Question: does the water vary sideways, or only with depth?
The station occupation in Dive 2 measured how the water changes with depth. This dive asks whether depth is the only thing it changes with.
This is a deployment, not an occupation. The array returns to the same station at the same depths, but its readings serve as a reference rather than a profile, and they do not enter the Dive 2 / 4 / 5 comparison. The array is deployed at the station and remains stationary throughout the dive, recording the water at a fixed point so that any reading taken elsewhere can be compared against it at matched depth and matched time. Nothing is left in the water between dives — the array is recovered with everything else and redeployed at the same depths.
Briefing
Explain what the array is doing on this dive. It is not producing a profile for its own sake — it is the control. Because it records continuously at fixed depths, a diver's reading taken away from the line can be compared against what the line saw at the same depth at the same moment. Depth is therefore controlled, and so is time. Without both, a difference measured forty minutes into a survey could simply be the morning warming up.
Name the positions to be surveyed and why each was chosen. Suitable contrasts on most reefs include a surge channel against open water at the same depth, the sheltered side of a large structure against the exposed side, beneath an overhang against two metres clear of it, and inside a lagoon or back-reef pocket against the fore-reef.
State the working depth. All survey observations are made at a common nominal depth so that they can be compared with each other and with the line at that depth. Assign that depth to coincide with a logger depth wherever the site allows it.
Remind student divers of the mechanism from Topic 1: a colony's environment is the water immediately around it, and that water is renewed both by ambient flow and by the colony's own ciliary pumping — an interaction the field has not yet settled. Horizontal differences at reef scale are exactly what no satellite and no moored instrument can resolve, and they are the reason this dive exists. Be clear with the team that they are measuring an input to an open question, not confirming a known relationship.
Dry Session 3 — Before the Dive
Repeat the full dry session sequence. Clock synchronization matters more on this dive than on any other, because every survey observation will be matched against the stationary array by time. An offset of a minute puts the two observations in different water.
Record conditions as before.
Performance Requirements
By the end of this open water dive, student divers should be able to:
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Deploy the logger array as a stationary reference and confirm it is recording before leaving the line.
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Record temperature, depth and clock time at as many distinct positions at a common nominal depth as the dwell arithmetic in requirement 3 permits, including at least one sheltered and one exposed position. Four is the target where the instruments allow it; fewer is correct where they do not, and the number achieved is recorded with the reason.
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Remain at each position for at least three time constants of their own dive computer's temperature sensor, and for the same dwell at every position, and state why an immediate reading on arrival would describe the previous position as much as the current one.
Measure the time constant; do not look it up. Settle the computer in the cooler bath, note the reading, move it to a bath at least 5 °C warmer, and time how long it takes to cover 63 percent of the step. That is τ for that instrument, measured on that instrument, which is the same discipline the course applies to everything else. Published values for dive computers range from about 17 seconds to over 340 seconds depending on sensor placement and housing material, so an assumed figure is worthless.
Do not rely on air-to-water for the step. On a calm morning the deck air and the surface water can be within a degree, and a one-degree step gives no usable threshold. Arrange the step deliberately and record its size.
Three time constants brings the sensor to 95 percent of the step. Five would reach 99 percent, and the extra four percent is far below what the array can resolve — it is not worth 2τ of bottom time per position.
The equal-dwell requirement is the one that matters most. This dive compares positions against each other. If every position gets the same dwell, whatever lag remains is common to all of them and largely cancels in the comparison — exactly as a common offset cancels between two readings from one logger. A dwell that varies between positions puts a varying error into the very differences being measured.
Plan the number of positions from τ, dry, before the dive. The arithmetic is n × 3τ, plus transit between positions, plus deployment and recovery — and the result must fit the gas plan and the no-decompression limit with margin. For a fast computer, τ = 20 s gives a one-minute dwell and six positions are comfortable. For a slow one, τ = 340 s gives a seventeen-minute dwell and one position is all that fits. Reduce the number of positions, never the dwell.
Where fewer than four positions can be surveyed, record that the target was not met and why. A survey of two positions properly settled is a result; a survey of six positions smeared by an unsettled sensor is not.
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Describe each surveyed position sufficiently for another diver to return to it, referenced to the Reef Cartographer survey of the site.
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Hold the common working depth within ±0.3 m / ±1 ft at each observation, where conditions permit.
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Time-match each observation to the stationary reference logger at the same depth, and record the reference sample's actual clock time alongside the observation, so that the gap between the two is visible in the report rather than buried.
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Recover all deployed instruments, leaving nothing in the water.
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Complete the survey without contact with the substrate.
Dry Session 3 — After the Dive
- Repeat the cross-calibration bath; record offsets and drift.
- Offload the array. For each survey observation, extract the reference logger's reading at the matched depth and matched clock time.
- Compute the difference between each survey observation and its matched reference.
- Identify which differences exceed the applicable survey threshold C(survey) = 2√(u²obs + u²ref), computed from the uncertainties of the travelling observation and its matched reference reading and which do not.
- Record the survey as a table of position, depth, time, observed value, reference value, difference, and whether the difference is resolved.
Writing the Prediction
This is done after the Dive 3 analysis and before Dive 4. In writing, dated, and signed by the team.
Two predictions are required, because Day 2 makes two comparisons.
First, the repeatability prediction. Dive 4 occupies the same station as Dive 2, at the same hour, one day later. State what you expect the difference to be, and why. Most teams should predict no resolvable difference — and should say what magnitude of difference would surprise them.
Second, the change prediction. Dive 5 occupies the same station hours after Dive 4. State what the re-occupation will show, at which logger, in which direction, and by what mechanism. State what result would show the prediction to be wrong, and what other mechanism could produce the same result.
A prediction that no observation could contradict is not a prediction, and the instructor should return it.
This is not a requirement that either prediction be wrong, or likely to be. A prediction that is properly falsifiable and then holds is the best outcome there is — it means something precisely because it could have gone the other way. And one that fails is informative too: it says the named mechanism was not the one operating, or that something suppressed it, or that the array could not resolve the change. Day 2 asks the team to work out which.
The predictions are recorded before Dive 4 takes place. They are not revised afterward under any circumstances. If an outcome contradicts one, the contradiction is the result.