Hi, so ive been diving for a little over a year and have quite a few dives under my belt now. however I am starting to do some deeper dives (apx. 100ft) and I would like to have a better understanding of the dive tables. when looking at the SSI tables and just for an example if I dive to 100ft for 20 minutes I would be f group designation. so being an f group with apx. 1 hour surface interval should mean I do my next dive as an e group. now this is where I get a little confused. so for my second dive being an e group if I was to dive to 80 ft does this mean I am limited to a 7 minute dive? any clarification on this matter would be greatly appreciated. I ask this because I am doing 2 wreck dives tomorrow and I want to make sure I'm doing this right.
Is the surface interval really only one hour? That is quite short. Tables use rounded values to remain manageable, so you may find that an extra few minutes of SI will let you go down a tissue code and have a longer second dive.
Different tables use different trade offs. Early tables used by navy divers typically tried to maximise single dives, but for fun diving they would give short second dives. Some only allow three dives a day.
The main disadvantage of tables is that they have to assume a square profile to the maximum depth. So your 100ft wreck dive is the same a 100ft maximum on a sloping reef as far as the table is concerned. Also any time you spend above 100ft is the same as at 100tt as far as it is concerned.
Using a dive computer will mean that your exposure is properly tracked and the NDL or stop times will reflect what you actually did. During the SI you would hope to be able to get an NDL time for the second dive but you need a computer which allows you to plan for a dive following your actual SI, not just the dive you would make if you got in the water immediately.
Be aware that depending on the computer and table combination and how 'square' your profile is you may find that the computer gives you less time on the second
dive than the tables, but this is unusual.
I suggest you get some dive planning software and play with the different profiles, surface intervals and so forth to get an idea of how each factor influences the NDL times such that you get dive times you like, the Suunto planner (DM5) is free, or you could try subsurface or pay for MultiDeco.
I would look at the plan for the day. I would want a much longer surface interval for a 80ft second dive. If the SI is fixed then a shallower second dive would be sensible.
Using nitrox would help a lot. These depths are the sweet spot for nitrox. The NDL time for the 100ft dive would increase from 20 to 28 minutes by my tables.
