Check-mark profiles and repetitive dives

[TSandM]

I have a feeling this is a very stupid question, but I don't have anything at hand that will answer it. It kind of has a couple of parts:

If you plan your dives to use the check-mark profile that UP describes, spending time shallow for time deep, do you still use the same basic tables for planning subsequent dives, or do you get some "credit" for having done what amounts to in-water off-gassing? If you get "credit", how do you figure out how much and where to apply it (surface interval, length of subsequent dives)?

As an extension, how do people doing decompression diving account for a previous dive when planning a subsequent one the same day?

Maybe the answer is that I should download some decompression software and play with it and it will answer my questions, and that's a fine answer for somebody to give me.

 

[ScubaSixString]

I have a feeling this is a very stupid question, but I don't have anything at hand that will answer it. It kind of has a couple of parts:

If you plan your dives to use the check-mark profile that UP describes, spending time shallow for time deep, do you still use the same basic tables for planning subsequent dives, or do you get some "credit" for having done what amounts to in-water deco? If you get "credit", how do you figure out how much and where to apply it (surface interval, shorter subsequent dives)?

As an extension, how do people doing decompression diving account for a previous dive when planning a subsequent one the same day?

Maybe the answer is that I should download some decompression software and play with it and it will answer my questions, and that's a fine answer for somebody to give me.

If you aren't deco diving, you don't get to count deco credits IMHO. Do you give yourself "points" for your safety stop?

You may not be "off-gassing" at all. If you dive to 120 ft for 3 minutes then come up to 80, you might be on gassing still....

What you describe is a multi-level dive, and tables aren't ideal for those types of things.

Unless you are diving deco tables, you kinda have to play by the NDL table rules (computers, however, will give you a little "credit"). That's my take on it....unless i misunderstand the question.....

 

[Diver0001]

I have a feeling this is a very stupid question, but I don't have anything at hand that will answer it. It kind of has a couple of parts:

If you plan your dives to use the check-mark profile that UP describes, spending time shallow for time deep, do you still use the same basic tables for planning subsequent dives, or do you get some "credit" for having done what amounts to in-water deco? If you get "credit", how do you figure out how much and where to apply it (surface interval, shorter subsequent dives)?

As an extension, how do people doing decompression diving account for a previous dive when planning a subsequent one the same day?

Maybe the answer is that I should download some decompression software and play with it and it will answer my questions, and that's a fine answer for somebody to give me.

Well.....it was only a matter of time before you came with this question.

It's time you downloaded vplanner or gap and started playing with them.

I guess you can see this a couple of ways. If you're doing two checkmark (CM) profiles on one day and you stayed within the NDL's on the first one but you want to treat it like a multi-level profile then you'll be off the tables in terms of bottom time but you can still fall back to the Z group (on the PADI table), do a 3 hr SIT and go from there.

Proof that this works will keep you off the streets for a couple of evenings.

The other thing that you can do is treat 2 CM profiles like one big one and pick the deepest depth and the combined BT of both dives and plug them into a deco program as a multi level profile. What you'll find is that within recreational depths you can go for ridiculously long bottom times and the CM method still works to deco you out (gap works better for this kind of fiddling than vplanner does). Corollary to that is that if you do them one after the other with a zero SIT it works too. Risk you run in not planning your gas increases with repetitive dives because you can lose access to the surface and since you're applying a sort of ratio deco you don't know for sure that you can surface until you're done. What you *can* do, however, is re-work the adjusted NDL using Z as above and keep that in mind for the global picture when you do your second dive.

If you're skeptical about whether or not this works then you have the right feeling. The only way you can really convince yourself of it is to work out dozens and dozens of dives like this on something like gap to see the pattern emerge. I think UP would tell you just to run the profiles and convince yourself that it's working.

As for the other question is that deco software all has SIT functionality.

R..

 

[O2BBubbleFree]

...If you plan your dives to use the check-mark profile that UP describes, spending time shallow for time deep, do you still use the same basic tables for planning subsequent dives, or do you get some "credit" for having done what amounts to in-water off-gassing?...

I think UP answered this here. If I understand it right, he treats the second dive like the first (no allowance for residual nitrogen). If there's a significant SI between #2 and #3, he treats #3 the same way. In other words, full credit for in-water deco, no allowance for residual nitrogen.

 

[Uncle Pug]

Keep in mind that what I described is:
1) Only what I do... and even though I am over half way to a century old... I'm in fairly good physical condition... cough, cough... hack, hack... and now that the fever is broken I think my pneumonia will get better.

2) What I do on recreational depth dives.

3) Using EAN32.

Lynne... yes, get GAP and play with it if you are the visual kind... or Vplanner if you can *see* numbers better.

 

[Doc Intrepid]

Here, Lynne, try this one:
http://www.hhssoftware.com/v-planner/index.html

Its written by Ross Hemingway, who is active on TDS and will answer both questions in threads and PMs.

Short answers are -

1. You use the software to plan your dives, and they needn't be 'deco' dives - e.g. you could be planning a multilevel dive that is pushing NDL but that does not 'require' a hard deco stop.

2. You use the same software to plan dive #2 - you indicate your SI, etc. The algorithm does indeed provide for residual nitrogen (helium, etc.) computation and allows "credit" (or debit ) for the effects of your first dive profile when planning the second. BUT...

Theoretically speaking, (and I'm open to criticism from those more knowledgeable or literate than I) at the end of a 'decompression dive', the software brings the diver up relatively clean based on the parameters of the dive (or with very little residual offgassing taking place on the surface). While there may be indeed residual offgassing occurring during the SI, if you look at the profiles generated for follow-on dives (in V-Planner) there are not significant penalties placed on the repetitive dive in terms of RNT added to BT (as would be the case if using tables).

You'll need to play with the software, as Diver0001 suggested, to begin to get a feel for whats happening. It isn't as simple as UP's method, but the software will provide you with a more in-depth understanding of the consequences of changes in one parameter or another.

Hope this helps,

Doc


Oops - sorry UP - you responded while I was pontificating...

 

[Bottom Scratcher]

You may, or may not find your answer using the deco tables. Check out the link and down load the PDF file.

http://www.schmarsow.net/divetab.html

 

[TSandM]

Bottom scratcher, thank you for the link. I have printed out the document and it looks like very interesting reading.

Time to start playing with software, I guess.

It's a little confusing to me how this works. For example, if you do a dive to 85 feet for 20 minutes (one I did last week), and spend 20 minutes above 30 feet, weighted to spend more time as you get more shallow, even if you counted the whole 20 minutes as surface interval, you'd still have more to do to be pressure group "A" by tables. Since the gradient for off-gassing is greater at the surface than it is anywhere underwater, you really haven't done 20 minutes of "surface interval" offgassing. (If you're using O2, that's different.) Maybe watching the loading and unloading on the software will make this clearer to me.