Whether you choose to dive off a boat or from the beach, you'll probably make more than one dive during the day. But with each dive you complete, the amount of nitrogen you ingas increases.
Divers spend time on the surface between dives, and during this time, nitrogen is offgassed. This increases the amount of time you can spend underwater during consecutive dives.
You use Table 2 to determine how much nitrogen you've offgassed while on the surface. In this lesson, you'll learn how to calculate the reduction in nitrogen levels.
Surface Interval Terms
Before we begin using Table 2, let's cover some important terms used throughout this chapter.
- Repetitive Dive
- A dive performed before offgassing all residual nitrogen from previous dives.
- Surface Interval Time
- The time spent at the surface between dives to allow the body to offgas nitrogen from previous dives.
Your body ingasses nitrogen during a dive, and offgasses nitrogen while on the surface. Unfortunately, it takes several hours to fully offgas nitrogen after diving. This means that if you complete more than one dive per day, your body still has residual nitrogen from previous dives when you begin repetitive dives. As a result, nitrogen levels continue to increase with every dive, which reduces the time you can spend underwater during repetitive dives.
Surface intervals help you safely complete more than one dive per day. During the surface interval, your body offgasses a significant amount of residual nitrogen, which increases your maximum dive time for repetitive dives.
The chart to the left illustrates the benefit of surface intervals. As you can see, after the first dive the diver has ingassed nitrogen. But after a surface interval the diver has offgassed a significant amount of that nitrogen, which allows a longer dive time for the second dive.
As you can imagine, it would be impossible for this diver to complete these three dives without taking surface intervals between those dives.
Table 2 Overview
As you learned during the previous lesson, the amount of residual nitrogen in your body is represented by a letter group. A higher letter group indicates you have a higher amount of nitrogen in your system, and a lower letter group indicates a lower amount.
You offgas nitrogen during your surface interval, so your letter group will change as well. To determine your new letter group following a surface interval, you use Table 2.
The letter groups at the top of Table 2 are your end-of-dive letter groups from Table 1. The cells below those letter groups contain time spans in hours and minutes. The letter groups at the left side of the table are the new letter groups following the surface interval.
Surface Interval Rules
Before we begin using Table 2, let's look at some rules to follow that increase your safety.
- Surface intervals must be at least 10 minutes long
- It's common for divers to surface during a dive for a brief moment. If the time you spend at the surface is less than 10 minutes, your dive time continues when you descend.
- A surface interval of 1 hour or longer is recommended
- A significant amount of nitrogen is offgassed during the first hour of your surface interval. In many cases, your letter group will not even change after less than one hour.
- All residual nitrogen is considered offgassed 24 hours after your last dive
- No matter what your end-of-dive letter group is following a dive, it takes 24 hours to completely offgas all residual nitrogen.
Determining the Letter Group After a Surface Interval
To determine your new letter group following a surface interval, you begin by locating your end-of-dive letter group from your previous dive. Next, you follow that column down to the cell that contains the time span that includes your surface interval time. Finally, follow that row to the left to find your new letter group.
For example, a dive team dove to 46 feet for 52 minutes, then took a surface interval of 1 hour and 32 minutes. Looking at Table 1, we can see that their end-of-dive letter group is "H" following the dive.
Next, go to Table 2 and follow the letter group's column down until you locate the time span the surface interval falls into. In our example, the divers spent 1 hour and 32 minutes at the surface. That falls between 1:07 and 1:41, so stop at that cell.
Now you follow the row to the left until you find the new letter group, which is "F" in our example.