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newSchoolWorks.com › VoTechWorks › Agriculture, Environment & Public Service › Aquaculture & Marine Science Tech › Commercial Diving Introduction for Aquaculture and Marine Science › Lesson 2
AGR-AQUA50-06 · Lesson 2 · 14 clock hours · theory

Decompression Theory and Dive Table / Computer Use

Objective. Students will plan single and repetitive air dives to recreational no-decompression limits using PADI Recreational Dive Planner tables and a recreational dive computer, identifying pressure groups and required surface intervals.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Why Nitrogen Matters

When you breathe compressed air underwater, nitrogen dissolves into your body tissues under pressure. The deeper you go and the longer you stay, the more nitrogen accumulates. When you ascend, that nitrogen must leave your tissues slowly through your lungs. If you ascend too fast, or if too much nitrogen has accumulated, bubbles can form inside tissues and blood vessels — a condition called decompression sickness (DCS). DCS ranges from joint pain and skin rashes to neurological injury and death. Preventing it is the central purpose of dive planning.

No-Decompression Limits

Recreational diving is planned to stay within no-decompression limits (NDLs) — the maximum time at a given depth that allows a direct ascent to the surface without mandatory decompression stops. The PADI Recreational Dive Planner (RDP) expresses these limits for air dives. For example, the NDL at 18 m (60 ft) is 56 minutes for a first dive starting with no prior nitrogen loading. Exceeding the NDL requires in-water decompression stops that recreational equipment and training do not support. Always plan to stay within NDL limits with a comfortable margin.

The PADI Recreational Dive Planner (RDP)

The RDP exists in two formats: the wheel (a circular slide-rule card) and the eRDPml (an electronic table). Both use the same underlying model. The RDP has three tables:

Adjusted No-Decompression Limit (ANDL) = NDL from Table 1 minus the RNT from Table 3. Your actual bottom time on the repetitive dive must not exceed the ANDL.

Total Nitrogen Time (TNT) = RNT + actual bottom time. Use TNT (not actual bottom time alone) to find your ending pressure group after a repetitive dive.

Planning a Single Dive

1. Choose the deepest point of the dive as your planning depth — always round up to the next line in the table.

2. Look up that depth in Table 1 to find the NDL.

3. Decide your planned bottom time (must be ≤ NDL). Find the pressure group you will be in at the end of the dive.

4. Plan your ascent rate: the RDP requires no faster than 18 m (60 ft) per minute. Make a safety stop at 5 m (15 ft) for 3 minutes on every dive as a standard practice — it meaningfully reduces residual nitrogen.

Planning a Repetitive Dive

1. After your first dive, note your ending pressure group.

2. Decide your surface interval. Use Table 2 to find your new pressure group after that interval.

3. Go to Table 3: find your new pressure group in the left column, then find your planned depth across the top. The intersection gives your RNT.

4. Calculate your ANDL (Table 1 NDL minus RNT). Stay within ANDL on the second dive.

5. Add actual bottom time to RNT to get TNT; use TNT to find your ending pressure group from Table 1.

Dive Computers

A dive computer measures actual depth and time continuously and recalculates nitrogen loading in real time, using a built-in decompression algorithm. The computer displays your remaining no-decompression time, current depth, maximum depth, and ascent rate warnings. Computers are more conservative or more permissive than RDP tables depending on the algorithm and settings — always note which system you are using and do not switch mid-dive-day from tables to a computer or vice versa without following the manufacturer's guidance.

For aquaculture work — such as net-pen inspection at 18 m — a computer adds an important safety margin because actual work depth may vary during the dive. Always plan conservatively with tables first, then verify with the computer.

Safety Rules — Non-Negotiable

Key terms9

Decompression Sickness (DCS)A condition caused by nitrogen bubbles forming in body tissues and blood when ascent is too fast or nitrogen loading is excessive; ranges from joint pain to life-threatening neurological injury.
No-Decompression Limit (NDL)The maximum allowable bottom time at a given depth during which a diver can ascend directly to the surface without mandatory decompression stops.
Pressure GroupA letter designation (A through Z on the PADI RDP) representing the amount of residual nitrogen in a diver's tissues at the end of a dive or surface interval.
Residual Nitrogen Time (RNT)The additional time, expressed in minutes, added to a repetitive dive's actual bottom time to account for nitrogen still dissolved in tissues from a previous dive.
Adjusted No-Decompression Limit (ANDL)The maximum actual bottom time allowed on a repetitive dive; calculated by subtracting the Residual Nitrogen Time from the Table 1 NDL for the planned depth.
Total Nitrogen Time (TNT)The sum of Residual Nitrogen Time and actual bottom time on a repetitive dive; used to determine the ending pressure group after a repetitive dive.
Surface IntervalThe time spent on the surface between two dives; a longer surface interval allows more nitrogen to off-gas and results in a lower (less nitrogen-loaded) pressure group.
Safety StopA precautionary pause at 5 m (15 ft) for 3 minutes at the end of every recreational dive to allow additional nitrogen off-gassing before surfacing.
Dive ComputerAn instrument worn by the diver that continuously tracks depth and time, calculates real-time nitrogen loading using a decompression algorithm, and displays remaining no-decompression time and ascent-rate warnings.

