Another GF Question

**Serge** · 6th February 2011, 01:06

So, my buddy and I have been diving with factory default GF settings of 35/80 a few years now. We have been using a Doppler and never noticed any deviations from the norm. So, I'm thinking about tweaking up the low GF. Anyone use higher limits of say 50/80, and any experienced issues?

jkaterenchuk · 6th February 2011, 01:20

I have not used that setting....I am using 20/85 for all Trimix deep dives.

I think you will need to explain what gases you are using and dive depths and times for someone to provide useful comment.

John

**Dsix36** · 6th February 2011, 02:12

I have used 45/90 but my profile was so different from my buddies that I went back to 25/85 to stay more inline with the others.

Deep dives with 10/50 for dil. I know from experience that my knee gets a bit of a niggle at ??/100, so 90 for the high GF is as high as I like to push it unless an emergency comes up. 45 for the low GF, makes sense to me, as it gets me shallow sooner.

This post is worth what you paid for it!

**Serge** · 6th February 2011, 12:27

The dive computer accounts for the gas breathed, depth and time to calculate the M-value, these parameters are already accounted for and I don't see any good reason to adjust GFs to my dive profile. These settings are more based on individual physiology and tolerance to bubble formation and DCS. There have also been rumors that deep stops are not really optimal. So, my rationale is if the Doppler results are clean, there is real, individual-specific data to tweak the low GF up. My question is how high up do folks typically push the low GF?

Janos · 6th February 2011, 18:08

Realistically I struggle to make a 12m/min ascent rate so it's rare that I hit the GF low I use of 30.

That said, if I were you I'd keep it the same. I have a conservative attitude to deco. As someone once said to me - what are you going to do in the ten minutes you save? Invent a cure for cancer?

Janos

Packhorse · 6th February 2011, 23:17

Quote: (Originally Posted by Janos)

That said, if I were you I'd keep it the same. I have a conservative attitude to deco. As someone once said to me - what are you going to do in the ten minutes you save? Invent a cure for cancer?

Janos

That 10 minutes that you could spend diving!

**Serge** · 7th February 2011, 00:32

Quote: (Originally Posted by Janos)

Realistically I struggle to make a 12m/min ascent rate so it's rare that I hit the GF low I use of 30.

That said, if I were you I'd keep it the same. I have a conservative attitude to deco. As someone once said to me - what are you going to do in the ten minutes you save? Invent a cure for cancer?

Janos

Quote: (Originally Posted by Packhorse)

That 10 minutes that you could spend diving!

Let me get this straight... you guys dive super conservative so that you could get more bottom time?... Build more deco to get more bottom time?... Interesting approach

Johan Pretorius · 7th February 2011, 05:49

I always wonder... (baring emergency) what is the rationale for getting out of the water sooner?
Does "less time on deco = Bigger / better / superior diver?
Sounds pretty much like GI3's type of argument

I am pretty sure the skipper would save you some chocolate, or lollipops or whatever dive treat is usually in store for you after the dive, or you could just ask him nice to keep some goodies for you of your matres are the hungry types

Mark Chase · 7th February 2011, 14:15

My personal Voodoo is to run 10/90GF with rich trimix and 30/80 with nitrox

Why?

Deep stops manage helium rather than nitrogen so I cant see much point in very deep stops running nitrox profiles.

GF deco for Nitrox is well tested and well proven so running 80% GFs is a conservative approach to a well established minimum deco requirement.

GF for trimix is widely believed to over compensate for the helium on the shallow deco. So if you are a believer in this you can cut short the shallow stops by setting a more aggressive GF shallow.

But as i said its just voodoo

I am 47 and 95kg 5'10" ex smoker with slightly above avg levels of fitness. No bends to report so far running 10/90GF with dives mainly avg 65-75m for 120 -180mins.

