Repetitive Deco Dives w/IANTD tables

[Luci]

Need some help figuring out repetitive dives using IANTD table. Here is a dive plan - I need to confirm how to calculate deco time!

First dive : 100 feet for 18 min.
Diver Group : D
Surface Interval : 30 min
Second dive : 90 feet for 40 min.
RNT : 9 min. at 90 ft

Question :
The usual no deco time at 90 ft is 22 min.
Using IANTD tables, which of the following second dive deco time formula is correct?

Solution one :
(22 – 9 = 13 min no deco) + (40 – 13 = 37 min. deco -> Deco table : 90 ft for 40 min. = stop 1min/20ft and 12min/10ft)
Or
Solution two :
(40 + 9 = 49 min deco -> Deco table : 90 ft for 50 min. = stop 4 min/20ft and 22min/10 ft)

Gotta be sure! Much thanks!

 

[nova]

Need some help figuring out repetitive dives using IANTD table. Here is a dive plan - I need to confirm how to calculate deco time!

First dive : 100 feet for 18 min.
Diver Group : D
Surface Interval : 30 min
Second dive : 90 feet for 40 min.
RNT : 9 min. at 90 ft

Question :
The usual no deco time at 90 ft is 22 min.
Using IANTD tables, which of the following second dive deco time formula is correct?

Solution one :
(22 – 9 = 13 min no deco) + (40 – 13 = 37 min. deco -> Deco table : 90 ft for 40 min. = stop 1min/20ft and 12min/10ft)
Or
Solution two :
(40 + 9 = 49 min deco -> Deco table : 90 ft for 50 min. = stop 4 min/20ft and 22min/10 ft)

Gotta be sure! Much thanks!

what's the backgas? what's the deco gas? these things will make a differance. If this is all done on air your going to get hit (hard)

 

[mer]

I've done non-deco dives like that on 32 (or 30/30), maybe with a slightly longer SI.

 

[Luci]

Thanks for your concern Nova. I realize this plan is not suitable for an air dive - I usually use EAN 32 for first blend and EAN 40 for deco.

FYI - My query refers to the calculation of the RNT for the second dive rather than actually plan to use this scenario as a dive plan.
Do I add the RNT to the no-deco time allowable OR do I subtract it from the no-deco time allowable then consider the rest of the dive as deco time...? That is my concern.

 

[mer]

I'm not familiar with these tables in specifically, but there is no way you would add the RNT (Residual Nitrogen Time).

A second dive will always have a shorter no deco time than a first dive.

 

[nova]

I'm not familiar with these tables in specifically, but there is no way you would add the RNT (Residual Nitrogen Time).

A second dive will always have a shorter no deco time than a first dive.

maybe I'm confused (not the first time) I think by adding the RNT to the bottom time of the second dive you could take the totall and work a deco schedule from that. and I'd get on the 40 as deep as possible( if that's all ya got to deco on ) (quick math .40 x 4atm = 1.6 po2) starts deco at depth,

 

[Spectre]

Solution one :
(22 – 9 = 13 min no deco) + (40 – 13 = 37 min. deco -> Deco table : 90 ft for 40 min. = stop 1min/20ft and 12min/10ft)
Or
Solution two :
(40 + 9 = 49 min deco -> Deco table : 90 ft for 50 min. = stop 4 min/20ft and 22min/10 ft)

I don't know IANTD specifically, so you should run it by your instructor.

However... the deco schedules take into consideration the time you spent within an NDL limit. Remember, it's not "No Deco", it's "No Stop" and is based on reasonable ascent rates to get the minimal deco done on your ascent. You can't just subtract out the NDL time and play with it.

Now the math on "Solution one" is incorrect, and you'd figure that one didn't make any sense if the math was right. 40-13 = 27, not 37.

Now, following the logic I think you were trying to use for solution 1. 40 minutes would be 22 of "NDL" and 18 on top of that. subtracting 9 from the 22 gives you 13, so it'd be 27 minutes above the "NDL". 22+27 =... yep... 49.

But in a nutshell... You wouldn't do less deco then you would have if you hadn't done a previous dive... so solution one is flawed from the get go. You don't get -credit- for previous dives, you take a penalty for previous dives.

 

[Luci]

But in a nutshell... You wouldn't do less deco than you would have if you hadn't done a previous dive... (...). You don't get -credit- for previous dives, you take a penalty for previous dives.

Thanks for the advice, Spectre. I like your logic: it makes good sense. That builds in a greater safety margin as well.