Gradients

[Green_Manelishi]

several of the more well known decompression planning software packages allow the user to adjust gradient
factors when calculating the deco obligations.

using GUE "DecoPlanner" as the example, a dive to
100 feet for 15 minutes on AIR (yeah, i know, not
particularly challenging) with a lo-gf of 30% and
hi-gf of 85 (both are defaults) yields stops of:

40 ft for 1 min
30 ft for 1 min
20 ft for 1 min
10 ft for 1 min

reducing the hi-gf to 70% causes all stops to stay at 1
minute EXCEPT 10 feet which is now 4 minutes.

increasing the lo-gf to 40% and reducing the hi-gf to 60%
causes the stop at 40 ft to be removed but yields a
20 ft stop for 3 minutes and a 10 ft stop for 6 minutes.

decreasing the lo-gf to 10% and increasing the hi-gf to 90%
causes the stops to begin at 60 ft for 1 minute per at
60, 50, 40, 30, 20, 10.

conventional wisdom holds that starting stops "deep" is a safe way to dive. without attempting to explain the conclusions i am drawing, other than it is 'better' to start
deep and make several stops of short duration rather than
a few of longer could you elaborate on the benefits/dis-advantages of adjusting gradients rather than staying with
the defaults.

Thanks

 

[DiverDave]

Eric Baker has an article at http://www.abysmal.com called “Clearing up the Confusion about Deep Stops” that explains deep stops using both the Pyle method and gradient adjustments. At the same site the other papers are worth a read, particularly “The Importance of Deep Safety Stops” on this subject.

Basically, the lower Gf-lo is set, the deeper will be the first stop. The first stop will occur when the leading tissue hits this percentage of the available decompression window. Gf-hi relates to how close to Mo for the leading tissue you can be on surfacing. The lower each is, the more conservative the deco profile will be. The article above gives a far more robust description.

If both factors are set to 100% then the model will behave exactly as an unadjusted Buhlmann model. If you set both very close to zero, you would have an enormous number of small stops (or your computer model will collapse and die!)

As for the benefits and disadvantages of various levels, remember that all deco models are just that, models. Altering the model will adjust your deco profiles using nothing but mathematics. You are not a model (You’re not Cindy Crawford in disguise are you?) and have to decide on a suitable balance between increasing safety and reducing hang time.

 

[Green_Manelishi]

Hi DiverDave,

thanks for your input.

i've read the articles etc and i assume that your reference to "Mo" is the M-values. i realize that deco is theory / models etc etc and you've stated the conclusions i was drawing after reading the articles and playing with
the profiles.

My question, perhaps unclear, is at what point does your
'safety factor' become so conservative as to be ridiculous.
Why 30% and 85% rather than 30 and 80 or 25 and 75 etc? Playing with the profiles shows that some modifications to the GFs make no difference to the number and duration of stops so is the physioliogical effect so insignificant as to
be inconsequential or is it a case of the programming rounding numbers?

Also, if you were planning a repetitive dive would you want to make the initial GFs lower/higher and how much based on the depth and duration of the dive(s)? Obviously, I could play around with the profiles, draw conclusions and then do the dives to 'see how i felt' afterward but if there
is a guideline that is applicable it would be useful.

It will be interesting to read the Doctor's response.

G_M

ps. some of the other members of this board have met me and
can testify that no matter HOW much i was made up i would
never be confused with Cindy Crawford ;-)

 

[Dr Deco]

Dear G_M:

The method of programming a “safety factor” into deco algorithms by adjusting the allowable supersaturation is quite interesting. This adjustment in some models is the “Gradient Factor” to which you allude. For the benefit of readers unfamiliar with this, allow me to say a few words.

BACKGROUND

. The M-value method was introduce by Robert Workman to allow for the changes in allowable supersaturation at different depths as one progresses to the surface by means of the decompression stops. These allowable supersaturations were at (or just below) the metastable limit, that supersaturation in the Haldane scheme whereby gas bubbles do not FORM in the tissue fluids. In terms of tissue micronuclei, the supersaturations are now considered (by some barophysiologists anyway) the levels of gas pressures that are too small for micronuclei to expand. That is to say, the surface tension (the water molecules’ inward tug on the bubble surface) on the microbubble creates an internal pressure (the Laplace pressure) that exceeds the supersaturation. When the supersaturation is small, only a very few micronuclei exceed the “Laplace cutoff” (as I term it).

The M-values allowed Workman to create an easy algorithm for table calculation. It represented the supersaturation without bubble formation. On a graph, it represented a point of pressure greater than ambient. That is, gas at ambient pressure is saturated, and at greater than ambient is supersaturated. One method to modify the supersaturation is to reduce the “allowable gradient.” This can be done without recourse to any mechanistic reasons, simply saying that less is better.

The amount of the gradient, the degree of allowable supersaturation, is adjusted by a factor that varies from 0.0 to 1.0. Zero is no supersaturation and 1.0 is whatever the maximum would be for that model (any type of Haldane model). These allowable supersaturations will vary for a Buhlmann, Workman, Roger (PADI) or whatever.

In some cases, the variations of the gradients will not produce differences that are particularly large - - a few minutes here and a few minutes there. The model will, however, be touted as being very significant by the model developer. I personally am unconvinced hat this has physiological significance, except that deep stops do seem to be of value when neurological problems such as fatigue are considered.

IN PRACTICE

When we look at recreational diving and DCS, the data indicates that DCS problems do not occur while the divers are in the water (or very seldom). With the exception of fatigue, it could be argued that DCS is brought on by the stress of exertion from leaving the water, climbing the ladder to the boat, moving around on the boat, and lifting heavy equipment between dives. These are all activities that are known to participate in stress-assisted nucleation . In my opinion (and it is only that, the differences in final results with dives can more often be traced to divers’ activity after the dive than while in the water.

That is not to say that anything goes while you are in the water, and it is only what happens topside that matters. Rather I am implying the avoidance of musculoskeletal stress after leaving the water is important - - very important. These things are not currently addressed in dive computers, and they could not be since it is not a gas-loading question. Nor is it a question of the size-number distribution of tissue micronuclei during decompression since that cannot be programmed into a computer with confidence.

Since tissue micronuclei are [probably] formed during the inter-dive interval, the adjustments of the computer parameters may not be as important as it would seem. I would advise against generating the nuclei on the surface and carrying them with you on the next descent.

They are not good dive buddies.

___________
Dr Deco [sp][sp] :doctor: