Question Iso-risk decompression schedules

[LFMarm]

I just watched the great presentation of David Doolette at RB4 on alternative decompression strategies (Advances In Decompression Theory And Practice). He shows a Navy algorithm that provides decompression schedules that are iso-risk. Interestingly they are similar to GF 50/85 for shallower dives but become more conservative for deeper dives. Does anybody know what these algorithms are and how to replicate them with Bulhman GF as a function of depth?

 

[LFMarm]

As an example for the 250ft / 76m dive on the right the profile from GF50/85 and that from XVal-He-9_040 are basically the same but for the 400ft / 122m dive on the right the iso-risk profile requires significantly longer deco (~60 min longer). Interestingly, the depth of the first stop is practically the same but the duration of the stops is longer.

 

[Whitrzac]

wonder how close a 50/70 would line up.

 

[L13]

You can replicate the algorithms by using a non-linear interpolation of GF Low/High as you surface. If the first stop is the same, then the GF Low replicates the beginning of the alternate algorithm. For the graph shown, the later stops are longer, which is equivalent to a lower GF at each of those depths (and a lower GF High).

The first step would be to reduce GF high to produce the same total deco time as the iso-risk profile. By itself, that might get you very close. If not, add a GF Middle interpolation point. It shouldn't take very many intermediate interpolation points to produce a result that is the same withing measurable tolerances.

You could write the target GF's (maybe planed using something like SubSurface) per depth in your wetnotes and use the GF99 feature (or even SurfGF though less convenient) on your dive computer to implement. A GF+3m or GF+10ft feature would make it even easier, but doesn't exist.

In general, much of what makes the current GF values significantly non iso-risk is the the relatively large difference between GF Low and GF high settings currently in vogue.

 

[L13]

wonder how close a 50/70 would line up.

Modeled in SubSurface, 50/70 would produce the same first stop depth and similar total time as XVal, so the curve would be much closer to the same.

Here is a proposed algorithm that will get even closer across short and long deco's:

• GF High = 120% GF Low
• Iterate to determine GF low such that:
• GF Low = 70% - 1% * (deco time/20 minutes)
• The three parameters above (120%, 70%, 20 min) may need a little tweaking to zero in even better.

The result will be close to Iso-Risk with a shape close to XVal above.

 

[LFMarm]

This is exactly the type of comparison I would like to do. Do you have a way to run XVal profiles for different depths and bottom times,

 

[Matan]

I just watched the great presentation of David Doolette at RB4 on alternative decompression strategies (Advances In Decompression Theory And Practice). He shows a Navy algorithm that provides decompression schedules that are iso-risk. Interestingly they are similar to GF 50/85 for shallower dives but become more conservative for deeper dives. Does anybody know what these algorithms are and how to replicate them with Bulhman GF as a function of depth?

No.

but looking at the graph for the deep 400' dive he presented - it seems the total deco difference was +1h for the safer algorithm


Matan.

 

[Underwater Tourist]

If the algorithms dont have the same total dive time, they cant be really compared

You can make any algo safer just by spending more time on deco

 

[LFMarm]

If the algorithms dont have the same total dive time, they cant be really compared

You can make any algo safer just by spending more time on deco

Off course. The point is to know how safe a certain profile is. The Algorithm presented by Doolette tells you the probability of getting DCS so it would be very useful to be able to compare to Buhlman to know how safe your GF are. It seems from his presentation that a given GF has different safety margin depending on the depth (safer with smaller deco dives, less safe with big dives).

 

[L13]

It seems from his presentation that a given GF has different safety margin depending on the depth (safer with smaller deco dives, less safe with big dives).

Not just depth, also bottom time. For two dives with the same GF's but different deco times, risk goes up as deco time goes up. But it isn't a linear function.