Computers Vs RDP and bottom times

[kstakem]

Hi everyone,

I have a small problem that I was hoping some people could shed some light on. I would appreciate if the flamers would hold their tongues please. I’m trying to learn here and don’t need computers rot your brain comments.

I am a new diver, as some of you know. I also have been diving with a Cobra dive computer since my OW dives. Since my number of dives is getting up there and I haven’t been good about logging the dives right away, I decided to bring my logbook up to date today. In doing so, I discovered a problem in how I am recording the dive data.

A little background info. I dive at the local quarry. From what I have seen I would say the style of diving there is very loose. Dives aren’t planned, you just swim out and figure what you will do. IE: drop to a platform to practice skills or swim to some of the lovely underwater stuff. Since I am new I can’t comment on if more “Surface Planning” happens on ocean / real diving environments. I would hope so

I do know how to use the RDP and fill out my logbook. I understand that bottom time consists of the time you descend to the time you start your ascent. The problem is the dive computer only logs total dive time, unless I am missing something with the Cobra. I also remember the question coming up in OW class if you can use the entire dive time as your bottom time. The answer was yes, as it creates a larger safety margin. So I figured I would just use total dive times for my bottom times and calculate pressure groups based off that.

One reason, whether it’s a good one or not, that I haven’t been worried about remembering all logbook info is because of task loading. I was more concerned with getting comfortable and relaxed in the water as well as learning my new gear. I have 16 dives logged so far and feeling pretty relaxed while I dive now. So now I’m working on increasing my tasks with whatever data I need for my logbook, if that makes sense.

So now to the meat of the post…

First dive of the day was to a max depth of 85 feet. Total dive time was 38 minutes. We dropped to the bottom, swam around some then found a wall and followed that up. The following is a break down of the depths we lingered at and the time into the dive. I was always good as far as the dive computer cared.

Depth – Dive time
0ft to 85ft – 4 min
80ish ft – 10 min
60ish ft – 18 min
30ish ft – 21 min
15 ft – 35 min
0ft - 38 min

So here I was writing 85 feet for max depth and a bottom time of 38 minutes into my log. I then checked my RDP for pressure group and uh oh . Max NDL at 90 feet is 25 minutes. So looking back via the profile on my Cobra I see that we really started to ascend after about 11 minutes.

So I guess my question is would you really count the bottom time as 11 minutes for a 38 minute dive? This is more like a tiered dive and I realize that but since I am not using the wheel, I don’t have that option.

Thanks for the help

Kevin

 

[MikeS]

Kevin,

I also have a Cobra. Because of the very issue you’ve raised I don’t even keep a traditional logbook. I down load the dives from the Cobra, add the pertinent information and print the dives, which serve as my logbook. It contains much more detailed information than a traditional log.

Mike

 

[O-ring]

The computer is calculating a "real-time" profile and crediting you for any time you don't spend at your max depth while the RDP (table version) only accounts for a square profile dive and counts the total dive time at the maximum depth. Even with a wheel it will round and not come out exactly like the computer...but will be closer than the RDP tables.

Bottom line, you can't log computer dives with computer data with an RDP...you will blow NDL all the time. Well, you can log max depth and dive time, but your pressure groups on the NDL will be way off. MikeS is right...easier to do it that way if you use the computer for all your dives..and much more accurate. It also calculates your SAC for you, etc. so you don't have to mess with nasty formulae

You can do a little experiment to play with it...look at your NDL for an 80' dive on your table. Now, set your Cobra in SIM mode and do an 80' dive. When you start to go down, watch what happens to the NDL numbers. When you start to come up, watch the numbers again.

 

[landlocked]

If you don't have the computer hardware/software to download and print your dives yet, you may want to consider building yourself a different log page until you do. The pages published by my cert agency didn't have the information that I wanted (nitrox, etc.) so I built my own. Takes about an hour. I print blank pages, punch them and put them in my log book to be filled out as needed. I have also started sketching scenes from the dives on the back.

 

[Spectre]

Personally, I log my underwater time, since that's what I'm keeping the log for. If you're using a computer, and diving the computer, than don't worry about filling in the pressure groups, 'cause you're going to blow them all the time.

Remember, it's your log, and log what you want to log. My log has only been 'checked'once... and all he did was: look at # of dives, flip back 1 year, look at # of dives. Flip through the dives in the past year and look at the max depths.

However, you might just want to download your computer and print 'em out. There is more details you can see when you download than you can get with your display [at least for mine that's the case].

Log how you want to log, but don't worry about filling out the pressure group stuff unless you're using that when planning your dives...

 

[O-ring]

...for once, Spectre and I are in agreement. I use standard log sheets, but I don't fill in any of the nonsense. Just depth, time, all the site information that I want, buddies, and CBT.

 

[Spectre]

O-ring once bubbled...
...for once, Spectre and I are in agreement.

There has been other times... I just try not to let on about it (don't want you to loose your edge, ya know! )