Petrel users have you been able to closely match NDL times with other PDC''s?

[Nasser]

Like some others, I also do not understand why you would want to tinker with GFs so hard, until they "match" your buddy's, I would never do that. The computer should be tracking whatever you think is appropriate for your body, and the conservatism should reflect how you feel on that day, how fit you are, and how much sporty you want to get. To me, this takes precedence over matching GFs. If you are doing an NDL dive, then as said earlier, whoever gets close to the NDL first, or whoever's computer fails first, simply aborts the dive, period. This should ensure that when you begin ascent, even if neither computer works, nobody's in deco, and a basic 30fpm ascent strategy will suffice. If this is not an NDL dive, on the other hand, then you should have redundant computers (or tables, or a rule of thumb, or redundant whatever sources of deco information float your boat), and either a concrete plan you will closely follow, or at least agreement on the basic dive parameters, like max bottom time, max deco, etc.

I myself have definitely been guilty of getting complacent. On a recent dive with a very experienced buddy, we got lazy, and we did not agree on the maximum amount of deco (and, to preempt a certain type of criticism, we did not set a fixed plan and fixed deco schedule, because this really was hardly more than a simple rec dive). Both of us had enough redundancy to do the dive solo, and both of us had plenty of gas. We just agreed we will stick around at the end of the dive, and do whatever deco was required in the shallows. At the end of the dive, I was happily doing my deco with a little under 10 minutes on my Suunto and 20 minutes on my Petrel, while my buddy was getting all excited, and kept poking me to try and get out. Turns out, we used wildly mismatching conservatism settings, we had slightly different gases, his obligation cleared up, and he was frantically trying to dell me that my computer is ****ed. We had more than enough gas to stay longer, but he was cold and in pain. After my Suunto cleared as well, we went up. On a subsequent dive together, I finally gave up and cranked down my excessive conservative settings from 30/70 to 20/80. But, more than anything, I think what went wrong is not so much the difference in conservatism, but the lack of prior communication, and setting boundaries for the dive... which I accept as my fault, because the parameters I was diving with were pretty off from one could consider standard, and it was my responsibility to recognize that, and to either change the parameters I was using, or establish sensible limits to preempt possible problems.

If you read my original post I stated that I feel fine on a 45/95 GF setting for the vast majority of my NDL dives so any changes made would end up towards the more conservative side of the spectrum. For tech dives my settings will vary considerably depending on the depth, bottom mix, bottom time, work load, and diving conditions etc… I may even tweak the Hi GF’s and pad the shallower stops during deco if it’s been a pretty busy working phase - but GF’s on a tech dive is a whole other extensive discussion.

The question here is simply to see how these two computers compare in an actual NDL dive scenario rather than just reading the NDL charts. Given that the Petrel was originally designed primarily as a tech computer and only later was the recreational mode added, it seems that at least a few folks on SB have discussed this and have been interested in seeing how the Petrel behaves in a “typical” NDL dive compared to other popular sport diving computers - such as the Suunto.

Indeed the NDL planner doesn’t even exist in OC or OC/CC Tech modes and was only added when they did the firmware update with the OC Rec mode. And yes... I’m quite aware of following the more conservative of a buddy team's computer NDL/conservative settings, or the diver with the least amount of gas, higher SAC rates, etc.. and other variables that will dictate a dive/buddy team's profile on an NDL or Tech dive.

Anyway thanks for your feedback... your comment on how your Suunto ran up against the Petrel with the GF settings you had … and compared to your buddy’s deco obligations was interesting.

 

[ajduplessis]

The Suunto algorithm seems to give a lot of weight to surface interval, and unless one takes a long surface interval it will compute less no-deco time for the subsequent dive than a Buhlmann model. I've been using my old D6 as a backup since I got a Petrel.

Suunto algorithm also give a lot of weight to ascent rates and deep stop if the latter is enabled. Surface intervals should be at least 2 hours between dives to maximize NDL on next dive.

 

[tbone1004]

you can't do it, I don't understand why you'd want to, nor are you running 95gf's on repetitive dives, but to each his own. You have your NDL's, they have theirs, they thumb the dive on their NDL and you follow them up and be a good buddy and hang out with them. Your lower GF isn't going to be tripped due to NDL diving, but the higher one has you with quite a bit of tissue loading, which for me doesn't leave me with enough of a margin in case of something going wrong. That said, if you're diving with a Suunto computer you aren't going to be anywhere near your NDL's especially at the end of the day.

