What Is Residual Nitrogen Time (RNT) In Scuba Diving?


While you may have a knack for natural scuba diving, the activity has its own technical aspect. Among the crucial skills that you need to learn is the diving tables. The 2-sided table is generally daunting to most scuba diving beginners. However, understanding it boosts both your scuba diving safety and experience.

One of the most important numbers on the dive table is the residual nitrogen time. Maybe you are currently finding it difficult to understand both residual time and residual nitrogen. In this article, we will focus on residual nitrogen time. Reading through this article should make the dive tables less confusing for you.

Why is the Residual Nitrogen Time Important?

Residual nitrogen time becomes crucial if you intend to dive more than once. It helps track your nitrogen absorption over a series of dives.

Most scuba divers find it easy to calculate nitrogen absorption for a single dive. However, things get more complicated when calculating nitrogen absorption for more than one dive.

Surprisingly, you only need the backside of the scuba diving table and some relatively simple calculations to figure out nitrogen absorption for multiple dives. To get everything right, however, you must understand the idea behind the involved calculations.

What Does Nitrogen Absorption Mean for Scuba Divers?

Simply defined, residual nitrogen time is a theoretical mathematical representation of the nitrogen amount your body tissues absorb after your scuba dive. The residual nitrogen time is expressed in minutes (more on this later) on diving tables.

For you to get a better idea of what the residual time is, you must understand underwater nitrogen absorption. When underwater, the diver’s tissues will absorb nitrogen from both the breathing gas and the environment.

Time limits, referred to as no-decompression limits (NDLs), exist. Simply defined, NDLs refers to the maximum amount of time the diver can stay at a certain depth. The no-decompression limits help minimize scuba diving dangers and risks.

If the diver exceeds the no-decompression limit, his/her body may absorb too much nitrogen. This raises the chances of developing decompression sickness. The no-decompression limits are largely dependent on the scuba diving depth.

Nitrogen absorption is directly proportional to the diving depth. This simply means that the rate at which the body absorbs nitrogen increases as the diving depth increases. Hence, if a scuba diver dives deeper, he/she will approach his/her no-decompression limit more quickly.

Does the Diver’s Body Have Nitrogen Even After Surfacing?

As you ascend, your body will start releasing the nitrogen your tissues had absorbed during your dive. The absorbed nitrogen does not just leave the body at once. Nitrogen release is generally a slow process. It happens gradually.

Even after navigating out of the water and staying out for a while, you will still have some nitrogen in your system. If you decide to use your scuba fins to dive for the second time, the nitrogen in your system will reduce the no-decompression limit.

Measuring Nitrogen in the Diver’s Body

Residual nitrogen, which is often referred to as left-over nitrogen, is generally measured in time units. What we mean by “time units” is that residual nitrogen is measured in minutes.

If you are a night scuba diving beginner, this may be confusing to you. It makes much more sense when you consider the fact that time is necessary for nitrogen absorption to occur.

To give you an example, let’s assume that it will take approximately 10 minutes for your body to absorb a certain amount of nitrogen. This nitrogen amount will be referred to as “10 minutes of nitrogen”.

As we had mentioned earlier, both depth and time influence nitrogen absorption. The rate at which the diver’s body absorbs nitrogen increases as the diving depth increases. Therefore, at a shallow depth, a diver’s body may take 6 mins to absorb a “Y” amount of nitrogen. However, if the diver decides to go deeper so that he/she can use his/her diving camera to take more interesting photos, the diver’s body may absorb “Y” amount of nitrogen in 3 minutes.

Since depth is a crucial factor, it is necessary to mention it when stating the “minutes of nitrogen”. To give you an example, let’s assume your body absorbs a certain amount of nitrogen in 8 minutes at 20 feet depth. This will be “8 minutes of nitrogen at 20 feet”. Now, this is your residual nitrogen time.

Since nitrogen absorption is largely dependent on dive depth, keeping track of depth when diving is crucial. With the best scuba gauge, measuring dive depth should be easy.

Residual Nitrogen Time Makes Tracking Nitrogen Absorption Over Multiple Dives Easier


When diving for the second or third time on the same day, your body will still have left-over nitrogen. The residual time has to account for the residual nitrogen.

When diving for a second, third, or fourth time, using both depth and time to calculate a nitrogen absorption is no longer an option. Basically, this is because you already have some residual nitrogen in your system. Adding the actual diving time to the residual time will, however, give the actual “minutes of nitrogen” in your system.

When calculating the nitrogen absorption for a series of dives, you need to add both the residual time and the actual diving time. Next, you will need to use the sum and the depth to calculate the nitrogen absorption. You can use the 2 numbers on the dive tables.

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Calculating the Residual Nitrogen Time

On the general dive tables, you should find the diver’s pressure group after the diver’s depth and surface interval. To calculate your nitrogen absorption for a series of dives, you need to do the following:

  • On the row/column listing the pressure group after the diver’s surface interval, run down until it intersects the column/row that lists the maximum dive depth.
  • Your residual nitrogen time should be listed in the box.
  • If you find 2 numbers in the box, the legend on your dive table should help you determine the residual time number.

Calculating your residual nitrogen time correctly and using the best diving safety gear will help you avoid diving problems while making your sea exploration more enjoyable.

Globo Surf Overview

If you intend to scuba dive more than once, the residual nitrogen time should come in handy. It will help you track your nitrogen absorption. Calculating your residual nitrogen time is not necessary on your first dive.

Residual time calculations help divers account for the residual nitrogen in their system. Adding the dive time and the residual nitrogen time will help you figure out the nitrogen amount you currently have in your body, after multiple dives. You can use your calculated dive time and the diving table to figure out your pressure group.

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