Tech diving - definition?

[EllieCee]

(24-11-2008 05:17 PM)aquafan Wrote:  For me I can't see that I would ever, unless my finances change use trimix/helium. I would look down the deep air dives.

I suppose it all depends on your Narcosis resistance. If you can do 55m without turning ga-ga, then it's certainly a much cheaper option!

Ellie

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As I obviously have nothing better to do at work, I've just been looking at the figures for deep air diving, and it seems that with a partial pressure for oxygen (PPO2) of 1.4 would give a depth limit of 56.6 metres or 66 metres at PPO2 of 1.6.

But then looking at the figures for Nitrogen, this would give PPN2 figures of 5.2 Bar & 6 Bar at the same depths. Which seems pretty high.

I was led to believe that modern thinking says that you should limit PPN2 to a maximum 4 - 4.5 bar which would give a limiting depth of between 40m and under 47m. So I can see the 55m air limit comes from the PPO2 figures, but shouldn't the PPN2 figures also be used?, or is this purely down to a tolerance thing?

Answers on a postcard please....

Ellie

[John_c]

(24-11-2008 05:13 PM)bottlefish Wrote:  Tell me about it, paying 5 cents a litre for helium over herethat eqautes to over 100 euros for just the helium for each dive on the Southwold

But Trimix/helium diving isn't mandatory in tech, many people stick to air/nitrox and keep it shallower then 55 metres....

Time for a YBOD.....

[bottlefish]

(24-11-2008 05:29 PM)EllieCee Wrote:  

(24-11-2008 05:17 PM)aquafan Wrote:  For me I can't see that I would ever, unless my finances change use trimix/helium. I would look down the deep air dives.

I suppose it all depends on your Narcosis resistance. If you can do 55m without turning ga-ga, then it's certainly a much cheaper option!

Ellie

---

As I obviously have nothing better to do at work, I've just been looking at the figures for deep air diving, and it seems that with a partial pressure for oxygen (PPO2) of 1.4 would give a depth limit of 56.6 metres or 66 metres at PPO2 of 1.6.

But then looking at the figures for Nitrogen, this would give PPN2 figures of 5.2 Bar & 6 Bar at the same depths. Which seems pretty high.

I was led to believe that modern thinking says that you should limit PPN2 to a maximum 4 - 4.5 bar which would give a limiting depth of between 40m and under 47m. So I can see the 55m air limit comes from the PPO2 figures, but shouldn't the PPN2 figures also be used?, or is this purely down to a tolerance thing?

Answers on a postcard please....

Ellie

I think it's more down to choice, trimix and the training is added expense, helium is a far less forgiving gas then nitrogen, so there are also additional risks, plus trimix is not as easy to get hold of as air or nitrox.

Personally I dont like diving air below 40 metres, it makes everything much harder to manage, especially with the added complication of the video camera - so anything below that, I'll be adding in a bit of helium to bring down the narcosis. I tend to work to a PN2 of between 3.16 and 3.95 (i.e equivalent to an air dive of between 30 and 40 metres), depending on how challenging the dive and conditions are.

GUE have a much shallower limit on air (30 metres if I remember correctly, NeilH will confirm), after which it's trimix, IANTD also push normoxic trimix above deep air, although they still offer both courses. So, yes the benefits of trimix on relatively shallower dives is being accepted, however it's certainly not the norm...

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(24-11-2008 06:20 PM)John_c Wrote:  

(24-11-2008 05:13 PM)bottlefish Wrote:  Tell me about it, paying 5 cents a litre for helium over herethat eqautes to over 100 euros for just the helium for each dive on the Southwold

But Trimix/helium diving isn't mandatory in tech, many people stick to air/nitrox and keep it shallower then 55 metres....

Time for a YBOD.....

Mine'll be a black one... one eye firmly on the Sentinel at the mo, although there's no way I can afford one for a while....

[Penguin]

(24-11-2008 05:29 PM)EllieCee Wrote:  

(24-11-2008 05:17 PM)aquafan Wrote:  For me I can't see that I would ever, unless my finances change use trimix/helium. I would look down the deep air dives.

