Dive Problem: Dive Shallow then deep

[DiveLvr]

My dive partners and myself, all experienced divers, are going diving at Sombrero Reef, average depth 25 ft. We also want to dive the wreck Thunderbolt. Unfortunately the Thunderbolt dive is in the afternoon. The boat also has a shallow reef trip to Sombrero that morning. I know the rule of thumb is to dive deep first but in this case the reef dive (2 tanks) will be on air with the dives separate with an hour SI. It is a 45 minute boat ride back, and hour delay from when we get back to the dock and the boat leaves to the Thunderbolt. There is another 45 min boat ride to the new dive site. On the Thunderbolt, max depth (at the sand) 120' with average depth at the bridge about 80'. We plan to use a 30% mix for both wreck dives. We will be diving with computers.

Just be going from PADI tables I don't see we have any more of a deco issue as we would anyway. Any thoughts?

Doug

[Tortuga68]

http://www.daneurope.org/web/guest/r...EMPLATE_NO_IMG

[HowardE]

You should not have any problems.

A) you are taking a long SI so its mostly irrelevant your AM dive.

2) Reverse profiles are a red Herring.

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[awap]

shallow followed by deep will get you a little less allowable bottom time on the deep dive but, with a long si, the difference should be quite small.

[DiveLvr]

I really appreciate the responses guys. I have been diving since 1973 and some rules just die hard.

[Jim Lapenta]

I agree with all the above. Where you run into problems with reverse profiles is when you are doing one dive after another with limited surface intervals. Just for illustration purposes I always work several profiles with students during class that illustrate the differences. Taking two indentical profiles, one shallow, one deep and flip them around and vary the SI's and also toss in some snags such as adding 5 minutes or 10 feet to each profile to show the variations. Sometimes there is no huge penalty. Other times, depending on the profiles, one could incur more than a little deco. This is a good opportunity to work on planning your dives and sticking to that plan as closely as possible. If you allow yourself say 15 minutes of bottom time on the deep dive at 15:01 you are heading up the line. Not just getting to it for example.

[DivemasterDennis]

I concur. Jim is spot on. I think DAN did a reverse profile study or article a couple of years back. You might search their web site for it.
DivemasterDennis

[DiveLvr]

Actually, one of the responders above, Tortuga68 provided a link for a DAN study. Informative.

[boulderjohn]

I really appreciate the responses guys. I have been diving since 1973 and some rules just die hard.

That year is interesting in regard to reverse profiles. The major reverse profile workshop a decade ago included an investigation into the history of the prohibition of reverse profiles. It turns out the first example that could be found was in a 1972 PADI OW manual. That manual suggested that divers do the deepest dive first. PADI participated in the conference, and they had no record of who wrote that suggestion or why. I believe the "why" is answered in the posts above--shorter surface intervals by the tables. (This was before the invention of the PADI RDP, so they were going off Navy tables.) In each succeeding edition of that manual, the language got stronger until it became a strict prohibition--again with no explanation of why.

In that conference, Bruce Weinke argued that because of the effect of depth on existing bubbles, the prohibition still made sense for deeper, technical diving. The conference agreed, but otherwise concluded that there was no reason to prohibit reverse profiles in recreational diving.

So you started diving right when the concept started, and the fact that "some rules die hard" seems to be the only real reason for its existence today.

If you have had enough of a surface interval to allow the dive you are planning, there is no reason to worry about the reverse profile rule.

[windapp]

I noticed you were using computers. Had you said you were using the RDP, caution would probably be advisable as the RDP calculations for multiple dives assume that you are doing your deepest dives first. Your computer, on the other hand is essentially creating the table as you go.