OW RDP Confusion!

[EFX]

I converted to imperial and ran it through the PADI tables and came up with 4 minutes minimum SI. This took me from N on the first dive to M on the second. Here is the procedure I used. Can someone cross check on the metric tables to see what the SI is? Table 1 is NDL (NDL), table 2 is surface interval credit (SIC), and table 3 is repetitive dive timetable (RDT).

1. Find first dive depth on NDL. Go down column to time. Go across to get group and land in SIC table.
2. Use same group (0 minutes surface interval) and go down SIC to RDT.
3. Go down column on RDT to second dive depth.
4. Scanning across the row, locate time of second dive using bottom number (this is maximum bottom time).
5. Now go up that column back to the SIC table.
6. Go up the column and stop at the box that is on the row of the group from the first dive.
7. Read the top number. This is the minimum surface interval time.

 

[dfx]

I converted to imperial and ran it through the PADI tables and came up with 4 minutes minimum SI. This took me from N on the first dive to M on the second. Here is the procedure I used. Can someone cross check on the metric tables to see what the SI is? Table 1 is NDL (NDL), table 2 is surface interval credit (SIC), and table 3 is repetitive dive timetable (RDT).

1. Find first dive depth on NDL. Go down column to time. Go across to get group and land in SIC table.
2. Use same group (0 minutes surface interval) and go down SIC to RDT.
3. Go down column on RDT to second dive depth.
4. Scanning across the row, locate time of second dive using bottom number (this is maximum bottom time).
5. Now go up that column back to the SIC table.
6. Go up the column and stop at the box that is on the row of the group from the first dive.
7. Read the top number. This is the minimum surface interval time.

In group N, it still shows 36 minutes adjusted NDL at 50 feet with no SI.

 

[EFX]

Yes, making the second dive with only 0-3 minutes of SI you would have to add 44 minutes (this is RNT) to the actual bottom time of the second dive. On the NDL table the NDL for 50 feet is 80 minutes. Subtracting 44 minutes from 80 gives you only 36 minutes of actual bottom time on this dive. But, we need 39 minutes! On the RDT going one group to the right at 50 feet, lo and behold, we find 39 minutes which just happens to be the bottom time we need for the second dive (how convenient, but it is a classroom problem so the numbers need to work out exactly). M is the new group we need to be in. Going up the column back to the SIC table where the M (our new group) and N (the first dive group) intersect is our required SI. To check this work it from the beginning using the newly found SI to see if the second dive bottom time is 39 minutes.

Can someone work the metric tables to see if the SI is the same or close?

 

[dfx]

Yes, making the second dive with only 0-3 minutes of SI you would have to add 44 minutes (this is RNT) to the actual bottom time of the second dive. On the NDL table the NDL for 50 feet is 80 minutes. Subtracting 44 minutes from 80 gives you only 36 minutes of actual bottom time on this dive. But, we need 39 minutes! On the RDT going one group to the right at 50 feet, lo and behold, we find 39 minutes which just happens to be the bottom time we need for the second dive (how convenient, but it is a classroom problem so the numbers need to work out exactly). M is the new group we need to be in. Going up the column back to the SIC table where the M (our new group) and N (the first dive group) intersect is our required SI.

Can someone work the metric tables to see if the SI is the same or close?

Right, of course. Somehow it slipped my mind that the second dive actually was supposed to be longer than 36 minutes

But as mentioned above, on metric tables you get an adjusted NDL of 51 minutes for the second dive without SI. I didn't think the rounding (about 5 feet for both dives) would make that much of a difference, but apparently it does.

 

[EFX]

That can't be right. On the imperial RDT 51 minutes puts you between group I and H. Getting to group I would require a minimum of 25 minutes of SI. Can you work it in metric using my procedure. Then, we're comparing apples to apples and oranges to oranges. Thanks.

 

[dfx]

That can't be right. On the PADI RDT 51 minutes puts you between group I and H. Getting to group I would require a minimum of 25 minutes of SI. Can you work it in metric using my procedure. Then, we're comparing apples to apples and oranges to oranges. Thanks.

20 meters for 29 (30) minutes puts you in group M. Group M has an adjusted NDL of 51 minutes at 14 meters, or a penalty time of 47 minutes. There's not much else to do.

 

[EFX]

51 - 39 = 12 minutes. I wouldn't think the difference would be that great. I'm working with version 1.2 which is rev. of 02/03. What version are you using? I can't find the metric pdf files. Of the two sites I could get through to one was actually imperial tables, the other the pdf files were broken. Oh well.

 

[dfx]

51 - 39 = 12 minutes. I wouldn't think the difference would be that great. I'm working with version 1.2 which is rev. of 02/03. What version are you using? I can't find the metric pdf files. Of the two sites I could get through to one was actually imperial tables, the other the pdf files were broken. Oh well.

I used whatever I posted above. It's Russian or something, but usable.

 

[rick00001967]

sorry. didn't read all responses but here is my take. i am an ssi diver but have the padi rdp in imperial

20mtrs = 66 feet, round up to 70 feet, 70 feet for 29 minutes = "N", need to be "M" to then dive 14mtrs (46 feet rounded to 50) for 39 minutes, therefore a 4 to 8 min SI would be required.

two things.
1st - with SSI we are required to take a minimum 10 min SI or the second dive is considered to be part of the first ie: it becomes one continuous dive. so i think this example is not a good one.
2nd - if you were planing a dive for 39 min you would never do it if it was the max time allowed. you would take a longer SI to give you more time as a safety margin. so in this example i would take "at least" a 14 min SI to be "K" which gives me a 44 min ndl (5min safety margin)

just for kicks i looked at the ssi rdp and here is how it works out using them
20mtrs (round up to 21mtrs) for 29min = F, to go to 14mtrs (round up to 15mtrs) for 39 min you must be "D", therefore you would have to take a 90min SI. and this would only give you a 2min safety margin (41 min ndl). so in the real world you would need a 150min SI giving you a 49min ndl (10min safety margin).

wow.thats a huge difference to the padi tables

 

[EFX]

I finally got the metric tables and the version is the same as the imperial. I followed my procedure and came up with what was posted earlier: 0 minutes of SI. You're correct. Staying in the same initial group M the max ABT for the 14 m dive is 51 minutes which exceeds the required 39 minutes in the problem!!! This is not a good problem. I think I know what happened. There is no 39 minute ABT on the RDT as there is on the imperial table. I bet whoever created the problem using the metric tables didn't change the BT for the second dive from the imperial problem.

---------- Post added July 22nd, 2014 at 01:13 PM ----------

just for kicks i looked at the ssi rdp and here is how it works out using them 20mtrs (round up to 21mtrs) for 29min = F, to go to 14mtrs (round up to 15mtrs) for 39 min you must be "D", therefore you would have to take a 90min SI. and this would only give you a 2min safety margin (41 min ndl). so in the real world you would need a 150min SI giving you a 49min ndl (10min safety margin).

wow.thats a huge difference to the padi tables

One of my buddies had the SSI tables. After two dives we compared the PADI and SSI tables. Using PADI we came within the NDL for the second dive. According to the SSI tables we were just over into deco. The difference is that SSI has made their tables more conservative to minimize bubble size and their quantity from recent doppler studies. I believe there is an actual reference to that on the tables.