What's your Low Gradient Factor Selection

[jturner]

What's the "consensus" on GF settings?

I'm broadly following this table from the APD manual, BTW:

Cheers
Matt.

I don't think there is a consensus, that's the problem! But FWIW, I came up with my figures using this table and the following highly scientific method - I typically would be diving mainly nitrox dil but sometimes ~30min bottom time on trimix, and not that deep. Accordingly, I took the figures 90/95 and 15/85, picked something in the middle then softened the top down a little as it seemed a little frantic & run time wasn't a problem, and ended up with 40/80. Spookily enough that was before ther whole deep stops thing kicked off too! Leading the way (by pure chance!)...

 

[rossh]

And a post by me attempting to summarise important points and direct the reader to key posts can be found here:

http://www.rebreatherworld.com/447844-post131.html

Simon M

Its amazing, reading that post again, just how many insults, mis-information, self serving inverted interpretations and one sided arguments, etc, that you can pack into one post.

 

[Simon Mitchell]

Interesting discussion, but I don't intend to change my decompression style. I'd rather base my descision on my own first hand experience, rather than experts, no matter how well qualified they are.

Entirely appropriate. I have no problem with that. I am just making people aware of the information that is out there.

Its amazing, reading that post again, just how many insults, mis-information, self serving inverted interpretations and one sided arguments, etc, that you can pack into one post.

Whatever Ross. The arguments are all there, and people can read them and judge for themselves who is making most sense. I'm not going to relitigate the issue with you here.

Simon M

 

[garethingham]

Simon, what is your take on the helium penalty most models incur? I am no expert but lightly skimming the recent documentation infers that helium moves about the same speed as nitrogen. Seems we have rightly focussed on the GF vs Deep Stop but have we somewhat ignored what could be just as bigger topic of discussion.

Thanks in advance.

 

[rossh]

Looking at the Poll results so far, 62% use a GF Lo in the 21 to 40 range. Its 73% for the 0 to 40 range.

:chin: There seems little doubt that the "deep stop" is in use by the majority.

Interestingly, if we cross check the 1st stop depth of a ZHL-C with GF Lo in the 21-40 range, to a matching VPM-B or RGBM plan, we find the 1st stop depth is about the same depth.

Perhaps that surprises some who steadfastly keep to a ZHL-GF planning system only. They have been happily using a deep stop profile all along, and not known it.


The closing slide on the recent TekDiveUSA deep stop presentation was: "Deep stops can work but require longer decompression time".

Now the essence of any decompression model is to account for on/off gassing. VPM-B does track the on gassing during the deeper deco, in all the same ways that ZHL with GF does. Those both use the same gas tracking mechanism, and provide this feedback loop into the deco calculations.

And for most dives, VPM-B is longer than ZHL-C. So I guess VPM-B is on the right track after all. :yippee:

 

[Mark Chase]

Looking at the Poll results so far, 62% use a GF Lo in the 21 to 40 range. Its 73% for the 0 to 40 range.

:chin: There seems little doubt that the "deep stop" is in use by the majority.

Interestingly, if we cross check the 1st stop depth of a ZHL-C with GF Lo in the 21-40 range, to a matching VPM-B or RGBM plan, we find the 1st stop depth is about the same depth.

Perhaps that surprises some who steadfastly keep to a ZHL-GF planning system only. They have been happily using a deep stop profile all along, and not known it.


The closing slide on the recent TekDiveUSA deep stop presentation was: "Deep stops can work but require longer decompression time".

Now the essence of any decompression model is to account for on/off gassing. VPM-B does track the on gassing during the deeper deco, in all the same ways that ZHL with GF does. Those both use the same gas tracking mechanism, and provide this feedback loop into the deco calculations.

And for most dives, VPM-B is longer than ZHL-C. So I guess VPM-B is on the right track after all. :yippee:

Ross 10 years ago when there was a big moove in the deep diving comunity tawards VPM it was for one reasion and one reasion only. To greatly reduce deco but to maintain the risk exposure.

It was widley beleived VPMB could get us out of the water 20%+ quicker than GFs for the same level of physical risk

I dived with divers who had switched and they were getting out of the water 20 -30mins before me and laughing at me using old fashoned GF profiles which were "penalising helium unnesseraly"

No one switched to change the profile but stay just as long if not longer in the water.

ATB

Mark

 

[Igor P]

Some believed that and some other were comparing the profiles and did not believe to the mantra you describe (the DIR guys mantra - not ours)... But need to agree, plain VPM was shorter .. not in use for a long time already.... from VPM-B implementation on ...

