Computer help

[emttim]

they say that the computer should stay at 21% for conservatisim, but then the amounts arent right at all are they because im breathing in more o2 arent i?? im diving mainly 1 dive a day but today 2 and then 1 again tomorrow, how deep is really deep and how long is really long

Honestly if you're doing 1-2 dives a day, I can't possibly see any way that you would even go near your limit of O2 exposure....that being said, the computer staying at 21% O2 when you're breathing 28% O2 is not conservative and they're FOS if they're saying it is. If anything, ignoring oxygen exposure when using a nitrox blend is liberal as all hell. As other people have mentioned, it's probably a very, very unlikely chance that you'll run into trouble, but again, I'd prefer to have accurate information on my computer to make decisions from instead of some dive op saying "trust me".

 

[buleetu]

ill tell them im unhappy with the 21% setting and why and see what there explanation is thanks guys ill get back to u in a bit

 

[buleetu]

hi guys i had a great dive today, the site was called the tower about half way from the naama bay jetty to ras mohammed i think, it has this deadly looking funnel area that drops to the depths and some great overhangs with soft corals and gorgonians

i told my instructor about my feelings on having the computer at 21% instead of 28 and he said its best to have it at 21 for conservatisim concerning, i said that i think it would be best if the computer knew what i was breathing o2 wise and he said that i wouldnt be doing the kind of depths that would make o2 toxicity a problem because it only begins to be a problem at 45m and as the deepest we go is 25m its nothing to worry about, he is a nitrox instructor so what do u think?? is this really too liberal an attitude

 

[kadleck]

Well, as has been stated many times in this thread, SUBEX are right in that the MOD for 28% is below the max depth for recreational diving. This is no doubt what drove them to only offer that mix.

The instructor is also correct, in that setting the computer for 21% is more conservative in terms of nitrogen exposure.

But if it was me, I would still set my computer for 28% nitrox if I was diving there. They can insist that I'm wrong all they like! I would just prefer that my computer was basing its calculations of what I'm actually breathing; even though I am never likely to be subject to oxygen toxicity. I would rather do that and end the dive based on the conservative figures of other computers, set for air than have my computer set for the incorrect mix.

It is interesting that SUBEX 'invented' air28, and in many ways I commend them for the great idea. But I still feel that their computer advice is flawed. If/when you do your nitrox class (which does not necessarily include any dives!), you will understand where I (and some others) are coming from.

 

[GSDMan]

he said its best to have it at 21 for conservatisim

This sounds to me like setting your watch 15 minutes ahead so you are never late. I never have understood that kind of logic. What's the point of carrying a computer at all if you're going to purposely set it incorrectly?

It all makes me wonder if there isn't some other reason for this. Let's look at it from a business standpoint. I suspect that, given that they likely only have the one gas, it doesn't cost them a whole lot more to bank EANx 28 than air. Computers set to air will end dives sooner assuming NDLs are the limiting factor. Short dives mean most tanks will only be partially depleted (which equals less total gas used, offsetting the more expensive gas mixture). Shorter dives on 28 EANx means shorter SIs. So, your 2 or 3 dives are over and done with that much sooner which means lower costs for personel, fuel, boat time, etc.

Being conservative is not a bad thing but when a business does it as a matter of policy it is likely going to benifit their bottom line, not your bottom time. (I made a funny)

Jim

 

[buleetu]

Being conservative is not a bad thing but when a business does it as a matter of policy it is likely going to benifit their bottom line, not your bottom time. (I made a funny)

hahah i like it

well i dont mind if it helps there bottom line at all, as long as im safe

i do understand what ur all saying but i have never even used a computer up until this trip so its all new to me, the last few days have been a massive learning curve

do you think i should just set the computer myself and to hell with what there telling me??

 

[Kern]

Being conservative is not a bad thing but when a business does it as a matter of policy it is likely going to benifit their bottom line, not your bottom time. (I made a funny)

hahah i like it

well i dont mind if it helps there bottom line at all, as long as im safe

i do understand what ur all saying but i have never even used a computer up until this trip so its all new to me, the last few days have been a massive learning curve

do you think i should just set the computer myself and to hell with what there telling me??