ATB

Mark

John Liddiard · 7th February 2011, 14:21

Quote: (Originally Posted by Serge)

The dive computer accounts for the gas breathed, depth and time to calculate the M-value, these parameters are already accounted for and I don't see any good reason to adjust GFs to my dive profile. These settings are more based on individual physiology and tolerance to bubble formation and DCS. There have also been rumors that deep stops are not really optimal. So, my rationale is if the Doppler results are clean, there is real, individual-specific data to tweak the low GF up. My question is how high up do folks typically push the low GF?

I remember a lecture by Max Haan about 15 years ago (or maybe longer). He explained that the risk of a straight Buehlmann model increased significantly with depth and time, showing graphs of risk to support his point, based on doppler scans. So whilst a straight Buehlmann profile (GF 100/100) may be ok for shallower dives with minimal decompression, any calibration of 'personal tolerance' based on doppler scans from such dives cannot be interpolated to deeper and longer dives.

6th February 2011, 01:06	#1 (permalink)
Serge RBW mEmber rEvo II Evolution Join Date: Dec 2007 Location: NY Posts: 923	Another GF Question So, my buddy and I have been diving with factory default GF settings of 35/80 a few years now. We have been using a Doppler and never noticed any deviations from the norm. So, I'm thinking about tweaking up the low GF. Anyone use higher limits of say 50/80, and any experienced issues?
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6th February 2011, 01:20	#2 (permalink)
jkaterenchuk RebreatherWorld Sponsor HammerMeg Kiss Classic, Inspiration Join Date: May 2006 Location: PA Posts: 2,625	Re: Another GF Question I have not used that setting....I am using 20/85 for all Trimix deep dives. I think you will need to explain what gases you are using and dive depths and times for someone to provide useful comment. John
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6th February 2011, 02:12	#3 (permalink)
Dsix36 Going Down? rEvo Optima Dolphin Join Date: Aug 2007 Location: Deerfield_Beach, Florida Posts: 3,030	Re: Another GF Question I have used 45/90 but my profile was so different from my buddies that I went back to 25/85 to stay more inline with the others. Deep dives with 10/50 for dil. I know from experience that my knee gets a bit of a niggle at ??/100, so 90 for the high GF is as high as I like to push it unless an emergency comes up. 45 for the low GF, makes sense to me, as it gets me shallow sooner. This post is worth what you paid for it! __________________ . Quote: (Originally Posted by kwinter) I could just be blowing smoke out my butt. FOR ALL YOU IDIOTS THAT DO NOT KNOW WHAT SATIRE IS, STOP READING MY FRICKIN' TRIP REPORTS AND KISS MY ARSE!!!!!! MY ADVICE AND POSTS ARE WORTH EXACTLY WHAT YOU PAID FOR THEM!!!!!! *POLITICAL CORRECTNESS IS SO YESTERDAY AND I AM DONE WITH IT!!!!!*
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6th February 2011, 12:27	#4 (permalink)
Serge RBW mEmber rEvo II Evolution Join Date: Dec 2007 Location: NY Posts: 923	Re: Another GF Question The dive computer accounts for the gas breathed, depth and time to calculate the M-value, these parameters are already accounted for and I don't see any good reason to adjust GFs to my dive profile. These settings are more based on individual physiology and tolerance to bubble formation and DCS. There have also been rumors that deep stops are not really optimal. So, my rationale is if the Doppler results are clean, there is real, individual-specific data to tweak the low GF up. My question is how high up do folks typically push the low GF?
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6th February 2011, 18:08	#5 (permalink)
Janos &quot;Two Sheds&quot; JJ Rebreather Classic Kiss Join Date: Feb 2005 Location: East Surrey Posts: 950	Re: Another GF Question Realistically I struggle to make a 12m/min ascent rate so it's rare that I hit the GF low I use of 30. That said, if I were you I'd keep it the same. I have a conservative attitude to deco. As someone once said to me - what are you going to do in the ten minutes you save? Invent a cure for cancer? Janos
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6th February 2011, 23:17	#6 (permalink)