You can't predict or match Sunnto, it's a different algorithm, it is proprietary, and there isn't a person outside of that company who has any clue what the actual algorithm is. There have been tons of threads on how to match it, how to predict what it is going to penalize you for, etc etc, and without the algorithm you can't predict it. If you are really adamant about trying, you can download Suunto's dive planner and run simulated dives against your petrel, but I can tell you with 100% certainty that if you're doing more than 1 dive per day you won't get anywhere close by trying to change the GF's on your Petrel and if you actually understood how they worked, you would know that already. Your top GF is your tissue loading when you get out, it is a fixed point and your NDL's are based exclusively on your tissue loading when you get in, plus the added nitrogen time, less the decompression time during your safety/deco stops. It doesn't care if you blew stops before, it doesn't care if you had fast ascent rates, it really doesn't care, the only thing it cares about is trying to predict how much N2 is in those tissue compartments before you break the surface and keeping you down long enough to get the loading down to whatever number you set it to. Suunto's algorithm factors in all of that goofy stuff, and because it does and penalizes you for it, and we don't know the extent to what the penalties are, you can't possibly match a straight GF profile to theirs. Don't try, it's a waste of your time, either follow Suunto's algorithm and get over it or dive the same dive tables and put computers in gauge mode.

FWIW I leave mine in 30/80 all the time, worst case is I have a "mandatory" 5 minute backgas decompression on repetitive dives, but if I was running higher GF's then it would have been within NDL's. I wouldn't run anything more aggressive than 80 on the top side because there are too many factors for repetitive dives that can screw you up running steep gf's, dehydration, thermal discomfort, even taking a wrong step on the ladder or shore getting back to the truck can spur bubbles when the gf's are too high, not worth the risk for me.

 

[victorzamora]

Yeah....I run mine on 30/70. 30/80 is a little aggressive for my blood. 45/95 is mucho aggressive.

 

[RickyF]

Where you dialing in your own custom GF's in tech mode or using one of the fixed GF presets in rec mode?

I am using the REC mode on the Petrel and 100% RGBM on the D6.

 

[Lorenzoid]

Suunto algorithm also give a lot of weight to ascent rates and deep stop if the latter is enabled. Surface intervals should be at least 2 hours between dives to maximize NDL on next dive.

I dunno about "2 hours" being a threshold, but as far as the rest of the factors that Suunto RGBM apparently gives more weight to than other algorithms, Suunto phrased it this way in one document (which I can't seem to find the original link to):

The Suunto RGBM addresses a number of diving circumstances that have not been considered by previous dissolved gas models, adapting to:
- continuous multiday diving
- closely spaced repetitive dives
- dives deeper than the previous dive
- rapid ascents which produce high microbubble buildup


I kind of like Suunto's use of "adapting to" instead of "penalizing you for." You can sure tell what side of the safety fence a diver's mindset is on by whether he uses the word "penalizes" in describing what a computer is doing.

 

[Nasser]

I don't understand why you'd want to, nor are you running 95gf's on repetitive dives, but to each his own.

The 95GF was simply a reference to the fixed conservative settings that Shearwater provides on the Petrel in OC Rec mode, not real time GF99 values during a dive.

Frankly I don't understand why you don't understand? I'm interested in seeing how these two computers compare in NDLS and in conservatism and if it's possible to bring them closer together... what's the big deal? Of course I can just hang and ride my buddy's Suunto if the Petrel is more liberal but you said it yourself... many people have tried to come close with the NDLs so obviously it's of some interest.

You have your NDL's, they have theirs, they thumb the dive on their NDL and you follow them up and be a good buddy and hang out with them.

Uhmm yeah... see my earlier post:

And yes... I’m quite aware of following the more conservative of a buddy team's computer NDL/conservative settings, or the diver with the least amount of gas, higher SAC rates, etc.. and other variables that will dictate a dive/buddy team's profile on an NDL or Tech dive.

You can't predict or match Sunnto, it's a different algorithm, it is proprietary, and there isn't a person outside of that company who has any clue what the actual algorithm is. There have been tons of threads on how to match it, how to predict what it is going to penalize you for, etc etc, and without the algorithm you can't predict it.

Yup, again... see my earlier post:

I understand that computers using different algorithms will have different NDL times and levels of conservatism, but I'm trying to dial in GFs on my Petrel to come close to the NDL's of a Suunto Vyper/Zoop.

