What Is a Pressure Group on a Dive Table?
If you've ever looked at a recreational dive table, you've seen the row of letters — A, B, C, and so on. Those letters are pressure groups, and they exist to answer one question: how much leftover nitrogen is this diver carrying right now?
The letter is a nitrogen label
During every dive, your body absorbs nitrogen from the compressed air you breathe. When you surface, some of that nitrogen is still dissolved in your tissues — what tables call residual nitrogen. A pressure group is simply a shorthand label for how much of it you have.
- A means very little residual nitrogen — a short, shallow dive, or a long time spent at the surface.
- Letters deeper into the alphabet mean progressively more residual nitrogen.
- The deeper and longer the dive, the further down the alphabet you land.
The U.S. Navy, whose tables shaped most recreational versions, uses the same idea under a different name: the repetitive group designator in the U.S. Navy Diving Manual. Recreational training agencies borrowed the concept and renamed the letters, but the logic is identical.
How the letter changes
Your pressure group isn't fixed. It moves in two directions:
- It gets "worse" (later in the alphabet) when you dive. At the end of a dive, the table assigns you a letter based on your depth and bottom time.
- It gets "better" (earlier in the alphabet) while you rest at the surface. During a surface interval, you breathe off nitrogen, and the table lets you slide back toward A. Wait long enough and you drop off the table entirely — the table considers you "clean."
When you start a second dive, your current letter converts into a time penalty: the table adds residual nitrogen time to your planned bottom time, shrinking how long you can stay down. That's the whole machinery behind planning a repetitive dive on paper.
Letters are not interchangeable between tables
This trips people up. A pressure group "F" on one agency's table does not mean the same thing as an "F" on another agency's table, because each table is built on its own math and its own assumptions. You cannot end a dive on one table and start the next dive on a different one. If you're working a table by hand, you stay on that table for the whole day.
Why you may never use one
Dive computers made pressure groups largely invisible. A computer tracks your dissolved nitrogen continuously instead of rounding you into a letter category, which is one reason computers usually allow more bottom time than tables do for the same dives. Most new divers today are certified with a computer and may only meet pressure groups in the classroom.
They're still worth understanding, though. Organizations that run working dives, like the NOAA Diving Program, still train divers on table-based planning as a foundation, and exam questions about pressure groups remain standard in entry-level courses. If you know what the letter represents — a rough category of leftover nitrogen — the rest of the table stops looking like a spreadsheet and starts making sense. For the step-by-step mechanics, see how to read a recreational dive table.