Packhorse RBW Member rEvoIII mini mCCR RG-UMF,IDA 71 Join Date: Jan 2008 Location: Auckland New Zealand Posts: 1,088	Re: Another GF Question Quote: (Originally Posted by Janos) That said, if I were you I'd keep it the same. I have a conservative attitude to deco. As someone once said to me - what are you going to do in the ten minutes you save? Invent a cure for cancer? Janos That 10 minutes that you could spend diving!
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7th February 2011, 00:32	#7 (permalink)
Serge RBW mEmber rEvo II Evolution Join Date: Dec 2007 Location: NY Posts: 923	Re: Another GF Question Quote: (Originally Posted by Janos) Realistically I struggle to make a 12m/min ascent rate so it's rare that I hit the GF low I use of 30. That said, if I were you I'd keep it the same. I have a conservative attitude to deco. As someone once said to me - what are you going to do in the ten minutes you save? Invent a cure for cancer? Janos Quote: (Originally Posted by Packhorse) That 10 minutes that you could spend diving! Let me get this straight... you guys dive super conservative so that you could get more bottom time?... Build more deco to get more bottom time?... Interesting approach Last edited by Serge; 7th February 2011 at 00:35..
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7th February 2011, 05:49	#8 (permalink)
Johan Pretorius RBW Member Inspiration Classic IDA-72, IDA-71 Join Date: Mar 2006 Location: Cape Town Posts: 276	Re: Another GF Question I always wonder... (baring emergency) what is the rationale for getting out of the water sooner? Does "less time on deco = Bigger / better / superior diver? Sounds pretty much like GI3's type of argument I am pretty sure the skipper would save you some chocolate, or lollipops or whatever dive treat is usually in store for you after the dive, or you could just ask him nice to keep some goodies for you of your matres are the hungry types
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7th February 2011, 14:15	#9 (permalink)
Mark Chase So many CCR So little etc JJ Hybrid Inspo, Hammer Head, KISS rEvo Join Date: Jan 2005 Location: Kent Posts: 4,362	Re: Another GF Question My personal Voodoo is to run 10/90GF with rich trimix and 30/80 with nitrox Why? Deep stops manage helium rather than nitrogen so I cant see much point in very deep stops running nitrox profiles. GF deco for Nitrox is well tested and well proven so running 80% GFs is a conservative approach to a well established minimum deco requirement. GF for trimix is widely believed to over compensate for the helium on the shallow deco. So if you are a believer in this you can cut short the shallow stops by setting a more aggressive GF shallow. But as i said its just voodoo I am 47 and 95kg 5'10" ex smoker with slightly above avg levels of fitness. No bends to report so far running 10/90GF with dives mainly avg 65-75m for 120 -180mins. ATB Mark __________________ See my "Doing It Chasey" video where I'm locked into a padded room, naked, with two ball bearings and within an Hour, I manage to lose one and break the other!!! Kevin Juergensen 16/11/08 [/QUOTE]
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7th February 2011, 14:21	#10 (permalink)
John Liddiard Photographer & Journalist rEvo II Mini Hybrid Join Date: Dec 2007 Location: Bristol Posts: 106	Re: Another GF Question Quote: (Originally Posted by Serge) The dive computer accounts for the gas breathed, depth and time to calculate the M-value, these parameters are already accounted for and I don't see any good reason to adjust GFs to my dive profile. These settings are more based on individual physiology and tolerance to bubble formation and DCS. There have also been rumors that deep stops are not really optimal. So, my rationale is if the Doppler results are clean, there is real, individual-specific data to tweak the low GF up. My question is how high up do folks typically push the low GF? I remember a lecture by Max Haan about 15 years ago (or maybe longer). He explained that the risk of a straight Buehlmann model increased significantly with depth and time, showing graphs of risk to support his point, based on doppler scans. So whilst a straight Buehlmann profile (GF 100/100) may be ok for shallower dives with minimal decompression, any calibration of 'personal tolerance' based on doppler scans from such dives cannot be interpolated to deeper and longer dives.
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