CNS and OTU answers how much oxygen exposure is this? Give it a ppO₂ and a time and it returns the two oxygen clocks the planner tracks on a dive: the CNS percentage and oxygen tolerance units (OTU).

What you enter

• From: ppO₂, to type the partial pressure directly, or Mix and depth, to have the tool work it out.
• ppO₂ (bar): the oxygen partial pressure breathed, for example a rebreather setpoint.
• Oxygen (%) and Depth (m): shown instead with Mix and depth. The ppO₂ assumes salt water at sea level.
• Time (min): how long the exposure lasts.

Reading the result

• CNS (%): the main figure, a share of the NOAA single-exposure limit at this ppO₂. Its rail has a dashed mark at the 80 advisory and a solid mark at the 100 % limit. Amber above 80 %, red above 100 %.
• OTU: pulmonary oxygen dose. Dashed mark at 200, solid mark at 300, the daily limit. Amber above 200, red above 300.
• NOAA limit (min): the longest single exposure the table allows at this ppO₂, or None below 0.5 bar.
• ppO₂ (bar): the partial pressure used, useful when it came from a mix.

The advisories and limits are the ones the planner raises warnings at on a plan. The verdict takes the worse of the two clocks:

• Over the oxygen limit. Shorten the exposure or lower the ppO₂.
• Past the advisory. Allow a long surface interval before the next dive.
• Within both the CNS and OTU limits, with the time and ppO₂.
• Below 0.5 bar, oxygen exposure does not accumulate.

CNS follows the NOAA table, read between its rows. OTU follows the standard UPTD formula, about one unit a minute at 1.0 bar.

What it does not count

• Anything but this one exposure. Earlier dives the same day, the surface interval, and the descent and ascent are not included. The planner tracks the whole dive.
• ppO₂ above 1.6 bar. The NOAA table stops at 1.6 bar. Above it the tool holds the 1.6 bar figures, 45 minutes to 100 % CNS, so it understates the exposure.
• You. The limits are population figures. Exertion, cold and CO₂ retention change your tolerance and do not enter the numbers.

Next

• Oxygen time limit gives the time allowed at one ppO₂.
• Oxygen exposure explains both clocks on a plan.