My little bend.................again

[Nick_lincoln]

Originally Posted by Steve S

The Dive – I would like to say that there was something of a smoking gun found in the dive that could be attributed to my subsequent problem but none could be found. The dive itself was conducted on a 33.7% nitrox mix with my alladin Pro set to 32%. The max depth of the dive was at 31.7 metres. With the majority of the bottom time at 29-30 metres. After 31 minutes we slowly ascended and took 9 minutes to reach our stop depth. By this time I only had 3 minutes of mandatory deco. I padded it out to 10 minutes; I then surfaced with a total dive time of 50 minutes.

Got to admit, after reading this dive profile in a thread about getting a bend I went and double checked the thread title! Seems highly unlikely that a bend could occur from this dive, but it did.

If I was asked to assess any risk from this dive it would be an underestimation of CNS / UPTD O2 loading from breathing a 33.7% nitrox mix with a computer thinking its a 32% mix. I assume this is because the Aladin uses pre-set nitrox mixes? and you cant increase or decrease the O2 amount in 1% increments?

Anyway, glad you are safe and back in the water.

Nick

[GLOC]

Originally Posted by Nick_lincoln

If I was asked to assess any risk from this dive it would be an underestimation of CNS / UPTD O2 loading from breathing a 33.7% nitrox mix with a computer thinking its a 32% mix. I assume this is because the Aladin uses pre-set nitrox mixes? and you cant increase or decrease the O2 amount in 1% increments?

Nick, I just ran a 50min BT-30m dive through Decoplanner (because I couldn't really simulate Steve's profile) with 32% and 34% mixes and there is 3 %CNS difference (29 to 32) between the dives. I very much doubt that this would be any contributory factor.

Unfortunately, I think it might be one of those things that happens. There has been a case recently of a very experienced GUE trained diver repeating a dive which he had done many times before without issue and ending up in hospital unconscious with possible brain damage from DCI. Fortunately, it appears he has made a full recovery with no apparent memory loss (short, med & long) and there does not seem to be a reason why he got hit.

Regards

[Nick_lincoln]

Originally Posted by GLOC

Nick, I just ran a 50min BT-30m dive through Decoplanner (because I couldn't really simulate Steve's profile) with 32% and 34% mixes and there is 3 %CNS difference (29 to 32) between the dives. I very much doubt that this would be any contributory factor.

Unfortunately, I think it might be one of those things that happens. There has been a case recently of a very experienced GUE trained diver repeating a dive which he had done many times before without issue and ending up in hospital unconscious with possible brain damage from DCI. Fortunately, it appears he has made a full recovery with no apparent memory loss (short, med & long) and there does not seem to be a reason why he got hit.

Regards

Definately not a contributary factor to the DCI hit that Steve got - I was making a point that the risk (although minor) from this dive appears to be from excess O2 due to underestimation and not excess N - which of course is the main contributory factor to DCI. This is why I was surprised to find that a DCI hit had occurred.

I agree, not a huge risk of O2 tox from that dive alone, but if done on a day with several repetative dives on nitrox, it could lead to a a larger margin of error if consistently telling your CPU you are on a slightly less rich mix than you are actaully breathing. Especially if the diver is doing deco on a rich O2 mix.

[Steve S]

Originally Posted by Nick_lincoln

Got to admit, after reading this dive profile in a thread about getting a bend I went and double checked the thread title! Seems highly unlikely that a bend could occur from this dive, but it did.

If I was asked to assess any risk from this dive it would be an underestimation of CNS / UPTD O2 loading from breathing a 33.7% nitrox mix with a computer thinking its a 32% mix. I assume this is because the Aladin uses pre-set nitrox mixes? and you cant increase or decrease the O2 amount in 1% increments?

Anyway, glad you are safe and back in the water.

Nick

Hi Nick,

Thanks for that and that's why I posted it in the hope that it may make a diver realise that you can get bent from even the most innocuous dive, and to not assume as I did initially that I couldn't possibly be bent from such a dive.

You are quite correct that on paper the risk would be from the computer underestimating 02 loading. That said I track my CNS/UPTD loading from my v planner beforehand and don't really use my pro to do that.

The pro can be set to individual mixes, but I choose 32 as my buddy was diving 32% that day.


Safe diving,
Steve

[Nick_lincoln]

Originally Posted by Steve S

Hi Nick,

Thanks for that and that's why I posted it in the hope that it may make a diver realise that you can get bent from even the most innocuous dive, and to not assume as I did initially that I couldn't possibly be bent from such a dive.

You are quite correct that on paper the risk would be from the computer underestimating 02 loading. That said I track my CNS/UPTD loading from my v planner beforehand and don't really use my pro to do that.

The pro can be set to individual mixes, but I choose 32 as my buddy was diving 32% that day.


