Dive Plan Help/ Question

I finished my OW certification in Hawaii a few days ago and want get get a few more dives in before I leave.

I did two private dives this morning and have a few questions for you guys. The dive master knew I was new at this so we planned the dive to 45ft but would go deeper if I felt comfortable. He had a computer and I had the standard rental gauges.

Most of the dive was spent above the 50ft level but we did go to 75 ft for a few minutes then we started our ascent. Total time down was 25 minutes.

After a 67 minute surface interval we did the next dive. The max depth was 62 ft for 42 min. After we surfaced I started doing my dive log using the rdp and he said that I could not use those figures because they were not correct, he said to use his computers figures.

When I got home I used the rdp just to check the numbers and now I'm questioning the dive. These are the numbers I got: First dive 80 ft / 25 min puts me in PG N. After a 67min SI I'm in D.

Second dive 70ft for 42 min. Adding my RNT of 13 from dive 1 to my 42 min on dive 2 gives me a TBT of 65.

If I'm reading the RDP right I should have never been down for 42 min on dive 2. The longest I should havevbeen down was under 27 minutes.

Is my math correct or am I missing something?




John

Sorry just want to add the the times given are not exactly correct, if anything the actual dive times are less. I used my watch timer, I started the timer when I jumped off the boat not actual time of decent and stopped when I surfaced.
Thanks
John


John

I don't have my tables handy and will assume your figures are correct. I did almost the same thing on my very first dive. Down to 54', but only for minutes. Tables provide for square profiles--that is, you calculate the whole dive as if you were at your 75 max depth the whole time. If you were only down there briefly, table calculations pretty much go out the window. The computer tracks your total bottom time (thus, theoretical nitrogen loading) according to exactly how deep you were every second. Thus, your 42 minute 2nd dive was probably very safe. After that first dive I made, I did my tables and said ^&%$!!! Started looking at my arms for rashes indicating the bends. Tables can be very usesful in a LOT of cases, such as most of the diving I do. I go down to the bottom and stay there at one depth the whole time looking for shells. But for multi-level diving the only real way to go is the computer.

The problem is not whether the math is correct or not. And your may well be right on.

The problem is you did a "trust me" dive by following someone else's dive plan with no planning on your part. In this case, you could redo your first div on the tables as a multi-level dive and be going into the 2nd dive in a lower PG (C or B) But the 2nd dive is still off the table which only allows 40 minutes at 70 ft with no residual loading. You should have know your status before you started the 2nd dive and you could have simply stayed 2 ft shallower or called the dive sooner and be fine on the tables.

Was your dive safe in terms of N2 loading - probably, almost surely, YES. But this is something you need to take responsibility for and not leave it up to some other diver regardless of that diver's qualifications. Good lessons in your taking responsibility for your dives and in your need for a dive computer if you want to get more bottom time.

this is a perfect example why you should have your own dive computer and not put your life in the hand of other people (my friend you only have one life). dive tables are made to represent a single level dive meaning you descend stay at one depth then ascend. If he was using a dive computer you should be fine (that is if he was using it lol). I hope you guys did a safty stop.


This should be a lesson learned!!

Sorry if i come off in a bad way, i dont mean too. glad your still hear to dive again

I'm sure you did the table calculations correctly. Your DM told you to use his computer's figures because your guy's dive plan would not work on Tables. He did a bad thing because he did not follow the more conservative dive planner in the group.

- A buddy team, no matter what certification or rating should follow the most conservative planner in the team. In this case that would be your Tables.

Recap of your Dive plan
Dive 1: 75ft / 25min
SI: 67min
Dive 2: 70ft / 42min

I don't know what tables you used so I'm going to plan off of NAUI just as a reference.
Dive 1 puts you as an G diver. After SI you're now in letter group F.
After Dive 2 you're now off tables.
On tables you're max dive time at 70ft is 45mins. Your bottom time with RNT included was 72 mins.
On NAUI tables 70ft @ 70mins requires 14mins of deco. NAUI Tables do not list time higher than 70mins for 70ft.
This is the same case you got planning off your tables.

So why didn't you take a DCS hit? Why are you ok? The answer is complicated but the gist is this:
Tables only take into account a square profile, which is not what you did. Computers calculate your dive profile and give you credit for ascending and take away credit for descending. Credit is your NDL.
Tables are also a lot more conservative in dive plans than computers.
Computers are more accurate than tables however. They give you your current NDL on the fly. This isn't to say that you should not plan your dive ahead of time however.

By not spending all your time at your max depth, you did not follow a square profile, which is what your Tables plan for. You DM's computer was calculating your NDL based on your actual dive profile and gave you credit, or more NDL time when you ascended early.
I'm sure your DM's computer was well within the NDL for "your" dive plan.

If you were wearing your DM's computer and planning off of that you would have seen that you were well withing your NDL, based on the computer's algorithm. However this was a trust me dive in a sense.
You did a trust me dive in the sense that you were following a dive plan based on a computer you were not wearing.

I appreciate all the feedback. After the first dive we did the dive profile from the dive masters computer then planned and dived the second dive. Both were well with in the computers limits, I only questioned him because of the "PADI" rule of not sharing computers. On both of the dives he was slightly deeper than me because I still struggle with buoyancy and tend to float up.

Thanks for all the comments, keep em coming.
John


John

How did your dive profiles look like? Square or multi-levels?

That could be quite a difference in N2 loading

Im not sure what u mean by square. John


John

same issue i had with my OW course.. got all the info post-dive from the instructors computer, but when i plug that in to a table it brings me way off.. we were doing 55 minute dives almost every time hitting around 18meters max depth. Basically comes down to the square profile vs. using a computer to calculate based on actual depths i guess..

Question for everyone though, when you do your dive plan lets say for a new dive site, would you do it based on a square profile to be safe (rather than trying to calculate a multi-level dive that you may not stick to exactly)? Assuming you plan based on a square profile, once the plan is made would you dive your plan or just go by your computer and stay within limits? Gonna be hitting some new dive sites soon and had been wondering about this as i started to plan some of the dives..