In order to understand how to properly use these dive tables, imagine yourself on a dive trip to the Great Barrier Reef with Reef Experience.
There will be a morning dive at around 9.30, snorkel until 12.30 then move to the second barrier reef location, arriving at approximately 1 pm for a repetitive dive.
You have just arrived at the first location, and your experienced dive guide has informed you that the reef location is 18 meters (60 feet) deep.
Using Dive Tables 1
View the ‘No-Decompression’ time limit for 18 meters (60 feet) by reading down the depth column to your desired depth, and across to the time limit given. Here you see you can remain at 18 meters for a maximum of 50 minutes.
In order to be conservative, you plan with your buddy to dive to 16 meters for 35 minutes. As there is no row for 16 meters or no 35 minute time option, you always round UP to the next greater depth and time measurement in order to be conservative.
The depth options given are 15 meters (50 feet) or 18 meters (60 feet), so we would need to round our 16 meter planned depth up and follow the 18 meter (60 feet) row. As 35 minutes is not located in 18 meter row, being less than 40 minutes and more than 30 minutes, you round up to 40 minutes. This places you in the G column; G is your Group Designation Letter for this dive. As this letter is used to represent how much nitrogen remains in your tissues, you are said to be a ‘G Diver’.
Now that you know your dive limits, you descend into the lush tropical waters of the Great Barrier Reef, surrounded by beautiful fish and amazing coral as far as the eye can see. After reaching 16 meters, you continue your dive before ascending in an Actual Bottom Time (ABT) of 35 minutes as planned.
Pop up link to: ‘What is Actual Bottom Time?’ Actual bottom time is the time from the beginning of your descent, all through the dive, up until you make your direct ascent to the surface at the end of the dive.
The Dive Master informs you that you exited the water at 11:10 hours and the next dive will begin at 13:00 hours. Your dive guide tells you the maximum depth of the next dive will be 12 meters (40 feet) and the maximum time for this dive will be 40 minutes.
Therefore, you now know that after a surface interval of 1 hour and 50 minutes the maximum depth of the next dive will be 12 meters. In order to remain within the No-Decompression Limits, you begin to plan your next dive.
Dive Table two allows us to find out what Group Designation Letter we would begin our repetitive dive as, if we finished your last dive as a ‘G Diver’ and had a Surface Interval of 1 hr50.
Continuing down from Dive Table 1, you trace Group Designation Letter G’s arrow down until it reaches the G row. Look across the row, and find the bracket of time that 1hr50 fits into.
Here, you can see that 1hr50 fits into the bracket between 1hr16 and 1hr59. Follow this down and you’ll see that after your Surface Interval you have gone from a ‘G Diver’ to an ‘E Diver’, at the beginning of your subsequent dive. (During your surface interval you have breathed out some nitrogen, so you have dropped down a pressure group by the beginning of your next dive).
This time, your Surface Interval brings your Group Designation Letter to an E at the beginning of your repetitive dive.
Dive Table 2 determines the level of residual nitrogen remaining in your system before a subsequent dive. This allows you to calculate your residual nitrogen time (RNT) using Dive Table Three.
Pop up Link to; ‘What is Residual Nitrogen Time?’ Residual Nitrogen time is the amount of additional time you have to add onto a repetitive dive, to account for the fact that you started with some residual of nitrogen in your body from the previous scuba dive(s).
Remember, your dive guide has informed you that the maximum depth of the next dive will be 12 meters (40 feet) and the maximum Actual Bottom Time for this dive will be 40 minutes. Let’s see if this is possible…
Using Dive Table 3
At the beginning of this repetitive dive we have already calculated that we start as an ‘E Diver’. Using our planned depth of 12 meters (40 feet) read down the E column until you reach the 12 meter (40 feet) row.
You will see that there are two numbers in the box where your depth intersects with ‘E’; A white top number ‘49’ and a green bottom number ‘81’.
First, let’s look at the bottom green number, which is 81 minutes. This number is your new, adjusted ‘No Decompression Time Limit’ for this dive. We only wish to dive for 40 minutes, so we know that as an ‘E-Diver’ who wishes to dive to 12 meters (40 feet) we would safely be within the 81 minute time limit.
(Look back to table one and notice that if this was a first time dive to 12 meters (40 feet) our time limit would be 130 minutes. Now, because we begin this dive with ‘E’ amount of nitrogen in our bodies, our time limit has been adjusted to only 81 minutes, to account for the residual nitrogen left in us from the previous dive).
Working out our Group Designation Letter after a repetitive dive:
Now, let’s look at the top white number, which is 49. This is the amount of time you have to add on to your Actual Bottom Time to get your Total Time for your repetitive dive. In other words, if you are an ‘E Diver’ who wants to go to 12 meters (40 feet) you must add on 49 minutes Residual Nitrogen Time.
If we add our Actual Bottom Time of 40 minutes with our Residual Nitrogen Time it gives us a Total Bottom Time of 89 minutes.
Total Bottom Time = Actual Bottom Time + Residual Nitrogen Time
TBT = 40 + 49
Total Bottom Time = 89 minutes
REMEMBER always use Dive Table One to calculate your Group Designation Letter at the end of every dive
You know that for this dive your maximum depth is 12 meters and your Total Bottom Time is 89 minutes, so you can work out your new Group Designation Letter, which would be ‘I’.
Third and even more repetitive dives can be planned by repeating the process, as long as you remain within the No-Decompression limits of the dive tables.