No Technical Training for Me.

[Charlie99]

I'll have to pay closer attention the next time I'm out in sloppy weather, but my recollection is that swells have little effect of measured depth/pressure at safety stop depths, while waves, particuarly very short period sharp fronted ones, do.

OTOH, if I'm deeper and stationary with respect to the bottom, it's just the opposite --- waves don't have any effect and swells do.


By swell I mean the nice smooth sinusoidal shaped patterns that come from far away storms. By waves I mean the shorter period things are are in response to the last few minutes or hours of wind. Neither waves nor swells are the same as the surf or shorebreak that occurs when waves and swells hit shallower water.

 

[MikeFerrara]

I'm not sure how that data will be useful because your inherent errors are not predictable and thus it will be difficult if not impossible to decipher what is a technique issue and what is a wave-related change in pressure. Maybe you have thought of a way to correct for that, but I can't think of any. I'm no engineer, though.

The right way to do it is to use a gauge R&R or other guage study method. But, since no one is going to inspect his records to make sure that his gauges were in "calibration" at the time of the study, he can just informally conpare the results he gets in rough water to the results he gets in calm water.

One of those dive loggers that was posted about a while ago has a pretty fast sampling rate and lots of memory. I'd use one of those and then the only real question is how well you dod holding your stop.

If you riged up a fixture such that the guage depth was referenced to the bottom (distance) and fixed in place, you could remove that variable as well....like maybe take a depth finder xducer, put it on a long cable and hold it with the pressure gauge...just thinking on my key board but using the depth finder and a digital data logger, you can pretty much take all the human variability out of it.

 

[wedivebc]

I've never actually measured it - I've always subscribed to the theory that if I can feel the depth (pressure) change in my ears then it's enough to be "less than optimum" - and to consider being deeper for a good deco stop.
But now that it's come up, I'll make a gauge and test it out the next time I can get offshore.
Rick

You know what I admire about you Rick. You have, on several occasions, stepped away from the keyboard and tried to verify a theory or idea in the water. I wish more internet divers were as willing to wet test their ideas.

 

[Soggy]

You know what I admire about you Rick. You have, on several occasions, stepped away from the keyboard and tried to verify a theory or idea in the water. I wish more internet divers were as willing to wet test their ideas.

Yes, Rick's testing of gas mixing within a set of isolated doubles has changed my analysis procedures. I now always check from the left post when picking up the tanks, and the right post before diving, in addition to checking the isolator both before and after dropping off.

 

[Mike Edmonston]

Is someone here arguing that poor buoyancy skills is a reason to use 80%? I seem to have missed that. Could you kindly cite the post?
Or are you questioning the physics that it takes more deviation in depth on 80% (and more still on 50%) to get the same change in PO₂ that you get on 100%? That physical fact - that the lower the oxygen percentage of a deco mix the more forgiving it is of depth deviations w/r/t PO₂ changes - doesn't say anything about buoyancy skills requirements or desires. Indeed, as decompression offgassing is mostly (almost entirely) a function of PN₂ gradient rather than PO₂, and bubble growth threshhold a function of dissolved gas pressures (or micro-bubble interior pressures, depending on which theory you want to use) and ambient pressure, and as decompression algorithms are formulated with an expectation of tight depth control during both ascents and at deco stops, any supposed benefit in the reduced sensitiity of PO₂ to depth changes w/r/t the overall decompression problem is a red herring.
As a matter of fact, I have never, ever heard the "poor buoyancy skills" argument mentioned by anyone in support of using a lowered oxygen content in a deco gas, other than those who claim it is someone else's excuse for using 80% vice 100%. Never. If you can show me the "someone else" by name, or a credible source (agency document or scientific study) that makes such a claim I'd like to see it. The only proponents of "80% as a cover for poor buoyancy skills" that I can think of right off hand are you and GI3
Rick

OK guys, I think this thread is getting a tad off topic.

I would like to clear up my intentions in my original post.

1. my original comment was in response to an instructor not being competent because he uses 80% DECO GAS. I use 80% in my classes for many reasons, not just because of buoyancy issues.

When I mentioned Buoyancy issues, I was referring to a student that is taskloaded, and LEARNING to do things right. he has proper buoancy when he's in his training comfort zone, but during new drills, with new skills, he might not be paying complete attention to everything that is going on PERIOD! We as instructors are there to teach them the proper problem solving methods, in order to get them to that controlled focus that is required in technical diving.

Therefore 1 OF THE REASONS I like 80% is to give us a little more buffer zone. That's the way I teach, and I'm not violating standards. There is no agency specific doctrine that states that I must use 80% if swells are over 10" but less than 24" etc... I just choose to teach that way. I may be incorrect about the swells / pressure / depth argument, but is it wrong to err on the side of caution?

2. Another reason I use 80% is that I don't like to short fill a cylinder 2000 vs 3000 when I need as much gas as I can get for a full weekend of diving. We are frequently on Islands or expeditions that don't have o2 available, and I don't want to drag a Haskel on a jungle hop!

3. I will commonly have 2 or 3 divers in the water sharing a DECO bottle of the anchor line. Therefore I need the full 3000 fill, if I expect that tank to last the weekend. I also fill the individual ponies that the divers each carry with 80%. I could have 100% on the anchor line bottle, but that would mean that each diver would need a dive computer with a minimum of 3 gas capability. Not all divers have VR3's and down here, alot of divers really like the Dive Rite Nitek series of computers. It's 2 gas only. hopefully that explains it.

3. Cost. it is cheaper to dive 80% as a DECO gas than it is to top off 100%. Flame me if you want, but the o2 lasts longer, I can charge my divers a little less for it, and it saves me a trip or 2 a year to the fillers.

