Qs About Science of Diving Content

[boulderjohn]

I have never before heard the argument about moving from shallower to deeper during a dive. It makes no sense to me.

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

DAN USA recommends 18 hours for most multiple dive days, but DAN Europe recommends 24. DAN USA and DAN Europe do not like each other. Seriously.

A few years ago, DAN Europe performed an experiment on flying after diving that they proclaimed verified the 24 hour wait, but it did not. They tested people for VGE (bubbles in the veins) immediately after diving and then 24 hours later. The indications were that they were pretty much free of bubbles at that point and extremely safe to fly. DAN Europe proudly proclaimed that the results supported the 24 hour waiting period.

Anyone with a lick of scientific sense would see that while it showed that waiting 24 hours made you safe, it did not show that it was necessary. If they had instead tested them a week later and found them bubble free, would that mean you have to wait a week to fly? They could have tested at 12, 18, and 24 hours with very little additional effort or expense and learned something useful. It is very possible that those divers were all clean and safe earlier. The fact that they chose not to do the obvious makes me wonder about their motivation.

 

[boulderjohn]

The term "reverse profile" in scuba usually refers to doing 2 or more dives, with subsequent dives having deeper maximum depths than earlier dives. This was once not allowed, and my copies of the old PADI tables still say not to do it. This was the topic of a 2001 UHMS workshop, and it concluded that there was no reason for that rule. By then, however, it was so well established that the habit was hard to break. Almost all 2-tank dives today still follow the "deepest dive first" rule. When I recently dived in Australia, we were forbidden to do subsequent dives deeper than earlier dives because of that "rule." I have never before heard the term "reverse profile" refer to the profile of a single dive.

What follows is an explanation for the deepest dive first rule--if you don't care, don't bother going on.

The UHMS workshop tried to find the reason for the rule, and they determined that the earliest known reference to it was the 1972 PADI OW manual, where diving the deepest dive first was a suggestion. There was no explanation, and the PADI representatives at the conference did not know why it was made. In subsequent manuals, the suggestion eventually became a hard and fast rule, still with no explanation of a reason for it.

It is not hard to figure out why the suggestion was likely made. Back then all diving used the US Navy tables, which used the 120 minute compartment to determine surface intervals. Those were very long surface intervals when computed doing the deepest dive first. Do the shallower dive first, and the surface intervals were downright huge. If you wanted to do 2 dives with a reasonably short surface interval, it made loads of sense to do the deepest one first.

 

[Dan G]

Better for an agency to recommend a 24 hr interval to be more conservative. Less liability that way.
If you were to follow DAN recommendations they are less conservative.

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.

 

[BoltSnap]

It is not hard to figure out why the suggestion was likely made. Back then all diving used the US Navy tables, which used the 120 minute compartment to determine surface intervals. Those were very long surface intervals when computed doing the deepest dive first. Do the shallower dive first, and the surface intervals were downright huge. If you wanted to do 2 dives with a reasonably short surface interval, it made loads of sense to do the deepest one first.

This is the reason. If you use US navy tables (also tables based on US navy tables), and do two dives of different depths where you are approaching the NDL limits, you will find that doing the deeper dive first and then the shallower dive second gives you more dive time (bottom time). Doing the reverse, shallow dive and then deeper dive, will likely put you in a required-stop situation (required decompression stop).

This is the reason I understand it to be for doing deeper and then shallower profiles.


In regards to flying after diving, NAUI recommends the following:

"NAUI’s current recommendation is to wait 24 hours after the completion of your last dive before flying or ascending to altitude. If you make dives that require a decompression stop, or you omit a required decompression stop, wait more than 24 hours before flying. "

 

[boulderjohn]

This is the reason I understand it to be for doing deeper and then shallower profiles.

It does do that, which is about what I said, but if that is stated anywhere officially, I don't know about it, and the UHMS workshop did not know about it, either.

