Fiona Sharp death in Bonaire

[doctormike]

Maybe he meant set to air diluent...?

 

[CptTightPants21]

So that means 20/20 dil, right? So leaving aside the hot loop issues, that's still an END of 218 FSW.

What does "setpoint controller and computer set to air" mean? Not sure I understand that in the context of CCR. Her decompression stress was calculated with her computer set to air OC bailout? What was the setpoint?

To eliminate the helium penalty and reduce deco. Same thing Rob and Sotis supposedly did.

 

[doctormike]

To eliminate the helium penalty and reduce deco. Same thing Rob and Sotis supposedly did.

Yeah, I realize that he meant "computer set to air diluent". Wasn't originally clear.

 

[The Chairman]

Through this discussion, it's become painfully evident that the rules for this forum needed to be rewritten. Please take a moment to review them before posting in this thread: PLEASE READ FIRST: Special Forum Rules

 

[rjack321]

Right, I understand, I was just saying that would involve a lack of PO2 monitoring.

But not diving an mCCR, I probably don't realize just how fast it can drop if you don't pay attention, if you start from a lower setpoint, and if the orifice is set for a lower workload and/or metabolic rate.

Thanks!

The orifice will never raise the SP. The only thing raising a setpoint on mCCR is descending or manually adding. The only thing lowering SP is time, typically more than 5+mins at a constant depth. Also lowering SP is working hard, ascending, and a clogged orifice.​

to eliminate the helium penalty on the deco algorithm maybe?

Yup. What the computer was set at and what was in the dil tank are not necessarily the same.

 

[CptTightPants21]

​

Yup. What the computer was set at and what was in the dil tank are not necessarily the same.

I have been guilty of doing a shallow shore dive on 32% and just kept my computer set to air from time to time, but never have I done a dive that would generate any kind of deco obligation and deliberately lied to the computer. I think a lot of people (myself included) are really thrown by this, but these two recent fatalities show that it is clearly prevalent in the CCR community.

I think a lot of us would say, if you want a shorter deco schedule then you should change your GF. Any idea why it is so prevalent?

I know some of the extremely deep dives are done with higher GFs. I haven't played with the numbers, but is it something along the lines that they are unwilling to openly admit to themselves that they want to dive what is essentially a 95 or 100 GF with actual gas mix?

For people that have done this practice or know of people doing this practice, what is the rationale? How did it develop in your diving style?

 

[doctormike]

The orifice will never raise the SP. The only thing raising a setpoint on mCCR is descending or manually adding. The only thing lowering SP is time, typically more than 5+mins at a constant depth. Also lowering SP is working hard, ascending, and a clogged orifice.​

I don't dive an mCCR, but interested in the terminology. I'm using the term "setpoint" to mean your target PO2, but it seems that you are using it to mean current PO2, right? Help me understand...

On either eCCR or mCCR, if you decide that you want to be at a PO2 of 1.3, that's your setpoint. On eCCR the solenoid makes up for the continual drop from metabolism, whatever your workload is, based on the sensors. On mCCR, your orifice is chosen to let O2 into the loop at just below metabolic rate, and it's up to the diver to use the MAV to maintain the setpoint. If you don't pay attention and use the MAV, the PO2 will drop, as will hard work, ascending or a clogged orifice. But the setpoint doesn't change, even if you don't maintain it, by my definition of the term.

 

[taimen]

I think a lot of us would say, if you want a shorter deco schedule then you should change your GF. Any idea why it is so prevalent?

Omitting the "helium penalty" in Buhlmann algorithm by setting lower helium percentage cannot be achieved by simply changing GFs with set helium content.
The reason why it's prevalent is that many think helium penalty is possibly a flaw in the Buhlmann algorithm and leads to suboptimal Deco profiles. Thus it is done to modify profile, not to shorten deco schedule.
This approach is very cautiously supported even by some researchers.