My original thought was not to slam instructors. PERIOD! as there is plenty of missinformation and not enough facts.

Thanks for listening

Cheers

Mike

 

[wedivebc]

Yes, Rick's testing of gas mixing within a set of isolated doubles has changed my analysis procedures. I now always check from the left post when picking up the tanks, and the right post before diving, in addition to checking the isolator both before and after dropping off.

OK so he didn't need to get wet for that one but I agree it was an eye opener.

 

[wedivebc]

OK guys, I think this thread is getting a tad off topic.

I would like to clear up my intentions in my original post.

1. my original comment was in response to an instructor not being competent because he uses 80% DECO GAS. I use 80% in my classes for many reasons, not just because of buoyancy issues.

When I mentioned Buoyancy issues, I was referring to a student that is taskloaded, and LEARNING to do things right. he has proper buoancy when he's in his training comfort zone, but during new drills, with new skills, he might not be paying complete attention to everything that is going on PERIOD! We as instructors are there to teach them the proper problem solving methods, in order to get them to that controlled focus that is required in technical diving.

Therefore 1 OF THE REASONS I like 80% is to give us a little more buffer zone. That's the way I teach, and I'm not violating standards. There is no agency specific doctrine that states that I must use 80% if swells are over 10" but less than 24" etc... I just choose to teach that way. I may be incorrect about the swells / pressure / depth argument, but is it wrong to err on the side of caution?

2. Another reason I use 80% is that I don't like to short fill a cylinder 2000 vs 3000 when I need as much gas as I can get for a full weekend of diving. We are frequently on Islands or expeditions that don't have o2 available, and I don't want to drag a Haskel on a jungle hop!

3. I will commonly have 2 or 3 divers in the water sharing a DECO bottle of the anchor line. Therefore I need the full 3000 fill, if I expect that tank to last the weekend. I also fill the individual ponies that the divers each carry with 80%. I could have 100% on the anchor line bottle, but that would mean that each diver would need a dive computer with a minimum of 3 gas capability. Not all divers have VR3's and down here, alot of divers really like the Dive Rite Nitek series of computers. It's 2 gas only. hopefully that explains it.

3. Cost. it is cheaper to dive 80% as a DECO gas than it is to top off 100%. Flame me if you want, but the o2 lasts longer, I can charge my divers a little less for it, and it saves me a trip or 2 a year to the fillers.

My original thought was not to slam instructors. PERIOD! as there is plenty of missinformation and not enough facts.

Thanks for listening

Cheers

Mike

I agree with you that there is a time and place for 80% and an instuctor does not have to be a complete idiot to use said gas. I also agree that poor bouyancy is a lame excuse for that gas choice although even though I can't find the exact reference I do recall Tom Mount claiming that as one reason for his selection with students, and I believe that was the impetus behind GI3s infamous "bakers dozen" rant.
I really wish Tom hadn't said that because it has stuck in the craw of every kool-aid drinker since and there are some good reasons outlined above but sorry, bouyancy is not a really good one.:no
I have played with plenty of profile/gas combinations on v-planner when planning a certain dive and sometimes 80% gets me out of the water a little quicker than O2. Sometimes I need a full 40cuft of gas to complete my deco and I don't feel like carrying an 80cuft so a full 3000psi of 80% does nicely.

 

[Thalassamania]

You know what I admire about you Rick. You have, on several occasions, stepped away from the keyboard and tried to verify a theory or idea in the water. I wish more internet divers were as willing to wet test their ideas.

Or were as open and honest.

 

[Soggy]

When I mentioned Buoyancy issues, I was referring to a student that is taskloaded, and LEARNING to do things right.

That student should not be USING deco gases until the taskloading has been reduced to a manageable level! Anything else is irresponsible on the instructor's part. That's the whole point. Shallow water, no decompression dives, until they can do what they need to do BEFORE you start adding in decompression gases.

I could have 100% on the anchor line bottle, but that would mean that each diver would need a dive computer with a minimum of 3 gas capability. Not all divers have VR3's and down here, alot of divers really like the Dive Rite Nitek series of computers. It's 2 gas only. hopefully that explains it.

No, nobody needs a computer. Some people might like a computer, but there is not a need for one. Do you teach people how to calculate their decompression profiles ahead of time using software or tables? Why does 100% require a 3 gas computer?

3. Cost. it is cheaper to dive 80% as a DECO gas than it is to top off 100%. Flame me if you want, but the o2 lasts longer, I can charge my divers a little less for it, and it saves me a trip or 2 a year to the fillers.

O2 is pretty bloody cheap relative to helium. You can use that as an argument, but it's pretty weak, especially since you are talking about only a few cubic feet difference per fill.

 

[Mike Edmonston]

That student should not be USING deco gases until the taskloading has been reduced to a manageable level! Anything else is irresponsible on the instructor's part. That's the whole point. Shallow water, no decompression dives, until they can do what they need to do BEFORE you start adding in decompression gases.





No, nobody needs a computer. Some people might like a computer, but there is not a need for one. Do you teach people how to calculate their decompression profiles ahead of time using software or tables? Why does 100% require a 3 gas computer?



O2 is pretty bloody cheap relative to helium. You can use that as an argument, but it's pretty weak, especially since you are talking about only a few cubic feet difference per fill.

Good god man, give it a rest!

Fine you're right about everything, your way is the only way, everyone else is an incompetent instuctor.

Let's just drop the instructor bashing shall we? (I leave it as a question mark so that you can have the last word):mooner:

Cheers

Mike