Worked examples4

1. Plan a single first-of-the-day air dive to 18 m (60 ft) for a net-pen inspection lasting 40 minutes. Find the ending pressure group.
  1. Step 1 – Identify planning depth: The dive reaches 18 m (60 ft). Use 18 m (60 ft) in Table 1 (round up to the next table line if needed; 18 m/60 ft is a listed line on the PADI RDP).
  2. Step 2 – Check NDL: Table 1 shows the NDL at 18 m (60 ft) is 56 minutes for a no-prior-exposure diver. A 40-minute bottom time is within the NDL — the dive is within recreational limits.
  3. Step 3 – Find pressure group: Locate 18 m (60 ft) in Table 1 and move across the row to 40 minutes bottom time. The corresponding pressure group is Q.
  4. Step 4 – Confirm ascent plan: Ascend at no faster than 18 m/min and make a 3-minute safety stop at 5 m (15 ft) before surfacing.
Answer: The dive is within NDL limits. Ending pressure group: Q.
2. After the first dive (ending pressure group Q), the team takes a 1-hour surface interval. What is the new pressure group before the second dive?
  1. Step 1 – Enter Table 2 with starting pressure group Q in the left column.
  2. Step 2 – Move across the row to find the time bracket that includes 1:00 (one hour). Read the resulting pressure group at the bottom of that column.
  3. Step 3 – Using the PADI RDP, a 1-hour surface interval from pressure group Q yields pressure group J.
Answer: New pressure group after 1-hour surface interval: J.
3. The team now plans a second dive to 18 m (60 ft). Their pre-dive pressure group is J. Find the RNT, ANDL, and the maximum allowable actual bottom time.
  1. Step 1 – Enter Table 3 with pressure group J in the left column and depth 18 m (60 ft) across the top. The intersection gives the Residual Nitrogen Time (RNT) = 27 minutes.
  2. Step 2 – Find the NDL for 18 m (60 ft) from Table 1 = 56 minutes.
  3. Step 3 – Calculate ANDL: ANDL = NDL − RNT = 56 − 27 = 29 minutes. The team's actual bottom time on the second dive must not exceed 29 minutes.
  4. Step 4 – If the team dives 25 minutes actual bottom time, then TNT = RNT + actual = 27 + 25 = 52 minutes. Return to Table 1 at 18 m (60 ft) and find the pressure group for 52 minutes = P.
Answer: RNT = 27 min; ANDL = 29 min; if actual bottom time is 25 min, ending pressure group is P.
4. A diver plans a dive to 20 m (70 ft) as the first dive of the day. They want to be in pressure group L or lower at the end. What is the maximum bottom time they should plan?
  1. Step 1 – Enter Table 1 at 20 m (70 ft). Check the NDL (40 minutes on the PADI RDP).
  2. Step 2 – Scan across the row to find which bottom time corresponds to pressure group L. On the PADI RDP at 20 m (70 ft), a bottom time of 30 minutes yields pressure group L.
  3. Step 3 – Confirm 30 minutes is within the NDL of 40 minutes — it is. Plan bottom time at 30 minutes or less to end in group L or lower.
  4. Step 4 – Plan ascent: no faster than 18 m/min; 3-minute safety stop at 5 m (15 ft).
Answer: Maximum planned bottom time to remain at or below pressure group L: 30 minutes.
In class: Instructor-led walkthrough of the PADI RDP table structure; students plan five progressive dive scenarios (increasing depth and repetitive exposure) on paper, recording pressure groups and surface intervals. · Pair exercise: one student plans a two-dive day using RDP tables while the partner programs the same profile into a dive computer (Suunto or equivalent), then both compare results and reconcile any differences. · Small-group scenario: plan a dive day for an aquaculture net-pen inspection at 18 m (60 ft), accounting for surface interval between a morning and afternoon dive, and present the plan to class with safety justification.
Materials: PADI Recreational Dive Planner wheel and eRDPml, recreational dive computers (one per pair), PADI Open Water Diver Manual, dive planning worksheet templates
Check: Five dive-planning problems completed on paper without errors in pressure group or surface interval determination; 100% accuracy required on any problem involving nitrogen loading (retest permitted once).

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Dive Planning and Decompression Theory
1. What does a pressure group letter represent on the PADI Recreational Dive Planner?
Dive Planning and Decompression Theory
2. A diver ends a dive in pressure group N and rests on the surface for 45 minutes. Which table is used to find the new pressure group?
Dive Planning and Decompression Theory
3. Before a repetitive dive to 18 m (60 ft), a diver's pressure group is H, giving an RNT of 21 minutes. The Table 1 NDL at 18 m (60 ft) is 56 minutes. What is the Adjusted No-Decompression Limit for this dive?
Dive Planning and Decompression Theory
4. A diver completes a repetitive dive with an RNT of 18 minutes and an actual bottom time of 22 minutes. What value is used to find the ending pressure group?
Dive Safety and Emergency Procedures
5. What is the recommended maximum ascent rate for recreational air diving according to the PADI RDP?
Dive Planning and Decompression Theory
6. A dive computer and RDP tables are used to plan the same two-dive day. The computer shows slightly more remaining NDL time than the tables for the second dive. What is the safest approach?
Dive Safety and Emergency Procedures
7. An aquaculture team plans a net-pen inspection at 18 m (60 ft). The first diver's computer fails underwater. What should the diver do?
Dive Safety and Emergency Procedures
8. Which of the following best explains why a safety stop at 5 m (15 ft) for 3 minutes is performed on every recreational dive?
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