Error in SSI course materials still not fixed

[Ken Kurtis]

The original 24h was just borrowed from the US Navy tables.

No.

The original Navy tables outgassing was 12 hours. That's because the model they used had 120-minute (2 hour) tissue as the slowest tissues. 6 half-times is consider saturated or desaturated so 6 x 2 +12, ergo 12 hours.

The NAUI version of the Navy tables, published in the late 80s or early 90s, I think was the first to put outgassing at 24 hours. As far as I can recall, PADI has just spent a bunch of money and done research on their new tables and NAUI didn't have the budget for that but wanted to keep pace with PADI. So NAUI took the existing NAVY tables, reduced all of the max no-deco times by one group, and doubled the max outgassing time to 24 hours. The thought was that the combo of less time underwater and more time to outgas would overall be a good thing.

But I'm also sitting here on a Sunday morning looking at my copy of "Decompression Tables taken from US Navy Diving Manual of June 1978 prepared by NAUI" (these were also known as "paper tables" because they're printed on tri-fold paper) and point #8 simply says "After diving, do not fly." Now if you take that literally, it means once you dive, you should never ever again get on a plane.

I was at a UHMS regional conference here at UCLA around 1992 when someone from DAN presented their then-still-evolving research into flying-after-diving. I think the general consensus in the industry back then was 24 hours. The DAN research, which they pointed out was done in dry chambers and not after actual underwater dives, showed that the incidence of bends-after-ascent-to-altitude statistically didn't exist after hour 17. I think they went with 18 because it was an easier number to remember/calculate (when thinking of clock time it's easy to visualize 6 hours and then add 12 more to it).

But the thing to remember is that flying-after-diving is pass/fail and you won't know that you're failing until it's too late to prevent it from happening. More is likely better. Less is likely more risky. By the same token, when I got certified in 1978, the standard to fly was to wait at least 2 hours after you last dive or until you became a D dive, which ever was greater.

 

[ginti]

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.

Caves, for example - where you have no freedom in deciding the profile

I can think of other examples, although mostly are related to exploration, not sure how many people do this type of thing

 

[tursiops]

No.

Yes

The original Navy tables outgassing was 12 hours. That's because the model they used had 120-minute (2 hour) tissue as the slowest tissues. 6 half-times is consider saturated or desaturated so 6 x 2 +12, ergo 12 hours.

By this argument, the PADI RDP tables would desaturate in 6h, and the Buhlmann ZHL (with a slowest compartment of 635 mins) would take 2.6 days.

The NAUI version of the Navy tables, published in the late 80s or early 90s, I think was the first to put outgassing at 24 hours.

The 1979 NOAA Dive Manual (2nd edition) gave 24 hours, based on Navy.

I was at a UHMS regional conference here at UCLA around 1992 when someone from DAN presented their then-still-evolving research into flying-after-diving. I think the general consensus in the industry back then was 24 hours. The DAN research, which they pointed out was done in dry chambers and not after actual underwater dives, showed that the incidence of bends-after-ascent-to-altitude statistically didn't exist after hour 17. I think they went with 18 because it was an easier number to remember/calculate (when thinking of clock time it's easy to visualize 6 hours and then add 12 more to it).

Then, after all the dry-chamber tests, they actually looked at real dives, those without DCS problems and with problems, And found nothing to change their minds. See Flying After Diving Report.

But the thing to remember is that flying-after-diving is pass/fail and you won't know that you're failing until it's too late to prevent it from happening. More is likely better. Less is likely more risky. By the same token, when I got certified in 1978, the standard to fly was to wait at least 2 hours after you last dive or until you became a D dive, which ever was greater.

This "D Diver" thing depends on which tables you are using. And with ew people using tables today anyway, is not very useful.

 

[Ken Kurtis]

By this argument, the PADI RDP tables would desaturate in 6h . . .

That's correct.

Look at those RDP tables and you'll see that the longest surface interval listed is 6:00 hours. That's because Rogers/DSAT felt the 120-minute tissue wasn't applicable to recreational divers. So the shortest compartment they used was 60 minutes (1 hour). Again going to 6 half-times equals saturation or desaturation, 6 x 1 hour = 6 hours, so 6 hours to desaturate on the RDP. (That doesn't mean they endorsed flying after 6 hours, even though you may be mathematically desaturated.)

The 1979 NOAA Dive Manual (2nd edition) gave 24 hours, based on Navy.

Not to split hairs here, but that's NOAA. I think the Navy still held to 12 hours, based on their algorithm and tissue compartment times.

 

[tursiops]

Not to split hairs here, but that's NOAA. I think the Navy still held to 12 hours, based on their algorithm and tissue compartment times.

The Navy Dive Manual Rev 4 does not agree with you.

 

[Ken Kurtis]

The Navy Dive Manual Rev 4 does not agree with you.

Then we'll leave it at that.