Gradient Factors - What is Everyone Using?

[CWK]

This is not correct. There are no gradient factors in pure Buhlmann. Gradient factors 100/100 only will give a profile identical to pure Buhlmann; as soon as you set GF-Lo to be less than 100, you will generate stops that are deeper than pure Buhlmann, and are therefore 'deep stops'.

Incidentally, Prof. Buhlmann himself didn't suggest using "pure Buhlmann", he suggested using 95% of the m-value (...for young, fit, expendible military divers), which would be equivalent to gradient factors 95/95.

I do think that gf high setting allows us to nominate a % of m value that we would like to dive to, and in the sense that we follow the Buhlmann algorithm for calculating ongassing, offgassing, ceilings, and stop times, then we are following a pure Buhlmann algorithm.

The last time I looked, my computer does not allow me to set a gf low value that exceeds the gf high value. If, as you say, any gf low value below 100 starts generating deep stops, then it would follow that any gf based computer would not be pure Buhlmann, but the way to minimise the deep stops would be to set gf low to the same value as gf high.

I note that the NEDU study compared the Buhlmann algorithm against a bubble algorithm. There has been no study on whether gradient factors offer any benefit over pure Bulhmann. Is 10/100 better than 100/100?

 

[KenGordon]

I note that the NEDU study compared the Buhlmann algorithm against a bubble algorithm

No, NEDU did not use Buhlmann for either side of the deep stops study. They were both Navy profiles.

 

[CWK]

No, NEDU did not use Buhlmann for either side of the deep stops study. They were both Navy profiles.

Is Navy not based on Buhlmann? I thought it was. But I’m happyto stand corrected.

Nevertheless, the question regarding gradient factors remains. Is 10/100 better than 100/100?

 

[KenGordon]

Is Navy not based on Buhlmann? I thought it was. But I’m happyto stand corrected.

Nevertheless, the question regarding gradient factors remains. Is 10/100 better than 100/100?

Not this navy profile

“candidate shallow stops dive profiles followed decompression schedules prescribed by the Thalmann Algorithm with the VVal-18 parameter set,3,8 which underlies the N2-O2 decompression tables in the U.S. Navy Diving Manual, Revision 6. The Thalmann Algorithm is a classical “deterministic” algorithm in which decompression is determined from the calculated compartmental dissolved gas contents and a table of M-values. In the Thalmann Algorithm, when a compartment is supersaturated, compartment gas kinetics may switch from exponential to linear, slowing gas washout and prolonging decompression stop times. “

But most fun of all, the way of calculating the compartment pressures is different to more old fashioned models such as Bühlmann. See the bold bit above.

It is still dissolved gas, and still has M values so is similar In general principle to Buhlmann.

100/100 vs 10/100? Nobody can tell you that either. And it likely depends on the depth, length of dive etc.

 

[huwporter]

The last time I looked, my computer does not allow me to set a gf low value that exceeds the gf high value. If, as you say, any gf low value below 100 starts generating deep stops, then it would follow that any gf based computer would not be pure Buhlmann, but the way to minimise the deep stops would be to set gf low to the same value as gf high.

Correct, anything with GFs is NOT pure Buhlmann. Gradient Factors are a completely separate add-on to Buhlmann, developed years later by a different dude (Eric Baker). However, I don't believe you'll find an implementation of the Buhlmann algorithm that doesn't also include gradient factors nowadays, the two have become pretty much synonymous. (Shearwater, for example, describe their computers as using a "Buhlmann GF algorithm")

Nevertheless, the question regarding gradient factors remains. Is 10/100 better than 100/100?

I think you'll find general consensus that GF-Hi of 100 is too aggressive for old, fat, recreational divers, who don't like to see themselves as expendible (compared to navy divers, anyway), and current best wisdom has that GF-Lo is best somewhere in the gap between 10 and GF-Hi rather than at either extreme, but I doubt you'll find anything concrete on exactly what the 'ideal' is, other than many opinions, from people who have varying levels of being informed.

 

[Mike1967]

I do think that gf high setting allows us to nominate a % of m value that we would like to dive to, and in the sense that we follow the Buhlmann algorithm for calculating ongassing, offgassing, ceilings, and stop times, then we are following a pure Buhlmann algorithm.

Anything other than 100/100 isn't "pure buhmann".

 

[cathal]

60/70 for everything in open water, scuba and wrecks with up to 3 1/2 hours of deco so far.

 

[leadduck]

Anything other than 100/100 isn't "pure buhmann".

The question is what is pure Bühlmann. When creating tables, to add a safety margin, he added a 2m (7ft) safety margin on the max.depth and used max.depth also during the descent. When using the table you have to round up bottom time, altitude, and depth to the next value listed in the table. In this way, using the tables in Bühlmann's book "Decompression - Decompression sickness" is somewhere between GF70/70 and GF80/90.

 

[huwporter]

The question is what is pure Bühlmann. When creating tables, to add a safety margin, he added a 2m (7ft) safety margin on the max.depth and used max.depth also during the descent. When using the table you have to round up bottom time, altitude, and depth to the next value listed in the table. In this way, using the tables in Bühlmann's book "Decompression - Decompression sickness" is somewhere between GF70/70 and GF80/90.

Interesting - got any more detail on this? Are these tables intended for recreational or for military use?

 

[PfcAJ]

In having success with 60/85. Shallow, deep, long, short, whatever. Seems to work well for me.