How's this, you're diving to no more than 25mts. That's an ambient pressure of 3.5 bar, maximum. The partial pressure of of O2 is a maximum of 0.98 bar.
FO2 (fraction of O2) X Ambient pressure = PPO2 (partial pressure of O2).

According to the NOAA CNS oxygen chart, with a PPO2 of 1 bar you have a single dive limit of 300 minutes. The daily limit is also 300 minutes. The convention I use is not to exceed 80% of my oxygen clock. That means I would be happy to dive EAN28 at 25mts to the NDL until I had accumulated 240 minutes of dive time for the day. That's 4 hours diving with a 20% safety margin left to avoid CNS oxygen toxicity.

Unless you are going to exceed 4 hours of diving a day, setting your computer to 21% O2 is quite conservative when it comes to you're N2 uptake, & you are not going to absorb enough O2 for CNS to be a problem.

In fact other things come into play that keep you even safer from CNS. O2 like N2 is washed out of our system during the surface interval. O2 has a 90 minute half life, so if I come up from a dive with 15% of my oxygen clock used up, after an hour surface interval it will reduce to 10%. We also have to take account of our N2 on & off gassing. We usually do this by doing dives in a repetitive series to ever shallower depths. If you have a look at the equation above it is obvious that this will decrease our PPO2 & so reduce our O2 uptake thus increasing the time it will take to reach the limit of our oxygen clock. There's more, square v's multy level profiles etc. etc., but that's more than enough.

I think it's great that you're thinking & not just following. It's a strange one really. I'd be happy to do what they say. I'v a bit more experiance & training than you so it wouldn't really be a trust me dive. Without that training & experiance I'd be asking all the questions you are & might not like doing a trust me dive. Catch 22 really. If you're not going to follow their instructions though, be upfront with them about it.

PS. have fun, catch 22 or not.

 

[buleetu]

i think that im gonna be fine when im diving with it set to 21, what kinda gets me about this shop is the fact that they took my card when i arrived so they know full well im an open water padi diver and am not trained in anything got to do with nitrox, the guy i done my check dive with didnt tell me anything about the gas at all, just that it was 28%o2 and it makes us have less nitrogen in our bodies there for longer dives, thats all, and then when my computer starts beeping that warning i got a bit of a shock, so i think they need to review the way they dive with newer people to the community, in fairness to them though they do kind of have a lot of more advanced divers with them this week so perhaps there just assuming were all the same, they are also an ssi centre so maybe they have some basic nitrox theory in there entry level course and they also assume that openwater divers have the same ??

what does the CNS in oxygen toxicity actually stand for?

and why doesnt everyone dive with 28% enax or even proper nitrox? is it the fact that theres a course and that before its safe to use maybe?
it is an awful lot better though, i done a dive today on normal 21% air with a different comapany just to try it out and i only got 38 minutes bottom time with a 12 litre tank, that is a big differance when ur paying more or less the same price for the dive

 

[stefo2]

it is an awful lot better though, i done a dive today on normal 21% air with a different comapany just to try it out and i only got 38 minutes bottom time with a 12 litre tank, that is a big differance when ur paying more or less the same price for the dive

Err - I think you are mixing something up here, or I am misundertanding you. Diving Nitrox doesn't give you longer bottom times by reducing your air consumption, but it extends your no decompression limit so that you can stay longer at a certain depth - if you have enough air.
At the kind of depths you are diving as a new open water diver, your dives probably will be ended by air supply (or shop / group regulations) rather than NDL.

If you ran low on air sooner on this dive today than usual, is was most likely because of the leaking BCD you mentioned in your other post, but had nothing to do with the gas mix you were breathing.

 

[bleeb]

what does the CNS in oxygen toxicity actually stand for?

Central Nervous System, i.e. brain and spine; excludes peripheral nerves; some definitions (all?) include cranial nerves as part of the CNS