A close call

[fisheater]

The problem with ending the "no reverse profiles" advice is that no one knows if the original advice was actually wrong. They've only determined that there is neither theoretical or practical evidenciary support for it.

Plus, if an authority comes out and says that reverse profiles are just fine and someone diving one gets bent, then . . . well, you know.

 

[Bubbletrubble]

The problem with ending the "no reverse profiles" advice is that no one knows if the original advice was actually wrong. They've only determined that there is neither theoretical or practical evidenciary support for it.

Instructors should stop doling out the "no reverse profiles" advice.
Instead, they should be teaching their students the following:

• For a long time, reverse profiles were discouraged by instructional agencies.
• On account of these old guidelines, more repetitive dives have been conducted safely according to forward profiles than reverse profiles.
• There now exists scientific evidence suggesting that reverse profiles do not pose any more risk than comparable forward profiles.
• It's probably best to dive conservatively.

Neglecting to inform their students of these recent findings is, IMHO, an oversight and certainly short-changes the students. At the very least, it would make for an interesting topic of discussion in class.

I'd be willing to bet that Jloren9's instructor is blissfully ignorant of the cited research.

Plus, if an authority comes out and says that reverse profiles are just fine and someone diving one gets bent, then . . . well, you know.

This statement greatly oversimplifies things. I have never heard anyone say that reverse profiles are flat-out "fine" or "safe." A relative comparison is being made between reverse profiles and forward profiles. Nothing more, nothing less.

 

[bsee65]

I would believe reverse profiles are fine as long as nothing goes wrong. If something does, then you're deeper with less margin for error because of a non-zero starting blood gas level. Is that a higher risk than being more shallow but with an even higher starting level from a deeper first dive? Good question, but probably a fair amount of study away from a proven answer.

 

[Puffer Fish]

Instructors should stop doling out the "no reverse profiles" advice.
Instead, they should be teaching their students the following:

• For a long time, reverse profiles were discouraged by instructional agencies.
• On account of these old guidelines, more repetitive dives have been conducted safely according to forward profiles than reverse profiles.
• There now exists scientific evidence suggesting that reverse profiles do not pose any more risk than comparable forward profiles.
• It's probably best to dive conservatively.

Neglecting to inform their students of these recent findings is, IMHO, an oversight and certainly short-changes the students. At the very least, it would make for an interesting topic of discussion in class.

I'd be willing to bet that Jloren9's instructor is blissfully ignorant of the cited research.

This statement greatly oversimplifies things. I have never heard anyone say that reverse profiles are flat-out "fine" or "safe." A relative comparison is being made between reverse profiles and forward profiles. Nothing more, nothing less.

There are times that doctors should not be allowed to interpret data.

Saying there is no evidence that indicates there is any issue, is not the same as it being as safe. The amount of data is very limited, and there is a very large difference in how tables versus various computers handle this issue. It is possible it is overly safe with some computers...ok with a couple more and not safe with one or two. How would anyone know, when they were not designed and people don't test them for that condition?

It would also be reasonable for a student to ask "So what happens at 40 ft, that does not happen at 39 ft?

While I have never taught "you should never", I have taught that one should avoid doing reverse profiles, and if you do them, be aware that any error (depth or time) on the first dive will make the second dive potentially dangerous.

Just check out the following differences:

1 Dive to 38 ft for 45 minute (when the depth should have been 45 ft), followed by a one hour surface interval and then a 59ft for 32 minutes (where the depth was actually 67ft).

Note: due to gauge error of 18%)

2. Reverse the dives.

In the first, had you had accurate information, you would have gone into deco. In the normal case, You would have been tremendously safe.

Note: Understand this is using a table, but in a class, for the most part, that is what people are using.

Fact is, doing a reverse profile has less tolerence for errors. Seems doctors don't know how to do math.

I would think that any reasonable instructor would teach that doing reverse profiles is more sensitive to errors and reduces your total available dive time, and so is generally not recommended. Don't see how that report would change that, even if they got 100 experts to agree with it.

 

[Bubbletrubble]

@Puffer Fish: Have you taken the time to read the papers published on reverse dive profiles?

Fact of the matter is that recreational divers are taught to avoid going into deco. I think it's admirable that you teach your students about the possibility of gauge error. It's yet another reason to dive conservatively (with respect to NDLs and gas management).

I agree that, when using dive tables, total bottom time might be decreased if a reverse profile is undertaken (compared to doing a comparable forward profile).

I also wanted to let you know that I found your following comment:

Seems doctors don't know how to do math.

to be very amusing. Coming up with a practical reason to do a forward profile does not necessarily invalidate the work on reverse profiles. Nor does it call into question the arithmetic skills of the scientists involved.

Please read the 2002 SPUMS letter written by Guy Williams that attempts to summarize the findings of the 1999 Smithsonian Institute Reverse Dive Profile Workshop. Perhaps it will address some of your concerns. I hope that you can approach the letter with an open mind.

I stand by my initial comment that instructors should discuss the topic of reverse dive profiles with their students. At the very least, you could bring up the many practical considerations in planning repetitive dives.

 

[Puffer Fish]

@Puffer Fish: Have you taken the time to read the papers published on reverse dive profiles?

Fact of the matter is that recreational divers are taught to avoid going into deco. I think it's admirable that you teach your students about the possibility of gauge error. It's yet another reason to dive conservatively (with respect to NDLs and gas management).

