USING HELIOX WITH A TRIMIX (LOWER FHe) DECO ALGORYTHM

[Seameadow]

Dear fellow divers,,,,

Please forgive this post, the last thing I want to do is start another Mitchell vs. Hemingway debate but I have a interesting discussion point I would like to get all your learned viewpoints on. My apologies if it is covered elsewhere, in which case please redirect.

In reference to the recent discussions on the fact that the Helium behaves similar to Nitrogen in terms of the decompression times, and the "Helium penalty" in deco algorithms work, although probably for the wrong reason, as well as articles such as this
Deep Helium • ADVANCED DIVER MAGAZINE • By B.R. Wienke and T.R. O’Leary
I am would like to suggest the following:

Using a Heliox gas on CCR, 10/90, but putting in a mix that equates to an END of 30m in my dive computer so that it calculates a "safe" but faster deco profile based on the depth that I am diving (So Something like 10/65 for a 90m dive). Assuming that the Helium penalty was an incorrect assumption a high helium content gas is the safer gas, (WoB, Gas density, Narcotic effect etc) and on a CCR not much more expensive. Therefore the assumption is using this gas but with the previous algorithms in the computer for an END of 30m would provide a relatively safe decompression profile. The O2 content being accurate of course.

What would the reasons be why this should not work? In other words besides the fact that there is no concrete data to support any of this why should this NOT work? (yes theoretical exercise only ) The only thing I can think of is perhaps IBCD if I bailout, but not if I do those mixes well? Assuming that the inert gas calculations done for decompression is pretty much a theoretical mix to provide a safe profile for a 30m end what would be the other reasons?

Cheers

PS- I am sure many would now grab the popcorn, but I assure you this is not an antagonistic post. Just trying to think of other reasons why that would not work?

 

[JohnnyC]

The point of the phrase, "we're doing the right amount of deco, but probably for the wrong reason," would indicate that the time added to the algorithm under the auspices of the "helium penalty" is necessary for safe decompression, albeit improperly named. If we were to call it the "ridiculously fun super happy joy penalty" and avoid associating helium content with it, would you still be looking for a way to trick your dive computer into creating a plan with less decompression than required? If dive computer manufacturers changed their algorithm implementation to where the helium content was not included in calculating decompression obligations, however required the same amount of decompression obligation as the prior "helium penalty" version, would you be seeking similar adjustments to shortern your decompression obligation?

 

[leadduck]

If we were to call it the "ridiculously fun super happy joy penalty" and avoid associating helium content with it, would you still be looking for a way to trick your dive computer into creating a plan with less decompression than required?

No. The "ridiculously fun super happy joy penalty" not associated with helium content would result in the same deco profile for 10/90 and 10/65, and that's what he proposed.

If dive computer manufacturers changed their algorithm implementation to where the helium content was not included in calculating decompression obligations, however required the same amount of decompression obligation as the prior "helium penalty" version, would you be seeking similar adjustments to shortern your decompression obligation?

I don't get that, seems to be contradictory. If the new dive computer ignored helium content, then it would generate the same profile for 10/90 and 10/65, and hence won't require the same amount of deco obligation compared to the old dive computer making a difference between helium and nitrogen.

 

[KenGordon]

It would be whatever the old algorithm gave for 10/90 for both 10/65 and 10/90.

The OP could do a plan with more aggressive setting but not lie about the gas. Lying about the gas seems less sensible that that. Or he could just ignore the plans and get out of the water in time for tea or whatever reason makes him want to do this.

 

[KenGordon]

http://www.dtic.mil/dtic/tr/fulltext/u2/1003571.pdf

I think the conclusion of this paper is that introducing nitrogen does not reduce deco time. This is what Johnny was refering to above.

 

[rossh]

In reference to the recent discussions on the fact that the Helium behaves similar to Nitrogen in terms of the decompression times, and the "Helium penalty" in deco algorithms work, although probably for the wrong reason, as well as articles such as this

If dive computer manufacturers changed their algorithm implementation to where the helium content was not included in calculating decompression obligations, however required the same amount of decompression obligation as the prior "helium penalty" version, would you be seeking similar adjustments to shortern your decompression obligation?

Perhaps its relevant to know how much the perceived difference is, and to test if any of this difference actually makes a minor or major change.


In MultiDeco and V-Planner, we have provided the tools to test and experiment with these changes as much as you like. You can adjust the model's gas kinetic parameters, or re-invent ZHL dimensions, and see what happens. Just press the Greek Tau (half time) symbol in the middle bar.


