Planning Your First Dive
Planning a day's first dive is accomplished using Table 1 of your dive tables. This table provides the maximum dive time for your planned depth and your letter group following the dive.
Dive Planning Terms
Before we begin using Table 1, let's cover some important terms used throughout this chapter.
- Dive Schedule
- The maximum depth and duration of a dive. For example, 42 feet for 28 minutes (or 42/28) is considered a dive schedule.
- Maximum Dive Time
- The time you can spend at a specific depth without being required to perform a decompression stop before surfacing.
- Residual Nitrogen
- Excess nitrogen in your body after a dive or series of dives that you have not offgassed.
- Letter Group
- A symbolic representation of the amount of residual nitrogen in your body.
The Dive Profile
When using dive tables, you need to keep track of each dive's depth and bottom time, as well as the surface time between dives. This information is used to make several calculations, which will then be used to plan your next dive. As you can see, organizing this information is an important step to ensure that your figures remain accurate.
A dive profile, such as the one pictured to the left, is a simple yet effective tool for organizing all of the information required to plan your dives. Dive profile worksheets and slates can be purchased at your local dive shop, or you can draw your own on a sheet of scratch paper.
You'll learn how to complete each section of a dive profile as you progress through this chapter.
Table 1 Overview
Table 1 is the first table in the set of dive tables. The rows contain dive times for a specific depth, with the maximum dive time for that depth indicated by a circle. The columns are used to determine letter groups, which are located at the bottom of the table.
Dive Planning Rules
- Make all calculations based on your deepest depth reached during a dive.
- No matter how much time you spend at your deepest depth, assume that the entire dive was spent at that depth. For example, if you spend 5 minutes at 90 feet, then ascend to 40 feet for 20 minutes, the dive should be considered a 90-foot dive.
- Round all numbers up
- Table 1 lists depths in 10-foot increments, and does not contain rows for depths shallower than 40 feet. If your depth is not listed in the table, round the depth up to the next greater depth. Additionally, all dives shallower than 40 feet are considered to be 40-foot dives. The same is true for dive times. If your exact dive time is not listed in a row, round it up to the next time listed.
- Ascend no faster than 30 feet per minute.
- A slow ascent allows the body to offgas nitrogen before you reach the surface, and significantly reduces the risk of decompression sickness.
- Perform a 3-5 minute safety stop at 15 feet.
- While this precautionary decompression stop is not mandatory, it reduces your risk of developing decompression sickness.
- Avoid diving to the maximum dive time for your maximum depth.
- If you accidentally exceed the maximum dive time, you are required to perform a decompression stop. This is a dangerous situation because missing the stop for any reason can lead to decompression sickness.
- Use the next greater time if you become chilled during a dive.
- Your blood and tissues absorb nitrogen faster when cold, so for increased safety, round your dive time up to the next greater time if you become chilled during a dive.
- Perform the deepest part of your dive first.
- Your body ingasses nitrogen faster at deeper depths than shallower depths, but it also begins offgassing a significant amount of that nitrogen when you ascend to a shallower depth.
- Do not use the standard dive tables when diving at altitudes higher than 1000 feet.
- Your maximum dive times at high altitude are lower than at sea level. To dive at altitudes higher than 1000 feet, you must use special tables designed for altitude diving or a dive computer equipped with an altitude compensation feature.
Maximum Dive Time
Before you begin a dive, it's important to determine how long you can safely remain at your planned depth without being required to perform a decompression stop before surfacing.
Maximum dive times are indicated by a red number enclosed in a circle. To determine the maximum dive time for a planned depth, you first locate the correct row for the depth, then follow the row to the right until you find the circled time.
For example, if a dive team is preparing to dive to 83 feet, the first step is to locate the correct row. Remember, if the exact depth is not included in the table, you round up to the next greater depth in the table. So for an 83-foot dive, the 90-foot row is used.
Now that the correct row is determined, you follow the row to the right until you locate the circled time. As you can see, 25 minutes is enclosed in a circle, so the maximum dive time for an 83-foot dive is 25 minutes.
Exceeding Maximum Dive Times
In the event you surface and realize that you exceeded the maximum dive time, you should take some preventative measures to reduce the risk of developing decompression sickness. These include:
- Avoid diving for at least 24 hours.
- Breathe 100% oxygen if available.
- Drink plenty of fluids to stay hydrated.
- Monitor yourself for symptoms of DCS.
- Seek medical attention and hyperbaric chamber treatment if you suspect you have DCS.
Under no circumstances should you return to the water in an attempt to decompress. Doing so places you at severe risk of drowning while serving little to no decompression benefits.
End-of-Dive Letter Group
As you learned earlier, your body ingasses nitrogen as you dive. The amount of nitrogen you ingas depends on the depth and duration of your dive.
You need to factor the amount of residual nitrogen in your body when planning your next dive. So let's learn how to determine your letter group following a dive.
To determine your letter group, you first locate the correct row for the maximum depth you dove to. Next, follow the row until you locate your dive time. Remember, you round numbers up, so if your actual dive time is not listed in the row, round up to the next dive time.
When you locate the correct dive time, follow the column down to the letter at the bottom of that column, and that's your letter group.
For example, a dive team has completed a dive to 83 feet for 14 minutes. The depth is rounded up to 90 feet, so follow the 90-foot row.
The dive time was 14 minutes, which is not included in the 90-foot row. So it needs to be rounded up to the next greater dive time, which is 15 minutes.
Now you follow the column containing the dive time straight down to find the letter group. And as you can see, after a dive to 83 feet for 14 minutes, a diver's letter group is E.