Dive Tables

[jim2386]

Hey guys,

I've been diving for a long time now but many of those years I have been diving computers. Tonight I decided to brush up on my SSI dive tables. I can use the thing and do the surface interval and the second repetitive dive, but what if I have a 3rd or 4th dive? How do I get back to the first table or get my group designation letter after my second dive? All I can find is how long I can stay at depth without going into decompression. Thanks

-Jim


Edit: Ok after a bit more staring, let me see if this is right. After my surface interval after dive 1, I get my new adjusted group designation letter, then I go to table 3 and it shows my Residual nitrogen time. So I take my RNT + my time spent at that depth. Lets say my second dive was at 60' for 10 minutes and going in I was a C diver. Therefore it'd be

17 RNT + 10 BT = 27 TBT

Take that back to the first table 60' for 27 min (round to 30 min) makes me an F diver. I'm using SSI tables.

So am I right!??! What do I win!!!?

 

[H2Andy]

i don't have an SSI table to check, but yes, that's the principle

 

[vshearer]

Refreshing to see someone take the time to review them. Take a look at this thread and you will see that understanding those tables can really be helpful. They had a computer and still got in trouble.

 

[ScubaSteve2000]

Is it a full moon? I'm sitting here comparing PADI and NAUI dive tables. (just for fun??) And I'm seeing a big difference in residual nitrogen levels and maximum dive time on the second dives. Here's the scenario. First dive, 60 feet, 35 minutes. Padi says I'm in pressure group N; NAUI says G (I don't care about the letter difference). Say, I wait 30 minutes before going on a dive to 50 feet. How long can I stay down on the second dive? Padi says after 30 minutes I am in pressure group I, have a RNT of 31 and can stay down for 49 minutes. NAUI says that after 30 minutes I'm still in pressure group G and I have a RNT of 56 and can only stay down for 24 minutes. That seems like a huge difference. Am I doing something wrong??

 

[jim2386]

Is it a full moon? I'm sitting here comparing PADI and NAUI dive tables. (just for fun??) And I'm seeing a big difference in residual nitrogen levels and maximum dive time on the second dives. Here's the scenario. First dive, 60 feet, 35 minutes. Padi says I'm in pressure group N; NAUI says G (I don't care about the letter difference). Say, I wait 30 minutes before going on a dive to 50 feet. How long can I stay down on the second dive? Padi says after 30 minutes I am in pressure group I, have a RNT of 31 and can stay down for 49 minutes. NAUI says that after 30 minutes I'm still in pressure group G and I have a RNT of 56 and can only stay down for 24 minutes. That seems like a huge difference. Am I doing something wrong??

I don't think so, my SSI tables agree pretty much with yours, except on my second dive my RNT is 56 and my NDL is 14 minutes.

From what I've heard over the years, PADI is more conservative.

 

[ScubaSteve2000]

Yeah but the PADI table says that I can stay down for 49 minutes on the second dive. NAUI says 24 minutes ans SSI saya 14 minutes. Padi's RNT is nearly half of both SSI and NAUI. So PADI says that I will burn off twice as much nitrogen as the other two in the same amount of time.

 

[vshearer]

Your calc's are correct and got me looking as well. Here is a good thread on the differences. Basically PADI is more liberal on repetitive dives. Also look at this. Also note PADI ends the dive when you start for the surface, NAUI when you hit 15' stop.

 

[Walter]

Is it a full moon? I'm sitting here comparing PADI and NAUI dive tables. (just for fun??) And I'm seeing a big difference in residual nitrogen levels and maximum dive time on the second dives. Here's the scenario. First dive, 60 feet, 35 minutes. Padi says I'm in pressure group N; NAUI says G (I don't care about the letter difference). Say, I wait 30 minutes before going on a dive to 50 feet. How long can I stay down on the second dive? Padi says after 30 minutes I am in pressure group I, have a RNT of 31 and can stay down for 49 minutes. NAUI says that after 30 minutes I'm still in pressure group G and I have a RNT of 56 and can only stay down for 24 minutes. That seems like a huge difference.

It IS a huge difference.

Am I doing something wrong??

I didn't check your work, but odds are you did nothing wrong. PADI's tables are probably the most liberal tables available. How do you feel about liberal vs conservative with regard to decompression issues? If you want to push the edge, use PADI's RDP, if you prever a wider safety margin, use the DCIEM tables, they are the most conservative I've found. YMCA tables are almost as conservative, NAUI is fairly middle of the road (except for the 24 hour clearance and all dives figured as 40 ft or deeper which is very conservative).

 

[Charlie99]

Padi's RNT is nearly half of both SSI and NAUI. So PADI says that I will burn off twice as much nitrogen as the other two in the same amount of time.

That's an astute and accurate observation. The USN table and those derived from it, which includes NAUI, YMCA, and SSI tables, have repetitive groups based upon the 120 minute compartment. The PADI table pressure groups are based upon the 60 minute compartment.

Normal open circuit scuba dives are controlled by compartments faster than 60 minutes, so basing repetitive dive calculations on the 60 minute compartment is both safe and more effective than basing the calculations on the 120 minute compartment, which controls very long, shallow dives. The USN-derived tables are optimal for the long 40' or shallower dives such as a surface supplied diver working on the bottom hull of a ship, but are not optimal for recreational scuba.

The downside to the PADI table basing pressure groups on the 60 minute compartment is that they had to add some ad hoc W,X,Y,Z rules for those unusual cases of 3 or more dives where any one of them reach pressure group W or higher. In those cases you have a minimum 1hr (W or X) or 3 hour (Y or Z) surface interval between each dive. Normal single tank diving won't invoke these rules.

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The format of the tables means that they are only a partial implementation of the underlying decompession model. To make a workable table, the designer must make certain choices. One major tradeoff is that table track one and only one of the many halftime compartments of the decompression model. PADI looked at recreational diving and chose the 60 minute compartment + some exceptions rules. The US Navy looked at ship husbandry and at deco diving and chose the 120 minute compartment for repetitive groups.

Charlie Allen

 

[Charlie99]

Also note PADI ends the dive when you start for the surface, NAUI when you hit 15' stop.

Only when you start a "direct ascent" to the surface or safety stop does the PADI bottom time stop. Safety stop doesn't count as bottom time on the PADI tables.

"Direct ascent" is vague, but the definition I use is that it includes the time needed for a 30fpm ascent, and any additional time, such as a multilevel dive, needs to be called bottom time.