RDP tables, why is RNT figure higher for shallow dives?

[peterpiper]

A question from a beginner, just going through PADI OWD e-learning.
Downside of e-learning is I'm coming up with a whole list of questions, though none are stopping me progressing (yet).
Most questions, I guess the diving school will be able to answer, But not sure about a tech question about RDP.

If I'm following it right, when using RDP tables to find your pressure group after a second (or subsequent) dive. You can end up in in the same group, after say a 1min ABT 2nd dive, whether it was to either 35ft or 70ft.
I'd like to understand that, or why RNT should go down as depth increases.

eg. From pressure group E, at end of surface time.
1 minute ABT dive to 35ft. RNT=36min. TBT = 37min. From table 1, 40min @35ft gives pressure group G
1 minute ABT dive to 70ft. RNT=16min. TBT = 17min. From table 1, 18min @70ft gives pressure group G

I guess 1minute ABT dives, wouldn't happen often, but I'm interested in the theory.

Also it's pretty unclear from e-learning exactly what the various times mentioned are referring to (ie when they start and stop). I've maybe missed it, but e-learning has no way to search so I'm not sure.
They need especially clear explanations of any (deliberately?) confusing terms. Like Actual Bottom Time (ABT), not actually being the time spent at bottom.

I eventually found (on this forum), that ABT is time from starting dive, until start of ascent. So I guess NDL is betweed those as well.

Also (from this forum) direct ascent time isn't counted towards ABT, as nitrogen is 'off-gassing' during the ascent, rather than accumulating. That makes sense.

 

[rjack321]

The shallower dive is loading up more intermediate and slow tissues with N2 before the controlling tissue forces you to begin your ascent. The deeper dive is so short that the slow tissues don't really have a chance to accumulate much.
Hence the longer shallow dive has a greater impact on your ability to do a 3rd dive as RNT than a shorter but deeper bounce. They both put you into group G which dictates how long it will take before you are back to baseline.

 

[steinbil]

You can end up in in the same group, after say a 1min ABT 2nd dive, whether it was to either 35ft or 70ft.
I'd like to understand that, or why RNT should go down as depth increases.

If I understand your questions correctly, you are confused about why:

1. A 1 minute dive to 35ft puts you in the same group as a 1 minute dive to 70ft
2. RNT decreases as depth increases

The answer to #1 is simply that the dive is so short (1 minute) that you don't really have time to load the tissues enough to push you into the next pressure group. If you did the same calculation with two 15 minute dives to 35ft/70ft, you would see that the 70ft dive would put you in a higher pressure group than the 35ft dive.

The answer to #2 is that you need to compare RNT to the NDL at the same depth - think of it as a percentage. The NDL at 35ft on air is 205min and at 70ft it's 40min. So an RNT of 36 minutes at 35ft is 17.6% of the NDL, while the RNT of 16min at 70ft is 40%. This shows the non-linear effect of deeper dives - not only are your NDLs drastically reduced at deeper depths, but your residual gas loading also decreases your NDLs more at deeper depths.

In short, deeper/longer = progressively higher gas loading and progressively increased risk.