Question Iso-risk decompression schedules

[dmaziuk]

And again, I will say this is not exactly correct. You are ignoring that the model (a specific model implied by your use of the term "GF") uses multiple compartments - each of which has its own GF and its own risk level.

The compartments are hypothetical. I strongly doubt Herr Doktor went so far into the weeds as to try and quantify risk for each individual unreal tissue. I believe the spacing/timing of tissue compartments is designed to approximate the overall picture -- like the integral is designed to approximate the area under the entire curve, not of each of its individual slices.

I think what you're talking about is the kind of DCS: skin bends vs. joint pains etc., and it's a very valid point, but the problem with it is you have to get bent to find out which one it is. Until you do it remains, like the tissue compartments: hypothetical.

 

[stuartv]

The compartments are hypothetical. I strongly doubt Herr Doktor went so far into the weeds as to try and quantify risk for each individual unreal tissue. I believe the spacing/timing of tissue compartments is designed to approximate the overall picture -- like the integral is designed to approximate the area under the entire curve, not of each of its individual slices.

I think what you're talking about is the kind of DCS: skin bends vs. joint pains etc., and it's a very valid point, but the problem with it is you have to get bent to find out which one it is. Until you do it remains, like the tissue compartments: hypothetical.

So, you think they used statistics from dives to work out some of the M-values and then just made up the others?

 

[dmaziuk]

So, you think they used statistics from dives to work out some of the M-values and then just made up the others?

I'm sure you remember that Haldane didn't even do that, he had the M-value of 2:1 (and made up 4 tissue compartments with half-times). Buhlmann, OTOH, made up M-values from tissue half-times: a = 2*T^-1/3, b=1.005-T^-1/2.

 

[LFMarm]

I realized that Shearwater computers are not limiting GFs to multiple of 5 so here is the more precise table of GFs corresponding to a DCS risk of 2.3% as a function of the TST:

TST (min)	GF Low	GF High
45	69	83
50	66	79
55	63	75
60	60	72
65	57	68
70	54	65
75	51	61
80	48	58
85	47	56
90	45	55
95	44	53
100	43	51
105	41	50
110-130	37	45
135+	35	42

 

[leadduck]

Please consider that iso-risk here means risk *per dive*. If you do three short recreational dives on one day with 2% risk each, then you have 6% risk on that day. Your friend did only one long tech dive on that same day with 6% risk for that one dive. Both of you had the same risk of DCS on that diving day.

You do accept that a diver with 100 dives a year has a higher risk than a diver with 10 dives a year (assuming the dives are all identical). You wouldn't ask the frequent tech diver to set the computer to an extremely conservative setting so that his DCS risk per year is the same as for a ten-shallow dives-per-year tourist. For the same reason we should accept that Pdcs *per dive* grows with dive time and not strive for iso-risk across dives with vastly different bottom time.

 

[dmaziuk]

EDIT: the risk on the day of 3 short recreational dives is 2%. Ask a statistician why.

 

[leadduck]

EDIT: the risk on the day of 3 short recreational dives is 2%. Ask a statistician why.

no, please see Doolette's paper https://apps.dtic.mil/sti/trecms/pdf/AD1215316.pdf p.28 last paragraph:

"(...) , before a diver sets out to perform a repetitive series of three dives, each with 2.3 % PDCS , the risk of DCS on at least one of the dives is the binomial probability of no DCS on all dives and is (...) 0.067. This is a higher risk than performing a single dive using an XVal-He-9_040 or XVal-He-9_050 Thalmann Algorithm (...)"

 

[LFMarm]

I did some more reading and PrT= (Depth [m]/10+1)*sqrt(time [min]) is a better parameter for deco risk exposure than TTS. Here are some iso-risk curves showing corresponding PrT and GF to have a DCS risk of 2.3%.

 

[Alekseolsen]

Sorry for going slightly off-topic, but this 2.3% chance of DCS sort of scares me. I've been well aware that deco diving does involve risk, but that ine statistically should get bent in a year seems a bit over the top.

Would "all types" of DCS be included here? As in, skin DCS would likely count for a large part of the cases?

 

[LFMarm]

Sorry for going slightly off-topic, but this 2.3% chance of DCS sort of scares me. I've been well aware that deco diving does involve risk, but that ine statistically should get bent in a year seems a bit over the top.

Would "all types" of DCS be included here? As in, skin DCS would likely count for a large part of the cases?

As far as I understand, 2.3% is the typical risk in all Navy tables, i.e., most tables used for recreational diving Deep in the Science of Diving