Six open water dives, in order. Each answers a question raised by the one before it, and a dive taken out of order tests nothing.
Two of the days are bound together; the rest of the schedule is not. Dives 2, 4 and 5 occupy the same station and their timing carries the whole comparison. Dive 3 follows Dive 2 on the same day, and Dives 4 and 5 fall on the day after.
Dive 2 and Dive 4 must each be the first dive of their day. This is why Dive 3 cannot be moved to the following morning, and why Dive 1 cannot share a day with Dive 2. The reason is worth stating because it is not obvious.
Matching the hour controls for time of day. Matching the position in the day controls for everything else that accumulates: repetitive loading, surface interval, how many times the calibration bath has been run that morning, and how tired and wet the divers are. A Dive 4 conducted after a morning Dive 3 differs from Dive 2 in several ways at once, and the comparison can no longer say which of them mattered.
So the two station days each open with a station occupation. Dive 3 follows Dive 2 on the first; Dive 5 follows Dive 4 on the second.
Only Dives 1 and 6 float: Dive 1 on any day before the sequence begins, Dive 6 on any day after it ends, subject to the three-dive daily limit.
The two days below are therefore the station days, not necessarily the first two days of the course.
The shape of the course is three station occupations and one survey deployment, all at the same station.
The distinction matters and is easy to blur. The array goes into the water at Station A on four dives, not three — but only three of them are occupations. An occupation profiles the column and produces a row in the comparison; Dive 3 deploys the same array at the same depths for a different purpose, as a stationary reference against which observations made elsewhere are matched. Its readings do not enter the Dive 2 / 4 / 5 comparison.
| Dive | Station | Role | What it establishes | |
|---|---|---|---|---|
| Earlier day | 1 | — | characterization | How good the instruments are, including the divers |
| Station day 1 | 2 | A | occupation | The reference occupation |
| 3 | A | survey deployment | Whether the water varies horizontally | |
| Station day 2 | 4 | A | occupation | Repeatability — same station, same hour, one day later |
| 5 | A | occupation | Change — same station, hours after Dive 4 | |
| Later day | 6 | comparison station or condition | occupation | Transferability — different water |
Two comparisons come out of that, and the order matters. Dive 4 against Dive 2 measures how far the whole procedure repeats when nothing is expected to have changed — the instruments, the divers, the deployment, the kick cycles, all of it together. Dive 5 against Dive 4 then tests whether the water changed over the hours of a day, and it is tested against comparison thresholds the team measured rather than ones they assumed.
That order is the reason the course is worth teaching. A team that occupies a station twice and finds a difference has found nothing until they know how different two occupations are when nothing has happened.
Depth-holding is assessed twice on every dive: the instructor observes it in the water, and the student diver verifies it afterward from their own dive computer log. The first is the performance requirement. The second is the number that goes into the Station Report, and it is also the point at which most student divers first see the difference between how well they think they held depth and how well they did.
The dry session is repeated in full before every dive on which data are collected — clock synchronization, logger configuration, cross-calibration in shaded seawater from the site, disc inspection. It is not optional on the grounds that it was done yesterday. Offsets drift, and the uncertainty reported for a dive must be the one measured on that day.