What's your SurfGF and how does it compare to your (Rec) GFHi?

[passeparici]

After reading this interesting thread and some other one, I think I pretty much understand the concept of GFHi, SurfGF but I still have a problem with GFLo ! Very often, in Rec diving, we can read that GFLo is not important because we stay without deco but as someone stated earlier, in Europe rec divers, if they are trained and have the ability, can make deco dives. I am not against lite deco dives and I saw many of the divers here put their GFlo the same value as their GFHi (like 80/80). Shearwater, on its medium setting, puts 40/85. 40 should force deeper stops which nowadays seems not advised. A graph on a previous post shows that a GFLo of 40 could add a stop at 9 m compare to only 6 m with a GFLo of 60 for instance. Is that the reason why many divers here put their GFLo at the same value of their GFHi ? I am not sure there is a consensus on that but what are the general feeling about that ?

 

[inquis]

@passeparici - One of the major researchers, David Doolette, sets GFLow at about 0.8 x GFHigh. You might find his explanation interesting.

 

[dmaziuk]

Fraedrich follow-up – The Theoretical Diver is an argument for keeping GFs in 75-85 range -- although his analysis does not include DSAT that's been best tested with (and designed for) no-stop diving. DSAT is closer to 95/95 (of course none of them match one-to-one).

 

[scubadada]

@passeparici - One of the major researchers, David Doolette, sets GFLow at about 0.8 x GFHigh. You might find his explanation interesting.

Fraedrich follow-up – The Theoretical Diver is an argument for keeping GFs in 75-85 range -- although his analysis does not include DSAT that's been best tested with (and designed for) no-stop diving. DSAT is closer to 95/95 (of course none of them match one-to-one).

I do mostly aggressive no stop diving but do about 5% light back gas deco, generally <10 minutes. I dive an Oceanic VT3 running DSAT and a Teric running 80/95. I run the 80/95 based on the Doolette article. However, with my short deco, I've had no stops deeper than 10 feet, up to 14 min.

DSAT and 80/95 give me long bottom times without deco or just shallow stops with deco. I use SurfGF to adjust my safety stop or to pad my deco stop so that I surface with a GF of no higher than 80. SurfGF has taken the guess work out of the final stop before surfacing.

 

[BlueTrin]

After reading this interesting thread and some other one, I think I pretty much understand the concept of GFHi, SurfGF but I still have a problem with GFLo ! Very often, in Rec diving, we can read that GFLo is not important because we stay without deco but as someone stated earlier, in Europe rec divers, if they are trained and have the ability, can make deco dives. I am not against lite deco dives and I saw many of the divers here put their GFlo the same value as their GFHi (like 80/80). Shearwater, on its medium setting, puts 40/85. 40 should force deeper stops which nowadays seems not advised. A graph on a previous post shows that a GFLo of 40 could add a stop at 9 m compare to only 6 m with a GFLo of 60 for instance. Is that the reason why many divers here put their GFLo at the same value of their GFHi ? I am not sure there is a consensus on that but what are the general feeling about that ?

There is not an exact rule about how GFLo should be set, but putting a low GFLo will add a fairly deeper first stop.

The older trend was to prefer deeper stops, the idea was that this would reduce the big bubbles and reduce the chances of getting bent.

Recent studies have shown that it is possible that it is actually better to do shallower stops because fast tissues have less chances to make you bent, so if you did a deep stop your slow tissues would in gas and this would increase your chances of getting bent.

There are still a lot of proponents on each side and it is not a closed matter.

I’d say that not many people put GFLo equal to GFHi for deco diving though, although the trend is to make GFLo higher than before, or so it seems.

Also your example of 9m vs 6m is probably not exposing the issue, the differences will appear for deeper dives. At 9m you’d probably not on gas your slow tissues.

This is my (possibly wrong) understanding from having read a few articles and seems a few presentations.

 

[Wibble]

I’d say that not many people put GFLo equal to GFHi for deco diving though, although the trend is to make GFLo higher than before, or so it seems.

GF-lo cannot be greater than GF-hi, certainly not on a Shearwater computer.

There’s a big difference between discussing Gradient Factors for Recreational (no deco) and Technical diving (with deco). For decompression diving the Gradient Factors have settled around GF50:80. For recreational NDL (No Decompression Limits) diving, the Gradient Factors are generally much higher, something like GF95:95 but with an added "Safety" decompression stop.

