■Diving time management for repetitive dives per day

If you make a second dive with less than a 24-hour surface interval after the previous dive, oxygen exposure from the earlier dive remains in the body.
For the second and subsequent dives, you must take into account the oxygen exposure accumulated from the previous dive(s).
Because dive time and depth may differ on each dive, use the oxygen exposure table to estimate oxygen exposure before diving and determine the oxygen exposure limit for the next dive.

Example 1: What is the oxygen exposure limit if you use EAN36 to dive to 28 m for 30 minutes, take a surface interval, and then use EAN36 to dive to 28 m for the second dive?

On the first dive, the oxygen fraction of EAN36 is 0.36, so using the calculation formula:
Oxygen partial pressure = 0.36 × (28 ÷ 10 + 1) = 1.386
In the left column of the table, locate the value closest to but not less than 1.386.
1.40 is the next higher value.
The table lists exposure time in 5-minute increments, and the oxygen exposure ratio in the 35-minute* column is 23%.
This indicates there is still a 77% margin remaining before reaching the oxygen exposure time limit.

※ For safety, treat the exposure time as “less than 5 minutes,” “less than 10 minutes,” “less than 15 minutes,” etc.
   Therefore, if you dive for 30 minutes, use the next higher column “:35,” not the exact “:30” column.

The oxygen fraction on the second dive is the same as on the first dive, so follow the 1.40 oxygen partial pressure row across and find the time that corresponds to a remaining margin of 77%.
This table lists oxygen exposure ratios only up to 40%, so use the “40%” value.
The oxygen exposure limit time is 59 minutes* at the 40% oxygen exposure ratio.

※ For safety, treat the exposure time as “less than 5 minutes,” “less than 10 minutes,” “less than 15 minutes,” etc.
   Therefore, “:60” means less than 60 minutes, and the oxygen exposure limit is 59 minutes.

Example 2: What is the oxygen exposure limit if you use EAN36 to dive to 28 m for 30 minutes, take a surface interval, then use EAN36 to dive to 28 m for 30 minutes, take a surface interval, and then use EAN36 to dive to 28 m for a third dive?

On the first dive, the oxygen fraction of EAN36 is 0.36, so using the calculation formula:
Oxygen partial pressure = 0.36 × (28 ÷ 10 + 1) = 1.386
In the left column of the table, locate the value closest to but not less than 1.386.
1.40 is the next higher value.
The table lists exposure time in 5-minute increments, and the oxygen exposure ratio in the 35-minute* column is 23%.
This indicates there is still a 77% margin remaining before reaching the oxygen exposure time limit.

※ For safety, treat the exposure time as “less than 5 minutes,” “less than 10 minutes,” “less than 15 minutes,” etc.
   Therefore, if you dive for 30 minutes, use the next higher column “:35,” not the exact “:30” column.

Because the oxygen fraction on the second dive is the same as on the first dive, the oxygen exposure ratio for the second dive is also 23%.
The oxygen exposure ratio at the end of the second dive is the sum of the exposure ratios after the first and second dives:
The oxygen exposure ratio at the end of the second dive is 23% + 23% = 46%.
This indicates there is still a 54% margin remaining before reaching the oxygen exposure time limit.

The oxygen fraction on the third dive is the same as on the second dive, so follow the 1.40 oxygen partial pressure row across and find the time that corresponds to a remaining margin of 54%.
This table lists oxygen exposure ratios only up to 40%, so use the “40%” value.
The oxygen exposure limit time is 59 minutes* at the 40% oxygen exposure ratio.

※ For safety, treat the exposure time as “less than 5 minutes,” “less than 10 minutes,” “less than 15 minutes,” etc.
   Therefore, “:60” means less than 60 minutes, and the oxygen exposure limit is 59 minutes.

A specialized American agency recommends a surface interval of 90 minutes or longer to reduce the risk of oxygen poisoning.