 

[emmbee]

Looking at the Poll results so far, 62% use a GF Lo in the 21 to 40 range. Its 73% for the 0 to 40 range.

:chin: There seems little doubt that the "deep stop" is in use by the majority.

I certainly don't disagree.

It's worth pointing out, tho, that if we're looking at what people are doing, it might be worth looking at which way they're going. If I had had to guess the result of that poll, I would have said vast majority between 11-20 and 21-30. Because 15/85 and 30/80, that's pretty much all I read and heard about over the past few years. Turns out 11-20 isn't all that much, less than 41-50, and 31-40 is about the same as 21-30 (a bit more, as things stand).

Maybe it's because of the sample. Maybe it's because I don't have a clue. Probably, actually . Maybe both.

Or maybe it's because people are moving away from deeper stops.

Now the essence of any decompression model is to account for on/off gassing. VPM-B does track the on gassing during the deeper deco, in all the same ways that ZHL with GF does. Those both use the same gas tracking mechanism [...]

It's funny you should say that, because some time ago you took issue with me saying the exact same thing. But fair enough

Cheers,

Matthieu

 

[rossh]

Ross 10 years ago when there was a big moove in the deep diving comunity tawards VPM it was for one reasion and one reasion only. To greatly reduce deco but to maintain the risk exposure.

It was widley beleived VPMB could get us out of the water 20%+ quicker than GFs for the same level of physical risk

I dived with divers who had switched and they were getting out of the water 20 -30mins before me and laughing at me using old fashoned GF profiles which were "penalising helium unnesseraly"

No one switched to change the profile but stay just as long if not longer in the water.

ATB

Mark

Hi Mark, I can't speak for why people in your part of the world did the move 10 - 15 years ago.

Around my part of the woods in 2000 (when I released ZPlanner [wrapper of zplan: MS-DOS program with GF and Pyle Stops, or WKPP stops] ) , we were only just getting into GF, but agency or navy tables with Pyle Stops were just as common, or more so. A handful were doing early RD too. DCIEM was another. There were many other choices, and it really depended on which side of the tracks you dived on as to what you used.

But Deco was not conservative back then. Plain VPM was considered a straight swap for what ever it replaced. Interestingly when VPM-B arrived in 2003, it was considered by many as too much deco, as many were quite happy with plain VPM.

There certainly has been a lot of vodo ideas kicking about deco magically disappearing, but I had nothing to do with this, and have never subscribed to any. I could see the numbers on the inside of the model, and knew this isn't possible. Some of the silliest notions about deep stops (the ones you mention), I have noticed to come out of the DIR classroom or passed down as wisdom from the gods. I have kept a web page up that distances myself from some of these ideas since 2004.

When Erik Baker found and fixed the math problems in VPM-A, and came up with VPM-B in 2003, it now took on a more natural deco shape. I was very pleased to see that the VPM-B model had taken a big step forward and a we had a model that was able to make useful dive plans across the nearly the whole dive spectrum, with zero fudging required. From NDL to saturation, and all places in between.

The extra conservative dive plan that you are seeking in the posts above, is a trend that developed in the last few years. Its predominately a CCR thing, and today we find that the CCR diver is asking for more deco time than his under equipped OC counterpart. It's a demographic trend, and probably an age related issue too. Sorry if anyone takes offense to this, but its just how things are.


I addition to what I said above re the TekDiveUSA, the latest version of David's deep stop presentation, he actually says that deep stops do work, but also adds they just need to be used properly. I told Simon at the meeting there, I was a lot happier to here this new format and recognition of the status quo. (past presentations had said or implied that d stops don't work and don't us em).


Now VPM-B (like any model) has always been accounting for on / off gassing, during ascent and deco. VPM-B and ZHL both use the Haldane and Schriener equations for gas tracking formula. The on / off gassing that occurs in deep stops, has all the same considerations as on /of gassing in the shallow stops, or any part of the dive for that matter.

If David and Simon are ready to accept a model that follows the on/ off gassing changes that occurring during stops, and the model dynamically adapts to these changes, then just about every model already qualifies.


*****


Matthieu, I was disputing the implied origin of VPM and implied suggestion that it came from Buhlmann. As I said in the other post, the common part is the gas tracking to both (Haldane and Schriener equations). Buhlmann's model is limiting ascent by defined A/B coefficients that are matched to a cell half time. But VPM uses something different by calculating microbubble conditions for each dive. Also the half times in Haldane cells in VPM, have no real effect on the outcome.

The two models use the same gas tracking system only. And it reacts the same to on /off gassing changes in the same way in both models. But the ZHL to VPM-B difference comes from the way each defines the limits of ascent. Or more commonly - how much GF is added onto the ZHL stop calculation.

