Questions on RNT please help.

[redacted]

You need to adjust your thinking a bit. Pressure group is the measure of how much nitrogen your body is carrying after a dive. RNT is the measure of how that nitrogen will effect a subsequent dive. RNT is not defined until you plan a specific subsequent dive. Your post dive N2 loading status is defined by your PG at any post dive interval.

 

[Colliam7]

Still don't really get it.

Possibly that is because you are thinking of RNT as a direct representation of the physical amount of nitrogen in your body (e.g. your apples analogy). It is NOT. RNT is a parameter used to quantitatively express the potential effect of residual nitrogen on future dives, in terms of how long you can stay at a particular depth on those future dives. It is not a real number, in terms of how much nitrogen is left after the first dive (or a series of dives). Rather, it is an indirect method of expressing nitrogen amount, in terms of time. The tables represent that amount of nitrogen as 'Minutes deducted from the NDL', for the particular depth of the next dive. It could also be presented as a percentage. But, that would then require everyone using the tables to perform calculations involving multiplication.

Instead, virtually all tables attempting to gauge the impact of residual nitrogen distill that impact down to something that can be subtracted, from NDL. In fact, NDL itself is an attempt to represent the amount of nitrogen taken on during a dive. It, too, is an indirect method of expressing nitrogen amount, in terms of time.

Also, part of the question you raised earlier involved the apparent difference in residual nitrogen at different dive depths. Keep in mind that the NDL for deeper dives is already shorter, so the 'amount' of nitrogen is adjusted to reflect that lower NDL. Don't expect it to be a linear relationship.

 

[EFX]

I am learning on PADI's Recreational Dive Planner and I have difficulty understanding RNT.

Lets assume that I had surfaced after my first dive and I am going to do my 2nd dive.

From what I understand of Residual Nitrogen time is the left over nitrogen that is inside me after my first dive.

so lets assume that this residual nitrogen time is X now just before I make my 2nd dive.

Table 3: repetitive dive time table:
I am at Pressure Group E.
if I dive for 16m my RNT is 21 minutes
If I dive for 14m my RNT is 24 minutes.

Question:
Before my second dive, there is X mins of nitrogen time in my body.
why does the X mins of nitrogen time changes according to my depth during my second dive?
If there is nitrogen time left inside my body before my 2nd dive. Isnt the RNT suppose to be a fixed amount?

If anyone can clear my doubts. Thank you.

The RNT has nothing to do with the amount of nitrogen at the surface. It has to do with the amount of time remaining at depth because of previous nitrogen loading. You are correct in saying that at the beginning of your descent for the second dive there is certain amount of nitrogen in your tissues. It is called residual nitrogen. This nitrogen amount doesn't change during the second dive regardless of depth. However, to this amount we must add the nitrogen accumulated from the second dive which increases with increasing depth and time. Therefore, as we dive deeper the NDL time becomes shorter. So far so good but what about RNT?

We must account for the previous dives (residual) N2 loading. The RNT is the time we have already spent at that depth due to the residual N2. That amount of N2 is fixed but to get that amount of N2 requires less time (RNT) at deeper depths. Hence you will see on your table that the RNT decreases the deeper you dive.

 

[tbone1004]

Thank you!!! The bucket example helped a lot!

glad that helped, it's a fairly simple concept if you are able to think about the grand scheme and understand the hows and why's of the tables. Unfortunately deco theory is a bit of a dark art, and we still don't fully understand it. The tables are a best guess at it, but the bucket example is the rough approximation of how we think it works.