Diving myths taught for safety?

[swimmer_spe]

I'm not sure if this is still taught, but surely a once-prevalent myth was "deepest dive of the day should be the first dive of the day."

That is what I was taught. Is it not true?

 

[boulderjohn]

That is what I was taught. Is it not true?

PADI has finally abandoned it, although it still appears on the tables and eRDPml.

There was an international workshop on this topic more than a decade ago. It determined that there is no known safety-related reason for that rule. If you have enough surface interval to do the kind of dive you want to do on a second or third dive, then do the dive with confidence.

One of the findings of the workshop was that no one knows why that rule got started. The earliest reference to it they could find was a 1972 PADI manual that suggested it without explaining why. As time went on, it somehow became an ironclad rule. (Lots of suggestions in all walks of life go thorough such a process.) If you think about it, the reason it was first suggested seems obvious. If you plan two dives in different orders using any dive table, doing the deepest dive first allows you to do the second dive after a shorter surface interval. If your surface interval is adequate for a planned second dive, it makes no difference.

 

[nimoh]

PADI has finally abandoned it, although it still appears on the tables and eRDPml.

There was an international workshop on this topic more than a decade ago. It determined that there is no known safety-related reason for that rule. If you have enough surface interval to do the kind of dive you want to do on a second or third dive, then do the dive with confidence.

One of the findings of the workshop was that no one knows why that rule got started. The earliest reference to it they could find was a 1972 PADI manual that suggested it without explaining why. As time went on, it somehow became an ironclad rule. (Lots of suggestions in all walks of life go thorough such a process.) If you think about it, the reason it was first suggested seems obvious. If you plan two dives in different orders using any dive table, doing the deepest dive first allows you to do the second dive after a shorter surface interval. If your surface interval is adequate for a planned second dive, it makes no difference.

one other thing that was found during that workshop was that since it had become a rule (without reason) most people were doing their deepest dive first so there was very little empirical evidence to show that reverse profiles were just as safe. This is likely (IMO) the reason why agencies were slow to let go of the rule.

 

[boulderjohn]

one other thing that was found during that workshop was that since it had become a rule (without reason) most people were doing their deepest dive first so there was very little empirical evidence to show that reverse profiles were just as safe. This is likely (IMO) the reason why agencies were slow to let go of the rule.

I agree, and I think it goes even beyond that.

Once it became a well established practice to do the deepest dive first, it became a de facto standard. Let's say you are a dive operator and you know darn well that there is no safety reason to do the deepest dive first. You decide to ignore that "rule" because of it. Someone on one of your dives gets DCS and is seriously hurt. In the resulting trial for the lawsuit, how sure are you that you can prove that the dive was safe when the attorneys are showing that "deepest dive first" is the industry standard?

BTW, the shop with which I used to work regularly takes 4 day trips to Key Largo. They travel on the 1st and 4th day, and they dive on the middle two days. They do 4 dives per day. The do a 2-tank dive in the morning (deepest dive first), and a 2-tank dive in the afternoon (deepest dive first). That means that each day the 3rd dive is deeper than the 2nd. Think about all the operations around the world in which DMs do morning 2-tank dives and afternoon 2-tank dives. They are violating the "rule" every day they do it.

 

[XxMerlinxX]

However, I am not going to do anything I know will kill me.

The problem is when you don't know that something can kill you.

 

[flots am]

That is what I was taught. Is it not true?

Yes and no . . .

The answer is that from a physiology standpoint, it apparently doesn't matter which dive is deeper.

From a practical standpoint, I don't know of any method of dive planning (computer or tables) that doesn't penalize you for doing it, so at least for now even though it's not "true" you're still stuck with it.

flots.

 

[CT-Rich]

one other thing that was found during that workshop was that since it had become a rule (without reason) most people were doing their deepest dive first so there was very little empirical evidence to show that reverse profiles were just as safe. This is likely (IMO) the reason why agencies were slow to let go of the rule.

It is virtually impossible to prove a negative. The problem agencies faced was looking for evidence that supports the deepest first rule once it was established as a rule. The rule does make sense if you look at it from the standpoint of clearing pre-clinical micro-bubbles from your system. Of course gather data about anything that is not reported is expensive (you need to actually create the scenario and then test the divers for effect, usually using expensive imagers), as opposed to data mining medical/accident reports. Considering the vagaries of the NDL and who does and doesn't get bent, the benefit of this rule is dubious.

From a table perspective, you do get the benefit of planning the deep dive first.

 

[nimoh]

It is virtually impossible to prove a negative. The problem agencies faced was looking for evidence that supports the deepest first rule once it was established as a rule. The rule does make sense if you look at it from the standpoint of clearing pre-clinical micro-bubbles from your system. Of course gather data about anything that is not reported is expensive (you need to actually create the scenario and then test the divers for effect, usually using expensive imagers), as opposed to data mining medical/accident reports. Considering the vagaries of the NDL and who does and doesn't get bent, the benefit of this rule is dubious.

From a table perspective, you do get the benefit of planning the deep dive first.

all good points, but I think there is a legal perspective in addition to the scientific. Once agencies had the rule, if they remove the rule and someone gets bent doing a deep dive first within the confines of appropriate tables. Agencies would then have to be able to defend why they removed the rule, rather than why they implemented the rule in the first place. Wouldn't they?

I'm no legal expert, pure speculation

 

[AfterDark]

Two years ago down in NC we did just that, deep dive 2nd because the weather wouldn't let us do it any other way; nobody got bent.

 

[BDSC]

Two years ago down in NC we did just that, deep dive 2nd because the weather wouldn't let us do it any other way; nobody got bent.

Just this past Saturday off the coast we did two deep dives back to back. Both involved some deco. Once again we seemed to cheat death.