Whether you are new to SCUBA diving or are an experienced diver, refreshing your knowledge about dive tables is essential.
Dive tables allow you to plan single or multiple dives and are your best companion in case your dive computer fails.
This post is a complete guide on how to use PADI’s dive tables. After going through the theory and examples, you will be able to plan single, non-repetitive dives as well as repetitive dives. You will also enhance your knowledge about PADI’s dive tables and the main differences with other dive tables.
This guide is intended to supplement the instruction of a qualified SCUBA instructor, and I strongly recommend discussing any doubt with him. Knowing how to use PADI’s dive tables will help you prevent decompression sickness in case of dive computer failure.
Using PADI Dive Tables
Diving is an extremely rewarding and exciting activity that more and more people are learning to enjoy. Thanks to dive agencies like PADI, and SCUBA certifications such as the Open Water Scuba Diver, many divers can enjoy access to the underwater world safely, and in a relatively short amount of time.
One of the most challenging aspects for newly certified divers, and for aspiring divemasters and instructors alike, is how to plan a dive safely using dive tables. The use of dive computers and the growing tendency for SCUBA agencies to move away from traditional dive tables in favor of dive computers is leading to a growing void in diver education.
While dive tables are still commonly used in technical diving, many recreational divers only use dive tables to pass their open water diver certification exam. After buying their first dive computer, these very same divers tend to forget how to use dive tables rather quickly. This is particularly the case now that many dive computers have integrated alerts.
To enjoy scuba diving safely, I strongly recommend using dive tables from time to time. Instead of fully relying on your dive guide, take 10 minutes to plan some of your dives with dive tables. This will definitely help you become a better and more conscious diver.
Before we start with dive planning using dive tables, let’s get briefly acquainted with PADI’s RDP Dive tables and their general guidelines.
PADI Dive Tables Guidelines
Whenever using PADI’s RDP tables, keep in mind the following general guidelines, irrespective of the type of dive you are planning.
- Bottom time begins immediately upon descent and ends whenever you leave the bottom and start ascending.
- Time spent on your safety stop does not count towards bottom time.
- Any dive planned to 35 feet / 10 meters or less should be considered as a dive to 35 feet / 10 meters.
- If the depth of your dive is not shown in the table, use the next greater depth.
- If the time of your dive is not shown in the table, use the next greater time.
- Ascent rate should never exceed 60 feet or 18 meters per minute.
- When planning a dive in cold water or in harsh conditions, plan your dive assuming the depth is 10 feet or 4 meters greater than actual.
- Plan repetitive dives in such a way that the depth of your next dive is always shallower. Repetitive dives shall be limited to a depth of 110 feet or 30 meters.
- Limit your maximum depth to your level of training and experience.
- Every dive should end with a safety stop before emerging.
- In case of multiple dives, special rules apply, they are known as WXYZ.
- If you accidentally dive deeper than 130 feet or 40 meters, ascend immediately to 5 meters at a rate not exceeding 60 feet or 18 meters per minute, and make an emergency decompression stop of 8 minutes. Note that at such depths you are highly exposed to Nitrogen Narcosis (you can read more about Nitrogen Narcosis here).
- Any dive below 130 feet should be followed by a surface interval of at least 6 hours.
- PADI’s dive tables are not designed for stage decompression diving. If you exceed a non decompression limit by less than 5 minutes, you should do an emergency decompression stop at 15 feet for 8 minutes. You should then do a minimum surface interval of 6 hours.
- If you exceed a no decompression limit by more than 5 minutes, an emergency decompression stop at 15 feet of no less than 15 minutes is mandatory. You should subsequently wait 24 hours prior to flying or diving again.
With these guidelines in mind, let’s now take a closer look at PADI’s RDP Tables.
PADI Dive Tables Explained
To make dive planning easier, PADI’s RDP tables are composed of three distinct dive tables.
The first and second tables are located on page 1 of PADI’s RDP tables. The first table includes the No Decompression Limits section, and the second table includes the Group Designation section for no-decompression Air Dives. The third table is located on page two and includes the repetitive dive timetable.
The first PADI Dive Table provides you with information about single dive ending pressure groups and no decompression limits (NDL). Pressure groups are marked with letters and inform you about the amount of residual nitrogen retained by your body by the end of a dive.
Let’s walk through the PADI Dive Table with the help of an example. Assume you want to conduct the following dives:
- Dive 1 – 80 feet for 28 minutes
- Surface interval – 35 minutes
- Dive 2 – 50 feet for 35 minutes
- Surface interval – 49 minutes
- Dive 3 – 38 feet for 45 minutes
You should then use the PADI dive tables as follows:
1. Start from the top left corner of the first dive table and find your desired depth (80 feet) along the top.
2. Move down along the 80 feet column until you find a time that is equal or greater (rounding up) than 28 minutes. As you can see, 28 minutes are in a shaded box, meaning that the safety stop rule applies.
3. Scroll through the right until you find your pressure group letter. P in our example.
4. Once you have found your pressure group at the end of your first dive, continue scrolling to the right along the same row until you find a surface interval box that contains 35 minutes.
5. Now, move down along the same column to find your pressure group after the surface interval. In our example, this is pressure group I.
6. You may now turn to Table 3.
7. You want to do your second dive at 50 feet for 35 minutes, and you know that your pressure group before starting the dive is I. As next step, you should therefore find the crossing point between the pressure group column and the depth row. As you can see from the table above, you will end up in a box containing two numbers, 31 and 49. The top number, 31, is the Residual Nitrogen Time (RNT). The bottom number, 49, is the Adjusted Non Decompression Limit (ANDL).
