Multi Level Dive Logging?

[PfcAJ]

Average depth is useful for calculating gas usage. It is USELESS for determining inert gas tissue loading (and thus, table-based Pressure Groups).

There is a lot of discussion of algorithms, including some for using average depth for manually calculating decompression obligations. Those discussions are in other subforums and, in my opinion, are not appropriate to even bring up here in the Basic forum.

For purposes of your original question, AVERAGE DEPTH IS USELESS.

If you're going to use tables, stick to your training and base things on the maximum depth reached during the dive.

If you're just calculating a PG in order to record it in your log for historical purposes, don't bother. Nobody down the road is ever going to look at or care about that.

If you're actually going to use a PG to calculate your NDL for a second (or subsequent) dive, then, as has already been noted, you need to use table-based planning for your first dive and follow what it says (i.e. adhere to the table-based NDL, even though your computer would let you stay down longer).

Your best bet is to simply learn how to use the planning function in your dive computer to plan your dives. Dive planning using tables will really limit your bottom time - especially if you are normally doing multi-level dives. If your dive computer doesn't have a planning function, it might be time to think about getting a more fully-featured dive computer.

Absolutely incorrect.

Work it out yourself. Avg depth vs planning a multilevel dive with deco softwear produces results that are within a minute of each other in all but the wackiest (and unrealistic) profiles. And in those cases, it’s within a few minutes.

I’m pretty sure you and I have gone over this where you proposed some fantasy dive profile, I went and did the math, and the difference was minuscule to nonexistent.

 

[BoltSnap]

Where I got tripped up, prompting me to ask the original question was my ending pressure group. How do I figure that out? I wasn't at 80ft for very long, but my training tells me that I need to 'act as if" I was at 80ft for the whole dive. When I tried that with my RDP at home I found I was well over the table limits for DCI, but I was only at 80ft for a few minutes and began to slowly ascend and finish the dive much shallower. So I didn't understand what to do with my data.

I don't understand why you still don't get it about tables, max depth and NDL's after several folks have explained it to you in this thread. With Dive Tables, you use the max depth you reached during the dive even if it were for a mere minute, nothing else and no if's or but's about it. No extrapolation or guessing or anything else, just the max depth!!!

Read @stuartv posts above very carefully.

 

[TMHeimer]

Absolutely incorrect.

Work it out yourself. Avg depth vs planning a multilevel dive with deco softwear produces results that are within a minute of each other in all but the wackiest (and unrealistic) profiles. And in those cases, it’s within a few minutes.

I’m pretty sure you and I have gone over this where you proposed some fantasy dive profile, I went and did the math, and the difference was minuscule to nonexistent.

I'm not gunna do the math, but using the avg. depth on a computer multi-level dive as PfcAj mentions seems logical to me.

Jim L.'s gold bar example further points out the difficulty (or pitfalls if you will) of trying to use the eRDPml (or even RDP) to do a multi level dive instead of just following a computer.

I love JackD342's graph. It shows something very few talk about on SB--that with a computer, you lose all the "rounding up". I have put forth the point that if you are doing a square profile using tables and depth gauge you always get the rounding. -- Because your gauge will read say Max depth was 90', which is when you were right on the bottom as I may be, picking up a shell, or taking a picture. But most of the time you're really at maybe an average of 87'. You're not losing much bottom time because of no computer because you are practically at the max depth from start to finish. I still would use my computer at those depths, but actually pay just as much (or more) to my gauge and dive watch.
The computer (as with tables) of course only tracks the theoretical gas in your tissues, but if we all had exactly the same tissues and nitrogen in you was measurable, your computer's remaining bottom number would be exact. Not a bit conservative like with tables.

 

[stuartv]

Absolutely incorrect.

This is the Basic Scuba forum, so my reply will reflect that.

On-gassing (and off-gassing) is not linear. Calculations based on averaging depth are inherently linear. That is why average depth is not useful for calculating decompression obligations (at the level implied by being in Basic Scuba).

Furthermore, it seems pretty clear that the OP only has training corresponding to PADI Open Water. As such, a recommendation to do anything regarding dive planning that is based on average depth is specifically a recommendation to go outside his training.

As such, it is completely inappropriate and arguably irresponsible on your part. This is supposed to be a flame-free zone and I'm not posting this to flame you. I'm posting it (versus, for example, a PM) so that the OP and anyone else who reads this can understand how (I think) your posts here should be regarded. I do not agree with recommending to divers at the OW or AOW level to do dive planning in ways that are outside the scope of their current level of training.

To the OP (and anyone else), I say: Dive (and plan your dives) the way you were trained. If you are fuzzy on how to apply that training, there are numerous folks here on SB that are happy to help you with that.

 

[stuartv]

I love JackD342's graph. It shows something very few talk about on SB--that with a computer, you lose all the "rounding up". I have put forth the point that if you are doing a square profile using tables and depth gauge you always get the rounding. -- Because your gauge will read say Max depth was 90', which is when you were right on the bottom as I may be, picking up a shell, or taking a picture. But most of the time you're really at maybe an average of 87'. You're not losing much bottom time because of no computer because you are practically at the max depth from start to finish.

