Place of dive tables in modern diving (Split from the basic thread)

[tursiops]

Yes, as a computer geek I feel much safer whenever I get inconsistent results from performing basic arithmetics.

If only rounding and quantizing were basic arithmetic. They probably did the two in a different sequence, thus the different answer.

 

[KenGordon]

@dmaziuk Maybe this will help. Let's use a different example.
Using your metric table, let's look at a 25m dive for 22 minutes. You are now in PG "L". Follow row "L" over to a SI of 45mins (in the :43 to :50 column); you have dropped from "L" down to "E".
Now, just for fun, look back at the 25m column, and go down to PG "E"; you see the number 13. that means from PG "L" (22 mins of nitrogen in you) you have outgassed to PG "E" (13 minutes of nitrogen in you). If you look up and down that 25m column, you see the nitrogen levels correspondingly changing. If you wanted to do a second dive to 25m, and you had only that 45 minute SI, you'd have 13 minutes of nitrogen left in you; that is your Residual Nitrogen Time, RNT. Since the NDL for 25m is 29 minutes, you've got 29-13=16 minutes of dive you can do for your second dive to 25m.
Now, finally, turn the table over and look at Table 3. Find the 25m row. You find for each PG that the two number shown add to 29....the NDL for 25m. The top number (from L to R, 29-28-26-25 etc) is the same as the times in Table 1 for 25m, reading from bottom to top, i.e. the RNTs for each depth/time combo. Go all the way to the right in the 25m row on Table 3: the top number is 4 mins, corresponding to PG "A", the same as on Table 1. The bottom number in that row is the NDL minus the RNT; this is the maximum BT you can have for that PG and that depth.

Now let's go to your two dives to 40m case.
The first dive has a 9 minute NDL from Table 1. You end in PG "G". Floow the "G" row over to the right. After 45 mins you would have dropped to PG "C". Still on Table 1, you note that PG "C" means 6 minutes RNT, so that means after a 45 minute SI, you could only go to 40m for NDL-RNT=9-6=3 minutes. You decide that is not enough time, especially since your descent is part of your allowable Bottom time. So you look at the PG 'G" row, go all the way to the right, and note that after a SI of 4h you would be in PG "A." Still on Table 1, you see that PG "A" has a down arrow, which means use PG "B". (The down arrow is there because of the rounding and quantizing inherent in a table.) PG "B" says your RNT is 5 mins, so you only have 9-5=4 minutes of available BT. Still not enough. But if you wait in your SI longer than 4:42, you drop out of PG "A" and back to no RNT, so you get your full NDL you can spend at 40m, i.e. 9 minutes.
You may note that the RNT for PG "A" and a dive to 25m on Table 3 is shown as 4 mins; another anomaly of the rounding and quantizing of a table.....they didn't do quite the same rounding for Table 3 as they did for Table 1. This is a table and consistency problem, not an algorithm problem. You'll find several down-arrows on Table 1, where the actual number is half-way between the numbers above and below. They are effectively rounding up to a greater RNT for your safety.

Conclusions:
1. You start the day NOT in PG "A" but rather without any residual nitrogen.
2. There is NO NEW INFO on Table 3; all it does is summarize Table 1 and do a subtraction for you of NDL-RNT.
3. PADI does not generally refer to no-stop diving, they prefer to call it no-decompression diving, as far as I know. So your concern about how can a mandatory safety stop be part of no-stop diving is moot.

I think this post nicely illustrates the problem with tables.

An entertaining experiment at a dive show would be to grab random punters and ask them to plan some dives with tables, in return for beer or shiny things.

 

[Jay]

re Table 3.

I totally ignore it because all the necessary info is in Table 1.

The top number of Table 3 in white is just the same as what Table 1 gives (call it 'X'), and the bottom number in blue is merely a subtraction of the max NDL less 'X'.

 

[tursiops]

re Table 3.

I totally ignore it because all the necessary info is in Table 1.

The top number of Table 3 in white is just the same as what Table 1 gives (call it 'X'), and the bottom number in blue is merely a subtraction of the max NDL less 'X'.

Agreed; that's what I said in Post #162.

 

[dmaziuk]

re Table 3.

I totally ignore it because all the necessary info is in Table 1.

The top number of Table 3 in white is just the same as what Table 1 gives (call it 'X'), and the bottom number in blue is merely a subtraction of the max NDL less 'X'.

This is getting better with every post: in order to avoid confusion, ignore 1/3rd of the planner.

 

[tursiops]

ignore the redundant 1/3rd of the planner.

Fixed it for you.

 

[dmaziuk]

Redundant: not or no longer useful.

I.e. PADI RPD is only 66% useful. Could be worse...

IRL table 3 is the only one I ever look at, so it's not redundant for me. Because it's the one thing my computer's plan mode does not tell me.

 

[Kevrumbo]

Yes, as a computer geek I feel much safer whenever I get inconsistent results from performing basic arithmetics.

If only rounding and quantizing were basic arithmetic. They probably did the two in a different sequence, thus the different answer.

I think this post nicely illustrates the problem with tables.

An entertaining experiment at a dive show would be to grab random punters and ask them to plan some dives with tables, in return for beer or shiny things.

It's even more inconsistent and entertaining fun with a PADI analog Wheel ml-RDP, from the parallax error lining up the pointers especially with presbyopic old eyes. . .

 

[tursiops]

It's even more inconsistent and entertaining fun with a PADI analog Wheel ml-RDP, from the parallax error lining up the pointers especially with presbyopic old eyes. . .

LOL. I have TWO Wheels, which even when both are "calibrated," allow me to get lots of different answers and match almost anything. Kind of like Gradient Factors.

 

[dmaziuk]

Ah, yes, that... I can still read my computer without reading glasses. Tables: not so well, maybe that's why I can't get the numbers to add up.