Trim with 3 deco cylinders?

[stuartv]

I don’t know that the ease of blending of standard mixes has been mentioned in this thread at all. The gas input is directed at a selection of glasses that are grossly inappropriate for the dive outline as planned.

If the shoe doesn't fit anyone in this thread, then... it doesn't fit anyone in this thread, so nobody should be offended, right?

I think the observation about the gas density of 10/50 at 300' is a point fairly made. It is 7.4g/L. On open circuit. On what I expect to be a dive with a short amount of time at max depth and pretty much no work being done. Without knowing anything more about the dive plan than that, I'm not sure it is fair to classify it as GROSSLY inappropriate.

Ditto for the other gases. Personally, I don't use helium for dives to 150' or less. So, at first blush (i.e. before class has even started) I do not have an issue with a bottom mix with an END of 150. So, again, I'm not sure it's fairly labeled grossly inappropriate. Would I use a gas with that density or that END on my CCR? Absolutely not. On OC? I'm okay with it.

There is clearly a spectrum of tolerance for narcosis among even experienced tech instructors. Some won't condone a dive past 100' without helium. Some think it's fine to go to 180' on air. I feel reasonably safe being pretty well in the near-center of that spectrum. I've been to 180 on air in the same cold, dark lake where I'm going for training this weekend - and felt fine, had reasonable reaction times, and clear memories. I would not bother with helium in the vast majority of dives I could do down to 150. But, I'm not going to knock anyone that has determined that THEY want or need helium at shallower depths. I was helping to teach AN/DP/Helitrox weekend before last in Lake Jocassee. I got to see firsthand how narced some students got on air at 145'. I know it's real. I also know that it's something we all have to learn about for ourselves and determine what our own limits are. So, I don't judge others on that - unless possibly they get REALLY extreme. Thank you to everyone else who extends others the same courtesy.

 

[tbone1004]

Things took a typical SB Downturn from there...

at least the downturn hear is a LOT less bad than it is on facebook. You'd be crucified if you posted that on there.

also, important to note that the discussion of narcosis and END is heavily gas dependent. The 100ft rule for the DIR agencies has a lot more merit when looking at the max ppO2 and gas density since these are intended to be working dives. Also part of why DPV's are required at deep mix depths, so keep all of that in mind when you are diving with END/Density equivalent to 6ata's instead of 4.

 

[jale]

Hi
To all of you, don't forget the logistics side and the "psychology" of an Adv Tx class.
Even if the student does this kind of class for learning diving with hypo mixes, (s)he expects the "graduation dive" at 100m because that is just natural human nature
When I was teaching Adv Tx, if the student didn't fail the class, I was proposing, out of the official class, that dive and no one has ever refused.
Did I take people to 100m on 10/50 even if the gas density and the end are a bit off? Yes I did because 10/50 is a heliair easy to blend when you are in some "exotic" locations and that the "graduation dive" is basically a touch and go to get THE number on the computer and I did these dives in tropical warm clear waters. Will I do it again? Frankly, with a fresh minted diver, no (we didin't know about gas density before) and anyway RB is the way to go for these dives
So it is a bit tricky for us to "judge" what Stuart's instructor is going to do and I am sure that Stuart has enough knowledge and a good analytic sense to draw a line if he doesn't feel confortable.

 

[nadwidny]

Why is this in "advanced diving" and not "techincal diving"???

 

[cerich]

There is clearly a spectrum of tolerance for narcosis among even experienced tech instructors. Some won't condone a dive past 100' without helium. Some think it's fine to go to 180' on air. I feel reasonably safe being pretty well in the near-center of that spectrum. I've been to 180 on air in the same cold, dark lake where I'm going for training this weekend - and felt fine, had reasonable reaction times, and clear memories. I would not bother with helium in the vast majority of dives I could do down to 150. But, I'm not going to knock anyone that has determined that THEY want or need helium at shallower depths. I was helping to teach AN/DP/Helitrox weekend before last in Lake Jocassee. I got to see firsthand how narced some students got on air at 145'. I know it's real. I also know that it's something we all have to learn about for ourselves and determine what our own limits are. So, I don't judge others on that - unless possibly they get REALLY extreme. Thank you to everyone else who extends others the same courtesy.

with more task loading, many folks have more narcosis. I am more comfortable with high END's for low task loading dives (single set doubles/sm, single deco bottle) than I am more complex dives with 2-3plus stages/deco. YMMV.

As a trimix instructor, in the teaching environment, I see no upside to be at 200 plus feet with a student with each of us moderately impaired. I run shallow END's.