I suppose it all depends on your Narcosis resistance. If you can do 55m without turning ga-ga, then it's certainly a much cheaper option!

Ellie

---

As I obviously have nothing better to do at work, I've just been looking at the figures for deep air diving, and it seems that with a partial pressure for oxygen (PPO2) of 1.4 would give a depth limit of 56.6 metres or 66 metres at PPO2 of 1.6.

But then looking at the figures for Nitrogen, this would give PPN2 figures of 5.2 Bar & 6 Bar at the same depths. Which seems pretty high.

I was led to believe that modern thinking says that you should limit PPN2 to a maximum 4 - 4.5 bar which would give a limiting depth of between 40m and under 47m. So I can see the 55m air limit comes from the PPO2 figures, but shouldn't the PPN2 figures also be used?, or is this purely down to a tolerance thing?

Answers on a postcard please....

Ellie

Limiting PPN2 comes down to narcosis, limiting PPO2 comes down to oxygen toxicity. Oxygen toxicity is worse, so it's really the limiting factor. It used to be that air was diluted with nitrogen to reduce O2 percentage and thus lower risk of toxing in deep commercial/military diving. Obviously helium doesn't have the narcotic effect and is prefered now. It does however have drawbacks too.

It is possible that lots of lovely nitrogen in the breathing gas reduces the susceptibility to oxygen toxicity, whereas replacing the nitrogen with helium increases susceptibility to O2 toxing. So in theory you can tolerate higher PPO2 on nitrox mixes than trimixes. If you are still bored. Hence the general recommendations to more strictly keep trimix or heliox PPO2s to a limit of 1.3-1.4 BAR.

[Suggsy]

GUE's limit for air is 0m.

[Penguin]

(24-11-2008 08:34 PM)Suggsy Wrote:  GUE's limit for air is 0m.

I understand a GUE Rec 1 qualification will allow the use of 21% nitrox...

[divingdoc1]

(24-11-2008 08:37 PM)Penguin Wrote:  

(24-11-2008 08:34 PM)Suggsy Wrote:  GUE's limit for air is 0m.

I understand a GUE Rec 1 qualification will allow the use of 21% nitrox...

no no no no you got the decimal point wrong, its 2.1% and must come in black

[Bobco]

(24-11-2008 08:37 PM)Penguin Wrote:  

(24-11-2008 08:34 PM)Suggsy Wrote:  GUE's limit for air is 0m.

I understand a GUE Rec 1 qualification will allow the use of 21% nitrox...

But only if you take it down in a bucket, and breath it through a straw

[John_c]

(24-11-2008 06:20 PM)John_c Wrote:  

(24-11-2008 05:13 PM)bottlefish Wrote:  Tell me about it, paying 5 cents a litre for helium over herethat eqautes to over 100 euros for just the helium for each dive on the Southwold

But Trimix/helium diving isn't mandatory in tech, many people stick to air/nitrox and keep it shallower then 55 metres....

Time for a YBOD.....

Mine'll be a black one... one eye firmly on the Sentinel at the mo, although there's no way I can afford one for a while....

Then you need to drool over this then m8 ...

http://www.vimeo.com/2287372

[aquafan]

What is the make of this new rebreathers? Is it a new reliable system?

(24-11-2008 10:06 PM)John_c Wrote:  

(24-11-2008 06:20 PM)John_c Wrote:  

(24-11-2008 05:13 PM)bottlefish Wrote:  Tell me about it, paying 5 cents a litre for helium over herethat eqautes to over 100 euros for just the helium for each dive on the Southwold

But Trimix/helium diving isn't mandatory in tech, many people stick to air/nitrox and keep it shallower then 55 metres....

Time for a YBOD.....

Mine'll be a black one... one eye firmly on the Sentinel at the mo, although there's no way I can afford one for a while....

Then you need to drool over this then m8 ...

http://www.vimeo.com/2287372