Reverse Dive Profiles

[FPDocMatt]

During my open water certification I asked the instructor why the first dive of the day has to be the deepest dive of the day. She didn't know why. I didn't understand the concept; I just figured there must be a good reason that I hadn't yet learned about.

Imagine my surprise to read in Deco For Divers that reverse dive profiles may not be harmful after all!

 

[Scott]

It may not be harmful but it tends to decrease your bottom on the second dive as aposed conducting dives in a 'conventional' profile of deepest first. Run a few different profiles with the tables or a program and you'll see.
DAN has an arcticle that states that there is little increased risk for reverse profiles with no-decompression dives that are less than 130 feet and have depth differentials of less than 40 feet.
Pretty interesting reading if you're a member. Divers Alert Network

 

[drbill]

Early research on reverse profiles was conducted out here off Catalina Island by friend and diving pioneer Dr. Bob Given. As a videographer, I have done many reverse profile dive days when an unexpected subject determined by maximum depth. Quite a number of those involved significant depths (to 200 ft) and deco. Current research does indicate that reverse profiles may not be as problematic as formerly thought. While doing the "extreme" diving a few years ago, I was very careful to include several deep stops as well as extended deco times (often double the required and sometimes triple) for added safety. I no longer do such dives though.

As for recommendations to others, I don't make them. Each of us differ in our physiology and other parameters. What may work for me may not for another diver.

 

[t-mac]

In doing my deep specialty my instructor made us plan dives a bunch of different ways, including pulling back out the RDP tables. There was one that was problematic. I don't remember the exact scenario, but it was something like a dive to 130 for 8 or 10 minutes (28% EAN) and then after an hour SI and 85 foot dive also near the RDP max (32-33% EAN). We had to plan the normal and reverse profiles. Due to the rounding, the forward profile was possible without exceeding NDL but the reverse was not. That doesn't address potential physiological concerns, but it may explain a historical factor. (Of course, the computer would allow the dive within NDL.) If you are interested, I could probably dig out the exact scenario, but the point is if all you have is the RDP and you are near the limits, there are some profiles that only work one direction.

 

[flots am]

During my open water certification I asked the instructor why the first dive of the day has to be the deepest dive of the day. She didn't know why. I didn't understand the concept; I just figured there must be a good reason that I hadn't yet learned about.

Because the tables and all the computers I've see penalize you for doing the shallow dive first.

There may or may not be actual science behind the practice, but unless you can find a better way to calculate your gas loading, you're kind of stuck with it.

flots.

 

[Dirty-Dog]

+1 for Dr Bill. I think this is essentially a SCUBA urban myth, without any solid science behind it.

However, I wouldn't worry about it much. You're a relatively new diver, and from what you've posted your plans seem to primarily include diving in places where guided dives are the norm. You can be quite certain that because this particular "rule" has been so mwidely accepted, the dive ops will invariably take you to the deeper of the days sites first.

Personally, I've only made one reverse profile dive. Last December, we took the ferry from Coz to the mainland for a day of cenote and shark diving. Since the best time to play with the sharks is mid-afternoon, we spent the morning doing two cenote dives (40FFW max, 24FFW average, 70 min runtime on air), followed by lunch (2:13 SI) and then the shark dive (70FSW max, 62FSW average, 40 min runtime on 32%). Running that through your favorite dive planning system will show that we didn't really need to feel nervous.

 

[FPDocMatt]

In doing my deep specialty my instructor made us plan dives a bunch of different ways, including pulling back out the RDP tables. There was one that was problematic. I don't remember the exact scenario, but it was something like a dive to 130 for 8 or 10 minutes (28% EAN) and then after an hour SI and 85 foot dive also near the RDP max (32-33% EAN). We had to plan the normal and reverse profiles. Due to the rounding, the forward profile was possible without exceeding NDL but the reverse was not. That doesn't address potential physiological concerns, but it may explain a historical factor. (Of course, the computer would allow the dive within NDL.) If you are interested, I could probably dig out the exact scenario, but the point is if all you have is the RDP and you are near the limits, there are some profiles that only work one direction.

Interesting! I wonder if this is just an arithmetic phenomenon.

 

[HowardE]

Reverse Profiles are a red herring.

 

[Doppler]

Reverse Profiles are a red herring.

Several of my favorite dives were red herrings, Howard... you know that.


For the record, the original guideline was cooked up before nitrox, before recreational divers (sport or tech) learned about decompression, and before 4th generation PDCs. This is from the era when we learned "You cannot get bent diving a steel 72"; and likewise is bunkum!


Matt: not sure if you're getting this, but the chances are that A LOT of the stuff you were force-fed on your open-water program has dubious value.

 

[Rhone Man]

Matt: not sure if you're getting this, but the chances are that A LOT of the stuff you were force-fed on your open-water program has dubious value.

Lots of good advice above, but just wanted to echo Steve's comment above. I remember when I started dabbling my toes in tec diving, it reminded me of the comparison between college education and primary school.

In primary school you are just trying hard to remember that it was William Shakespeare that wrote Macbeth.

In college, you are busy learning why it wasn't William Shakespeare that wrote Macbeth!

Sort of similar with "received wisdom" on an OW course and more advanced diving knowledge. In the case of both reverse profiling and Shakepspeare's authorship: we have a good idea, but we just can't be sure.