Error in SSI Science of Diving online materials?

[tryerson64]

I was browsing through the SSI "Science of Diving" online course materials today, and found this in their "decompression theory/dive computers" section:

"The mathematical model (they're referring to Haldanean models run by dive computers) only works with the correct dive profile, which is a multi-level profile with the deepest depth first, followed by subsequently shallower depths." They have a following section on dive computer "Limitations" which says "Going deep, then shallow, then deep again will yield unreliable results" in the computer's calculation of nitrogen absorption and desorption.

This just can't be correct. Computers integrate over actual depth and time, which will be accurate (well, as long as the model is accurate, that's a different topic) regardless of the ordering of depths on a dive.

What am I missing? Did I mis-read the materials somehow?

 

[scubadada]

You are correct, this is wrong, very wrong.

 

[Mobulai]

That sounds like experiences I read of sunntos rbgm (not a Gas content model)

Sounds off to me as well

 

[tryerson64]

I kept re-reading in hopes of figuring out how it might have been unintentionally garbled, but the same assertion is repeated on the following pages as well. Seems like they really meant it. I'm going to contact SSI and see about getting it changed. wish me luck

 

[Mobulai]

You should ask for a bug finders fee

 

[tursiops]

There was a consensus conference on this, and a critique offered, and that critique rejected.
Here they are:

 

[tryerson64]

Thanks - I hadn't seen the rejection, will read with interest; I was aware of the earlier discussions. Those specifically focused on repetitive dives, while the SSI materials are talking about a single dive, but I think they're similar enough.

I think @boulderjohn started a thread somewhere that talked about the history of the "deeper then shallower" guidance for repetitive dives. Maybe he could link that thread here? If I recall, his history suggested that it was canonified without any real concrete basis, other than it does minimize the surface interval between dives.

In any case, the SSI assertion that dive computers can't track nitrogen loading accurately if you go deep, then shallow, then deep again on a single dive just isn't right.

 

[tryerson64]

Here’s the thread with BoulderJohn’s history of the deeper-before-shallower repetitive dive topic.

I just finished Section 3 of the Science of Diving course, which is "Decompression Theory." I read some content I have questions about.

From the "Dive computer" section:

The mathematical model works only with the correct dive profile, which is a multi-level profile with the deepest depth first, followed by subsequently shallower depths.

Dive computers were designed for multi-level diving and are an excellent tool if they are used properly and conservatively. Use the computer with deep-to-shallow profiles, not deep-shallow-deep profiles. The computer continuously computes nitrogen absorption based on the actual depth and time actually spent there.
Therefore, if divers go deep then shallow, their nitrogen absorption is computed at a slower rate at the shallow depth. However, on descending again to greater depth, the theoretical nitrogen absorption is increased accordingly. As indicated earlier, the residual nitrogen affects that process, yet the computer does not accurately account for this affect. Therefore, the computer is not calculating absorption the same way your body is, which is potentially hazardous.

Is this a fairly common pov from certifying agencies? I do not remember seeing anything about this in the directions for my dive computer. I was unaware that reverse profiles were an issue for computers and thought that this theory would be thought of similarly to needing the first dive of the day to be the deepest.

This section is in regards to diving after flying:

SSI’s recommendations are:

1. A minimum surface interval of 24 hours is required to be reasonably assured a diver will remain symptom free upon ascent to altitude in a commercial jet airliner (pressurized to an altitude of 2500m).
2. A surface interval of more than 24 hours is recommended after daily, multiple dives over several days or dives that require decompression stops. The greater the surface interval before flight the less likely decompression sickness will occur.

Are most agencies still using 24 hrs despite DAN's more recent 18 hr recommendation?

And the thread starts with DanG noting the exact same thing about the SSI course that I did - just two years earlier! Remarkable that all three of us happen to live in the same area. And that SSI has not updated the online materials to fix this error.

Edit: mods, in your spare time, please feel free to move, combine, or untangle these threads.

 

[happy-diver]

When you walk into a Titanic exhibition you follow a prescribed path and then you exit
Just as when you visit the Titanic you first go down to the bottom, and then you go up

"Otherwise there is just CHAOS, right Max?"

 

[tryerson64]

Or you are forced to follow a deep-shallow-deep profile that is dictated by a cave.

My point was that any dive computer will accurately calculate the modeled nitrogen saturation on a single dive for any arbitrary profile, contrary to what SSI appears to be saying.