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

[stuartv]

perhaps someone can explain how one can get a a surface GF of less than 60.

If you were saturated at 20 ft the ratios would be 1.6:1.6 or 1:1 and then when you hit the surface you would now be 1.6:1? unless you safety stop was at 10 ft. I can see that being able to happen but not probably by tech divers but not rec divers..

This is just a quickie, off the top of my head, so I may not have thought it through all the way. But...

If you breathed O2 for long enough, I reckon you could get to a SurfGF of 0 (less, actually, if such a thing were "allowed"). I mean, you could get out and be actually on-gassing, instead of off-gassing. If you breathed O2 for long enough....

 

[dmaziuk]

perhaps someone can explain how one can get a a surface GF of less than 60.

If you were saturated at 20 ft the ratios would be 1.6:1.6 or 1:1 and then when you hit the surface you would now be 1.6:1? unless you safety stop was at 10 ft. I can see that being able to happen but not probably by tech divers but not rec divers..

You have to be saturated to begin with. Which you could do easily enough in the fastest compartment, but it'll off-gas fast too, so it'll cancel out.

E.g. if you take 5 minutes to come up from 5 msw, your 5-minute TC will go from saturated to GF 50.

 

[NAM001]

This is just a quickie, off the top of my head, so I may not have thought it through all the way. But...

If you breathed O2 for long enough, I reckon you could get to a SurfGF of 0 (less, actually, if such a thing were "allowed"). I mean, you could get out and be actually on-gassing, instead of off-gassing. If you breathed O2 for long enough....

I agree totally under those circimstances. but with in the rec,,, non technical common ow aow methods of diving.

 

[stuartv]

I agree totally under those circimstances. but with in the rec,,, non technical common ow aow methods of diving.

My point was that a higher (than air) FO2 could get the SurfGF lower.

Maybe calculate for yourself what you would have if you were on EAN32, doing your stop at 15', and saturated? My gut says that right there would result in less than GF60.

 

[NAM001]

My point was that a higher (than air) FO2 could get the SurfGF lower.

Maybe calculate for yourself what you would have if you were on EAN32, doing your stop at 15', and saturated? My gut says that right there would result in less than GF60.

I agree

 

[stuartv]

I agree

Actually, I think this whole thing is ignoring a fundamental point. The actual M value. GF60 means you are 60% of the way to the M value. Not 60% greater than surface ambient. If you surface with tissue tension at 1.6 ATA (i.e. saturated at 20'), and the M value for that compartment is 3.2 (I think! Not sure off the top of my head), then you're only at GF50.

 

[rsingler]

@EFX ?

 

[stuartv]

You have to be saturated to begin with. Which you could do easily enough in the fastest compartment, but it'll off-gas fast too, so it'll cancel out.

E.g. if you take 5 minutes to come up from 5 msw, your 5-minute TC will go from saturated to GF 50.

If you assume you're breathing air, then your inspired PPN2 is 1.19ATA. If you're saturated, that is also your tissue tension of N2.

If you instantly surface and then wait 5 minutes, the tissue tension will drop halfway from 1.19 to .79. So, you'll be at 0.99 ATA tissue tension of N2.

The ratio of tissue tension to ambient is 0.99 / 0.79 = 1.25. That is your pressure gradient 1.25:1. So, if the M value for the 5 minute compartment is 2.5, then yes, you'd be at GF50 in that compartment.

And if you are breathing something other than air, then you would be at a different GF. All the way up to if you're breathing pure O2, you'd be at GF0.

Is 2.5 the M value for the 5 minute compartment?

Disclaimer: I'm no expert on the Buhlmann algorithm. I think I have a reasonably layman's understanding, but what I just wrote could be as full of holes as Swiss cheese.

 

[Jay]

FWIW, @Jay already commented on this here: What's your SurfGF and how does it compare to your (Rec) GFHi? , calling it "trivial". Just wondering if that's been tested.

At the moment, I think it's small, but perhaps not as small as I thought previously because .... playing around with SubSurface, on a 30m dive for 15m on air, surfacing at 9m/min versus 99m/min (max permitted) produces a difference of 9GFs. (using 16C).

 

[NAM001]

Actually, I think this whole thing is ignoring a fundamental point. The actual M value. GF60 means you are 60% of the way to the M value. Not 60% greater than surface ambient. If you surface with tissue tension at 1.6 ATA (i.e. saturated at 20'), and the M value for that compartment is 3.2 (I think! Not sure off the top of my head), then you're only at GF50.

ratios dont always work like we want them to b if you have a 1:1 ratio at 20 ft then it is really 1.6 to 1.6 then gotothe surface and it is now instantaniously 1.6 to .

Yes if you had tissues at 3.2atm and you were at 20 or 1.6 atm then the ratio would be 2:1 goto the surface and you get 3.2 to 1 SS is to lower the 2:1 to a lower figure so you can surface and not exceed 2 to1 minus the level of conservatism. that gets messed up with who is the controlling tissue and things. but in its most basic function it would be correct. thew slower tissues are so much not the problem.