8. By adding the RNT to the Actual Bottom Time or ABT (as planned in our example) you will have a Total Bottom Time (TBT) of 66 minutes ( = 31 + 35). To find out your pressure group at the end of the second dive, move back to Table 1.
9. Given that your second dive will be at 50 feet, start from the 50 feet column and move down until you find your Total Bottom Time. In our example we round up to 67 as 66 is not in the table. Note that 67 is in a grey box, meaning that you will have to do a safety stop.
10. Moving on the right along the same row, you will find the pressure group at the end of your second dive, U.
11. Continue on the right along the same row, until you find the surface interval box that includes 49 minutes.
12. Move down along the 49 minutes surface interval column to find the pressure group at the end of the surface interval. J in our case.
13. You may now turn to Table 3.
14. Remember that you want to conduct your third dive at 38 feet for 45 minutes. You therefore need to identify the crossing point between the J pressure group and a depth of 40 feet (note that we rounded up as 38 is not in the table).
15. Your RNT is 44 minutes and your ANDL is 96 minutes.
16. Add your RNT to your ABT to find out your total bottom time. 44 +45 = 89, which is your Total Bottom Time
17. To find out your pressure group at the end of your third dive, move back to Table 1 and identify the highest closest number to 38 feet.
18. Follow it down to the higher closest number to 89 minutes (your TBT).
19. You are in pressure group T after your third dive.
To wrap up, in our example, we saw how to find out your pressure group at the end of a dive, after the surface interval, and at the end of multiple dives.
In particular, to find out your pressure group at the end of multiple dives, you will need to sum your Residual Nitrogen Time and your Absolute Bottom Time in order to find out your Total Bottom Time.
In our example, we already knew the profile of the dives and surface intervals for the three repetitive dives.
PADI’s RDP tables can however be used to find out and plan way more than the pressure group at the end of a dive.
For instance, you can find out the minimum surface interval between two dives, the adjusted no decompression limit for a repetitive dive, or the maximum allowed depth for a repetitive dive.
How do you find the minimum surface interval between two or more dives?
Suppose you want to do two repetitive dives. The first dive at a depth of 73 feet for 27 minutes and the second dive at a depth of 45 feet for 48 minutes. How do you find the minimum required surface interval between those two dives?
- Find the pressure group at the end of your first dive. Using the First Table, and starting from a depth of 80 feet (73 feet is not available, therefore we round up), we move down along the 80 feet column until we find the higher number closer to 27 (if 27 is unavailable). Moving to the right, we find the pressure group at the end of our first dive, P.
2. Now, you know that your next dive is at a depth of 45 feet for 45 minutes. Move on to table 3 and identify the higher depth closest to 45 feet (that’s 50 feet). Move to the right, reading the ABT (i.e. blue) row, until you find the higher number closest to 45 minutes (that’s 47).
3. Move up along the 47 minutes column to identify your pressure group at the beginning of the second dive.
4. You now have your pressure group at the end of the first dive, P, and the pressure group at the beginning of your second dive, J.
5. To find the minimum duration of your surface interval, move to Table 2 and find the crossing point between pressure group P and J.
6. Your minimum surface interval will be 28 minutes.
How do you Find the Maximum Allowable Depth for A Repetitive Dive?
Suppose you would like to know the maximum allowable depth for a 60 minute dive following a dive to 50 feet for 45 minutes and a 30 minutes surface interval.
1. Let’s start by finding our your pressure group at the end of your first dive. Using Padi’s first table, the pressure group for a 45 minute dive to 50 feet is O. Note that we have rounded up to 47 minutes as 45 is not in the table.
2. As a second step, you need to determine your pressure group at the end of the 30 minute surface interval. Using PADI’s second table, you will find that you are in pressure group I.
3. Next, you need to find out the maximum allowable bottom time for your second dive. You can do this by using Table 3.
4. Start from Column I, your pressure group at the end of your surface interval, and look for the blue box with the ABT equal or greater than 60 minute.
5. Once you have identified it, scroll to the left and read the corresponding depth. In our example that’s 40 feet.
How do you Find the Adjusted No Decompression Limit for A Repetitive Dive?
Suppose you would like to know the maximum time allowable for a second dive to 80 feet if your first dive was to 100 feet for 16 minutes followed by a 50 minute surface interval. Your Adjusted No Decompression Limit will tell you just that.
1. Let’s start by finding out your pressure group at the end of the first dive, using the first table. Your pressure group at the end of a 16 minute dive at 100 feet is K.
2. Next, let’s find out your pressure group at the end of your 50 minute surface interval. Using the second table, that’s D.
3. Next, move on to table 3, and start from the column with pressure group D.
4. You need to identify the crossing point between pressure group D and 80 feet. The blue box at the crossing point will display the Adjusted Non Decompression Limit, which is the maximum time allowable for your second dive.
5. Your maximum allowable time will be 19 minutes.
Final Thoughts on PADI Dive Tables
In this blog post, you have learned how to use PADI’s dive tables. Whether you are taking your PADI Open Water, Divemaster or Instructor Development courses, it is always a good idea to refresh your knowledge and skills with PADI’s dive tables.
Using dive tables is the only option you have left in case your dive computer fails, they are designed in an extremely conservative way and will therefore prevent you from experiencing Decompression Sickness on recreational dives.
Work out the examples above at least three times, using your own PADI dive tables, this will help you create a mental habit and memorize the different types of uses.