Shearwater computers show the SurfGF which is the current Gradient Factor if you instantly appeared on the surface. For NDL dives this would always be below 95 (assuming GF95:95). With decompression dives the SurfGF will always be higher than GF-hi, many hundreds for longer and deeper dives (a 60m/200ft dive for 40mins bottom time last weekend showed SurfGF over 400)

 

[passeparici]

It seems to me that 95/95 is really pushing the limit just not to have a deco. Is that safe to try so hard not to have a deco when you are a rec diver ? In Europe, once again, with adequate training, rec divers can have decos. For a "young" rec diver it might be safer to have the GFHi 85 or 80 for instance and have a few minutes of deco, or none (he has already a safety stop normally) than to have his GF 95/95 and be on the limit of NDL...
SurfGF is nice but the risk is having people using SurfGF to control the GF at the surface but if you have your SurfGF at 150 for instance at 5 m, if you wait for it to be around 95, as I read some people doing that, it may be around 27 mn wait ! (around 2% every mn it seems). You'd better have a normal deco then, planed by your computer and following tasted rules than just making your owns no ?
Back to GFLo, we can read different opinions , it is not clear.

 

[BlueTrin]

GF-lo cannot be greater than GF-hi, certainly not on a Shearwater computer.

My wording was confusing. It was two separate statements, not that GFLo > GFHi

 

[Wibble]

It seems to me that 95/95 is really pushing the limit just not to have a deco. Is that safe to try so hard not to have a deco when you are a rec diver ? In Europe, once again, with adequate training, rec divers can have decos. For a "young" rec diver it might be safer to have the GFHi 85 or 80 for instance and have a few minutes of deco, or none (he has already a safety stop normally) than to have his GF 95/95 and be on the limit of NDL...

There's a big difference between a decompression dive and a NDL dive.

A decompression dive means you are going well above the no-deco limits, as illustrated by the "SurfGF" which represents the instantaneous GF value if you were on the surface.

An NDL dive means the SurfGF has not gone above the GFs and therefore you do not have a decompression obligation. Added to that there is the "highly recommended" safety stop which adds 3 minutes of decompression which will act to bring down the current SurfGF.

A quick perusal of the, for example, PADI RDP (recreational dive planner) tables shows this as they recognise that deeper dives could stray into decompression and thus add the safety stops as a general precaution.

SurfGF is nice but the risk is having people using SurfGF to control the GF at the surface but if you have your SurfGF at 150 for instance at 5 m, if you wait for it to be around 95, as I read some people doing that, it may be around 27 mn wait ! (around 2% every mn it seems). You'd better have a normal deco then, planed by your computer and following tasted rules than just making your owns no ?

Let's be clear here. If you have a SurfGF above 95 then you are doing a decompression dive.

A SurfGF of 150 is most definitely a decompression dive (probably 20+ minutes of breathing an oxygen rich gas). There's no way that is an NDL dive which has strayed into a little bit of decompression; it's a technical dive where you must have gas redundancy and probably advanced nitrox gas switches.

Again, the Surface Gradient Factor is displaying the calculated gradient factor at depth if you were to be instantaneously transported to the surface. If you're doing a dive which is deep, say 40m/130ft, and you're there for a few minutes and then come "up the reef" to shallower depths, say 10m/33ft, the SurfGF would reduce as you're effectively decompressing as you are no longer on-gassing.

Back to GFLo, we can read different opinions , it is not clear.

In essence GFlo is not relevant to NDL diving. It is most definitely relevant to decompression diving as it affects the depth of the first decompression stop. Typically (for most people) this will be set to GFlo of 50. The deep stop proponents would use a much lower GFlo, but the argument went that if you stop deep, you're still on-gassing in the slower compartments and thus needlessly extending your decompression time.

This is clearly seen if you play with a dive planner such as MultiDeco and play with the gradient factors for the same dive profile.

 

[hammet]

That’s a percentage of M-value in the leading compartment if you surface directly at that moment (using standard ascend rate) without doing safety/deco stops

You can use it eg. for padding until certain Surfacing GF during safety stops, evaluating rick of breaking deco stops if needed, etc

Haven't read the whole thread yet but this is wrong. According to the teric manual, it is assumed you surfaced INSTANTLY not after a standard ascent rate. (Apologies if this was corrected later on).