Residual Nitrogen Time (RNT) and Scuba Diving

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Were you confused the first time that your scuba instructor explained the recreational dive tables to you? If you were, you are not alone. Many divers find the two-sided table covered with seemingly random numbers rather daunting. However, once a diver understands what the numbers signify, the recreational dive tables become clearer are more intuitive to use. This article focuses on "residual nitrogen time" -- perhaps the most confusing of all the numbers on the dive tables.

Tracking Nitrogen Absorption Over a Series of Dives

Residual nitrogen time is used to track nitrogen absorption over a series of dives. Most divers have no issue tracking nitrogen absorption for a single dive, but when it comes to calculating nitrogen absorption for repetitive (or multiple) dives on the same day, their skills are a bit rusty. Tracking nitrogen absorption for the second, third, or even fourth dive of the day requires using the back side of the dive table and doing some simple addition. As with most mathematics, understanding the theory behind the math helps to keep the procedures and calculations clear.

A Simplified Review of Nitrogen Absorption When Scuba Diving

To understand residual nitrogen time (RNT), a basic understanding of nitrogen absorption underwater is necessary. When a diver is underwater, his body absorbs nitrogen gas from the air (or other breathing gas) that he uses. Time limits (called no-decompression limits) exist to reduce the chance of a diver absorbing so much nitrogen that he runs an unacceptable risk of decompression sickness. These time limits are based on depth -- the deeper a person dives, the more rapidly his body absorbs nitrogen, and the more quickly he approaches his no-decompression limit. Nitrogen absorption is (simplistically) proportional to depth.

Nitrogen Remains in a Diver's Body Long After He Surfaces

As a diver ascends, his body begins to release the nitrogen gas he absorbed during the dive. However, the release of nitrogen from a diver's body is a slow and gradual process. Even after surfacing and spending time out of the water, some nitrogen remains in his system. If a diver makes another dive on the same day, the left-over nitrogen from the first dive will reduce his no-decompression time limit.

How Do We Measure Nitrogen in a Diver's Body?

This is where dive theory gets pretty interesting. Left-over nitrogen (or residual nitrogen) in a diver's body is measured in units of time. Yes, that's right, we measure nitrogen in minutes. This might seem illogical at first, but remember that time is required for a diver's body to absorb nitrogen. For example, it takes five minutes to absorb "x" amount of nitrogen. In diving, we can refer to that "x" amount of nitrogen as "five minutes of nitrogen". Almost...

Remember that two factors affect nitrogen absorption - time and depth. The deeper a diver descends, the more quickly he absorbs nitrogen. It might take him five minutes to absorb "x" amount of nitrogen at a shallow depth, and only two minutes to absorb "x" amount of nitrogen at a deeper depth. For this reason, when we refer to nitrogen in "minutes of nitrogen" we also have to state depth. If a diver's body absorbs "x" amount of nitrogen in five minutes at forty feet of depth, we can say he has "five minutes of nitrogen at forty feet." This is his residual nitrogen time.

Residual Nitrogen Time Helps Track Nitrogen Absorption Over a Series of Dives

At the beginning of a second, third, or fourth dive of the day, a diver still has some residual nitrogen in his body from his previous dives. Residual nitrogen time accounts for this left-over nitrogen. A diver descends to a given depth, and even though he just began his dive, he has the same amount of nitrogen in his system as if he had already been diving at the depth for some number of minutes -- the residual nitrogen time.

We already know that on a single dive, divers track nitrogen absorption according to minutes at a certain depth. A diver on a repetitive dive can no longer use his actual dive time and depth to calculate his nitrogen absorption because he already has some nitrogen in his body when he starts the dive. However, if we add his residual nitrogen time to his actual dive time, we come up with a time in minutes that is representative of the actual amount of nitrogen in his system.

For this reason, when determining a diver's nitrogen absorption after a repetitive dive, we add his residual nitrogen time and his actual dive time together and use the resulting number of minutes and his maximum dive depth to calculate his nitrogen absorption. These two numbers can be used on dive tables without any further adjustments.

How Does a Diver Calculate His Residual Nitrogen Time?

It is difficult to explain how to calculate a diver's residual nitrogen time without posting photos of the copyrighted dive tables online and breaking all sorts of laws. However, on every dive table, there is a section which has headings for the diver's pressure group after his surface interval and depth. To calculate nitrogen absorption on a repetitive dive:

  • Run down the column/row listing the diver's pressure group after his surface interval until it intersects the row/column listing the maximum depth of his dive.
  • The diver's residual nitrogen time is listed in this box.
  • If two numbers are listed in this box, use the legend on the dive table to determine which number is the residual nitrogen time.

The Take-Home Message About Residual Nitrogen Times

Residual nitrogen time is used when tracking nitrogen absorption on a repetitive dive. A diver does not need to calculate his residual nitrogen time on the first dive of the day. Calculating his residual nitrogen time allows a diver to account for the nitrogen left over in his system from previous dives. By adding the residual nitrogen time to his actual dive time, a diver can adjust his dive times to more accurately reflect the actual amount of nitrogen in his body after a series of dives. He can then use this adjusted dive time on the front of the dive table to calculate his pressure group after the dive.