Computer help

[kadleck]

what does the CNS in oxygen toxicity actually stand for?

To add to bleeb's answer: when Kern stated "In fact other things come into play that keep you even safer from CNS", he missed one key word! He meant to say, "...CNS toxicity". Perhaps that will make a little more sense to you.

Incidentally, I dragged out the (PADI) Enriched Air Diving course book, and they do say the following, which reinforces the advice of your SUBEX instructors! Note that PADI are talking about nitrox at higher O2 levels than your 28%, too (the italics were added by me):

You can make use of this [less nitrogen] benefit in two ways; either by extending your dive times beyond those you could have with air, or by staying within the limits of an air-based computer or dive table and viewing the subsequent reduction in your exposure to nitrogen as an additional personal safety margin.

As you will be aware from my previous posts, I set my computer for the mix I am using. Many do not!

So apologies to SUBEX. I should have re-read the course notes earlier!

 

[buleetu]

quote stefo2 "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."

when i was diving here in april i was using a 10 litre tank with normal air, i was getting dive times of 37 or 40 minutes, the first dive i done this time round was with a 10 litre tank with 28%o2 and i got a 67 minute total dive time and have had dive times similar to this for every dive since the check dive, thats 27 more minutes and im ending all the dives this time with 40 - 60 bar left in the tank, on one occasion in april i had to get the instructors alternate and breath off that for the safetly stop because i let the tank go to 20 bar, thats a major differance, i was convinced it was the air i was breathing helping me to consume less air, why am i getting these long total dive times then>???


so if someone was to get a say 90% cns warning on there computer would there body pack in or something, what does cns toxicity do to someone??

 

[stefo2]

why am i getting these long total dive times then>???

Because you are getting more experience, are more relaxed, more properly weighted, have better trim, maybe better diving conditions etc. Nothing to do with the air you breathe.

 

[kadleck]

so if someone was to get a say 90% cns warning on there computer would there body pack in or something, what does cns toxicity do to someone??

CNS toxicity is extremely serious. Worse than 'the bends' in some ways (because DCI tends to show up some time after surfacing). CNS toxicity can cause convulsions (whilst still diving), which can lead to rapid death by drowning. Not something to be taken lightly.

 

[Kern]

so if someone was to get a say 90% cns warning on there computer would there body pack in or something, what does cns toxicity do to someone??

Like DCS, CNS toxicity isn't an exact science. Every individual will have a different response & each individual may have a variation in their response from day to day. Many use the 80% convention as an added safety margin. If you reach 90% of your O2 clock it's unlikely you'll instantly tox., but there are no guaranties. Some day's you may get away with 110%, on others you might tox. at a much lower percentage. As a rec. diver you would have to be doing a lot of really silly things to worry about O2 toxicity really.

 

[katdiver]

So you see, you've had an introduction to nitrox and the benefits and the hazards. The whole lot can be summed up in that we control our PPO2 (partial pressure oxygen) we breathe along with the nitrogen for our deco obligation. From these we obtain our MOD (maximum operating depth), oxygen exposure and deco.

So there is a MOD for air, as we have to consider the PPO2. Extend this further when you talk about Tec deep diving, to limit the PPO2 to safe levels, the mix of O2 in the breathing gas might actually be hypoxic at the surface. I think it's required to have around 16% O2 to maintain consiousness at the surface, but as you go deeper the increased PPO2 comes within "normal" levels. So a TEC diver will replace oxygen and nitrogen for helium to reduce the PPO2 at depth and N2 narcosis. Commercial divers will use Heliox (helium and oxygen). Of course this is very expensive.

 

[buleetu]

i read a peice in one of the local mags here (its called h20) about divers from years ago that were experimenting with the different gases to get more depth or to work underwater for longer times, its crazy how some of them died because the idea they had was a tiny bit flawed in some way, one of the divers was trying out his theory that 16%o2 at a depth of lets say 60 metres would be fine to sustain life (just like u descibe katdiver) as long as he switched the mix properly on the way down and on the way back up, his idea worked perfectly but there was a problem with his buddy and he convulsed or something and then the surface crew rushed the deco to get the hurt diver out of the water and then the person who was trying out his idea died at the surface from dcs, so his idea worked perfectly and all his work was great but because of an emergency he died through no fault of his own

on our boat all week there was a couple diving nitrox, i think it was 32% it was really cool the way they were totally in control of what they were doing on the boat, they decided the times they were in the water where they went underwater,everything, i think they had thousands of dives though because they knew all the dives sites and what was there, it was really cool the way they tested the bottles at the start of the day and then planned what depth they were going to and that, i would love to be able to do it one day

 

[kadleck]

... i would love to be able to do it one day

You will, sir; you will!
Just don't try to run before you can walk.

 

[buleetu]

hahah i know i know,
i actually led a dive yesterday though, my guide/slash guy who done my check dive last week, said i had excellent buoyancy control, a great buddy awareness/system, good air consumption, confidence in the water (haha ye right) and then he said i was a serious diver and that if i wanted i could go down with a buddy group unguided, he would let me because he thought i had the skills, most divers in eygpt have to be guided so people dont get hurt and damage the tourisim indusrty, i would imagine thats why anyway
but thats when the fun started haha the buddy "team" i ended up were crazy

your right though i will of course walk before i can run, but i think that with time and a ton of dives i can be quite a good diver, it just means not being afraid of the cold or perhaps buying a dry suit when i get back to ireland

 

[kadleck]

You will probably get away with a 7mm full wet suit in the warmer months - Ireland's water temperature is similar to England. Though you might want a dry suit if you want to extend the season.