In that example you wouldn't lose hardly any bottom time. But, if the example were "at 89' all the time except for a moment at 92' to pick up a shell" then the difference would potentially be pretty big. Since, for use of tables you always round up. So, your max depth, on a table, would be 100' (since your max was 92 and you always round up to a multiple of 10). A table NDL for 100', compared to a computer NDL that was from spending 95% of your bottom time at 89' would potentially be around 5 minutes (or 20-25% of your total bottom time).

 

[Steve_C]

Ok, it is all nice but lets consider a local NC dive. We are diving on the hyde. An intact ship. It is 80 ft to the sand, 85 on the sand next to the ship in the washout, 60 on the deck but say 50 along the top part of the deck. We do not know the viz at the start. Turns out there is a haze layer in the bottom 10 ft. We may will spend the whole dive on the top deck. Or more typically we start on the sand. Some dives we come up a level pretty soon. Others there is a lot of life down there, we play with flounder, and pick up a lot of teeth. As we come along the mid deck, we are doing good on air. You are into a bit of fish ID, so we drop down to the sand in the hold to check out the spotted gobies. We are back to 80ft. Coming up we again have some options. Point is there is no way to say ahead of time I will spend x minutes at y and then x2 minutes at y2, etc. Where we go will be determined by what shows up, the water conditions, and how we are doing on air. I dive with lots of instabuddies but if somebody comes along with a rigid multilevel dive plan or relying on tables then I will politely decline to dive with them.

I keep a reaonably close monitor on the air of mine and dive buddies. If we have the air I will take a picture of the gold bars. If not, then we will do it on dive 2 or not at all.

 

[stuartv]

I just started diving with a new buddy, and on our last 2 dives (4 days apart), we went to about 80 or 90ft max. We adhered to the limits of the computer and stuck to our dive plan.

That's where your confusion is coming from. If you want to use a table to plan a second dive, you have to adhere to the limits of the TABLE on the first dive. By adhering to the limit of your computer, you were able to enjoy longer bottom time than what the table would have allowed. So, by the table, your first dive had you in deco. Thus, following the table for a second dive would actually mean that you would be required to sit out for 24 hours or something like that (because the tables don't like it when you go into deco).

 

[PfcAJ]

This is the Basic Scuba forum, so my reply will reflect that.

On-gassing (and off-gassing) is not linear. Calculations based on averaging depth are inherently linear. That is why average depth is not useful for calculating decompression obligations (at the level implied by being in Basic Scuba).

Furthermore, it seems pretty clear that the OP only has training corresponding to PADI Open Water. As such, a recommendation to do anything regarding dive planning that is based on average depth is specifically a recommendation to go outside his training.

As such, it is completely inappropriate and arguably irresponsible on your part. This is supposed to be a flame-free zone and I'm not posting this to flame you. I'm posting it (versus, for example, a PM) so that the OP and anyone else who reads this can understand how (I think) your posts here should be regarded. I do not agree with recommending to divers at the OW or AOW level to do dive planning in ways that are outside the scope of their current level of training.

To the OP (and anyone else), I say: Dive (and plan your dives) the way you were trained. If you are fuzzy on how to apply that training, there are numerous folks here on SB that are happy to help you with that.

“Dive the way you were trained”

Then why have a discussion board? Everyone apparently knows everything they need to know from their OW class.

 

[RainPilot]

If i understand the OP correctly, the question was not about planning/conducting the dives but about LOGGING the dives afterward, since they want to have all fields filled in and that pesky "End Pressure Group" box is like a missing tooth in a supermodel's smile.

As many have pointed out, EPG is totally irrelevant UNLESS the dive has been planned and conducted on tables. Even then, it will reflect what the tables say and not an actual indication of the gas loading accumulated during the dive. Using average depth to calculate an EPG after the dive should give a more accurate indication if it was a genuine multilevel dive.

I, for one, don't bother with the EPG even when I had a paper log. My suggestion would be to download Subsurface (free) and use that to log your dives. It won't ask you for EPG so no blank spaces, it will allow you to search your dives by depth (useful for later courses), equipment (nice for remembering weighting for suits etc) and if you later get the ability to download dives it will be a LOT simpler to populate the software. You can print out a paper log if you ever need one for IDC or such, but honestly next to nobody will ever care, like nobody cares about stamps.

The only times I have needed to show a log for a course, I emailed a PDF and we were good to go. Even when I had a (bunch) of paper logs, they only wanted to see the total dive number.

If you REALLY want to keep that paper logbook going, then just write "PDC" in the EPG box and move on, everyone who cares will know what that means.

 

[TMHeimer]

In that example you wouldn't lose hardly any bottom time. But, if the example were "at 89' all the time except for a moment at 92' to pick up a shell" then the difference would potentially be pretty big. Since, for use of tables you always round up. So, your max depth, on a table, would be 100' (since your max was 92 and you always round up to a multiple of 10). A table NDL for 100', compared to a computer NDL that was from spending 95% of your bottom time at 89' would potentially be around 5 minutes (or 20-25% of your total bottom time).

Good point. I agree. To eliminate that situation I suppose there could be a table in increments of 1 foot. Now there's a project (maybe that's been done somewhere)>