Dive Tables and Dive Log Question

[Ronniemu]

I did my AOW Deep and Boat Dive and have a question on my dive. It was a two tank dive. The first dive was 85 feet for 24 minutes. There was a surface interval of 1 hour and twenty seven minutes. The second dive was 87 feet for 26 minutes. Using my Padi RDP I had a Pressure Group of "P" after the first dive. With a surface interval of 1 hour and twenty seven minutes my Pressure group was now a C. After calculating my second dive of 87 feet for 26 minutes the planner shows my no decompression limit should not exceed 16 minutes and the second dive was for 26 minutes. I was using my computer and my deco remained max out on the green and did not go to yellow. We did our 3 minute safety stop and all was fine. My question is how do I log this dive, the computer gives credit as you change depth and is not rigid as my RDP. When I try to log diving profile it shows gas groups higher than the charts go.

 

[phoenix09]

I log my max depth and dive time as recorded by the computer, and also make sure to note the time in and time out for each dive, then do the same for a repetitive dive. That way, I have the surface interval recorded, even if it doesn't match up exactly with the RDP. I don't worry about the pressure groups, since the RDP is more rigid than a computer, and I don't know how (or if) you can compare the two when logging a dive. Oh, I also make sure to write down the brand and model of computer I'm using. I was actually curious about this issue as well...what does everyone else do about this?

 

[CAPNVINNY]

log it like you dived it,the rdp acts as if you were at the max depth the entire dive and doesn't give credit for any shallower portion of the dive unless you use the wheel ,your always gonna get a higher pressure group than your computer. you can log your final pressure group as a Z or just leave the pressure group off your log.THe tables are a nice conservative backup in case your computer dies,glad to hear some one is checking their dives with them.

 

[FJTrekr]

This is precisely why you invested in your computer. It tracks your actual profile during the dives, giving more time before reaching your NDL. The RDP assumes you went to max depth and stayed there the entire dive. Because you did not, the computer credits you for time at shallower depth. The RDP is also subject to conservative rounding, which further limits your allowable bottom time. To log it, make a note that you were using the computer. If you still want to compare computer data to the RDP, try using your average depth versus max. It should be closer.

 

[CrawfishDiver]

Glad you asked that I have been just logging with put pressure group

 

[wardric]

If your computer dies, you cant go right back to using tables since you dont know your pressure group so you gotta wait the full desaturation time before using the tables.

 

[trucker girl]

Unless you track your dives manually as well- my trip to Hawaii will be my first diving trip with a computer... but I plan to use the ol' handy dandy Timex and slate as a backup just in case... Everything should be in fine working order with my computer... but that's what I say about this damned laptop I'm typing on and I have to just walk away from it sometimes...

But that's not what this thread is about.
I, too, figure my pressure group based on the time my computer says I was down and the depth it shows. I've checked it against other divers' computers and watches, etc, and it seems to be accurate.

I don't like the 85 ft dive followed by an 87 ft dive. I'm not so concerned with the 2 foot deeper... but the two deep dives right in a row so close together. But, as I've said elsewhere, I'm a wimp. It works for me .

 

[wardric]

Unless you track your dives manually as well-

exactly. If you dive the tables and keep the computer as a backup, you can rely on the tables but once you "bail out" (not sure if it's the correct expression) using your computer (i.e. you cheat the tables coz the puter says you're ok), then the tables become useless.

I haven't log pressure groups for many years (since I got my computer). I Just write max depth and dive time along with all the other details since I'm a log maniac

 

[Charlie99]

I, too, figure my pressure group based on the time my computer says I was down and the depth it shows. I've checked it against other divers' computers and watches, etc, and it seems to be accurate.

Your method works only for dives that are either 1) relatively square profile, where you spend most of your time near max depth; 2) relatively shallow so that you don't approach NDL even assuming all time is at max depth, or 3) your dives are short, and therefore stay well away from NDL. As the orginal poster noted, a multilevel dive within no-stop limits can be way beyond no-stop limits when you try to calulate it using a square profile table.

If you dive the tables and keep the computer as a backup, you can rely on the tables but once you "bail out" .. using your computer .., then the tables become useless.

My fallback method, which I have not yet had to use, is to simply assume that I am at the maximum no-stop pressure group at the end of my last dive, and move onto the tables from there.

If using the PADI tables, one also has to make a judgement call as to whether the special "W,X,Y,Z" rules related with extremely long shallow dives and the minimum 1 hour and 3 hour SI periods apply. It takes some seriously long dives to activate those special rules, and on a practical basis, just assuming pressure group Z works.

 

[rottielover]

I'm glad somone else does this! I too dive with computers (2 on me, my wife has 1 and a watch)...

Thus far, we've always logged the dives using the depth and time from the computer, but then ran the tables with those numbers in order to write up our log entries.

We have not yet had the occation to "bust the tables" using the computers, but with our upcomming Bonaire trip next year, I'm sure we'll be using the computer numbers in order to increase our BT.

Can some more instructor's chime in on this? I'm sure you must have students asking about this all the time right?

I think I'm going to fire off an email to one of my instructors to see his take on this as well.