Doc Wong Getting Bent in Monterey!

[rhlee]

I've recently started using these tables as guidance, which allow for 50min@80' with a min-deco ascent with 3@20, 3@10 (which is what Doc did).

In my limited experience, these (or any) tables are terrible for planning the typical dive at Lobos, and this is somehting that we discussed in some amount of detail during my Fundies class. The message that I took home from Beto was that the tables we are using are reasonable, but that our best bet for planning dives of the sort I am talking about is to enter the segments into DecoPlanner (or some equivalent) and play around with the variables to see what gives you.

An example of a dive we did yesterday is shown here.

This is sort of on the borderline for me, but we ran the numbers ahead of time, and min-deco (just 1's up from 50%) was enough. We padded that by quite a bit though, as you can see.

I guess what I'm trying to say here is, the profile, as I've heard it, sounds reasonable to me, and is something that I would dive; I would think the bigger factors are the post-dive exertion, etc....

 

[paulwlee]

Apples and oranges. You either have to do the whole thing with the "real" tables or with the GUE tables, you can't mix them.

No, I'm not talking about the PADI table at all. I'm using AG/JJT's mindeco table for air and adapting that to 32% using simple math as you are supposed to do. (The 80% EAD method of using the air table for 32% is not a magic formula that they came up with, but just a mental shortcut for quick-and-dirty calculations. It's OK-to-conservative for recreational depths and great when you don't want to juggle with adding and subtracting 1 ata between the multiplication and divisions in the actual conversion, but it's still just an easy way of approximating the real calculation.)

The equivalent air depth for Nitrox is the same thing no matter what table you are using.

 

[DocWong]

I'm quite familiar with the area where Doc was diving. It would be a very square
profile. I just checked some recent dives there, and my max was 78, 80, 84 on
them. There's very little relief there. I'm guessing 75 was an average, but it ranged
from 70-80. (I'd let Doc answer this, but he's off diving Pt. Lobos.)

I don't think so. The DIR guys I know do a basic ascent rate of 10' per minute, plus some
stops, so Harry's 12 minutes would be about right. Interestingly, my DIR Fundies book
doesn't seem to mention ascent rates.

You're right Chuck, my depths varied from 70-80 and averaged 75 feet. It's a pretty sandy bottom without any pinnacles, etc. We searched for the Anchors with our scooters and didn't find them.

 

[Chuck Tribolet]

You're right Chuck, my depths varied from 70-80 and averaged 75 feet. It's a pretty sandy bottom without any pinnacles, etc. We searched for the Anchors with our scooters and didn't find them.

If it was a sandy bottom you were in the wrong place.

You shouldn't have any trouble laying your anchor about 40' down wind of the big
anchors at N36 36.874 W121 52.938. Just set a waypoint, set the GPS to navigate to
that waypoint. Stop there and let the boat drift. When the GPS says you are 40'
away, drop the hook.

 

[Chuck Tribolet]

This is correct, but for 75ft and using the more exact 68/79 ata air->32% conversion factor instead of the shortcut conversion (that's slightly more conservative) that's taught, 75ft on 32% is equivalent to 60ft on air, which is 50 minutes minimum deco time (a.k.a. NDL) according to their tables.

I think you need a new calculator:

75*(68/79) = 64.556962

not 60'.

And don't forget that he still had some loading from his prior dive.

 

[paulwlee]

Actually,

{ (75/33 + 1) * 0.68 / 0.79 - 1 } * 33 ~= 60

broken down,
75/33 + 1 : ambient pressure in atms at 75ft
* 0.68 : get PPN2 at 75ft on 32%
/ 0.79 : get the ambient pressure you would have to be at on air
to get same PPN2 (which means same N2 loading)
- 1 : get equivalent air depth (EAD) in terms of atms
* 33 : get EAD in terms of feet

Simplified, the EAD formula in feet for EAN32 would be
EAD ~= depth * 0.86 - 4.6
(Or you can take 80% of the depth for depths up to almost 100ft, which becomes more conservative as it gets shallower, but is close enough for the range where 32% is used and deco is an issue, namely 60~100ft.)

BTW, the way the tables under discussion here work is that for a second dive with surface interval over 2 hours the same minimum deco profile is used.
I believe this comes from simple rules that can be extracted from profiles run on decoplanner.

I think you need a new calculator:

75*(68/79) = 64.556962

not 60'.

And don't forget that he still had some loading from his prior dive.

 

[paulwlee]

For the tables that AG and JT taught, 80' would be 35 minutes on 32%.

Just realized while replying to Chuck's post that even using the 80% EAD approximation, 75' on 32% has EAD of 60ft (it's where the approximation is spot on), so our misunderstanding does not stem from the 80% EAD approximation.
80' on 32% is about 64ft EAD, so Mark's above statement is correct as I said before, but if you want to get numbers for a 75' dive you get the EAD for 75' and then apply it to a table, rounding up only then if necessary. No need to round up to 80ft first when getting the EAD and then rounding up again when applying that EAD to a table. (If that was what you were doing.)

 

[shakeybrainsurgeon]

Thanks for the info Doc,


This is where semantics get confusing for those reading SB who are simple grunt recreational divers like me and not technical or decompression divers. Doc said he was doing recreational diving which should mean NO DECOMPRESSION DIVES, at least that's what I was taught. If I am reading my nitrox NDL tables correctly, this was not the case. According to the EAD NDL tables, the second dive alone exceeded recreational limits.

This was a decompression dive, all well and good. However, many rec diving readers can read the initial post and think "heck, can I do a 30 ft reef dive, lift a scooter and get bent?" In fact, this profile is beyond the majority of people who dive or read this board, I suspect. While it's true that DCS can occur after innocuous dives, Doc's case doesn't really fit, at least from the perspective of those of us weekend quarry/reef divers who live above 60 feet and never do decompression dives.

As for being silly for requesting the profiles, the profiles indicate a) this is not no deco recreational diving but decompression diving, a more advanced and inherently riskier art; b) there appears to be some debate as to how conservative this profile was (again, I am not being critical...I know nothing about decompression diving personally...I am just observing posts by people who do know something about it and there seems to be differences in opinion as to how the profiles were calculated indicating that there might have been some possibility of bumping against the limits).

Posts describing DCS should make it clear if the profiles were recreational or decompression dives. Otherwise, people doing NDL dives within limits will fear DCS more than they should.

Again, I may be talking out my rear end as I am wont to do, but many SBer's like me lack the knowledge to understand how an experience like Doc's should influence their own behavior. Avoiding exercise after two shallow reef dives may not be as critical as avoiding exercise after two dives like Doc's.

 

[pilot fish]

I agree if it was a month betweeen dives but I said IN the last month. I sbould have said WITHIN the last month. Does that change the possibility?

I'm sorry, but this statement is erroneous. It IS possible to accumulate nitrogen in slow compartments over multiple days of diving, and get into trouble that way, but if you had a compartment or tissue with a five day half life (required to take a month to EMPTY it) it would load so slowly that it would take many, many consecutive days of diving to accumulate significant load there.

Trouble from diving from the previous few days, yes; from a month before, no.

 

[pilot fish]

.
Now I guess there is the possibility of latent tissue damage accumulating from silent hits which may tie in to the 170 dives per year...

Possible. That is something to look at