Embarrassing Naivety

[Coll]

I recently went on a 2 tank dive in San Diego and basically had to abort the 2nd dive due to ignorance and I was just looking for some clarification. First dive was at a max depth of 40 feet, average was about 30 ft. Down for 60 minutes and did not do a safety stop. Second dive was after a 1 hour SI. The bottom was at 60 feet which was issue one, I was taught not to dive deeper than your previous dive? The DM then asked me to check the anchor so I drop down to 50 feet and its not hooked in so I surface, tell him and then drop down to 15 feet and do a 3 minute safety stop. Now Im trying to figure out what to do since the 60 foot bounce dive confused my plans. I decide to spend 20 minutes at 30 feet and then do another 3 minute stop at 15 ft before surfacing. Basically Im wondering what my options were, can I dive deeper on the 2nd dive? How does the 60 foot bounce affect adjusted max bottom time? Was the bounce a bad choice? Any and all advice is greatly appreciated, even if you want to tell me how stupid I was.

 

[emoreira]

Hi Coll :

I see that you have more than 50 logged dives, so this should not be an issue for you to do a dive plan and dive what you planned.
Besides, the DM should have done a briefing before the first and second dive. A general day long briefing and a special briefing before the second dive, considering the Surface Interval you needed. If the DM is not doing it, you should do. This is not a matter of how strong or "macho" you are. It's a matter of your security. No one will say how macho you were with a bend.
The dive tables are conservative just to take into account those that do not follow them.
There is no warranty that if you follow the tables you will suffer no bend, but the chances are minimal. If you do not follow the tables, well, chances are much greater.
Yes, it's generally accepted that the second dive should be no deeper than the first of the day, That's not a rule of thumb. That comes from the tables and the Pressure Group you are and will be after the SI and second dive.
It's all about planning the dive and diving the plan.

 

[willgr1]

rule of thumb is to do deepest dive first...but a 40 then 60ft dive should be ok, 40 then 100ft dive, not wise.

Simple answer would be to see what your computer says (if your using one), or at a minimum, run the tables and find out what your allowable bottom time would have been after those two dives.

 

[LeadTurn_SD]

Hi Coll:

In the past, my understanding was it was best to avoid "reverse profile" dives like you describe.

However, my current understanding is that for typical "recreational depth" dives that remain within NDL's, the prohibition against diving deeper on the 2nd dive has been removed. I think that current data did not support the theory that you'd be more prone to DCS on a reverse profile dive at recreational depths.

While I still try to avoid "reverse profile" dives (old habits die hard I guess), what you did sounds fine to me. Out of curiosity, were you diving tables, or a computer?

Best wishes.

 

[Mr Carcharodon]

rule of thumb is to do deepest dive first...but a 40 then 60ft dive should be ok, 40 then 100ft dive, not wise.

Why? Can you point to anything other than conventional wisdom that supports this?

 

[svs]

Both computer and tables will provide you with all information you need. You may get slightly shorter combined bottom time doing reverse profile with bounce dive between. But there is nothing to prevent you from 60 ft after 1 hour at 40 ft and then 2-3 min at 50, just calculate bottom time correctly.

 

[Cave Bum]

In the old days, they taught that you shouldn't dor 'reverse profile' dives, but rather to do your deepest dive first getting gradually shallower.

Since the advent of computers, however, it has become more the standard to do dives in whatever order they come up and let the computer take care of the calculation, which it does just fine.

You didn't say if you were diving a computer. Were you?

 

[Henryville]

Recognizing that DCS is very uncommon on an incidents per number of dives basis, there is evidence that reverse profiles carry greater DCS risk, but not reverse dives. I.e. going deeper later in a dive is not recommended, but there is not strong evidence that a deeper dive after a shallower dive carries any greater risk assuming one follows the tables and that good ascent and other practices are observed.

J.P. Morgan said for every decision there's a good reason, and a real reason. I've always thought that the real (and unstated) reason for the deeper dive first practice had more to do with getting a dive boat part of the way home after the first dive, when weather and waves are more likely to get worse.

Bounce dives are clearly a bad idea after a dive as solution bubbles can recompress and pass the lung capillaries where they would ordinarily be trapped, to re-emerge on the arterial side. It may be less of an issue after a decent surface interval, but I wouldn't do one. The option would have been to check the anchor at the beginning of a dive, secure it if it wasn't, and signal the boat from the bottom (foam cup or smb.) IMHO, this should ordinarily be the divemaster's job, but maybe your circumstances were unique. In any case, I wouldn't have wanted to come up quickly.

 

[boulderjohn]

There was a conference on reverse profiles in 2001. In that conference, it was determined that the origin of the "deepest dive first" concept was a suggestion in a 1972 PADI manual. PADI did not know who made that recommendation or why. In subsequent editions, the wording was more strongly stated until it somehow became a rule.

If you play with the tables long enough, you will see that the original reasoning was probably because if you do two dives using the tables as your planning device, you will get a shorter surface interval if you do the deepest dive first. If you have had enough surface interval to do the dive you want to do, then there is no reason for it.

The conference concluded that there was no reason to require deepest dive first in recreational diving. The recommendation for deepest dive first remained in place for diving beyond recreational limits because of the theories presented at the conference by Bruce Weinke, creator of the RGBM model for decompression. Weinke made the argument because of issues with bubble formation on deep dives.

Quick Summary: If you have had enough of a surface interval to do whatever dive you want to do safely, then there should be no concern if that second dive is deeper than the first dive.

 

[Coll]

Thanks for all the information, I was diving without a computer and I knew what the tables told me, I just wasn't sure how a bounce dive would figure into the calculations. Do you just consider it an extra dive? The whole dive trip was very unsatisfactory, the DM was very lazy but that's beside the point.