You will find that adding new He times, makes only a small real difference. That is because the existing standard gas kinetic formula, automatically eliminates the "too fast" helium components from the calculations. The fastest He parts have little or no real affect on the plan now. The assumption that existing planning results is way off, seems to be wrong.

Further more, in private conversation with David, he said that his test data applies to the faster tissues end, while the slower ones are likely still correct and are well tested in various other applications under heliox conditions. Hence his suggestion of a merged transition from newer fast end tissue rates towards the existing slower older tissues. This change is incorporated under the "new helium rate" setting in MultiDeco and V-Planner.


In summary, this "helium penalty" concept / adjustment / changes, etc, is not really effective or affecting the current planning. It's mostly eliminated by existing formula, and has a diminishing effect as the dive gets longer.

.

 

[Seameadow]

Hi all,

Thank you for the responses so far, interesting read. Just to reiterate, I am not suggesting to do anything, this is a theoretical exercise / debate.

To clarify what I am suggesting is to do your gas planning as normal and determine your gas mixes for an END of 30m, so lets say the 10/65 for a 100m dive with a SP of 1.3. All we do differently is add a higher helium content in the mix ie 10/90), but not change this on the computer obviously. So the deco time, would be the same as for the former but we have a safer breathing gas. If the assumption is that the algorithms worked but probably because of the type of diving people do with helium rather than because of the gas, the END of 30m calculation should still give you a safe deco profile regardless of using higher He content correct?

Ross, running a quick profile (not changing the advanced cell half times, cool tool by the way), but just changing the gasses for a 100m dive, bottom time of 20min using GF 75/75, on 10/65 as opposed to 10/90 gives a 28 minute difference in runtime. (128 minutes for 10/65 and 156 minutes for 10/90).

The theoretical argument being that if I do the dive on 10/90 with the 128min runtime deco profile that should still be "safe" if the helium is not the factor requiring longer deco time.

Ross, your inputs on the data applying to faster tissue as opposed to slower tissue seems like a possible hiccup but if I recall the experiments concluded that there was no significant variance based on the different gasses that was used?

I am sure everyone can agree to not mess around with these as you become a test pilot, but where am I wrong?

Cheers,,

 

[KenGordon]

The paper supports the idea that the 10/60 plan ought to be 156 minutes not that the 10/90 plan should be 128.

END is about narcosis, not decompression.

 

[Seameadow]

The paper supports the idea that the 10/60 plan ought to be 156 minutes not that the 10/90 plan should be 128.

END is about narcosis, not decompression.

Hi Ken, Thanks for the reply, I do appreciate it, although I think you might be misunderstanding my point. Firstly the paper was just one reference the main one I was referring to was the recent studies by NEDU that suggest longer deco times for helium was unfounded and using helium or nitrogen with the same profiles produced similar results. (See Eliminating The Helium Penalty - Shearwater Research )

Putting the references aside bare with me for an assumption for a minute. I understand why traditionally you would use an END of 30m. To reduce the narcotic effect to levels akin to 30m of depth right? You do this by adding helium to the mix correct? So if I add a higher helium mix (the 10/90) the actual end is something ridiculous like 4m. Safer than 30m correct? Plus your gas density is now 3.3 g/l, much safer etc etc. The only down side is you will have a longer run-time based on the fact that you have a higher helium mix.

BUT, if the higher helium mix does not require longer decompression, as per the study, then what would be a safe (but not overly long) deco profile? Well the assumption is that the deco profiles generated by the algorithms worked NOT because of the helium content but because of the type of diving you do with helium, i.e. longer deeper dives requiring longer decompression etc and in general longer run-times. So we use the end of 30m not because of the gas mix but because we are looking for a commensurate dive profile for a typical dive to 100m.

Not sure if I am clear on this or not?

Anyway all inputs welcome

 

[Seameadow]

http://www.dtic.mil/dtic/tr/fulltext/u2/1003571.pdf

I think the conclusion of this paper is that introducing nitrogen does not reduce deco time. This is what Johnny was refering to above.

"Their findings: A bounce diver’s decompression requirements depend solely upon the time, depth and level of oxygen (PO2) over the course of the dive regardless of the fraction of helium and or nitrogen used in the breathing mix. In other words the so-called “helium penalty,” i.e. the extra stops and decompression time required when breathing helium mixes on a surface-to-surface bounce dive, does not exist."