technical diving. &nbsp;Looking at your depth gauge regularly is BSAC OD/PADI OW stuff. &nbsp;If you are on deep air or on a too lean mix then the narcosis may impair your ability to remember to look at your gauges, so don't dive deep air and dive the right mix for the depth.

Oh no not the 'I am perfict and never make a mistake aproach to diving '

Sorry, I am not perfict and do make mistakes.

Originally Posted by [b

Quote[/b] ]Again, if you are diving thirds and your shutdowns are rapid enough, then you should have enough gas to get you to your first gas switch, without even counting your buddy's gas supply. &nbsp;Even if you lose a lot of gas, I would not have a massive problem with busting some of my deep stops and extending my stops above 21m, in particular my 6m O2 stop. &nbsp;This is where your ingrained knowledge of deco (or the stuff written in your wet notes!) pays dividends.

I dont dive wreck dives on rule of thirds and neither do any of the other divers in my group. Bottom mix is generaly used up on the bottom. Redundancy is usualy carried in additional deco gas and or travel gas and drop tanks.

A rule of thirds dive to 70m would limit me to 15-20mins on the bottom on a £200 dive. Blow that for a game of tennis.
Rule of thirds is overhead environment stuff.

Originally Posted by [b

Quote[/b] ]The difference being that your twin set does not extend out beyond the width of your shoulders and your canister torch should be tucked away out of the slipstream under your right arm. &nbsp;If I carry a reel it is on my butt D-ring and my spools are in my drysuit pockets - all out the way and not a line trap. &nbsp;Deco or stage bottles are right in front of you and easily adjusted or disentangled, unlike a pony on the side of your twins, which is not.

Like I said if you are worried about it side sling it with the other stage bottles. I have never had a problem with it so i prefer to have it out of the way where it can not be confused with a stage.

Originally Posted by [b

Quote[/b] ]This is a good example of piss-poor diving practices having tragic but easily avoidable consequences.

I quite agree but our individual choice of how to avoid it are diferent. I chose a system that is independent and totaly in MY control, and you chose a system which places trust in your buddy.

Your faith in human nature is far greater than my own. I have seen people do things way out of charictor in high stress situations and I have seen people turn into jibbering wrecks or freze when it hits the fan. You can never tell what a person would actualy do untill that person is subjected to the realiaty of the event.

If my buddy on the day it hits the fan turns out to be profesionisum its self I will be plesently surprised. If he /she turns out to be a jibbering wreck I will be alive to tell the tail.

ATB

Mark Chase