Extended dive times at depth

I am currently taking a master scuba diver course. Our instructor wants us to dive an extended time at depth. Is that safe with regards to nitrogen buildup? I have an upcoming dive with the following profile:
130 feet: 20 minutes
98 feet: 2 minutes
65 feet: 2 minutes
40 feet: 4 minutes
30 feet: 5 minutes
20 feet: 6 minutes
10 feet: 16 minutes
Every time above 130 feet includes a one-minute ascent. Is this a safe profile, or should it be more conservative?


The profile you describe is within the limits of currently available dive tables and is slightly more conservative than the U.S. Navy dive tables. The dive profile is decompression (deco) diving, which requires decompression stops, and all these tables were developed for square profiles. I assume your instructor chose these times based on the dive tables of your training organisation or a commercially available dive planner.
If you are concerned about an increased risk of decompression sickness (DCS), you can consider options to make the dive more conservative, such as the following:
  •  Add time to the shallower deco stops (e.g., another two minutes at 20 and 30 feet), and follow the ascent rates that your instructor proposed. Ascent rates are one of the main keys to safe decompression.
  • Add enriched air Nitrox (EAN) as a decompression gas starting on the deco stops to add oxygen to your decompression phase and speed up nitrogen elimination. Keep the same times as for the air profile, even if you switch to EAN.
  • Decrease your bottom time by a few minutes.
It is always important to stay well hydrated before every dive, especially in deco diving.
— Frauke Tillmans, Ph.D.

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