Welcome to your deep dive into the numbers that keep you safe underwater. If you are taking your PADI Open Water Diver course, you have likely just been handed a blue-and-white grid full of tiny numbers or a gray calculator-like device and felt a wave of confusion. Don't worry—this is a rite of passage for every diver.
Today, we are going to demystify dive planning. We will walk through exactly how to use the physical Recreational Dive Planner—often called the "RDP Table"—and then we will look at its electronic cousin, the eRDPml, which is the "software" or calculator version you might be tested on.
Why Do We Need This?
Before we look at the rows and columns, let's understand the goal. When you breathe compressed air underwater, your body absorbs nitrogen. If you stay too long or ascend too fast, that nitrogen can form bubbles, causing Decompression Sickness, or "the bends."
These tables are simply a way to track that nitrogen. They tell you two main things:
- No Decompression Limit (NDL): How long you can stay at a specific depth without needing mandatory decompression stops.
- Pressure Group: A letter (A to Z) that represents how much nitrogen is left in your body after a dive.
Part 1: The Physical RDP Table Walkthrough
Let's plan a hypothetical day of diving.
Scenario:
You want to make a first dive to 60 feet (18 meters) for 30 minutes. You will rest on the boat for 1 hour, and then you want to see how long you can safely stay at 50 feet (15 meters) for your second dive.
Step 1: Table 1 - The Dive
Look at the front of the card (Table 1). Along the top row, find your depth. If your exact depth isn't there, always round up to the deeper number. We find 60.
Now, run your finger down that column. The big black numbers are times in minutes. Find 30. (Again, if 30 wasn't there, you'd round up to the next longer time).
Follow that row to the right to find your Pressure Group. You should land on the letter M.
Result: You are now a "M" diver. You have a moderate amount of nitrogen in your system.
Step 2: Table 2 - The Surface Interval
Flip the card over or move to Table 2 (the middle section). Find your letter M along the slanted diagonal edge.
Move horizontally to the right into the box of times. You are looking for a time range that includes your 1-hour rest (1:00). You will find a box that says something like 0:56 - 1:02.
Follow that column vertically down to find your new Pressure Group. You land on H.
Result: After resting for an hour, your body has off-gassed some nitrogen. You went from an M to an H.
Step 3: Table 3 - The Repetitive Dive
Now, continue down from that H into Table 3. Look at the row of depths on the left side. Find your second dive depth: 50 feet.
Where the H column and the 50-foot row intersect, you will see two numbers in a box:
- The top number (white area) is Residual Nitrogen Time (RNT). This represents the nitrogen you still have in your body, expressed as "penalty minutes."
- The bottom number (blue area) is your Adjusted No Decompression Limit. This is the actual time you can spend underwater on this second dive.
For an H diver at 50 feet, the Adjusted Limit is typically around 48 minutes. Even though a fresh diver could stay at 50 feet for 80 minutes, you only get 48 because of your first dive.
Part 2: The Software (eRDPml) Walkthrough
The eRDPml is the gray calculator-style device. It uses the exact same math but does the lookups for you. It removes the human error of tracing tiny lines with your finger. Here is how you would run that same scenario.
The Interface:
- ON/OFF: Turns it on.
- MODE: Cycles between "Dive Planning," "Surface Interval," and "Max Depth."
- ENTER: Confirms your choice.
- Green/Red Buttons: Yes/No.
Step 1: Plan Dive 1
- Turn it on. Press MODE until you see
DIVE PLANNING. Press ENTER. - It asks:
MULTILEVEL?Press NO (since we are staying at one depth). - It asks:
FIRST DIVE?Press YES. - It asks:
DEPTH?Type 60 and press ENTER. - It will show the NDL (55 or 56 mins). You plan to stay for 30.
- It asks:
ABT?(Actual Bottom Time). Type 30 and press ENTER. - It displays:
PG AFTER DIVE: M. (Matches our table result!)
Step 2: The Surface Interval
- Press ENTER to continue.
- It asks:
SURF INT?Type 1:00 and press ENTER. - It displays:
PG AFTER SI: H. (Matches our table result!)
Step 3: Plan Dive 2
- Press ENTER to continue.
- It asks:
DEPTH?Type 50 and press ENTER. - It will now display
ANDL(Adjusted No Decompression Limit). It will show 48. - This confirms you can safely dive to 50 feet for 48 minutes.
Summary
Whether you use the plastic card or the electronic calculator, the logic is identical:
- Dive 1 gives you a Pressure Group.
- Surface Interval lowers that Pressure Group.
- Dive 2 treats your remaining pressure group as "penalty time," reducing how long you can stay down.
Mastering this proves you understand the theory of gas loading, making you a safer, more competent diver—even if you eventually rely on a dive computer on your wrist!
Backgrounder Notes
As an expert researcher and library scientist, I have reviewed the article and identified several key concepts that are central to underwater safety and dive theory. Below are the backgrounders and definitions for these terms to provide you with a more comprehensive understanding of the subject.
PADI (Professional Association of Diving Instructors) PADI is the world’s largest and most recognized scuba diving training organization, providing standardized education and certification for recreational divers globally. The Recreational Dive Planner (RDP) discussed in the article is a proprietary tool developed by PADI based on specific decompression models.
Decompression Sickness (DCS) Commonly known as "the bends," DCS occurs when dissolved nitrogen—absorbed under high pressure—forms bubbles in the blood or tissues as a diver ascends too quickly. These bubbles can cause symptoms ranging from joint pain and rashes to neurological impairment and, in severe cases, paralysis or death.
Nitrogen Off-gassing This is the physiological process where the body slowly releases excess dissolved nitrogen through the lungs during a surface interval or a slow ascent. This process is the reverse of "on-gassing" and is why divers must wait between dives to reset their nitrogen levels.
No Decompression Limit (NDL) An NDL is the maximum amount of time a diver can spend at a specific depth while still being able to perform a direct ascent to the surface without a mandatory decompression stop. Exceeding this limit requires a series of timed pauses during ascent to prevent the onset of decompression sickness.
Residual Nitrogen Time (RNT) RNT is a mathematical "penalty" applied to a second or third dive to account for nitrogen remaining in the body from previous dives. It is expressed in minutes and added to the actual time of the next dive to ensure the diver does not exceed safe nitrogen limits.
Actual Bottom Time (ABT) In dive planning, ABT is the total time elapsed from the beginning of the descent until the start of the final ascent to the surface. It is the primary number used to calculate a diver’s Pressure Group at the end of a single dive.
Safety Stop While not a "mandatory" decompression stop, a safety stop is a recommended three-minute pause at 15 feet (5 meters) at the end of every dive. This pause allows for additional off-gassing and acts as a buffer against accidental depth or time limit violations.
Dive Computer A dive computer is a wearable electronic device that uses a pressure transducer and a timer to calculate a diver's nitrogen loading in real-time. Unlike static tables, a computer accounts for fluctuations in depth throughout a dive, providing a more precise and often more generous No Decompression Limit.
