The Rig
What you carry, and why
Three sensed quantities and one purchase. Temperature and light come from a small array of loggers hung on a mooring line; depth and temperature also come from the dive computer already on your wrist; horizontal clarity comes from a disc cut out of plastic sheet. Nothing is constructed beyond the disc, and nothing is left in the water.
The course names no product. Instruments change and prices change, and a guide that specifies a model becomes wrong the year that model is discontinued. What the guide states instead is what a logger must do — and any instrument that does it is acceptable.
The expensive part of this rig is not the instruments. It is the discipline of using them the same way every time: one bath before every dive that collects data, one clock for every device, the same logger depths at every occupation of a station. A station occupied three times with three different methods has measured nothing.
The kit
What goes in the water
Temperature and light loggers
One per 3 m of water column, minimum three, hung on the mooring line at nominal 3 m increments. Both channels in one unit and on one timestamp — two separate instruments cannot be relied upon to agree about when a reading was taken.
A black disc
Cut from plastic sheet. Two Teams swim apart until each Team's disc is indistinguishable from its background, and the separation at that moment is the extinction distance. The only thing in the course anyone has to make.
The cross-calibration bath
A container holding every logger and every dive computer at once. An insulated cooler rather than a bare bucket: it holds a steady temperature while the instruments equilibrate, which a bucket on a sunlit deck does not.
Attachments
Reusable bungee loops or marine clips rather than cable ties, which are consumable, make waste, and must be cut off at every station. Whatever is used must hold each logger in a fixed orientation with its light sensor facing up and unable to rotate.
Your dive computer
An instrument in this course, not just a safety device. It must record a depth and temperature profile and let you review or download the log afterwards — the depth-holding standard deviation comes out of that log.
Compass, slate, a measured course
A bearing for the disc swim, a slate drawn before entry, and a course of known length ashore or on the bottom for kick-cycle calibration — which is how distance is measured underwater here. Established before the course, not on the day.
A spare logger is recommended. A flooded or failed one otherwise ends the day's data collection, and the sequence of dives cannot simply be resumed a week later — a longer gap measures the season rather than the day.
Choosing a logger
The two that disqualify
Depth rating
At least 20 m, which is deeper than the course works with margin. “Waterproof” and an IP rating are not depth ratings — most consumer loggers sold as IP67 will flood at station depth.
A photopic light channel
Required, and not negotiable. The optical product pairs an attenuation coefficient with a distance judged by a diver's eye, and the pairing only means something because both are weighted roughly the same way. A team without a photopic channel cannot compute the course's central optical result.
Inexpensive temperature loggers are common. Inexpensive temperature-and-light loggers rated for depth are not — and a temperature-only array removes the optical half of the course.
And what matters less than you would think
Absolute accuracy. The course measures its own uncertainty and reports differences, so an instrument that reads two degrees high is usable as long as it does so consistently. Cross-calibration removes a fixed offset; nothing removes scatter.
Resolution of 0.1 °C or finer matters, though, and so does repeatability better than 0.1 °C. An instrument that steps in half-degrees cannot resolve most of what this course looks for; one that wanders by a quarter degree is not usable at all.
A published response time — any value. The number matters less than knowing it, because the staged descent and the survey dwell both depend on how long a sensor takes to settle. An instrument with no published time constant makes those procedures unteachable.
Brand, communication method and price are left open, as is absolute calibration. The logging interval must reach one minute or shorter, and 30 seconds for the horizontal survey.
Labeling
A coefficient without a weighting describes nothing
This course writes Kd(photopic) throughout, because a lux-weighted photodiode is what the commonly available loggers carry. A sensor with a different spectral response is acceptable — but the coefficient must then be labeled for that response: Kd(PAR), Kd(broadband), or whatever applies.
What is never acceptable is an unqualified Kd, or a photopic label on a sensor that is not photopic. Attenuation depends on wavelength, so a coefficient without a stated weighting describes nothing in particular — and a product built from two differently weighted quantities is not the quantity the course's expected range applies to.