Why 2 gradient factors ?

[Lorenzoid]

You can. Some do. Personal choice and need.
Baker could have had just one GF that applied to the entire depth range. He provided two so you could adjust the "conservatism" as you call it, to be different at depth than near the surface.

I don't think it can be emphasized enough (and it has, by you here and by others in a number of posts above) that there is no universally agreed-upon relation between "conservatism" (that is, risk) and the pair of gradient factors, GF_Lo and GF_Hi.

The additional one, GF_Lo, was added because some people believed that stopping earlier in the ascent decreased risk. But there was still no general agreement on how early in the ascent, and different people would set their GF_Lo to different values to reflect their different beliefs. The reason there are two GF values is to allow for this adjustability or customization. Nowadays, more people believe that stopping earlier in the ascent may actually increase risk. But people may still set their GF_Lo to different values to reflect their differing beliefs about its effect on risk.

 

[scubadada]

From my Teric owner's manual:

The default conservatism of the system depends on the dive mode

For OC Rec mode the default conservatism setting is medium (40/85)

For OC Tec and CC/BO modes where some decompression is presumed, the default is a more conservative 30/70. The system provides several settings that are more aggressive than the default.

Do not edit GF values until you understand the effects.

The GFs are described as adjusting conservatism without further explanation. The last line would appear to be good advice.

 

[scubadada]

Very clear! But, still no explanation why we use 2 different GF values...

...You're welcome to set them equal, if you like...

Some people do.

You can. Some do. Personal choice and need...

It is interesting to me that some computers that run Buhlmann offer GF pairs that are the same, in at least some of their presets.

Aqua Lung i330R: 90/90, 35/85, 35/70

Seac: 85/85, 80/80, 75/75, 70/70, 65/65, 60/60, this is just like having only a GF high

Ratio: 93/93, 90/90, 80/80, 75/75, their older computers also run 30/90 and 20/89, newest computers run 50/85 and 45/80 in keeping with an increasing GF low. Their older computers run ZH-L16B, newest also run ZH-L16C

 

[Nelson_]

Keep in mind the derivation of the Buhlmann algorithms. ZH-L16B had the a coefficients decreased from ZH-L16A, and thus the Mo decreased for compartments 6, 7, 8, and 13. This shifts the M-value line to the right as the slope of the line remains the same with the lower intercept. ZH-L16B is suggested for use in tables. ZH-L16C underwent further changes with additional decreases in the a coefficients and Mo for compartments 5-12 and 14. ZH-L16C is suggested for use in dive computers and is the version used by most of us. So, ZH-L16C is more conservative in the medium and many of the slow compartments compared to the original algorithm, ZH-L16A.

So, there is already more safety built in in ZH-L16 C. Maybe the use of a lower GF low dates from ZH-L 16 A?

And in the article from prof. Doolette I read: “Tyler Coen at Shearwater Research Inc. noted that GF settings recommended by Fraedrich modify ZH-L16 M-values so that approximately the same level supersaturation is allowed at all stop depths.”.’

Another argument for parity?

 

[Nelson_]

Aqua Lung i330R: 90/90, 35/85, 35/70

Seac: 85/85, 80/80, 75/75, 70/70, 65/65, 60/60, this is just like having only a GF high

Ratio: 93/93, 90/90, 80/80, 75/75, their older computers also run 30/90 and 20/89, newest computers run 50/85 and 45/80 in keeping with an increasing GF low. Their older computers run ZH-L16B, newest also run ZH-L16C

Wow! Very interesting!

 

[inquis]

GF settings recommended by Fraedrich modify ZH-L16 M-values so that approximately the same level supersaturation is allowed at all stop depths.”.’

Another argument for parity?

No, comparable levels of supersaturation argue for GFLow = 0.83 * GFHigh.

 

[scubadada]

So, there is already more safety built in in ZH-L16 C. Maybe the use of a lower GF low dates from ZH-L 16 A?

And in the article from prof. Doolette I read: “Tyler Coen at Shearwater Research Inc. noted that GF settings recommended by Fraedrich modify ZH-L16 M-values so that approximately the same level supersaturation is allowed at all stop depths.”.’

Another argument for parity?

I've never seen a computer running ZH-L16A. Most all computers running Buhlmann use ZH-L16C, Older Ratio is the only example I know of running ZH-L16B. I would imagine GFs were mainly aimed at ZH-L16C. Planning softeware, MuliDeco, allows you to apply GF to B or C.

Coen was referring to GF 55/70, consistent with Doolette's 83% recommendation

 

[tursiops]

No, comparable levels of supersaturation argue for GFLow = 0.83 * GFHigh.

Within the usual range of GFs high, and considering the accuracy of the equation, this can also be expressed as GFlow=GFhigh - 15. A little easier to do in your head!

 

[KenGordon]

It feels to me that people forget that the basic dissolved gas models explicitly allow for greater over saturation at depth. The GF multiplier is applied to that and off gf high is greater than gf low tends to flatten the limit line.

The manipulation (or equivalent as GF depends on the deepest ceiling) done by GF could also be done by picking different constants for each compartment.

Basically you have a line drawn by decompression scientists being flattened by a formula invented by some bloke off the internet.

On the other hand the scientists had navy divers as test subjects but the bloke off the internet had a representative sample of other (derogatory descriptions of fitness, age etc deleted) blokes off the internet, who may like to pretend they are hard as nails, as test subjects.

 

[BlueTrin]

Very clear! But, still no explanation why we use 2 different GF values. Forget Pyle and bubbles, why don’t we use the same value for both?

It’s really up to you, once you understand what it does …