 

[doctormike]

I have been guilty of doing a shallow shore dive on 32% and just kept my computer set to air from time to time, but never have I done a dive that would generate any kind of deco obligation and deliberately lied to the computer. I think a lot of people (myself included) are really thrown by this, but these two recent fatalities show that it is clearly prevalent in the CCR community.

I think a lot of us would say, if you want a shorter deco schedule then you should change your GF. Any idea why it is so prevalent?

I know some of the extremely deep dives are done with higher GFs. I haven't played with the numbers, but is it something along the lines that they are unwilling to openly admit to themselves that they want to dive what is essentially a 95 or 100 GF with actual gas mix?

For people that have done this practice or know of people doing this practice, what is the rationale? How did it develop in your diving style?

You probably know this, but for the non-tech divers reading the thread ...

I think that the lie to the computer in this case is different than the example you gave. For OC nitrox diving, if you dive 32 and set your computer to air, you are onloading less inert gas than the computer things you are, so you are building in a buffer. In this case, the lie was not how much inert gas was in the breathing mix, but how much of it was helium and how much is nitrogen. If the report is true, the dil was actually 20% O2, 20% He, and 60% N2, while the computer was told that it was 21% O2 and 79% N2.

So if your computer believes in the helium penalty and you don't, you might dive 21/20 dil while setting your computer to air, so you won't be penalized in terms of deco requirements because some of your inert gas is helium. If there really is no helium penalty but the computer algorithms haven't been updated to reflect that fact, then this might seem more reasonable than choosing a more aggressive GF.

Here's Shearwater's discussion of the issue:

"Virtually all of the existing tables, dive computers and deco software, used by technical divers today, particularly the Buhlmann ZH-L16 algorithm which fixes nitrogen halftimes as 2.65 times longer than those of helium, have a legacy bias that adds increasing decompression time the higher the fraction of helium in the mix. ...

The extra decompression time calculated by various algorithms when breathing a helium mix is a consequence of the long held belief that helium, which is lighter than air, enjoys faster uptake by the body than nitrogen (in the case of the Buhlmann algorithm 2.65 times faster). As a result, current algorithms prescribe a deeper first stop (to clear the helium supersaturation in fast compartments representing tissue like brain or spinal cord) resulting in continued gas uptake in slower compartments and consequently increased total decompression time. However, there have been some indications that the helium penalty was fictitious...

...the more pressing issue is, “What algorithm will tekkies use to make their heliox decompression calculations?” The Navy’s current Mk-16 MOD 1 tables, which are published in the U.S. Navy diving manual, are one possibility, but as noted above the current version contains a helium penalty. So tekkies will have to wait for NEDU or others like Duke University to publish a corrected algorithm, and then wait for dive computer manufacturers to implement them.

Unfortunately, that won’t happen overnight. “Because of funding cycles, it will likely take two to three years, for this work to hit the public domain,” Doolette explained."

But of course, nothing is ever that simple. If the Helium penalty is a myth, then why NOT just tell you computer that all of the inert gas in your dil is nitrogen? That is not recommended by Shearwater, because:

"The trouble is that while this helium penalty is fictitious, the “depth penalty,” i.e. most algorithms prescribe increasing risky schedules the deeper and longer the dive, is not. However there is no reliable way, a priori, to determine which is which, that is to know which schedules can be reduced and by how much."

 

[rjack321]

snip

I think a lot of us would say, if you want a shorter deco schedule then you should change your GF. Any idea why it is so prevalent?

snip

For people that have done this practice or know of people doing this practice, what is the rationale? How did it develop in your diving style?

I would say its not a CCR vs OC thing.
"lying" about your helium content is a holdover from the past (and it sounds like Fiona wasn't a new kid to deco or tech diving in general). The era of having an actual believable deco computer is pretty damn recent IMO. As recently as 8 years ago the options included units like the nitec-he which would generate 2 hrs at 10ft if it had the slightest wiff of helium entered.

I guess the point is that trimix divers of the 1990s and early 2000s eras have been "lying" to their computers for a long time and in many cases continue to do so because it has worked for them in the past.