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Reverse Profiles: An Urban Legend?

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Reverse Profiles: An Urban Legend?

Postby eDiver » Sun Mar 04, 2012 10:35 am

When you learned to scuba dive, most likely you were told that, when doing multiple dives, you should do the deep dive first and so avoid reverse dive profiles.

But, did your Instructor explain to you why reverse dive profiles are dangerous?
But, are reverse dive profiles really dangerous? Or is this an urban legend created by the recommendations of several Training Agencies?

In this article I am going to use divePAL to dig a bit deeper on this subject.

According to DAN's recent studies, the probability of decompression illness is linearly proportional to tissue nitrogen partial pressure and depth and, according to the results of the symposium on reverse dive profiles held in October 1999 in Washington DC, at the Smithsonian Institute, there is no convincing evidence that reverse dive profiles within the no-decompression limits lead to a measurable increase in the risk of DCS.

In decompression theory we use tissue nitrogen partial pressure, depth and time to estimate the "nitrogen loading" of a specific dive profile. Then you can use the nitrogen loading end value to estimate how close to the limits (how risky) that specific dive was.

In the same fashion you can use the nitrogen loading end value to compare dives or even series of dives.

So, let's use divePAL to compare a series of 2 dives (deep first) with its correlated reverse series.

I put into divePAL a series of 2 dives (I called them: Deep first - Urban legen #1 and #2)
Dive #1
Max depth: 120ft
Bottom time: 19 minutes.
Descent rate: 60ft/min
Ascent rate: 30ft/min
Safety Stop: 3min at 15ft

Surface Interval: 60min

Dive #2
Max depth: 80ft
Bottom time: 25 minutes.
Descent rate: 60ft/min
Ascent rate: 30ft/min
Safety Stop: 3min at 15ft

Analyzing the series with ZH-L16C Moderate, there was no deco situation and the Nitrogen loading at the end of dive 2 was 75%
Using the new feature Estimated Pressure Group, the EPG at the end of dive 2 is R
Here image of dive 2 of this series:

Image


Then I reversed the series.
I created the series "Reverse Profile - Urban Legen?" where dive 1 has the profile of dive 2 of the series above and dive 2 has the profile of dive 1. Surface interval is still 60 minutes.

Surprise surprise ....
Analyzing this Reverse Profile series with the same decompression algorithm, the Nitrogen loading at the end of the second dive is 78% - very very similar to the value we had for the previous series.
Basically it looks like that "reversing the profile" had little impact on the overall Nitrogen Loading.

But what happened to the Pressure Groups?

Here is where the decompression algorithms used in dive computers and the tables REALLY DIVERGE!

The EPG of dive 2 of the first series was R, but the EPG for dive 2 of the Reversed series is OFF THE CHART !
As you can see in the image below (image of dive 2 of the "Reverse Profile" series) the table went into deco in the middle of the dive and never came out of it.

Image


Basically it looks like that the Tables are indeed heavily penalizing reverse profiles.
We think that this diverging behavior of the table is because of their "asymmetric" nature.
Were the tables designed this way? Or it just happened?
The DiveNav Team
eDiver
 
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