Qs About Science of Diving Content

[L13]

That's what Wienke argued for why reverse profiles should not be allowed.

I never understood why bubbles that have formed and then shrunk to a smaller size during a later descent would be likely to to do this when the same size bubbles formed at any other part of the dive would not.

The idea is that existing bubbles, even shrunk, act as nuclei to promote buble growth sooner and faster on a latter ascent. They supplement the new bubbles generated on the later ascent, resulting in more total bubbles and bigger bubbles.

I believe this actually correlates with measured bubbles in chamber dives. However, bubble model based deco does not seem to be better at predicting DCS.

 

[L13]

Since every dive starts with a "reverse profile" would that thinking not lead to the requirement to descend as quickly as possible?

Things like waiting under the boat for the rest of the group to get in the water might be considered dangerous?

Not really, on the initial descent, there are no bubbles to compress.

 

[inquis]

I never understood why bubbles that have formed and then shrunk to a smaller size during a later descent would be likely to to do this when the same size bubbles formed at any other part of the dive would not.

Once a bubble of a given size has formed, I would expect the same ease of migration. It's whether or not a bubble forms that is the real question. With a bottom at, say, 80 ft, which is more likely: a) that size bubble will form going from the bottom to 60 ft or b) going from bottom to 20 ft (forming a larger bubble) then back down to 60 ft (compressing to the given size).

 

[L13]

Once a bubble of a given size has formed, I would expect the same ease of migration. It's whether or not a bubble forms that is the real question. With a bottom at, say, 80 ft, which is more likely: a) that size bubble will form going from the bottom to 60 ft or b) going from bottom to 20 ft (forming a larger bubble) then back down to 60 ft (compressing to the given size).

And, theoretically, the smaller bubble could migrate to somewhere more risky before growing again when you ascend again.

 

[tursiops]

There seem to be two different definitions of "reverse profile" on this thread. One definition is doing a shallow dive, then an SI, then a deeper dive. The other definition is a dive that gets progressively deeper. Avoiding these profiles does not seem warranted using modern computers. Here's the language from SSI regarding the second definition:

Going deep, then shallow, then deep again will yield unreliable results. Since a dive computer is an actual computer, an old axiom in the computer business is applicable here: "garbage in equals garbage out." In other words, the computer model must be supplied with the correct input to calculate decompression status properly. Since the computer gathers its input using a depth gauge and a watch, if a diver dives improperly the computer will calculate improperly (see Multi-Level Diving). This illustrates the third limitation, which is training.

Multi-level dives, in general, are those where divers spend time at progressively shallower depths during the dive, which allows extended bottom time. Typical multi-level dives are on walls, large coral formations and wrecks where the diver can descend to the deepest depth and work up to the surface.

If divers go to the deepest depth first, and proceed to successively shallower depths, then multi-level diving may be relatively safer. If divers alternate between deep and shallow depths, however, then it is less safe. The reasons why are complex and have to do with the solubility of gases under pressure, but to illustrate in simple terms, imagine a sponge absorbing water. Sponges absorb water in varying amounts, but at some point will be saturated—they will hold no more water. The tissues at increased pressure are much like sponges; they will absorb nitrogen until they are saturated (for that pressure). At reduced pressure, tissues release nitrogen. If they are re-subjected to increased pressure, they will again absorb nitrogen, but the residual nitrogen will affect that process.

It is best to think of the SSI Science of Diving as not always right, but never in doubt.

 

[boulderjohn]

Since every dive starts with a "reverse profile" would that thinking not lead to the requirement to descend as quickly as possible?

Things like waiting under the boat for the rest of the group to get in the water might be considered dangerous?

If that were the only factor, sure. Lots of people do descend as fast as they can. There are, however reasons not to descend to quickly, including the fact that vigorous activity upon descent can raise CO2 levels and help cause narcosis.

Second, you have no choice but to descend at the beginning of a dive, All of your tissues are on-gassing, and one are off-gassing, so there is no comparison.

I said it is safer to do a multi-level dive with the deepest part first. I did not say it was unsafe to do it the other way. . If you have a dive situation that requires you to be shallower for a while before reaching a point where you descend to a deeper target location, than that is what you are going to do.

Finally, I did not say how much safer it is. The answer to that obvious ranges quite a bit, depending upon the profiles being compared. You could create a comparison between two profiles where there is barely a difference. As long as you are diving within limits, the difference should not matter.

 

[rick00001967]

This is what I figured, but I do think it is a bad idea to teach something in such a definitive way that will likely be contradicted later. It can lead to students putting much more up for debate and wondering what else wasn't "really true" but taught to them for liability purposes.

There's far better language SSI could use to explain that for some divers the "dive after flying" rule is up for debate but by and large 18 hrs is the standard for multi-day repetitive non-decompression diving.

whenever this topic comes up i always wonder why so many people get so wrapped up in what i would consider to be such a minor detail.

is someones holiday time planned that tight that they cannot manage to wait 24 hrs? will their vacation be ruined over a measly 6hrs?

pls remember that the DAN recommendations are "minimum" times required. people seem to over look that.

i don't know about you but i try to exceed minimum standards with most things in life and i have very rarely been diving where i pushed my limits. when it did happen it was due to my own poor execution of the dive plan. it was not because i planned it that way.

if the max i can dive today is 100 feet, i don't dive to 100 feet. if my max time at a depth is 60 min, i don't stay down 60 min. but thats me. many others will disagree.

so when we dive on holiday, we plan our time so we have at least 24hrs before we fly. its not that hard. why not be conservative. there are other things on holiday we like to enjoy.

if you did one dive on a trip that was no where near your limits, would it be safe to only wait 12hrs? if you did 5 days of 2 dives per day, would it be safe to only wait 18hrs? i would respectfully say the answer is somewhere between maybe and probably.

are you in peak physical condition? did you consume large amounts of booze that week? are you dehydrated most of the time? was your last dive on the verge of requiring an added deco stop? etc etc there are so many variables.

if many divers choose to follow the 18hr rule and nothing bad ever happens, then thats great. but that does not mean it is a guarrantee all divers will have the same results. i choose to wait the 24hrs.

 

[BoltSnap]

18hr rule

It isn't a "rule" as much as a minimum recommended SIT before flying. When DAN changed it from 24 to 18 hrs, it had a lot to do with politics as well way back then.

 

[tursiops]

It isn't a "rule" as much as a minimum recommended SIT before flying. When DAN changed it from 24 to 18 hrs, it had a lot to do with politics as well way back then.

The change had mostly to do with the data showing no DCS for preflight times over 17h.

 

[BoltSnap]

The change had mostly to do with the data showing no DCS for preflight times over 17h.

Were you an instructor and involved in the debate back then?