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Shearwater Petrel GF Settings

Posted: Thu Apr 17, 2014 2:03 pm
by mheyns
I was diving down to the I-Beams yesterday and by the time I arrived there, pretty much a straight swim to the top of the I-beams too at around 100ft, my Petrel informed me that I had 2 minutes at 100ft before I exceeded my NDLs.

Given that my buddies had 15+min on their Oceanic puck computers I was a little perplexed.

My GF setting is 30/70, and I'm wondering what folks around here use? I don't generally exceed 120ft, and haven't gone down to >100ft since getting the computer (it's new).

Should I revert to 30/85 or some other setting? Start using the full functionality of the Petrel and taking the deco in stride? Upgrade to VPM?

Thoughts?

Shearwater Petrel GF Settings

Posted: Thu Apr 17, 2014 2:17 pm
by AdrianSmith
mheyns wrote:I was diving down to the I-Beams yesterday and by the time I arrived there, pretty much a straight swim to the top of the I-beams too at around 100ft, my Petrel informed me that I had 2 minutes at 100ft before I exceeded my NDLs.

Given that my buddies had 15+min on their Oceanic puck computers I was a little perplexed.

My GF setting is 30/70, and I'm wondering what folks around here use? I don't generally exceed 120ft, and haven't gone down to >100ft since getting the computer (it's new).

Should I revert to 30/85 or some other setting? Start using the full functionality of the Petrel and taking the deco in stride? Upgrade to VPM?

Thoughts?


Did you by chance have it set to 21% while your buddies' computers were set to a higher EAN blend? That would explain the NDL time.

The GF shouldn't have any impact on the NDL. It determines how deep your first deco stop should be, not how much NDL time you have left.

-Adrian

Re: Shearwater Petrel GF Settings

Posted: Thu Apr 17, 2014 2:59 pm
by mheyns
That makes sense as far as GF and NDL influence. Thanks for the clarification. I was on 21, but everyone was on 21 as well...

Re: Shearwater Petrel GF Settings

Posted: Thu Apr 17, 2014 6:18 pm
by Joshua Smith
mheyns wrote:That makes sense as far as GF and NDL influence. Thanks for the clarification. I was on 21, but everyone was on 21 as well...


Oceanic comps have notoriously liberal NDLs. And I wouldn't go playing with the GF settings until you understand what they mean.....

Re: Shearwater Petrel GF Settings

Posted: Thu Apr 17, 2014 9:26 pm
by spatman
At the very least read up on gradient factors before you mess with anything.

http://www.diverite.com/education/rebre ... 20factors/
http://www.rebreatherworld.com/general- ... mmies.html
http://njscuba.net/zzz_gear/deco_m-values.pdf
Etc

Just out of curiosity, what level of training are you at?

Re: Shearwater Petrel GF Settings

Posted: Fri Apr 18, 2014 7:49 am
by Marc
Joshua Smith wrote:
mheyns wrote:That makes sense as far as GF and NDL influence. Thanks for the clarification. I was on 21, but everyone was on 21 as well...


Oceanic comps have notoriously liberal NDLs. And I wouldn't go playing with the GF settings until you understand what they mean.....



wuss

Re: Shearwater Petrel GF Settings

Posted: Fri Apr 18, 2014 7:53 am
by Joshua Smith
Marc wrote:
Joshua Smith wrote:
mheyns wrote:That makes sense as far as GF and NDL influence. Thanks for the clarification. I was on 21, but everyone was on 21 as well...


Oceanic comps have notoriously liberal NDLs. And I wouldn't go playing with the GF settings until you understand what they mean.....



wuss


Yes. But I am trained up to technical wuss levels. So I really know what I'm talking about, wuss wise.

Re: Shearwater Petrel GF Settings

Posted: Fri Apr 18, 2014 8:04 am
by Marc
Joshua Smith wrote:
Marc wrote:
Joshua Smith wrote:
mheyns wrote:That makes sense as far as GF and NDL influence. Thanks for the clarification. I was on 21, but everyone was on 21 as well...


Oceanic comps have notoriously liberal NDLs. And I wouldn't go playing with the GF settings until you understand what they mean.....



wuss


Yes. But I am trained up to technical wuss levels. So I really know what I'm talking about, wuss wise.



Hey, I am trained up, too.... Maybe we should get "technical know it all wuss" patches made.

Re: Shearwater Petrel GF Settings

Posted: Fri Apr 18, 2014 11:18 am
by H20doctor
seems like a strange question if your trained in nitrox ?

Re: Shearwater Petrel GF Settings

Posted: Fri Apr 18, 2014 12:07 pm
by pensacoladiver
AdrianSmith wrote:
mheyns wrote:I was diving down to the I-Beams yesterday and by the time I arrived there, pretty much a straight swim to the top of the I-beams too at around 100ft, my Petrel informed me that I had 2 minutes at 100ft before I exceeded my NDLs.

