Have Diving Standards Changed?

[10X]

When I learned to dive, only 3 years ago, I was taught that you do the deepest dive first, with the second dive being more shallow. However, on a cruise I took this month I encountered just the opposite.

In St. Lucia, with a dive master, we did a two tank dive, both fairly shallow, with the first at 39' and the second at 46'. For the 7' difference I didn't think too much about it, but I did wonder why the deeper dive was second, especially since the dive sites were so close together that it easily could have been reversed.

Our next dives were in St. Vincent, with a dive master, with the first dive going to 64' and the second dive to 85'. Again, these two dives could have been easily reversed since we sailed past site #2 to get to site #1. My bottom time was max-ed out according to my computer.

Our third set of dives was in Bonaire, which we did on our own. We did 92', followed by 64'.

However, I met a diver from England onboard our cruise ship and invited him along on the Bonaire dives. I explained what happened on the first two dive trips and he informed me that there is now a school of thought that says bottom time alone is the deciding factor on dives, not how deep the dives were or the order in which they were taken.

Anyone familiar with this?

 

[StSomewhere]

There are recent threads about this here on SB.

The reason to to the deep dive first is that you'll get more underwater time that way if you are doing NDL dives. You can do it the other way, but the total dive times will be shorter. Not to be cynical, but the DM wasn't in any hurry to get you back to your boat, was he?

 

[Walter]

You were not in any increased danger, but your bottom time was reduced.

 

[Damselfish]

yes, you'll get more time doing the deeper dive first. Given the exact 2 dives, doing the deeper one first will be more conservative than the other way. But if you're within your table or computer, I don't see a problem, and I especially wouldn't worry about 7 feet or even 20. I would say it makes sense to avoid doing a 40' followed by a 100' when there is no really good reason to do so.

(Some DMs at Dive Saint Vincent have an annoying habit of doing a reverse profile on a single dive for no apparent good reason, which is stupid. It seems someone there has heard doing the deeper dive first is no longer a big deal, and somehow interpreted that to mean shallow followed by deep on the same dive is fine. I gave up arguing about it.)

 

[eponym]

The more conservative authors on this subject say that the tables (and the computers that internalize them) are known to work well for single, non-reverse-profile dives, and that their extension to activities like reverse dive days, as it were, is less certain.

I won't try to find quotations right now as I'm sure Dr.Deco can amplify if you post in the diving medicine forum.

Bryan

 

[Skeeter1097]

If you think about it it makes a lot of sence. If you louse a few minets by diven short first. The dive trep is shorter and that get back faster. That get more trips in and make more money.

 

[drbill]

Reverse profile diving has become much more accepted than it was in the past. As others have pointed out, it does reduce bottom time but is not unsafe as long as it is done within the appropriate limits. A friend of mine now deceased, Jon Hardy, did some "pioneering" work on reverse profile diving. I've been doing it for years, if appropriate to my dive purposes, without any problem.

As a general rule, it probably makes sense to teach new divers not to do reverse profiles initially. I was amused when a diver who came to dive with me e-mailed back after her visit to say her dive instructor said I had put her in some danger by doing a reverse profile series. As I remember, the second dive was about 5' deeper than the first!

 

[RonFrank]

If you think about it it makes a lot of sence. If you louse a few minets by diven short first. The dive trep is shorter and that get back faster. That get more trips in and make more money.

I guessing that it's not that they make more $$$, but that they do so in less time.

DiveOps routinely schedule 2 trips of 4 tanks, and a night dive, and reversing profiles is not going to get them 3 trips and 6 tanks + the night dive. But it WILL provide them with more time inbetween dives :11doh:

OTOH if a dive op did this to me on a trip I paid dearly for, I'd likely never use them again. I want max Bottom time on my tropical vacations, or at LEAST reasonable BT.

So 10X, what were the bottom times on those first two dives? If the answer is much under 60 minutes each, then that's an issue as shallow reef dives IMO should allow 60 minutes of diving per tank or more.

 

[RonFrank]

I was amused when a diver who came to dive with me e-mailed back after her visit to say her dive instructor said I had put her in some danger by doing a reverse profile series. As I remember, the second dive was about 5' deeper than the first!

Your a danger to others Dr. Bill with those 5' reverse profiles

I guess there is always going to be a difference between knowing the *rules* and understanding the reasons behind them. Obviously that instructor lacks a lot in the way of understanding...