Question 1

What is the residual nitrogen penalty (RNT)?

Accepted Answer

After a dive, nitrogen stays dissolved in your body. On a repetitive dive, that leftover nitrogen 'eats into' your bottom time at the new depth. The penalty (also called Residual Nitrogen Time, RNT) is the number of minutes, at your second dive's depth, that represent the nitrogen still in your body from the first dive. We add it to your planned bottom time to check the pressure group and remaining NDL.

Question 2

How much bottom time do I really have on my second dive?

Accepted Answer

At any depth there is a no-decompression limit (NDL): the maximum bottom time before a mandatory decompression stop. On a repetitive dive you subtract your residual penalty from that NDL: the result is the bottom time you actually have available. The tool shows both the NDL at your depth and the time left after your residual penalty.

Question 3

What is a pressure group?

Accepted Answer

A pressure group (or repetitive group) is a letter from A to Z that represents how much residual nitrogen remains in your body after a dive. 'A' is the least nitrogen loading and 'Z' the most. You end dive 1 in a group, the surface interval drops you to a lower group, and that is the group you start dive 2 with before you reach a new end group.

Question 4

Total time or bottom time: which one should I enter?

Accepted Answer

Enter the total dive time, exactly as your dive computer shows it: it includes descent, time at depth, ascent and safety stop. The tool estimates the bottom time at your max depth by removing the ascent to 6 m (at 9 m/min, rounded up), a 3-minute safety stop and the final 2-minute ascent to the surface. For example, a 50-minute dive to 18 m becomes a 43-minute bottom time.

Question 5

Can I use Nitrox for repetitive dives?

Accepted Answer

Yes, and it helps. Nitrox reduces the nitrogen fraction in your breathing gas, which lowers your Equivalent Air Depth (EAD). A lower EAD means less nitrogen absorbed, so you end the dive in a lower pressure group, carry less residual nitrogen into the next dive, and keep more bottom time on dive 2. Select your mix to see how your penalty and remaining time change.

Question 6

Why is my second dive shorter than the first?

Accepted Answer

Your second dive carries residual nitrogen from the first. That residual is expressed as a penalty (RNT) added to your actual bottom time. As a result you reach a higher pressure group and your no-decompression limit sooner than on the first dive, effectively shortening the bottom time available on the second dive.

Repetitive Dive & Surface Interval Planner

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