Given that my buddies had 15+min on their Oceanic puck computers I was a little perplexed.

My GF setting is 30/70, and I'm wondering what folks around here use? I don't generally exceed 120ft, and haven't gone down to >100ft since getting the computer (it's new).

Should I revert to 30/85 or some other setting? Start using the full functionality of the Petrel and taking the deco in stride? Upgrade to VPM?

Thoughts?


Did you by chance have it set to 21% while your buddies' computers were set to a higher EAN blend? That would explain the NDL time.

The GF shouldn't have any impact on the NDL. It determines how deep your first deco stop should be, not how much NDL time you have left.

-Adrian


That statement is true for the first (low) GF number, BUT, the last (high) number will absolutely have a direct effect on NDL time.

Re: Shearwater Petrel GF Settings

Posted: Fri Apr 18, 2014 12:15 pm
by pensacoladiver
H20doctor wrote:seems like a strange question if your trained in nitrox ?


???? Gradient factors are gradient factors, whether being used with air, nitrox or trimix.

You can use/adjust them on air just as well as you can with any other gas.

Re: Shearwater Petrel GF Settings

Posted: Fri Apr 18, 2014 12:24 pm
by pensacoladiver
This is nothing more than my experience... So take it with a grain of salt.

Shearwater defaults used to be 30/85. They changed it to 30/70 about 2 years ago after some deep diving bubble research that was done... Or so I understand that to be the case.

I used to dive 30/85 on all my dives, rec and tech. And I started to notice a small bit of tingling in my left shoulder after the dives.

As such, I switched to 30/70 for all dives and have had no further issues.

I am also 43 years old. If I was younger and experienced no issues, I would dive 30/85 with no worries.

If you think GF won't extend your bottom time, next time you are diving somewhere and your NDL is something lower than 99, increase your hi gradient factor in the water and watch what happens to your NDL

I'll bet you lunch it goes up. :-)

You can always adjust it right back as well.

Once your NDL gets pretty low, you can change your high GF to a lower setting... Say 60 and sure enough... You will show a deco obligation. You can set it back and the deco obligation will disappear.

It is a great computer, but as stated, read up on what the GF's mean and what it means to adjust them.

Re: Shearwater Petrel GF Settings

Posted: Fri Apr 18, 2014 12:43 pm
by Jeff Pack
30/70 is what DAN reccomends, more conservative than VPMB+2

Re: Shearwater Petrel GF Settings

Posted: Fri Apr 18, 2014 1:59 pm
by H20doctor
GF stands for girlfriend...his girlfriends computer was set differently.. Lol you need to retake your tech certification class

Re: Shearwater Petrel GF Settings

Posted: Fri Apr 18, 2014 2:45 pm
by pensacoladiver
There is absolutely no end to your numbskullery

Re: Shearwater Petrel GF Settings

Posted: Sat Apr 19, 2014 10:04 pm
by CaptnJack
pensacoladiver wrote:There is absolutely no end to your numbskullery


And coming from an old fart like you that's sayin' somethin'!

Re: Shearwater Petrel GF Settings

Posted: Sat Apr 19, 2014 11:44 pm
by pensacoladiver
Aren't you a 60s child?

Re: Shearwater Petrel GF Settings

Posted: Sun Apr 20, 2014 4:57 pm
by CaptnJack
pensacoladiver wrote:Aren't you a 60s child?


yeah your 'generation'

Re: Shearwater Petrel GF Settings

Posted: Sun Apr 20, 2014 5:04 pm
by pensacoladiver
No way buddy. I was hatched in 1970

Re: Shearwater Petrel GF Settings

Posted: Sun Apr 20, 2014 6:58 pm
by CaptnJack
pensacoladiver wrote:No way buddy. I was hatched in 1970


awww a puppy!

Let the numbskullery continue ;)

Re: Shearwater Petrel GF Settings

Posted: Mon Apr 21, 2014 1:17 pm
by mheyns
I guess I'm just perplexed because my Petrel definitely told me that I had 2 minutes before I hit NDL at 100ft (or in that low vicinity) and we had dropped almost straight to that depth. Took about 6-7 minutes to get there dropping from 0-30-40-60-80-100 ft increments.

GF was set to 30/70.

The PADI recreational table says that even at 100ft I should have plenty of time at that depth.

http://www.dolphinscuba.com/media/catalog/product/r/d/rdp.jpg

That said, when I use multi-deco to plan the dive it does tell me to make a deco stop as well. But why do the PADI recreational tables tell me I don't need to?

When I run a simulated multi-deco dive to 60ft even, I'm told I need a safety stop. The PADI tables don't even tell me I need a safety stop in that case so I'm perplexed.

