Multilevel Recreational Dive Planner vs the RDP for multilevel dive

[YvonneV]

As a Divemaster Trainee I got the question: “Is it possible to plan a multilevel dive to three different depths as three different dives, with a surface interval time of 0:00? Why do you need the Multilevel Recreational Dive Planner?”
My first response was 'it's possible, but your dive will be much shorter since it's more conservative.'
Then I planned a dive to 30 meter for 10 minutes, 20 meter for 10 minutes and 10 meter for 20 minutes with the RDP and with the Multilevel Recreational Dive Planner. Surprisingly both gave the same pressure group: O
It seems that it doesn't make a difference. Yet I'm not convinced.
Do you know?

 

[divebunnie]

I too have seen that method of planning multilevel dives using the normal RDP, and a zero surface interval, however there are some fundamental safety aspects missing from this method, that are addressed by using the E-RDP ML (multilevel electronic RDP).

The main point that is not addressed by using this (unapproved by PADI) method is the fact that on a multilevel dive (planned on the E-RDP ML), you cannot simply shallow from 30m to 28m to 20m, you need to ensure that there is a reasonable reduction in depth between each level of your dive.. this is indicated by the depths in yellow in the centre, on the, now defunct, wheel, and would trigger an alarm on the electronic version.

You are also not allowed to perform a dive with more than three different depths on a multilevel dive if you plan it on the E-RDP ML, however again, this would not be addressed if you were using the normal table and a zero surface interval. It may be that it works much of the time, but it doesn't have the safety factors built in that you would find if you were planning your dive the recognised, way.

Also, unless you were to double check every single possible dive you could do, you cannot be absolutely sure that this method does actually work every time. And if you are going to do that, you may as well use the E-RDP ML in the first place anyway. I must admit, I prefer some aspects of using the wheel, instead of the electronic version, however that is a whole different argument altogether

I hope that helps.

 

[Diver0001]

Also, unless you were to double check every single possible dive you could do, you cannot be absolutely sure that this method does actually work every time.

That's true and I would bet that there will be many cases where you will see one or two groups difference between the tables and the wheel using the technique the OP described.

The reason I think there could be a group or so difference in some cases has to do with the assumption (calculation) of the amount of off-gassing that will take place during the ascent.

If you plan a dive on the table, it calculates your group immediately after the dive INCLUDING the ascent (but not including the optional safety stop). What happens if you don't make that ascent and just go to a different depth is that you didn't get the benefit of the ascent and therefore you didn't off-gas as much as you might have thought. Ergo.... you might be a group or two higher if you work it out on the wheel than using "zero SI" would give you on the table, especially after the 2nd dive/leg.

I would also think that this effect would be exacerbated in relation to the depth of the first dive/leg.

Obviously there is a difference between saying "you don't need to make a surface interval between two dives" and "you don't have to return to the surface between two dives".

Hope that isn't too confusing.

R..

 

[mjatkins]

If you plan a dive on the table, it calculates your group immediately after the dive INCLUDING the ascent (but not including the optional safety stop).
R..

Do you know that to be true, or is that an assumption? The RDP only counts bottom time until you begin a direct ascent to the surface (or to your safety stop), I have not heard before that they give off gassing credit for the time spent ascending. Or am I misunderstanding your point?

Either way, If you accounted for the ascent time between levels and included it in your bottom time, you should end up with a slightly/somewhat more conservative dive plan. I believe that is your point.

 

[knotical]

If you're using the flat table and the wheel (as opposed to the eRDP's), the question might have been asked simply to get you to discover one significant difference between the two.
In the Imperial versions, the flat table RDP depths are in 10-foot increments. The wheel's are 5-foot. I suspect something similar in the metric versions.

 

[Diver0001]

Do you know that to be true, or is that an assumption? The RDP only counts bottom time until you begin a direct ascent to the surface (or to your safety stop), I have not heard before that they give off gassing credit for the time spent ascending. Or am I misunderstanding your point?

Think it though.... why do you think the NDL's (ie, the maximum "formal" bottom times) are set to "assume" or "demand" a certain ascent rate?

What do other deco programs (like Vplanner) do with the ascent rate? Do you think the deco-program that DSAT wrote to calculate the table would be the only one in existence to ignore the ascent?

Either way, If you accounted for the ascent time between levels and included it in your bottom time, you should end up with a slightly/somewhat more conservative dive plan. I believe that is your point.

Partly. But the point I made in my last post is that there will be a difference in the amount of residual nitrogen in your body if you do these two things:

• Ascend from depth X all the way to the surface and then descend back to depth Y
• Ascend from depth X only as far as depth Y.

In the first case I would expect some offgassing to occur during the period of time you were above depth Y. IN the second case you never went above depth Y so your body will still have more residual nitrogen in it.

The difference might not be huge, but still that's why I was saying that I would expect the wheel to give one or two groups higher on many scenarios than it would on the table, especially if you use the table incorrectly and dno't actually *make* the ascent, but only pretend you did. IIRC the wheel also has different NDL's for multilevel dives so that would be as sort of confirmation of this effect.

I have nothing special planned tonight. When I get home from work I'll try to dream up a scenario to illustrate this.

R..

 

[Blackwood]

I have not heard before that they give off gassing credit for the time spent ascending.

That's exactly how it works.

 

[Crowley]

Also worth pointing out that if you multi-level a dive then you are still "on-gassing", even as you ascend between levels. Unless you're completely saturated (impossible on a recreational dive), you will still be taking on nitrogen in some tissue compartments during an ascent from say 30 metres to 20. The eRDP-ML and the (sadly) obsolete wheel take this into account, hence the shorter Multilevel no decompression limits.

So for that reason (and of course the extra rounding inherent in the RDP table) no, it is not possible to calculate multi-level dives using the RDP table, even though from time to time you might arrive at the same answer.

Cheers

C.

 

[YvonneV]

Thanks to you all for your answers and help! I really appreciate it! Yvonne