Place of dive tables in modern diving (Split from the basic thread)
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@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.
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 can only find "no stop" in two places in the Instructor Manual (under supervising Scuba Divers and under the Deep Adventure Dive), and cannot find it at all in the Guide to Teaching (but I don't have the latest edition). It is certainly in the current OW manual. They also say, "A few computers and tables have a 'required' safety stop. With these, because you are nearing the limits, they call it 'required' to put more importance on being conservative." So, I suppose someone looking for an argument might argue that PADI's no-stop diving actually has stops, but that really does try and make gray things black and white, and ignores all the waffle words.
nolatom
Contributor
I confess to not having read the whole 17 pages of this thread.
Let's just say I "know a guy" who dived anyway, even though his computer went dead (battery) just before dive dive dive on a drift dive. The boat limited nitrox divers to 45 minutes and "this guy" had a very good buddy who used air much more sparingly than he, and was aware of the no-computer situation, "no problem, gas will limit you much earlier than nitrogen".
first dive about 70 on the reef, 90 at the sand. Second dive about ten feet less on both. Warm water, good viz.
Back to the stone age! Wristwatch and pressure gauge only. Guy stayed mostly near the top of the ledges, each dive was right around 36 minutes. Coming up by agreement earlier than long-breathing buddy who had way more gas while guy had 900, he guessed at 15-20 feet depth, 4 minutes each safety stop. He did not use tables because he hadn't done it since forever and, uh, what the hey. Guy had in mind the 120 rule, or more conservatively the 100 rule, and basically stayed well within them for first and second dives respectively. Long surface interval between dives.
Guy feels fine. It was fun. couple of lemons and huge sea turtles. Don't ask me how I know all this.
Let's just say I "know a guy" who dived anyway, even though his computer went dead (battery) just before dive dive dive on a drift dive. The boat limited nitrox divers to 45 minutes and "this guy" had a very good buddy who used air much more sparingly than he, and was aware of the no-computer situation, "no problem, gas will limit you much earlier than nitrogen".
first dive about 70 on the reef, 90 at the sand. Second dive about ten feet less on both. Warm water, good viz.
Back to the stone age! Wristwatch and pressure gauge only. Guy stayed mostly near the top of the ledges, each dive was right around 36 minutes. Coming up by agreement earlier than long-breathing buddy who had way more gas while guy had 900, he guessed at 15-20 feet depth, 4 minutes each safety stop. He did not use tables because he hadn't done it since forever and, uh, what the hey. Guy had in mind the 120 rule, or more conservatively the 100 rule, and basically stayed well within them for first and second dives respectively. Long surface interval between dives.
Guy feels fine. It was fun. couple of lemons and huge sea turtles. Don't ask me how I know all this.
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As I said earlier, when I was doing shallow muck dives in the Philippines, it occured to me that I could do the dives with pretty much no instrumentation at all. No depth gauge, no SPG, no computer, no tables--no problem. I just needed a watch because we were supposed to be done at a certain time. If those are the dives you are doing, pretty much anything will work.
If you are doing other kinds of dives, you need other kinds of devices.
If you are doing other kinds of dives, you need other kinds of devices.
nolatom
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