Safe diving,
Steve

A good thread mate - we have the theory rammed into us that diving nitrox considerably reduces DCI risk, your case proves that this is not always true. I am new to Nitrox diving (Qualififed Jan 07') and can honestly say this thread has convinced me to be as careful with managing DCI risk on nitrox dives as I am on air dives. And that my plan to use 50% mix in a deco cylinder is definately a good idea.

Nick

[Steve S]

Originally Posted by Nick_lincoln

A good thread mate - we have the theory rammed into us that diving nitrox considerably reduces DCI risk, your case proves that this is not always true. I am new to Nitrox diving (Qualififed Jan 07') and can honestly say this thread has convinced me to be as careful with managing DCI risk on nitrox dives as I am on air dives. And that my plan to use 50% mix in a deco cylinder is definately a good idea.

Nick

Yep, the only way to 100% avoid DCS is not to dive, all you can do is stack the odds in your favour.

Safe diving,
Steve

[Mal Bridgeman]

Originally Posted by Nick_lincoln

I am new to Nitrox diving (Qualififed Jan 07') ........my plan to use 50% mix in a deco cylinder is definately a good idea.

Nick

What ? Now ? or some time in the future when you've been trained?
Mal

[Nick_lincoln]

Originally Posted by Mal Bridgeman

What ? Now ? or some time in the future when you've been trained?
Mal

I am trained.......... I passed the theory assessment - got one answer wrong - which wasn't one of the calculation questions. Then I passed the practical assessment on the second day. BSAC Advanced nitrox diver qualification allows divers to obtain up to a 50% nitrox fill.

My plan is to side sling a 3L pony with 50% and use this during deco' on air dives or even during mandatory safety stops on air dives - but following the deco requirements as if I was on air - this builds in an extra safety margin. 50% mix has an MOD of 18m, I use a vytec DS and usually use deep stops so for extra O2 safety I plan to use the 50% mix from 15m upwards.

Depending on the depth / mix used I may use the 50% mix on certain nitrox dives for deco / safety stops also to build in an extra safety margin. But this will depend on O2 loading throughout the day.

This practice was taught on my Adv' Nitrox course and then also independently recommended by a dive leader in my club and an Adv' nitrox instructor in my club -NOT the instructor who taught me, these are buddies I dive with.

I calculate my CNS and UPTD loads after each dive using BSAC tables to make sure the 50% mix isn't taking me too close to the daily max.

I plan to use this set up with twin 12's in Weymouth next month. Why should I not use a technique that I've been trained to do just because I'm new to it??

If this method is considered un-safe please tell me, I am open to all opinions, thats what this forum is about for me - to learn.

Nick

P.S. - Steve sorry to hi-jack your thread.

[Mal Bridgeman]

Originally Posted by Nick_lincoln

I am trained.......... I passed the theory assessment - got one answer wrong - which wasn't one of the calculation questions. Then I passed the practical assessment on the second day. BSAC Advanced nitrox diver qualification allows divers to obtain up to a 50% nitrox fill.

My plan is to side sling a 3L pony with 50% and use this during deco' on air dives or even during mandatory safety stops on air dives - but following the deco requirements as if I was on air - this builds in an extra safety margin. 50% mix has an MOD of 18m, I use a vytec DS and usually use deep stops so for extra O2 safety I plan to use the 50% mix from 15m upwards.

Depending on the depth / mix used I may use the 50% mix on certain nitrox dives for deco / safety stops also to build in an extra safety margin. But this will depend on O2 loading throughout the day.

This practice was taught on my Adv' Nitrox course and then also independently recommended by a dive leader in my club and an Adv' nitrox instructor in my club -NOT the instructor who taught me, these are buddies I dive with.

I calculate my CNS and UPTD loads after each dive using BSAC tables to make sure the 50% mix isn't taking me too close to the daily max.

I plan to use this set up with twin 12's in Weymouth next month. Why should I not use a technique that I've been trained to do just because I'm new to it??

If this method is considered un-safe please tell me, I am open to all opinions, thats what this forum is about for me - to learn.

Nick

P.S. - Steve sorry to hi-jack your thread.

Nick...when you said you were new to Nitrox I made an assumption that you had done the Basic Nitrox course and were now proposing to jump straight into deco'ing on 50%. I apologise.

I know little of BSAC training so could not comment about its safety.

Rgrds
Mal

[Nick_lincoln]

Originally Posted by Mal Bridgeman

Nick...when you said you were new to Nitrox I made an assumption that you had done the Basic Nitrox course and were now proposing to jump straight into deco'ing on 50%. I apologise.

I know little of BSAC training so could not comment about its safety.

Rgrds
Mal

No worries dude . I thought you were going to reply with some complex new theory about deco on rich O2 mixes and how I was gonna get myself killed!!

I intend to write a trip report about Weymouth for all on YD to read, I'll put a bit in that about this deco' / safety stop method. If its good or bad I'll include a thread in the 'I learned about diving from that...' too.

Nick