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

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Residual Nitrogen Time (RNT) is the time that a diver can remain underwater with a given set of scuba tanks before returning to the surface. It is directly proportional to how deep the diver goes and how long they stay at their destination. The more nitrogen in their blood, the higher their risk for decompression sickness; therefore, it’s crucial for divers to be aware of this metric and avoid decompression sickness risks by not staying underwater any longer than necessary.

A diver’s RNT can be estimated by the following equation, where “X” is the maximum depth in feet and “Y” is the average depth in feet.

The above formula assumes that a diver starts diving with a completely full tank, then spends an entire dive at the given depth while using compressed air or oxygen (there’s no need to add nitrogen). At the end of this dive, as long as it was not deeper than “X”, then that tank would still have some nitrogen left in it.

Why is the Residual Nitrogen Time Important?
Since diving is an activity that often involves long periods underwater, it’s natural to wonder how safe a given dive really is. This metric, the Residual Nitrogen Time, can help a diver know how safe their dive is by determining how much nitrogen their body will contain on the end of it.

Determining one’s RNT for a given dive then allows them to decide whether or not to continue that dive because it takes into account all of the factors that could result in increased decompression sickness risks.

For instance, if a diver goes to a depth of 60 feet and spends an hour there before going back to the surface, they will have a residual nitrogen time of two hours. On the other hand, if that same diver went to 90 feet and stayed there for two hours, they would have a residual nitrogen time of three hours. That’s because decompression sickness risks increase as the amount of nitrogen in their bodies increases: The deeper one goes, the longer one stays and vice versa.

If a diver goes to a maximum depth of 100 feet and stays there for two hours, they will have a residual nitrogen time of three hours. If that diver then went to 110 feet and stayed there for another two hours, their residual nitrogen time would be five hours.

How Is Residual Nitrogen Time Calculated?
The most important factor in calculating RNT is the maximum depth at which the diver plans to spend their entire dive. This is easy to determine since it’s just how deep the diver wants to go. The next factor to consider is the average depth at which the diver will spend their time, or how deep they’ll go once they’re at their destination. A good way to estimate this is to use a depth gauge that measures depth in tenths of a foot (instead of full feet).

In order to keep track of how long one’s been underwater, some divers rely on dive computers, which can also be a useful safety tool.

What Does Nitrogen Absorption Mean for Scuba Divers?
Nitrogen absorption is when a person’s body absorbs nitrogen into their blood at a rate that’s faster than they’re getting rid of it. Nitrogen absorption, which can occur over time, is risky because it leads to increased decompression sickness risks when the diver resurfaces.

For this reason, it’s crucial for divers to keep careful track of how much nitrogen they’ve absorbed in order to ensure that they stay within safe limits. Since RNT can help a diver know exactly how long they’ve been underwater and will be underwater, it’s an important metric to pay attention to when diving.

What Is Decompression Sickness?
Decompression sickness (also known as “the bends”) is a condition that results from too much nitrogen being absorbed over too short of a period of time. It’s something that can be avoided by safely planning and taking part in dives using the appropriate diving tables. A diver who follows a dive plan carefully can also minimize decompression sickness risks because they’ll be able to limit the amount of time they’re underwater at any point during their dive.

Divers who come up from a deep dive too quickly may experience decompression sickness because the pressure around them is low, causing nitrogen bubbles to form in their body tissues. As those bubbles form, they can lead to pain, numbness and paralysis.

Measuring Nitrogen in the Diver’s Body

Numerous tests are available that can be used to measure nitrogen in a diver’s body. However, the most common and least invasive of these is known as a nitroprusside test, which requires only that a diver breathes into a tube, which then measures the amount of nitric oxide in their blood.

This test is fairly simple since it requires no special equipment or other preparations. Because of this, it’s the most common test that a diver can have to measure their RNT.

Other tests provide a more in-depth look into how much nitrogen a diver has in their body. These tests may involve the use of electrodes and can provide just as much information as the nitroprusside test but require more testing equipment, which is why they’re not always recommended for scuba divers.

Residual Nitrogen Time Makes Tracking Nitrogen Absorption Over Multiple Dives Easier

Although the RNT can keep a scuba diver on top of their nitrogen absorption, it’s still important to know when they’re at risk because this will allow them to watch for problems or signs of their impending decompression sickness.

The RNT is also an important metric in determining how safe a dive is going to be since it allows divers to keep accurate track of how much nitrogen they’ve absorbed over the course of their entire dive.

Knowing how much nitrogen is in their body will help divers make better decisions when they need to dive again. They should aim for a RNT that’s as close to zero as possible, but sometimes this number can’t be reached.

This happens because of how diving works: the deeper one goes, the longer one stays down there and vice versa. So if a diver goes to a maximum depth of 100 feet and stays there for two hours, they will have a residual nitrogen time of three hours. If that diver then went to 110 feet and stayed there for another two hours, their residual nitrogen time would be five hours.

For this reason, it’s important to keep track of RNT so that one can adjust their dive plans properly. A good way to do this is through diving computers, which can also be a useful safety tool for divers.

Nitrogen Absorption and the Risk of Decompression Sickness for Scuba Divers

Many people underestimate the dangers of nitrogen absorption, thinking that since it only increases RNT by a few hours that it won’t be a big deal. This is far from the truth when diving since nitrogen absorption can lead to increased risks of decompression sickness.

Risks arise because when one goes deeper, their blood absorbs a higher amount of nitrogen faster than they’re able to remove it from their body. This can increase the risks of decompression sickness since it’s common for divers to have multiple dives over the course of a day, which means that they can absorb nitrogen at a faster rate than they’re able to remove it.

Conclusion

There are many factors that determine whether or not a dive will be dangerous for a diver. Scuba divers can take a number of steps to ensure that they’re well-prepared for their dives, which can make them safer. Residual nitrogen time is an important factor to consider when planning one’s dives since it allows divers to track how much nitrogen they’ve absorbed in order to avoid decompression sickness. This can be done by taking note of RNT and adjusting future dive plans accordingly.

For more information on dive computers, consult our article on dive computers and other tools that can help divers thrive in the water.

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