Multi Level Dive Plan Approximation (without the wheel, eRDPML, or app)

[ScubaForScience]

My first dive of the day is a multi-level dive and I will use a computer to stay within NDL limits on the dive. However, the unit doesn't plan multi-level dives. I want to make a back-of-the-napkin approximation of the tables to provide a general sense of what's possible before getting in the water with my computer. I'm looking for a solution that doesn't involve the wheel, eRDPML, smartphone app, or other software.

How accurate of an approximation is it to look at each of the depth levels for the dive on a dive table and calculate the percentage of the NDL for that particular depth, then add the percentages for all depths and make sure the number doesn't exceed 100%?

For example, start at 100ft and stay there for 10% of the NDL for that depth, then move to 80ft for 30% of that depth's NDL, then 60ft for 20% of that depth's NDL, and finish off at 40ft for 40% of that depth's NDL.

I want to reiterate that I plan to dive with a computer, so I'm only looking for a reasonable approximation before getting in the water. However, I'm not sure how close this approximation is to a table's corresponding algorithm.

 

[tursiops]

No, that will not work, in general, but it might for spurious examples, such as the one you've given. The problem is that "NDL" is tracking the tissue compartment that is closest to saturation, and which compartment that is will vary with the depth and time, so you are trying to add percentages of constantly changing numbers. The physics/physiology of nitrogen absorbtion is highly non-linear. What you are missing is that the impact of the residual nitrogen amount keeps changing with depth.

An alternative procedure -- which is also incorrect in general but less so -- is to follow the commont-but-wrong multilevel procedure reviewed in Why you can't use the PADI RDP table for Multi-Level dives. Using your example, with the PADI RDP:

100 ft for 10% of 20 mins = 2 mins​

80 ft for 30% of 30 mins = 9 mins​

60 ft for 20% of 55 mins = 11 mins​

40 ft for 40% of 140 mins = 56 mins >> TOTAL dive time 78 mins​

The better way to also do it incorrectly is:

100 ft for 2 mins = Group A​

Group A diver at 80 ft for 9 mins = Group E​

Group E diver at 60 ft for 11 misn = Group K​

Group K diver at 40 ft for 56 mins = Group V......could have stayed another 36 mins.​

Doing your planning this way requires you to say how long you want to stay at each depth, as opposed to some percentage of the NDL for that depth.
NOTE 1: your example dive cannot be calculated on the Wheel of the eRDPML because the 80 ft depth of your example is too deep for the Wheel/eRDPML; if the first level is 100 ft, the second level must be 70 ft or shallower.

By the way, using MultiDeco (GF 70/85, RMV 0.7 cuft/min) on your example dive says it is an OK dive, during which you will use 142 cur ft of air. Your table-based dive planners don't give you this critical information, no matter how incorrectly you use them.

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[tursiops]

No, that will not work, in general, but it might for spurious examples, such as the one you've given. The problem is that "NDL" is tracking the tissue compartment that is closest to saturation, and which compartment that is will vary with the depth and time, so you are trying to add percentages of constantly changing numbers. The physics/physiology of nitrogen absorbtion is highly non-linear. What you are missing is that the impact of the residual nitrogen amount keeps changing with depth.

An alternative procedure -- which is also incorrect in general but less so -- is to follow the commont-but-wrong multilevel procedure reviewed in Why you can't use the PADI RDP table for Multi-Level dives. Using your example, with the PADI RDP:

100 ft for 10% of 20 mins = 2 mins​

80 ft for 30% of 30 mins = 9 mins​

60 ft for 20% of 55 mins = 11 mins​

40 ft for 40% of 140 mins = 56 mins >> TOTAL dive time 78 mins​

The better way to also do it incorrectly is:

100 ft for 2 mins = Group A​

Group A diver at 80 ft for 9 mins = Group E​

Group E diver at 60 ft for 11 misn = Group K​

Group K diver at 40 ft for 56 mins = Group V......could have stayed another 36 mins.​

Doing your planning this way requires you to say how long you want to stay at each depth, as opposed to some percentage of the NDL for that depth.
NOTE 1: your example dive cannot be calculated on the Wheel of the eRDPML because the 80 ft depth of your example is too deep for the Wheel/eRDPML; if the first level is 100 ft, the second level must be 70 ft or shallower.

By the way, using MultiDeco (GF 70/85, RMV 0.7 cuft/min) on your example dive says it is an OK dive, during which you will use 142 cur ft of air. Your table-based dive planners don't give you this critical information, no matter how incorrectly you use them.

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ADDED:
Let's change your example to 100 ft for 90% of NDL, thus 18 mins.
Then go to 40 ft for the remaining 10% of NDL, thus 14 mins at 40 ft.
The eRDPML says at 40 ft you'd have 72 mins before NDL.

Alternatively, let's do 2 mins at 100 ft, as in your example, and then go to 40 ft. Your method would allow 90% of 140 mins = 126 mins.
The eRDPML says 118 mins allowable.

 

[ScubaForScience]

Thanks for the quick and thorough response! It sounds like it's a pretty poor approximation. I had a feeling that there's no reasonable alternative to dive planning software. Figured I'd ask.