The operative words being "come close" not "predict" or "match"

I can tell you with 100% certainty that if you're doing more than 1 dive per day you won't get anywhere close by trying to change the GF's on your Petrel and if you actually understood how they worked, you would know that already...

hmmm.. you seem a little agitated, is everything alright?
No need to be condescending or smarmy.

Your top GF is your tissue loading when you get out, it is a fixed point and your NDL's are based exclusively on your tissue loading when you get in, plus the added nitrogen time, less the decompression time during your safety/deco stops.

Changing my HI GF during the dive was reference to a tech dive during deco not an NDL dive. Once again...

For tech dives my settings will vary considerably depending on the depth, bottom mix, bottom time, work load, and diving conditions etc… I may even tweak the Hi GF’s and pad the shallower stops during deco if it’s been a pretty busy working phase - but GF’s on a tech dive is a whole other extensive discussion.

... and NOT to try and match the Suunto algo on a tech dive (or during an NDL dive). For tech dives I cut tables and dive the run times.

FWIW I leave mine in 30/80 all the time, worst case is I have a "mandatory" 5 minute backgas decompression on repetitive dives, but if I was running higher GF's then it would have been within NDL's.

I did say that these where NDL dives didn't I? That's pretty much why I stick within NDL limits.

I mean you make a lot of good points but like a said earlier, I really can't see why you find it difficult to understand that it's of interest to some how the Petrel compares and behaves in NDL dives with certain conservatism settings (read GF setting in this case) and how that compares with other computers.

Of course I can have more liberal settings on my Petrel and just ride my buddy's Suunto... but lets put it this way.. and I've already mentioned this - The Petrel started out as a tech computer for staged decompression dives... and many people, who wanted to use it for NDL dives, where trying to dial in GF settings to approximate some of the more popular sport diving computers on the market and to not necessarily have the need to follow a deco schedule for a typical multilevel NDL dive... while of course taking into account how they felt after a dive with the GF settings they chose.

They also wanted to have features like safety-stop countdown and an NDL planner.. again to bring the Petrel closer to a sport diving computer when used that way. Shearwater responded by introducing the OC Rec mode and 3 basic conservatism settings along with a few other feature like adaptive safety stops, countdown, etc.. etc..

So getting the Petrel to dive in "sport mode" is obviously in demand, and for some... to get it similar (not exactly) like a Suunto or other PDC is also of interest too...

There have been tons of threads on how to match it, how to predict what it is going to penalize you for, etc etc,

Have a nice day.

 

[tbone1004]

fair enough, so here's my thoughts

your high GF is what really matters for these types of dives, why would you change the high GF for recreational vs technical? I would be more apt to run a high GF for a single big technical dive per day than I would repetitive recreational dives because the single long technical dive is a lot easier on your system than doing a bunch of dives where each one you are pushing saturation limits of your tissues. So, I don't understand why you would want to exit at 95% loading multiple times a day and risk getting bent on multiple recreational dives, but you would lower it for technical diving due to a perceived increase in risk.

The Petrel itself is set on a fixed algorithm, but you can choose which gf's you want which are how conservative it is obviously.

Suunto uses modified RGBM, bubble model. Shearwater default is Buhlmann with GF which is a dissolved gas model. They are completely different. I just really don't understand why anyone would want to make them match, someone is going to call the dive, whether it is on gas, on not feeling well/equipment problems, or on NDL's. You follow your buddy, so since you're following your buddy to the surface and both using computers that are constantly tracking depth and time, why does whether your computer says you have 5 minutes left on your NDL matter? Why does your safety stop counter matter if you aren't going to ascend until the person running the most conservative algorithm comes up? All the rec nx mode does is fix the ndl's, and make the screen pretty, it doesn't actually change the computer, it was a 30 minute GUI setup for the programmers.

Suunto has software for the Eon steel
Suunto DM5 - Suunto
Plan some dives, and run the same planner in the decompression software of your choice and alter the gf's to find what matches if you want to waste your time. You claim you feel good coming up when you hit NDL's at GF95, any dives shorter than that are just a perk for your body, I just don't understand why you think it matters. If it isn't exact all the time for all dives, it isn't going to matter because whether you violate ndl's by 1 minute or 10 on the Suunto algorithms it is going to be a royal pita for you for the next 24 hours. FWIW I think the closest you'll get is 15/85. 30/70 is a lot healthier for you, but 15/85 is I think as close as you'll get at least for the first dive.

