Error in SSI course materials still not fixed

[tryerson64]

Today in the Computers forum I posted about an error in the SSI online Science of Diving materials - which I then saw was identified two years ago(!) by DanG; his original thread is copied below.

What would need to happen for SSI to fix this error?

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?

 

[rick00001967]

i cant speak to the computer question as i guess it would depend on the algorithm used, but i am not sure you are interpreting the statement accurately.
perhaps someone at ssi could clarify it.
but i read it as saying it is ok to do a dive starting deep and then moving shallow. which is what most dives are.
but they are recommending not to start a dive deep, move shallow, then go deep again during the same dive. not sure why anyone would do this anyway.
this is not the same as doing a shallow dive first, taking a surface interval, and then doing a deep dive. which is how i think you are interpreting it.
as far as the 24hr recommendation.....our local shop has always recommended this. they even recommend this after pool sessions.
i believe the pilots that were trained had said their standards for flying insist on a minimum of 24hrs after breathing any compressed gas for any duration of time.
depending if you have done a single dive or multiple days of multiple dives, DAN does have different recommendations. but people seem to often leave out the fact that DAN says those are "minimum" times before flying.
i dont know about you, but i try to avoid doing anything to a minimum standard.

 

[tryerson64]

I agree on doing things beyond any minimum standards. The flying after diving discussion was a digression in DanG’s post, and not the subject I was calling out.

My point was that modeled nitrogen loading on any arbitrary profile on a single dive will be accurately calculated by a dive computer, contrary to what SSI seems to be saying. A deep-shallow-deep profile is not unheard of; imagine a cave or wreck penetration…

 

[Dan G]

Saying " I avoid doing things to a minimum standard" is a bit of a red herring. At one point, the most up to date research by DAN stated that 24 hrs was the minimum a diver should wait after repetitive diving. Was waiting 24 hours OK or was that only following the minimum standard? Now that the minimum standard is 18 hours, how has 24 hours all of a sudden become better than it was pre-2002? Shouldn't 24 hours still be considered the minimum since it was the minimum about 25 years ago?

I dive with two second stages, one primary and one back up, which seems to be the minimum any cert agency recommends. Should I have three or four second stages to avoid using the minimum number of second stages SSI states to dive with?

 

[rick00001967]

Saying " I avoid doing things to a minimum standard" is a bit of a red herring. At one point, the most up to date research by DAN stated that 24 hrs was the minimum a diver should wait after repetitive diving. Was waiting 24 hours OK or was that only following the minimum standard? Now that the minimum standard is 18 hours, how has 24 hours all of a sudden become better than it was pre-2002? Shouldn't 24 hours still be considered the minimum since it was the minimum about 25 years ago?

I dive with two second stages, one primary and one back up, which seems to be the minimum any cert agency recommends. Should I have three or four second stages to avoid using the minimum number of second stages SSI states to dive with?

certainly a valid point in the larger context.

but i think my comments intention is what should matter. when it comes to deciding on how long to wait until we fly, i think more is better in that case.

was 24hrs the minimum standard many years ago? i assume it was. but nowadays the new minimum standard is 12 hrs after a single dive and 18hrs after multiple dives. it is only my opinion that for me personally, i prefer to wait.

as i have always said, there are plenty of other things to do on vacation besides diving. there really is no need to stress over trying to plan your last dive in such a way that the plane takes off 18hrs after you exit the water. just my personal preference.

besides, ya need time to dry out all your gear anyway right?

 

[tursiops]

DAN-Europe says 24h, based on flawed research.
DAN-US says 12h after a single dive, 18h after repetitive dives, based on this report from 22 years ago:

https://scubaboard.com/community/resources/flying-after-diving-dan-workshop-report-2002.19/

 

[Dan G]

certainly a valid point in the larger context.

but i think my comments intention is what should matter. when it comes to deciding on how long to wait until we fly, i think more is better in that case.

was 24hrs the minimum standard many years ago? i assume it was. but nowadays the new minimum standard is 12 hrs after a single dive and 18hrs after multiple dives. it is only my opinion that for me personally, i prefer to wait.

as i have always said, there are plenty of other things to do on vacation besides diving. there really is no need to stress over trying to plan your last dive in such a way that the plane takes off 18hrs after you exit the water. just my personal preference.

besides, ya need time to dry out all your gear anyway right?

Thank you for the thoughtful reply. I agree that I would never recommend to someone that they squeeze in an additional dive and just barely make it to 18 hours of off gassing before flying, as if 18 hours is some magic number. Regarding the change from 24 hours to 18 hours for repetitive NDL diving, the article @tursiops linked above is a good resource.

 

[tursiops]

Please note: 18h is NOT an arbitrary "magic" number; In the DAN report you will find that no DCS was found for pre-flight times of 17h. The flawed European study simply looked at Pre-flight times of 24h, and siad no DCS was found, so 24h is safe. But they did not look at shorter times.
There are situations where 18h (or even 24h) are NOT long enough, for example deco diving, flying to altitude (like Denver), and PFOs. So 18h really IS a minimum.....24h is better....and 48h if things are not "normal."

 

[Dan G]

Please note: 18h is NOT an arbitrary "magic" number; In the DAN report you will find that no DCS was found for pre-flight times of 17h. The flawed European study simply looked at Pre-flight times of 24h, and siad no DCS was found, so 24h is safe. But they did not look at shorter times.
There are situations where 18h (or even 24h) are NOT long enough, for example deco diving, flying to altitude (like Denver), and PFOs. So 18h really IS a minimum.....24h is better....and 48h if things are not "normal."

I can see why you might refer to my "magic number" description as if I thought of it as "arbitrary." However, I do not think that and know that it is researched based. The point I was trying to make is that getting to precisely 18 hours of off gassing before flying is not something I am going to do.

That being said, prior to approx 2002, DAN USA recommended 24 hours as the minimum. This means that almost 25 years ago, waiting 24 hours was only waiting the minimum. Today, with 18 hours being the recommendation, why does 24 become "better" when just 25 years ago it was only the "minimum?" After all, 24 hours was the minimum not that long ago.

 

[tursiops]

It is called "new data." The original 24h was just borrowed from the US Navy tables.
The new research said "most" of the time you need not wait that long.
24h is better in "some" circumstances, but less than 18h is NOT acceptable....it is the "minimum.
You are welcome to exceed the minimums, but not to subceed them, as people used to say.