I agree that, when using dive tables, total bottom time might be decreased if a reverse profile is undertaken (compared to doing a comparable forward profile).

I also wanted to let you know that I found your following comment:

to be very amusing. Coming up with a practical reason to do a forward profile does not necessarily invalidate the work on reverse profiles. Nor does it call into question the arithmetic skills of the scientists involved.

Please read the 2002 SPUMS letter written by Guy Williams that attempts to summarize the findings of the 1999 Smithsonian Institute Reverse Dive Profile Workshop. Perhaps it will address some of your concerns. I hope that you can approach the letter with an open mind.

I stand by my initial comment that instructors should discuss the topic of reverse dive profiles with their students. At the very least, you could bring up the many practical considerations in planning repetitive dives.

Bubble, several I could find were not available without paying a charge.. so no did not read them.

The one you linked to is interesting, in that it does point out the issue of available time... and Tom's comment on how that can get you into trouble (so some evidence was submitted, but ignored)

I believe this sort of sums up the issue:

"However no evidence was produced that showed that
reverse profile dives were safe. Only evidence which
suggested that they were not dangerous."

That is roughly like saying "I can find no evidence that jumping off this bridge is safe, only that it does not seem dangerous".

In my example, if the new diver has to make an emergency accent at the end of the second dive, in one case they should have very little risk, in the other they would very likely have a DCS hit (even without the gauge error). To me that still represents some increased risk.

Oddly enough, if you are diving deeper than OW, the issue is much smaller. Turns out that 40 to 70 ft range is the worst area to do a reverse profile. (my reason for the math comment, because if anyone had checked they would have seen that).

And there is a reason why I happen to know this, as I was around when it was established. If you think back, we were using 60 fpm accent rates and no safety stops and all on air. We had DCS hits from reverse profiles in both the recreational and military (happen to be both). Somewhere, the information got lost, so now we have "experts" saying that there is no data. (darn internet was not around)

As a big fan of slow accents and safety stops, I sure would never want to go back to those days, but just like DECO, reverse profiles can (not always) have a reduced safety margin. And to me, reduced safey margin = not as safe.

 

[Bubbletrubble]

Bubble, several I could find were not available without paying a charge.. so no did not read them.

Wow. So you are adamantly disagreeing with research that you haven't taken the time and effort to actually read. Hmm.
I'm going to have to direct you to the Rubicon Foundation Online archives. It's a wonderful resource that offers free access to downloadable, peer-reviewed articles. Gene Hobbs has done a fantastic job with adding content to the site.

The one you linked to is interesting, in that it does point out the issue of available time... and Tom's comment on how that can get you into trouble (so some evidence was submitted, but ignored)

I don't understand your comments at all. Who is Tom? What "evidence" is being "submitted, but ignored"? BTW, I wouldn't attach the label of "evidence" to any primary information that was shared in this thread.

I believe this sort of sums up the issue:

"However no evidence was produced that showed that
reverse profile dives were safe. Only evidence which
suggested that they were not dangerous."

That is roughly like saying "I can find no evidence that jumping off this bridge is safe, only that it does not seem dangerous".

There is absolutely no equivalence between jumping off a bridge and conducting a reverse dive profile. Making a hystrionic statement like this erodes the credibility that you deserve as an experienced diver, instructor, and SB participant.

In my example, if the new diver has to make an emergency accent at the end of the second dive, in one case they should have very little risk, in the other they would very likely have a DCS hit (even without the gauge error). To me that still represents some increased risk.

First of all, I don't know how one could reasonably speculate a "very likely" DCS hit from anyone staying within NDLs and skipping a safety stop. If, in your example, you have a diver who goes into deco on the second deeper dive of a repetitive series, then I don't see that profile as being equivalent or comparable to a forward profile in which the diver does not go into deco.

And there is a reason why I happen to know this, as I was around when it was established. If you think back, we were using 60 fpm accent rates and no safety stops and all on air. We had DCS hits from reverse profiles in both the recreational and military (happen to be both). Somewhere, the information got lost, so now we have "experts" saying that there is no data. (darn internet was not around)

Is it possible that you (and others) wrongly attributed the DCS hits to reverse profiles when it had more to do with 60 fpm ascent rates, no safety stops, and doing it all on air? You admitted yourself that those were the common dive practices decades ago.
If you have evidence that reverse profiles directly causes DCS hits, then you really should make this information available to others. It would be critical to include the conditions under which such DCS hits were sustained.

 

[Troutmaster]

Let me first say I do not know if reverse profiles are dangerous or not. I will say that when I got certified (21 years ago) "Deepest dive first" was written in stone. But I also read soon after I was certified about this crazy new concept of adding extra oxygen to breathing air to increase NDL limits. Alot of the mainstream thinking at the time was that this new "voodoo gas" called nitrox was dangerous and the "recreational" divers had no buisness using it. I think we can all agree that nitrox is not so dangerous that rec divers should stop using it.

My point is this, for along time "reverse profiles" were thought to be dangerous. Now there is some data that maybe they are not. I think presenting the information to new divers is important, and that it is worth some more study. But lets not get into the trap of thinking just because we have always thought reverse profiles are bad that they must be.

Popular thinking at one point was that the earth was flat. Turns out some new information is available.....