 

[Nasser]

fair enough, so here's my thoughts

your high GF is what really matters for these types of dives, why would you change the high GF for recreational vs technical? I would be more apt to run a high GF for a single big technical dive per day than I would repetitive recreational dives because the single long technical dive is a lot easier on your system than doing a bunch of dives where each one you are pushing saturation limits of your tissues.

Ok fine that's a very valid point... but on tech dives I do feel that you are at significantly more risk undertaking staged decompression and going to more extreme depths for longer - not to mention other issues like increased CNS loading at higher PP02s, ICD, etc.. etc.. But yes I do also take into consideration several aspects in NDL dives over several days... you are building up more Nitogen and I do in practice dial things back a bit.

So, I don't understand why you would want to exit at 95% loading multiple times a day and risk getting bent on multiple recreational dives, but you would lower it for technical diving due to a perceived increase in risk.

See above, I said I feel fine on the Petrels 45/95 OS Rec setting... but it doesn't necessarily mean that on multi-day trips I don't take precautions... I decide on my settings on what diving conditions dictate.

I was just pointing out that even when reading the Petrel's NDLs and finding that it was very close to the Suuntos NDLs (at its most liberal)... I felt ok on 45/95 but was not sure how over several dives the two computers would vary...even though on first glance they are very close at those settings.. and the Suunto actually seemed more liberal, at least on the first dive... and hence the root of my question to everyone.

The Petrel itself is set on a fixed algorithm, but you can choose which gf's you want which are how conservative it is obviously.

Yes I know ZHL-16C. .. a version of the original Buhlmann's algo with Eric Bakers GF's mods. I have used both ZHL-16C in Multi Deco and on the Petrel Deco planner as well as ZHL-16B in Baltic Deco. I understand that these algos are different from dual-phase bubble models and some would even argue that GF's are a way to emulate deep stops generated by bubble models... they do allow one to set arbitrary - and really that's what they are - levels of conservatism.

Suunto uses modified RGBM, bubble model. Shearwater default is Buhlmann with GF which is a dissolved gas model. They are completely different.

I understand and FWI I have owned a couple of Vypers and really had no complaints with the Suunto RGBM algo being overly conservative on NDL dives... even on liveaboards doing multi day dives over a week or 10 days. I'm also very familiar with Suunto's "continuous decompression" approach and the idea of doing deco within a "zone" with a floor and ceiling... rather than staged deco stops. I just haven't had the chance to compare the two side by side and see how they vary when it comes to doing these particular NDL dives.

I just really don't understand why anyone would want to make them match

Well like a said, it's of interest to me to see how they compare and if it's possible to get them close... if you can't understand, then I can't help you.

someone is going to call the dive, whether it is on gas, on not feeling well/equipment problems, or on NDL's. You follow your buddy, so since you're following your buddy to the surface and both using computers that are constantly tracking depth and time, why does whether your computer says you have 5 minutes left on your NDL matter? Why does your safety stop counter matter if you aren't going to ascend until the person running the most conservative algorithm comes up?

Again this was something that I already alluded to earlier... I understand that they are two very different algos and even if they were the same, there are several other factors that will call the dive. Like I said, I used to dive a Suunto and even with my other buddies diving the same units we came up when NDLs, gas limits etc.. were called. I am simply interested in seeing how the Petrel behaves as an NDL computer like the Suunto and what settings on the Petrel come close to that... it's really just that, nothing more nothing less.

Suunto has software for the Eon steel
Suunto DM5 - Suunto
Plan some dives, and run the same planner in the decompression software of your choice and alter the gf's to find what matches if you want to waste your time. You claim you feel good coming up when you hit NDL's at GF95, any dives shorter than that are just a perk for your body, I just don't understand why you think it matters. If it isn't exact all the time for all dives, it isn't going to matter because whether you violate ndl's by 1 minute or 10 on the Suunto algorithms it is going to be a royal pita for you for the next 24 hours. FWIW I think the closest you'll get is 15/85. 30/70 is a lot healthier for you, but 15/85 is I think as close as you'll get at least for the first dive.

Don't really follow you here. I'll just go back and say that I'm generally fine with the Petrel's most liberal 45/95 Rec setting but like all my dives (tech and NDL) I tweak things accordingly.. always have always will.

Thanks for sharing your thoughts.

Cheers.