Dec to 40ft (0) Air 60ft/min descent.
Level 40ft 1:20 (2) Air 0.46 ppO2, 40ft ead
Dec to 60ft (2) Air 60ft/min descent.
Level 60ft 29:40 (32) Air 0.59 ppO2, 60ft ead
Asc to 20ft (33) Air -30ft/min ascent.
Stop at 20ft 0:40 (34) Air 0.34 ppO2, 20ft ead
Stop at 10ft 1:00 (35) Air 0.27 ppO2, 10ft ead
Surface (35) Air -30ft/min ascent.

Maybe I'm missing something obvious here.

Re: Shearwater Petrel GF Settings

Posted: Mon Apr 21, 2014 2:11 pm
by lamont
recreational tables are sort of GF 100/100 -- pure buhlmann.

since that is overly aggressive, they typically include 'mandatory safety stops' (3 mins of deco) at 15 feet for 100-130 foot dives and similar. i believe the padi tables note that somewhere.

you can adjust the petrel to 100/100 in which case you'll just prevent the computer from telling you that you're in deco. 30/85 would also probably be a good intermediate choice -- 30/70 really is very conservative.

or you can go a few minutes into deco on the computer which is probably more accurate.

if you're nervous about the computer telling you that you're going a few minutes into deco at 100 feet, you probably should look at more training/experience.

32% at 100 feet also helps a lot.

Re: Shearwater Petrel GF Settings

Posted: Mon Apr 21, 2014 2:59 pm
by CaptnJack
In all seriousness, minus the numbskullery, the 70 of 30/70 is what is driving down your "NDL" to ridiculously low levels. BUT if you put it into plan mode, or the next time you are at the I beams take note of how long the new required stops are. Its going to give you 1 min at 10ft, then 2 mins at 10ft, the 3 at 10 and 1 at 20 etc. So ~15mins into the dive you end up with roughly the equivalent of a 3min safety stop. 30/70 is still about the most conservative GF setting commonly chosen so your dives will definitely be shorter than on the oceanic pucks. At least the bottom phase will be shorter, once you get up <40ft it won't matter at all.

Re: Shearwater Petrel GF Settings

Posted: Mon Apr 21, 2014 3:02 pm
by mheyns
It's not that I'm nervous about deco at 100ft, but more that I'm confused and trying to reconcile the apparent "extreme conservatism" of a technical dive computer against a recreational one or recreational tables (which one would expect to be even more conservative). I want to make sure that my petrel is working as it should be, and that I'm using it correctly.

GF100/100 does seem to conform better to recreational tables, very interesting. I did a little further reading and found some information which is beginning to make more sense.

It seems like normal Buhlmann tables basically took you straight up from a deep dive to the M value, and kept you there until offgassing was complete. They would get you shallow quickly, and offgas quickly, but at the expense of sitting on the m-value line. Just like you suggested, GF100/100 similarity. It sounds very similar to the RGBM model in Suuntos, or more like Oceanic's pure Buhlmann at GF100/100 which my buddy was diving, and are just very different from GF 30/70.

My takeaway is that I really need to think about what GFs to set per dive. It sounds like, within recreational limits/times, I can dive GF100/100 theoretically and be fine (given that I would trust my Suunto or Oceanic the petrel at those same settings). For long/deeper dives, more conservative GF settings would be better. I wonder when/if there will ever be a recommended set of GF guidelines for different types of dives. An interesting quote I found in an article, "if you use the same 20/80 Gradient factor for an 80m dive, on a 30m dive, you will have an excess of decompression in shallow water because we know that a safety factor close to zero (100/100) is applicable for this shallow water dive."

SOunds like I could have done that dive at GF 90/90 or GF 100/100 and would still have been fine/aligned with the Oceanic of my buddy. I wonder however about Suunto and Oceanic computers in that case...

I've done many dives as deep as 120ft with my Suunto Vyper and Oceanic and I've always been fine. If diving at what is effectively GF100/100 on those computers worked in the past, what is the benefit of more conservative GF settings. All food for thought I suppose.

Re: Shearwater Petrel GF Settings

Posted: Mon Apr 21, 2014 4:11 pm
by lamont
mheyns wrote:My takeaway is that I really need to think about what GFs to set per dive. It sounds like, within recreational limits/times, I can dive GF100/100 theoretically and be fine.


I would suggest not.

For "shallower" recreational dives it probably doesn't matter since you probably won't hit the NDL and will hit gas, time, cold, bladder limits first.

For "deeper" recreational dives, generally nobody recommends straight buhlmann now, particularly with no safety stop.

So you could, again dive 100/100 and then do a mandatory safety stop at 15 feet.

But what you're doing there is really diving something like X/85 and surfacing at something like 85% oversaturation, and lying to the computer about it so that you have it lying to you that you're not going into any decompression -- when you really are.

So you want to have the longer "NDLs", but you will encounter some decompression that you should treat as mandatory (just like the mandatory 3 minute PADI safety stop you get when you hit the "NDLs" at the limit of the PADI tables).