The Station Occupation Procedure

This procedure is used on Dives 2, 4, 5 and 6. It is set out once here. Each of those dives then states only what is specific to it — the question it asks, any additional requirement, and the analysis that follows.

It does not change between them, and that is the whole point. Dives 2, 4 and 5 occupy the same station; the comparisons between them are only meaningful if the method was identical. A diver who improves the procedure between occupations has destroyed the experiment rather than improved it.

Dive 1 establishes the ability to perform this procedure. Dive 3 uses the array as a stationary reference and follows a different procedure of its own.

The Dry Session — Before the Dive

Repeat in full before every dive on which data are collected.

1. Synchronize the clocks on all loggers and all dive computers to a single reference. Record the reference and the time.
2. Configure the loggers: sampling interval, start time, and logger identity recorded against serial number.
3. Fill the calibration container with seawater drawn from the site and place it in shade. An insulated cooler is preferred where the deck is hot.
4. Place all loggers and all dive computers in the container. Leave them until readings stabilise — approximately twenty minutes — and leave them there until entry.
5. Read the whole group three times, five minutes apart, recording every instrument at each reading.
6. Snapshot 1 gives the offsets; each instrument's departure from the group mean at that reading.
7. Snapshots 2 and 3 give u(T), the uncertainty of one reading. Apply snapshot 1's offsets to them; the spread of the residuals is u(T), the standard uncertainty of one corrected reading.
8. If the group mean moved between snapshots by more than that u(T), the bath is still warming. Wait and repeat the set.
9. Light intercomparison: loggers side by side on deck, sensors up and level, open sky, at least three sampling intervals; record the window's start and stop times. Gain offsets and u(lnE) are computed topside from the simultaneous samples.
10. Inspect the black discs for damage, fouling and gloss, viewing them at several angles. Check the hand loop and its attachment.
11. Record the conditions: sea state, swell direction and height, cloud cover, wind, tide state and direction, and time of local sunrise. These cannot be reconstructed afterward.

Briefing

Confirm the station position and its relationship to the Reef Cartographer survey of the site. Assign logger depths — one per 3 m / 10 ft of water column, minimum three — and the diver responsible for each. Assign the disc bearings and the background to be sighted against, and state the maximum separation for the conditions. Confirm that no reading will be used unless its logger has been at station depth for at least thirty minutes.

On Dives 4 and 5, confirm that every one of those assignments is identical to Dive 2. Read them from the Dive 2 record rather than from memory.

Performance Requirements

By the end of each station occupation dive, student divers should be able to:

1. Occupy a station: deploy the logger array on the mooring line at the assigned increments, recording each logger's actual depth from a dive computer at the moment of attachment, never from line length.
2. Verify that every logger has clear sky above it and that each light sensor is oriented upward and cannot rotate.
3. Remain on station for the full dwell time, holding assigned depth within ±0.3 m / ±1 ft where conditions permit.
4. Determine horizontal black disc extinction distance as a two-Team measurement, recording the clock time of each Team's extinction event, the kick-cycle counts at those moments, the bearings, the background sighted against, and the depth at which the measurement was made.
5. Record temperature and depth from their own dive computer at stated intervals through the dive.
6. Determine surge period by timing ten complete oscillations.
7. Recover all deployed instruments, leaving nothing in the water.
8. Complete the dive without contact with the substrate.

The Dry Session — After the Dive

1. Repeat the bath and take one reading per instrument. Compare each against its pre-dive offset; the difference is that instrument's drift. This is a drift check, not a second uncertainty estimate — the day's u(T) was established before the dive.
2. Offload the loggers and apply the pre-dive offsets to the station readings. The day's single-reading uncertainty u(T) was established from the pre-dive snapshots; this step applies the offsets and does not re-estimate it. The light gain offsets from the intercomparison are applied to the illuminance readings in the same step.
3. Download the dive computer logs and compute the achieved depth standard deviation for each station.
4. Plot temperature against depth, and the light readings against depth as the Technical Supplement describes.
5. Compute the attenuation coefficient from the gain-corrected shallowest and deepest loggers. Use the intermediate loggers to test whether the column is optically uniform, and record the result of that test.
6. Compute the beam attenuation coefficient from the two Teams' extinction estimates, using each diver's own kick-cycle value, and record their difference as the between-observer spread; half of it is the observer term in u(y).
7. Compute the optical product and compare it against the expected value.
8. State the appropriate uncertainty for every reported value and the applicable threshold for every comparison, and state whether a station mean is defensible for this column.