Rob Stewart Investigation

[wedivebc]

Wow! No switch to a bailout Deco Gas? Why was 10/20 programmed in if it wasn't used?
CNS 02 of 78-86%? 25 minute surface interval between the second and third dive to 220 fsw? Running that 90/90 GF on the last stop he had 1 stop calculated at 20 ft with an assent rate of roughly 60 ft/min (180ft/3min). Approximately 6 minutes of deco at 20 ft. I know where I'm placing my money. Type 2 DCS. His fast tissues never had a chance.

Why would someone switch off a functioning rebreather to a bailout gas?

 

[Rosborne]

Wow! Why was 10/20 programmed in if it wasn't used?
CNS 02 of 78-86%? 25 minute surface interval between the second and third dive to 220 fsw? Running that 90/90 GF on the last stop he had 1 stop calculated at 10 ft with an assent rate of roughly 60 ft/min (180ft/3min). Approximately 6 minutes of deco at 10 ft. I know where I'm placing my money. Type 2 DCS. His fast tissues never had a chance.

All very relevant questions.

 

[wedivebc]

And it looks like the He content of dil used was understated according to the profiles where 10/20 is listed as dil, an inappropriate gas for those depths, and will impact the amount of deco if He content of the dil was more like 60%, something more typical for that depth.

How do you even blend 10/20?

 

[Protondecay123]

Many people question that GF. But the Navy report described it as aggressive, but within acceptable parameters.

Here is a chart of Hi Gradient Factors from DCS Hits on Recreational Divers. This is a link to the paper Dive Risk Factors, Gas Bubble Formation, and Decompression Illness in Recreational SCUBA Diving: Analysis of DAN Europe DSL Data Base

 

[KenGordon]

The SI of 25 minutes between the second and third dives looks to me to drop them in with considerable bubbling going on. They’d all be squashed to get past the lungs and distribute them throughout the arterial system.

They did several things that increase the likelihood of a bend. High GF (more than 90 due to the lying to the computer about the helium - that alone would be an extra 10 minutes of stops), very fast ascent to the (relatively shallow) first stop, three long deco dives in a day, and that extremely short SI before the final bounce.

Given nothing else that was clearly the cause (bad gas, broken kit, hit by a boat etc) it seems to me that they were bent.

My normal settings (50/80) would have given me another half an hour (so 85 minutes rather than 55) of deco/ascent time.

 

[Tassi Devil Diver]

If I did the first dive according to my usual parameters 70m for 35min with TX 15/60 (I don't lie to my computer) and GF 50/80 my run time will be 142min a whopping 50min more deco!!

 

[KenGordon]

If I did the first dive according to my usual parameters 70m for 35min with TX 15/60 (I don't lie to my computer) and GF 50/80 my run time will be 142min a whopping 50min more deco!!

I assumed 65m rather than 70 as they didn’t spend long at max depth. I mostly did it to see how the helium lies changed the runtime.

 

[Protondecay123]

The actual He content was 50% if I'm not mistaken,

Do you know the content of Oxygen? I'm thinking his CNS O2 may have been much greater than the computer calculated.

 

[KenGordon]

Do you know the content of Oxygen? I'm thinking his CNS O2 may have been much greater than the computer calculated.

The computer was measuring ppO2 so the CNS numbers ought to have been ok. In any case people get away with taking much worse liberties with O2 exposure that they do shortening deco.

 

[Protondecay123]

The computer was measuring ppO2 so the CNS numbers ought to have been ok. In any case people get away with taking much worse liberties with O2 exposure that they do shortening deco.

Well Ken, I was looking at their mix and they had 10/20 entered which I figured was a little on the hypoxic side for 218 fsw. Rob says that the Helium content was actually 50% not 20%. So I thought that maybe the Oxygen content was more like 18% or 15%? Which should push up the CNSO2