stroke mix

[voidware]

I am not quite clear on this: Why is 80% referred to as stroke mix?

I can see the benefits of the wkpp's standardized gases and understand that it is not quite the best mix for 30ft. However, if one does not do deep dives requiring long decompressions (ie nitrox and your deco begins at 30 or 40) wouldn't it be more beneficial to spend more time decompression on the high ppo2 mix rather than back gas? why should you carry o2 if you can only use it until 20 ft and then you only spend maybe 10 minutes on it? (compared to a little more for 80) in fact, in playing with GAP you can get out of the water faster on such dives if you use 80% compared to o2. I am not tech trained, just interested, so if you spot problems please point them out.

thanks

brandon

 

[Lost Yooper]

Hey Brandon,

Here's a link with a dozen reasons or so why it's not recommended:

http://www.cisatlantic.com/trimix/gmism/gmisms.htm#Subject: WHY WE DO NOT USE 80/2

You have to scroll down and find the 80/20 section.

Mike

 

[Jonathan]

And when you get there read the next article as well.

IMHO (especially as I am just getting into the deco stuff) they both have pretty good arguments for and against.

I look forward to what the other tech heads have to say on this as I know I still have a lot to learn about this.

Jonathan

 

[Rick Murchison]

Originally posted by Lost Yooper
Hey Brandon,

Here's a link with a dozen reasons or so why it's not recommended:

http://www.cisatlantic.com/trimix/gmism/gmisms.htm#Subject: WHY WE DO NOT USE 80/2

You have to scroll down and find the 80/20 section.

Mike

Sorry Mike,
Since I have an old twist-on light, I started reading with the first thing that came up, and I can't get through all the insults and name calling to find the point - could you be so kind as to extract the infromation for us without the vitriol and present it in an emotionless, scientific fashion.
I'd greatly appreciate it.
Thanks,
Rick

 

[AquaTec]

Yooper
Thanks for the link
interesting reading, much more informative and scientific than this site. plus the people are well known respectable people in the diving comunity

From the home page How do I access the the pages you linked me to??

 

[Waterlover]

I prefer to have a 100% Gradient on deco vs. 80%. Doesn't matter if I can start at 30 with the 80%. I prefer to have no inert gas in my deco gas @20 and 10. Either gas requires good bouyancy control. IMO
Keneek:

 

[voidware]

of course, there is preference (which Full-DIR does not leave much room for, but, alas i digress).

response to the reasons:

1. If you are tech diving you should not have bouancy problems. If you do have problems, you are decoing on the anchor line or with an smb.

2. not a reason. high seas != diving

3. a possibility, but if everyone uses 80/20... and i don't think your deco would be successful trying to buddy breathe. and if you're only bringing one gas, standards are not a problem. Just breathe what you got.

4. point, but decoing in open water is not the same as a cave.

5. I pointed this out, but if it gets you out of the water quicker... (if it works use it, its better than using back gas)

6. what is this risk stuff? if you can't maintain depth, don't deco.

7. oxy clean is a myth (at least everyone on the board thinks so) o2 clean chicken anyone?

8. in my mind, i see lowered ppn2s as the purpose deco gas, not high ppo2s

To make matters worse, you can not get out from your 30 foot stop in an emergency (not doing the other stops) on the 80/20 mix without really risking a type 2 hit.

this may have a point, but it's the same with 50 or back gas, or even o2 at the beginning of your 20 stop.

9. sure your breathing improves with diving, but big, strong divers need more air than small ones. besides these dives won't have more than a few minutes deco.

10. I don't quite understand how they got this one, haldane deco shows that 80 at 30 will result in less time than 100 at 20.

11. sure having extra o2 at the surface is good, but there should be some on the boat if you are tech diving.

12. if you're going back in the water, maybe, but why are you going back in the water? go to the chamber and breathe the boats o2 until you get there.

now, before i get flamed (although, i don't see many flames here), i am not tech trained and i am only asking this in order to improve my understanding of deco. if you notice any problems in my logic, please point them out.

thanks,
brandon

 

[AquaTec]

Waterlover
Your point is well taken that high PPO2 offers a greater presure gradiant for off gassing all inert gasses from your system

But I also believe that a PPO2 of 1.6 is on the edge of CNS O2 Tox
To me this is a hard and fast rule you don't mess around with.
ON 100% at 1.6 you MOD equals 19.8 feet so at 20 you have allready exceded this limit even with perfect boyancy

What about air brakes when using 100%. I havent decoed on 100% in years but I remember 5 minute air brakes or something like that.

One conclusion I am comming to with all this discution about 100% etc. is that for deeper dives with a long stop at 15/10 feet which ever you preffer 100% would be very benificial

how about a 25 foot whip to a full face mask surface supplied with 100% from the boat. I think this would be very easy to accomplish. and add many benifits to your therories.

 

[jetblast00]

Originally posted by AquaTec

But I also believe that a PPO2 of 1.6 is on the edge of CNS O2 Tox
To me this is a hard and fast rule you don't mess around with.
ON 100% at 1.6 you MOD equals 19.8 feet so at 20 you have allready exceded this limit even with perfect boyancy

1.6 is a generaly agreed on limit by the diving community. Like tables and other 'rules' dealing with pysiology, they are not exact for any one person, they are (or rather, should be) conservative recomendations for the general whole.

I could be wrong, but I believe I read that the US Navy's published CNS Limit was a ppO2 of 2.0. (Navy seals are probably in better shape than average recreational divers though...)

 

[Waterlover]

Aquatec,
I have accepted the risk of the .2 of a foot on my deco putting me slightly over 1.6 po2. I prefer to have that 100% gradient. My choice. I'm not telling anyone else to do it. I do back gas breaks(forget the air)5 min out of every 20. I count the breaks as part of my deco. I won't use a FFM I have had problems with them in the past.
Ken