Question Why GF high not lower that GF low?

[EFX]

Hello

I wanted to know why the GF high is always lower or equal to the GF low.
the GF low is always lower or equal to the GF high.

Some studies tell us that deep stops are not that great compared to shallow stops.

Why not have a GF high lower than GF low to spend longer time on the last stop?

Some studies indicate that some deep stops were maybe too deep. So, the tendency now is to make the first deco stop shallower by setting GFLo higher. But, reversing GF's so that GFLo > GFHi is not the way to plan for shallower first stops. In fact depending on the GF's used could result in a large penalty in deco time.

I ran two identical dives in my spreadsheet: air to 180 ft, 20 mins, 60 ft/min descent rate, 30 ft/min ascent rate, and no deco gasses. One dive used a GF (GFLo/GFHi) of 30/80 and the other 80/30. The 30/80 dive yielded a total of 64 min of deco with the 1st stop at 80 ft while the 80/30 dive gave 178 minutes with a 1st stop at 50 ft. That's quite a difference. The advantage of the 80/30 dive is you will reach the surface with a GF (where GF99 and SurfGF will be equal) lower than the 30/80. A GF of 29 verses 78 respectively. This makes sense considering the additional amount of time spent in the shallows to allow enough off gassing to further reduce the GF. The main reason for the longer deco time for the 80/30 dive is that the controlling tissue compartment (CTC) at the 10 ft stop is 9 with a half-time of 109 minutes verses a CTC of 7 with a half-time of 54.3 minutes.

I find it helpful to look at a graph of tissue inert gas for the dive profile in terms of the %GF. I've included this graph. Here is the link to my spreadsheet. Download the scuba dive excel spreadsheet.

 

[rjack321]

You could do 90/70. It's like extra anti deep stop. I doubt there is any human testing of this kind of approach so best of luck with it

 

[EFX]

In my above post I chose two dives with GFLo/GFHi of 30/80 and 80/30 because the graph I included used those GF's. Using the same profile as above but changing the GF's to GFLo/GFHi = 70/90 and 90/70 gives these results for a profile using: air to 180 ft for 20 min, 60 ft/min descent rate, 30 ft/min ascent rate, no deco gasses.

70/90: deco = 45 min, surfGF = 88, 1st stop at 60 ft, CTC at 10 ft stop = 7 with HT = 54.3 min.
90/70: deco = 65 min, surfGF = 68, 1st stop at 40 ft, CTC at 10 ft stop = 7 with HT = 54.3 min.

As you can see the closer GFLo is to GFHi the smaller the difference in the overall deco time. I don't see any advantage of doing reverse GF's.

 

[scubadada]

In my above post I chose two dives with GFLo/GFHi of 30/80 and 80/30 because the graph I included used those GF's. Using the same profile as above but changing the GF's to GFLo/GFHi = 70/90 and 90/70 gives these results for a profile using: air to 180 ft for 20 min, 60 ft/min descent rate, 30 ft/min ascent rate, no deco gasses.

70/90: deco = 45 min, surfGF = 88, 1st stop at 60 ft, CTC at 10 ft stop = 7 with HT = 54.3 min.
90/70: deco = 65 min, surfGF = 68, 1st stop at 40 ft, CTC at 10 ft stop = 7 with HT = 54.3 min.

As you can see the closer GFLo is to GFHi the smaller the difference in the overall deco time. I don't see any advantage of doing reverse GF's.

I dive 80/95. On aggressive no stop dives or light deco, I simply spend more time at my safety stop or last deco stop to surface with the GF I want, as guided by SurfGF. I have not surfaced with a GF greater than 80 since buying my SW Teric in 2019. I've become more conservative after 2350 dives and making it to 70 years old.

 

[EFX]

If you are not surfacing with a GF greater than 80, why not just set GFHi to 80? Then the computer will adjust the NDL or deco depths and times to bring you to the surface at or below 80.

 

[scubadada]

If you are not surfacing with a GF greater than 80, why not just set GFHi to 80? Then the computer will adjust the NDL or deco depths and times to bring you to the surface at or below 80.

Hi @EFX, good question, long answer.

I have been diving a computer since 2002. My primary computer has always been an Oceanic, running DSAT. A Pro Plus 2 from 2002-2010 and then a VT3 from 2010-current. I have dived a second computer for most of that time. Initially, a Cochran, more liberal than DSAT, and then a Geo 2 running DSAT. I decided that I wanted to gain experience with Buhlmann and in 2016 started diving a Dive Rite Nitek Q. I did not replace that with my Shearwater Teric until 2019 so have only had Surf GF since then.