 

[Mark Chase]

Ross

I planned a dive for 45mins to 70m running VPMB 2 and 20/90GF

Running the two profiles gave a first stop depth of 48m and a proper 1min stop at 45m (I switched my ascent to a more conventional 9m/min)

However if you look at the shalow deco time from 9m and up the VPM profile had 18mins less deco than the 20/90 profile

When VPMB was making a big impact in my diving circles i was diving 20/80GF so for me it ment this dive would incure 28mins less deco above 9m

65mins shalow deco for VPM-B 2

83mins shalow deco for 20/90 GF

93mins shalow deco for 20/80GF

Switch to 20/100GF and the shalow deco is 74mins so 9mins more than VPMB2

Which to my humble understanding meens to match VPMB2 Id have to be running in excess of 100GF to acheive the same shalow stop times

Thats what I mean by greatly reduced shalow stops

ATB

Mark

 

[jptaylor9]

Simon, what is your take on the helium penalty most models incur? I am no expert but lightly skimming the recent documentation infers that helium moves about the same speed as nitrogen. Seems we have rightly focussed on the GF vs Deep Stop but have we somewhat ignored what could be just as bigger topic of discussion.

I'm most probably wrong, but VPM-B doesn't appear to penalize you for using Helium in the same way as ZHL does.... It at least appears that way in practise.

 

[Mark Chase]

I'm most probably wrong, but VPM-B doesn't appear to penalize you for using Helium in the same way as ZHL does.... It at least appears that way in practise.

Thats what I used to think as well,

We used to tell our VR3s fibs about how much He we were useing to cut the deco down to a level we felt was right for He rich dives. However I have asked the question many times "does Bhulman over compensate for He" and i have never had anyone say catagoricly yes.

I did have an interesting coversation with a chap from Comex many years ago who said Nitrogen had no place in deep diving and we should only dive a single inert (Heleox) He also said our recreational diving deco plans were no where near conservitave enough for the dives we were doing (we were discussing 80-140m dives)

We did then look at doing the Victoria on Heleox but the bailout gas logistics and costing were beyound prohibative

ATB

Mark

 

[Simon Mitchell]

The closing slide on the recent TekDiveUSA deep stop presentation was: "Deep stops can work but require longer decompression time".

I addition to what I said above re the TekDiveUSA, the latest version of David's deep stop presentation, he actually says that deep stops do work, but also adds they just need to be used properly. I told Simon at the meeting there, I was a lot happier to here this new format and recognition of the status quo. (past presentations had said or implied that d stops don't work and don't us em).

Ross,

While the quotations are approximately correct, the meaning you attribute to them ignores the context David provided at the time, is not accurate, and reflects your own considerable biases. Both David and I have consistently acknowledged that deep stop decompression can work; the central question is whether or not it is the optimal approach. On the basis of the best evidence currently available neither David nor I consider that an emphasis on deep stops is optimal. You infer a changing / more moderate stance on our behalf, but none of our views have changed since the extensive debate that took place on RBW to which I have referred (and linked) elsewhere; a debate that I am not prepared reprise here since it is completely unnecessary to do so.

For an accurate appraisal of our views I refer readers back to the RBW threads that I linked to in my earlier post.

Simon M

 

[Simon Mitchell]

Hello Gareth,

This is an interesting question which is still being studied. David has contributed much of the relevant contemporary data which we summarized in a recent review article:

DOOLETTE DJ, MITCHELL SJ. Recreational technical diving part 2. Decompression from deep technical dives. Diving Hyperb Med 43, 96-104, 2013

I would be happy to send it to you if you pm me an email address. You will see that we doubt that the kinetics of these gases need be treated as significantly different in relevant tissues during typical technical diving bounce dives.

I know that further relevant work is underway at NEDU but it would be inappropriate for me to discuss it further before its completion and publication.

Simon M

 

[Hydreliox]

Ross,

While the quotations are approximately correct, the meaning you attribute to them ignores the context David provided at the time, is not accurate, and reflects your own considerable biases. Both David and I have consistently acknowledged that deep stop decompression can work; the central question is whether or not it is the optimal approach. On the basis of the best evidence currently available neither David nor I consider that an emphasis on deep stops is optimal. You infer a changing / more moderate stance on our behalf, but none of our views have changed since the extensive debate that took place on RBW to which I have referred (and linked) elsewhere; a debate that I am not prepared reprise here since it is completely unnecessary to do so.

For an accurate appraisal of our views I refer readers back to the RBW threads that I linked to in my earlier post.