 

[kensuf]

OK, I agree with a lot of the comments here, but the approach is a bit harsh. Phrases like "grossly inappropriate" are a bit exaggerated, especially considering how common 10/50 has been (not saying it's correct, just saying it's not an uncommon gas). Let's try educating rather than simply being harsh.

Stuart, you may feel comfortable at 150' on OC, and I do too, but when diving deep there's several factors at play beyond simple narcosis. Time pressure is much more important at 300' than 150' -- the ability to solve problems quickly and efficiently is important because your RMV is 9x more than it is at the surface and your deco obligations are 4 minutes of deco for every minute farting around -- to be blunt, you don't have nearly as much time to deal with a simple issue at 300' as you do at 150', so you have to be 100% on your game when you're deep. Smooth and efficient are the keys to success, fumbling with a reel/spool because of a little impairment is not appropriate deep.

Personally, for a 300' dive, I want more helium in the mix for these very reasons. My PERSONAL opinion is my END's should become shallower the deeper I go simply because I *REALLY* want to be on my game when I'm deep. And, once you start squirting helium into the mix, the difference between 50% and 65% is not really that drastic in terms of cost, but it is dramatic in terms of utility.

 

[stuartv]

Thank you, @cerich and @kensuf!

You have both made some good points that totally make sense to me.

 

[stuartv]

Why is this in "advanced diving" and not "techincal diving"???

Fair question. I debated that before I posted.

It IS advanced diving. And, when I looked, there is not a "Technical Diving" subforum. There is a "Technical Diving Specialties" forum with subforums for Cave, etc..

My question is not specific to any tech diving specialty, so I had to make a judgment call and I decided to put it here. A moderator can move it and it certainly won't hurt my feelings.

 

[stuartv]

Hi
To all of you, don't forget the logistics side and the "psychology" of an Adv Tx class.
Even if the student does this kind of class for learning diving with hypo mixes, (s)he expects the "graduation dive" at 100m because that is just natural human nature
When I was teaching Adv Tx, if the student didn't fail the class, I was proposing, out of the official class, that dive and no one has ever refused.
Did I take people to 100m on 10/50 even if the gas density and the end are a bit off? Yes I did because 10/50 is a heliair easy to blend when you are in some "exotic" locations and that the "graduation dive" is basically a touch and go to get THE number on the computer and I did these dives in tropical warm clear waters. Will I do it again? Frankly, with a fresh minted diver, no (we didin't know about gas density before) and anyway RB is the way to go for these dives
So it is a bit tricky for us to "judge" what Stuart's instructor is going to do and I am sure that Stuart has enough knowledge and a good analytic sense to draw a line if he doesn't feel confortable.

FWIW, I don't think I have that issue of expecting a "graduation" dive to 100m. My instructor and I talked about whether we would dive to 330 and he explained how the dive site works. A wall that hits bottom at around 280 - 300, then the ground slopes away. So, to get to 330, we would have to swim away from the wall, following the bottom. I told him "no, thanks." I'm not interested in doing an actual swim at 300+ feet with 5 cylinders. Not at this early stage of my progress in hypoxic mix diving.

I have always maintained (to myself) that checkout dives for classes are not for "fun" and not for me to try and set any personal records. I just want to meet the requirements - fully - not (for example) hitting 101 in a class that requires a minimum of 100. But, I have no interest in trying to max the limits of a class while taking the class. My first time to 300' will be planned very conservatively (to the extent I can).

Don't get me wrong, though. If the wall went all the way down to 330, and the instructor said "let's go" and I already did some work-up dives, I would have no problem going to 330. I'm just saying that I have no problem NOT doing that dive, either. I want to dive. I like diving. But (despite how it may appear to some) I am in no rush to push limits. I mean, I need SOMETHING to look forward to, anyway, don't I?? LOL

 

[J-Vo]

I previously would have said the same as @stuartv about my narcosis at 150 not being problem (still wouldn't do it at 300' though). Suddenly as of last fall, I've been narced off my a$$ on every air/nitrox dive to those depths. You never know when this could happen to you. Also, at those depths the extra gas density is going to add CO2 on top of the N2 narcosis.

As for bottle handling, this is the class where I went from 2 to 4. I did some pre-class work on it, just like @stuartv , but I was not an expert going in.

Personally I would have been disappointed if my class dives where quick bounces just to say I got to depth. There is a big difference in bouncing to 300' and doing a "real" hypoxic dive where the first deco stop is below 100'. I felt confidant doing 300' dives without the instructor after my class.