Intermediate 7 minutes
The history of dive computers: from the table to the algorithm
How we went from calculating on a printed grid to wearing the model on your wrist
For decades, planning a dive meant looking up two numbers on a printed table and accepting that your real profile (which went up and down) was counted as if you'd spent the whole dive at maximum depth. It was conservative, it was inconvenient, and it was all there was.
In this article you'll see where decompression tables came from, what attempts were made to automate the calculation before electronics, how digital computers arrived, which model families coexist today, and what actually changed in practice.
The device on your wrist doesn't measure your saturation: it runs a mathematical model. Knowing where that model comes from and what it assumes is what lets you use it as a planning tool instead of an oracle.
The tables
The starting point is the early-20th-century work, mentioned in the article on diving helmets, that produced the first systematic staged-decompression method: the idea that different tissues absorb and release gas at different rates, and that there's a supersaturation margin you shouldn't exceed.
That produced tables relating depth and time to an ascent procedure, revised and expanded over decades by navies and research bodies, and later adapted for recreational diving.
Their structural limitation is that they only understand a rectangle: one depth and one time. A real profile (which descends, ascends, descends again, and finishes shallow) has to be counted at its deepest point for the whole dive, which penalizes the actual dive enormously.
For repetitive dives, letter groups and interval tables were added, with a manual procedure you had to execute without errors: the most dreaded part of any course.
Even so, they worked: for decades they were the foundation of all diving, and they're still the backup tool and the instrument through which you really understand what a computer does.
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