Many people assumed the reason related to safety, and in the workshop, Bruce Wienke argued that with such fervor that the workshop left the recommendation in for technical diving. Twenty years later, I find his argument unpersuasive. Reverse profiles are in fact common in technical instruction because the course requirements typically call for increasing depths as the students move through the course. Unless you do one dive a day, you are probably going to do some reverse profiles.

About 5-6 years ago, I participated in an online discussion among scuba instructors in another forum. The question was how they felt about doing reverse profile dives now that there was no prohibition on them, and the overwhelming majority said they would continue to follow the deepest dive first rule "for safety reasons." When asked what those safety reasons were, they could not name them, but they assumed there had to be safety reasons or the rule would not have existed in the first place.

In the age of computers, divers should check their computer planning feature to see if the dive they have planned can be done with the amount of surface interval they have done. If so, they can do it, with no concern as to whether it is deeper than an earlier dive.

 

[Dan G]

It does do that, which is about what I said, but if that is stated anywhere officially, I don't know about it, and the UHMS workshop did not know about it, either.

Many people assumed the reason related to safety, and in the workshop, Bruce Wienke argued that with such fervor that the workshop left the recommendation in for technical diving. Twenty years later, I find his argument unpersuasive. Reverse profiles are in fact common in technical instruction because the course requirements typically call for increasing depths as the students move through the course. Unless you do one dive a day, you are probably going to do some reverse profiles.

About 5-6 years ago, I participated in an online discussion among scuba instructors in another forum. The question was how they felt about doing reverse profile dives now that there was no prohibition on them, and the overwhelming majority said they would continue to follow the deepest dive first rule "for safety reasons." When asked what those safety reasons were, they could not name them, but they assumed there had to be safety reasons or the rule would not have existed in the first place.

In the age of computers, divers should check their computer planning feature to see if the dive they have planned can be done with the amount of surface interval they have done. If so, they can do it, with no concern as to whether it is deeper than an earlier dive.

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.

 

[BlueTrin]

As a side note, about the last paragraph, I am not sure if going up and down many times during a single dive is safe.

I heard anecdotal stories of people getting skin bends going up to the surface and down many times in training quarries but I do not have hard evidence either way.

Would be interested in knowing if there is some truth in this.

Also I think I had seen a similar topic about SSI during covid when they made this class free and we asked the same questions. Would be nice if SSI publishes the references to the sources they used for the material.

 

[Dan G]

I am not sure if going up and down many times during a single dive is safe.

I heard anecdotal stories of people getting skin bends going up to the surface and down many times in training quarries but I do not have hard evidence either way.

I have heard that lots of up and down in a single dive can be a problem, but I do not know if that is true. My guess would be that the more up and down a diver does, the more opportunities there are to ascend too quickly. However, that is just a guess. My primary wonderings are around a "multi-level" dive that begins shallow and gets progressively deeper.

I completed two such dives on a recent trip. I would need to download the data from my computer for details, but I do remember on one dive, we spent about 30 minutes at about 30-35 ft before going deeper to about 70-80 feet for a while. Perhaps such a profile has not been tested by my computer, but I did not get any unexpected results from it.

 

[BoltSnap]

I am not sure if going up and down many times during a single dive is safe.

I heard anecdotal stories of people getting skin bends going up to the surface and down many times in training quarries but I do not have hard evidence either way.

Yes, we simply don't know.

 

[boulderjohn]

Going up and down repeatedly on a dive is often called a sawtooth profile, and it is generally considered to be unsafe, although I have ever heard an explanation as to why. I remember looking at my computer profile after diving the Car Wash cave in Tulum. It was one of the worst sawtooth profiles you will see, but I had no choice because I was in a cave. It is indeed typically avoided.

As for multi-level dives, it is indeed better to do the deepest parts of the dive first, since the fact that you are slowly ascending aids in your off-gassing. It is better to have the final ascent from the shallowest part of the dive rather than the deepest part of the dive.

This does not, however, fool your computer, and I have no idea why it would.