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← All lessons Lesson 12 of 18 · 6 min

Dive 1 — Instrument and Observer Characterization

Question: how good are my instruments, including me?

No station-occupation data are collected for the Station Report on this dive. The kick-cycle calibration and the time constant are the exceptions: both are established here and both are used later, the first in the optical uncertainty and the second in the Dive 3 dwell. Its purpose is to establish that the student diver can operate the measurement system and knows what it can and cannot resolve. The performance requirements are procedural competence, not results.

Briefing

Cover the day's plan, the site, and the entry and exit. Then cover, in this order: clock synchronization across every instrument; the cross-calibration bath and what it is for; the logger array and how it is attached; the depth-holding standard and how it will be verified; the kick-cycle calibration run; the black disc procedure including the agreed maximum separation and the rule that a target still visible at that distance is recorded as "greater than"; and the recovery of every piece of equipment deployed.

Emphasise that no station-occupation result from today will appear in a Station Report — only the kick-cycle values and the time constant carry forward. Student divers who understand this dive as practice tend to rush it. Those who understand it as measuring the instruments — themselves included — tend to do it properly.

Dry Session 1 — Before the Dive

Conducted as a dry session and repeated before every subsequent data dive.

  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. Place it in shade. An insulated cooler holds temperature far better than a bare bucket and is worth using 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 rather than laying them out on a hot deck.
  5. Read the whole group three times, five minutes apart, recording every instrument at each reading. Three synchronized snapshots, not one.
  6. Snapshot 1 gives the offsets. Each instrument's offset is its departure from the group mean at that first reading.
  7. Snapshots 2 and 3 give u(T), the uncertainty of one reading. Apply the offsets from snapshot 1 to the readings from snapshots 2 and 3. The spread of what remains is u(T) for the array. It cannot be computed from snapshot 1, which by construction leaves nothing — Topic 3 note (f) explains why.
  8. Check the bath while you are there. If the group mean moved between snapshots by more than u(T) itself, the bath is still warming and the whole set must be repeated once it settles.
  9. Light intercomparison. Lay all loggers side by side on deck, sensors up and level, under open sky clear of any shadow, and leave them for at least three sampling intervals. Record the start and stop times of the window. Nothing is read in the field — the loggers are already logging, and the gain offsets and u(lnE), the light channel's single-reading uncertainty, are computed topside from the simultaneous samples, exactly as the bath produces the offsets and u(T). The Technical Supplement, section A.8, gives the arithmetic.
  10. Inspect the black discs for damage, fouling and gloss. View them at several angles. Any surface that returns a highlight is no longer a black target. Check the hand loop and its attachment.

Performance Requirements

By the end of this open water dive, student divers should be able to:

  1. Attach loggers to a mooring line at nominal 3 m / 10 ft increments and record each logger's actual depth from a dive computer at the moment of attachment, without reference to line length.

  2. Hold an assigned depth within ±0.3 m / ±1 ft for a continuous three-minute interval, where conditions permit.

  3. Determine their personal kick-cycle distance by swimming a measured course at a steady pace and counting cycles, and express the result in metres per kick cycle.

  4. Measure the time constant of their own dive computer's temperature sensor. Let it settle in the cooler bath — normally the shaded site-water bath it is already in — note the reading, move it to the warmer bath, and time how long it takes to cover 63 percent of the difference between the two.

    The step must be at least 5 °C, and the size of the step is recorded. Air-to-water is the obvious step and often the wrong one: on a calm morning the deck air and the surface water can be within a degree of each other, and a one-degree step measured by an instrument that resolves 0.1 °C gives a 63 percent threshold with no precision at all. Make the step deliberately — a second container left in the sun, or warm fresh water — rather than hoping the weather provides one. Where a 5 °C step cannot be arranged, record the value obtained and the step it came from, and treat the resulting τ as a lower bound: a shorter dwell than the true requirement is the failure mode, so round the dwell up.

  5. Demonstrate the two-Team black disc procedure, including reciprocal bearings, the vanish-and-reappear bracket, the agreed maximum separation, and the recording of a censored result.

  6. Record an observation independently of their buddy and compare the two records.

  7. Describe the procedure by which u(T) is established, and explain why a measurement without one cannot support a claim.

  8. Recover all deployed equipment, leaving nothing in the water.

  9. Complete the dive without contact with the substrate.

Dry Session 1 — After the Dive

  1. Repeat the cross-calibration bath. Record the offsets again. The difference between the before and after sets is the drift over the dive.
  2. Download each student diver's dive computer log. Compute the standard deviation of depth over the three-minute station interval. Compare it against the ±1 ft standard and against the student's own impression of how steadily they held.
  3. Compute each student diver's kick-cycle distance and record it. This value is used for the remainder of the course and is recorded in every Station Report that depends on it.
  4. Compare the paired independent observations from requirement 5 and compute the spread.
  5. Compute each light channel's gain offset and the array's u(lnE) from the intercomparison window, exactly as the bath yields the offsets and u(T).

Practice slate

This is the real recording grid you copy onto a slate before the dive — Slate B — Characterization (Dive 1). Fill it in here to get the feel of it. Nothing is saved, nothing is submitted, and there is no right answer to find; it is a surface to practise on. Print it blank if you want a slate to copy from.

If a field cannot be filled, write a dash rather than leaving it blank, so the reader knows it was missed rather than overlooked. Every blank is a question you will not be able to answer later.

Quick check

Three questions, drawn at random from this lesson's set, one attempt each. The right answer is shown either way. Two of three opens the next lesson; fewer and you get another three. Nothing is recorded anywhere but this browser, and this is not the Knowledge Review — it is a way of showing the lesson was read.

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