My diving is mainly no stop, some of it aggressive, and about 5% light back gas deco, almost always <10 min of deco. So, I have 2300 dives with DSAT and am very comfortable with using it for my no stop and light deco diving. I only have about 1150 dives with Buhlmann and just 550 with SurfGF. Though I have experimented, I have chosen to dive Buhlmann at a GF high of 95 to reasonably match DSAT.

DSAT/GF high 95 gives me a liberal NDL that I would be comfortable surfacing from. All the liveaboard diving and many of the day boat charters I have used prohibit deco diving, I have no problem complying with that policy. I have good gas consumption and appreciate the dive time. As I have grown older, I have become more conservative in my diving. Well before having SurfGF, I would do an adaptive-like safety stop from a dive to my NDL and would pad my last deco stop. SurfGF simply takes the guesswork out of the equation.

As it turns out, most all of my no stop diving has a surfacing GF of well less than 80, the average being in the 50s. It is a rare no stop dive or one of my light deco dives where I use SurfGF to lower my surfacing value. I dive my 2 computers reasonably conservatively. For no stop dives, the more conservative of the 2 computers controls the dive. I clear one or both computers from deco before using SurfGF. I have made it to 70 years old, 2350 dives, without an episode of DCS. I would like to keep it that way to the end of my diving days.

 

[bubblemonkey2]

Computers like the ScubaPro G2 were running like GF 90/90 at the "L0 conservatism" level. We were diving those profiles regularly on tec-rec 40 to 60 metre dives with no issues.

As @harvalen said, a customary few extra minutes past 'clear' on shallowest stops gives same effect as an 'inverted' GF. Everyone seemed fine with that. Could indeed bake that in with 90/70 or something (or 'SurfGF')

A high GF_low means you go right up to your 'real' shallower deco stops ('slower tissues') at a reasonable, continuous ascent rate of ~10 metres per minute or less. Countless dives indicate that this seems sufficient to avoid a fast-onset, 'fast tissue' CNS hit, which is also differently addressed by formalized deep stops or low GF_low values.

A clear benefit of setting a GF_low to 70 or less is that it formalizes more controlled ascents, reducing the chances of 'fast tissue' hits from excessive ascent rates and poor off-gassing up to and between incremental stops. This is believed to be increasingly important with helium in the mix, which is supposed to be a faster-diffusing gas.

It is very easy to run 50/70 or 65/80 rather than 90/90 or 90/70, why not spend those few extra minutes on a better controlled ascent?

 

[inquis]

Completely swapping GFHigh and GFLow values makes it very difficult to make a meaningful comparison. The GF inversion effects are obscured by the differing surfacing effects (and vice versa). A better example would be comparing 65/80 to 95/80. Obviously the latter is still inverted from tradition & current researcher practice, but the surfacing tensions are much more comparable.

The primary impact of the higher GFLow is to shorten/eliminate early stops, resulting in correspondingly higher tensions in the faster tissues. Current researchers and newer Navy algorithms clearly indicate this is not desired. For the average diver, the more controlled ascent afforded by a somewhat lower GFLow certainly gives more leeway against depth control errors.

 

[EFX]

Completely swapping GFHigh and GFLow values makes it very difficult to make a meaningful comparison. The GF inversion effects are obscured by the differing surfacing effects (and vice versa). A better example would be comparing 65/80 to 95/80. Obviously the latter is still inverted from tradition & current researcher practice, but the surfacing tensions are much more comparable.

Let's compare two profiles with these GF's: 65/80 to 95/80.
Dive parameters: air to 180 ft for 20 min, 60 ft/min descent rate, 30 ft/min ascent rate, no deco gasses.

65/80: deco = 58 min, surfGF = 77, 1st stop = 60 ft, CTC at 10 ft = 7 with HT of 54.3 min.
95/80: deco = 52 min, surfGF = 78, 1st stop = 40 ft, CTC at 10 ft = 7 with HT of 54.3 min.

With the CTC and GFHi being the same the difference in deco time is due to the depth of the first stop. The 95/80 dive has shorter deco because the shallower first stop allowed greater off gassing at the start of deco. The spreadsheet shows a tissue pressure of 131.85 fsw at the start of deco for the 65/80 dive verses 128.05 fsw for the 95/80 dive.

 

[dmaziuk]

You could do 90/70. It's like extra anti deep stop. I doubt there is any human testing of this kind of approach so best of luck with it

It doesn't matter because Erik Baker's GFs never cross the M-value line. If you consider 100/100 "safe enough" then 100/90 should be "safer than enough"; it is unlikely that you'd have more people bent on 100/90 that on 100/100.

The testing will only tell you something new if you push a(*) GF past 100.

*) either of them