Simon M

Simon,

Sorry to barge into this late and a little clumsy. I think given all the recent debate the question really at the front of my mind, is the inverse of your very statement. Given the current research, with which yourself and David are familiar and consider relevant to these discussions could you shed some insight into what you would consider to be an optimal approach to decompression. I think the most crucial point in all of this is exactly as highlighted on both sides of the fence - in needs to be expressed in a context that is relevant to the parameters of measurement with which we have common access and availability - principally the decompression models that we have available.

ZHL -x with extreme GF (Lo-Lo & Hi-Hi)
ZHL-x with mid GF (20-40Lo & 70-90Hi)
ZHL -x with High GF (Hi-Lo matching Hi-Hi)
VPM-B + (x) cons.
VPM-B + (x) cons. + GFS

To be overly bold and direct - which approach is closest to optimum? Is one better for air, is one better at dealing with mixed gases. Does one have a more optimal approach to on/off gassing modelling.

Or....to digress a little - are we all just making it up and what are the people around me setting up their deco against and why?

Parameters based on actual diving circumstances are vast, possibly too much so to quantify given physiological differential. Everyone is at the edge of their seats waiting to hear what the 'optimal' approach to decompression might be, whilst alongside still diving, adapting approaches and using different strategy and, comparatively, without incidence. So, you have to question - how relevant is all of this?

Just to clarify this is from a dunces point of view

 

[Simon Mitchell]

could you shed some insight into what you would consider to be an optimal approach to decompression.

Hello,

Your questions are addressed as best as they possibly can be at the present time on the deep stops threads that I have linked to previously. You could start here for a broad brush opinion:

http://www.rebreatherworld.com/439085-post132.html

To be clear, none of us pretend to know exactly what "optimal decompression" from deep mixed gas bounce dives is, but recent research has given some insight into what it isn't. Have a read of the threads. They are an incredibly valuable resource.

Simon M

 

[jptaylor9]

Simon, your not taking other factors into account, there is more to consider than deco theory in isolation. I usually dive with a 2hr max runtime, my optimal deco is that which maximises bottom time without taking a helicopter ride after the dive! It shouldn't be like that but it is, dives have become expensive so have to max bottom time, otherwise its not worth bothering. I can even live with sub clinical bends if that's what it takes.... As others diving 70/70 profiles have admitted, they've had to cut their bottom times, but that makes dive unacceptably short. In other situations this may not be an issue, but in my case that's the situation. What GF well maybe I'll try 90/95 ;-)

 

[iamyourgasman]

Hello Gareth,

This is an interesting question which is still being studied. David has contributed much of the relevant contemporary data which we summarized in a recent review article:

DOOLETTE DJ, MITCHELL SJ. Recreational technical diving part 2. Decompression from deep technical dives. Diving Hyperb Med 43, 96-104, 2013

I would be happy to send it to you if you pm me an email address. You will see that we doubt that the kinetics of these gases need be treated as significantly different in relevant tissues during typical technical diving bounce dives.

I know that further relevant work is underway at NEDU but it would be inappropriate for me to discuss it further before its completion and publication.

Simon M

Dear Simon, May I ask you to send me this paper. You know my email. Thanks Tamas

 

[matt]

What GF well maybe I'll try 90/95 ;-)

Why not? I tried it yesterday and did not die. But also did not go deep or for very long! I'm going to use it on my upcoming trip; if I do not post again then it is obviously not to be advised!

Matt.

 

[matt]

This thread got me thinking about something that's been going on for years but which people don't talk a lot about. Quite a number of divers switch from their He based diluent to a less-lean mix (or air) at or around 40m/first-stop. They don't do the flush (that trend died a decade ago), but just lie to the computer. I take this to mean that they don't believe the M-values for helium to be correct - they think they ought to be closer to that of nitrogen than the accepted values are.

So I wondered if the move away from Gradient Factor deep-stop is somehow related to this?

Sticking with the 45mins at 70m dive, and my favourite gas 10/52 we have my current 15/85 dive as:

70m, 45mins, 10/52, 15-85, SP=1.3, first stop 48m, RT=197.

Using the seemingly implausible 90-95:

70m, 45mins, 10/52, 90-95, SP=1.3, first stop 30m, RT=162. Last stop 61 mins.

And with telling the software the switch-lie of air at the first-stop using the 15-85:

70m, 45mins, 10/52, 15-85, SP=1.3, Switch to Air @ 48m (first stop), RT=163. Last stop 62 mins.

Not quite the same, but I found the comparison interesting at least.

Profiles from Projection.

Matt.