Documentation

PAR Measurement Methodology

How PAR is measured on this tank, and what the numbers do and do not mean.

Published

Purpose

To make every PAR figure published on this site checkable. If the method is stated plainly, a reader can judge the numbers rather than take them on trust, and can repeat the survey on their own tank.

Background

Published PAR figures describe a fixture, usually on a grid at a stated height over open water. That is a fair way to characterize a light and a poor way to answer what a specific plant is receiving, because it does not know where the plant is or how deep its canopy sits. On a 90 cm tank lit by one pendant the distribution across the layout is the interesting part. Measuring underwater matters too: refraction and absorption mean an above-water reading is not the light the plant sees.

Methodology

  1. Measure with an Apogee MQ-510 underwater quantum sensor, reading PPFD in µmol/m²/s.
  2. Take readings at each numbered plant position, nineteen of the original twenty currently live, so surveys can be compared to each other.
  3. Hold the sensor level at the plant or substrate location, since small tilts change the reading materially.
  4. Measure mid-photoperiod with the aquarium light on and the ADA Shade fitted, since the shade is part of the fixture as run and every published reading includes it.
  5. Minimize room lighting and take readings only when the water is clear.
  6. Record depth and fixture height alongside every reading, since neither is implied by the number itself. The August 8th survey is the first to carry both on every reading, and is the one the published figures are drawn from. The two earlier runs predate the requirement in different ways: the 1st of August covered seventeen positions and carried depth and fixture height on all of them, while the 2nd covered all twenty and carried neither. Neither was both complete and reproducible, which is why the survey was re-run rather than corrected.

Observations

  • Measured PPFD varies widely across the layout rather than tracking a single headline figure.
  • The center of the tank reads substantially higher than the outer positions under one pendant fixture.
  • Moving the fixture a small distance front-to-back changed the measured distribution noticeably.
  • Raising the fixture appeared to reduce measured light at the ends rather than spreading it more evenly. The comparison readings were not preserved as a standalone survey, so this is recorded as an observation rather than a result.

Scope

  • Instrument, sensor type and why underwater measurement is used
  • The numbered plant positions and how they are located
  • Conditions held constant between surveys
  • Why a working range is not a biological threshold

Related equipment

ADA Solar RGBApogee MQ-510

Related experiments

PAR mapping across the planted layoutThe first PPFD survey of the planted layout, since superseded by the 8 August re-run.CompletedSolar RGB mounting height testRaising the fixture was tried, PAR at the ends decreased, and the fixture was returned to its original height. It stays open because the before-and-after readings were never kept as a dataset, so the result is a recollection rather than a record.In progressCanopy versus substrate PARMeasuring the same positions at substrate and at canopy height as stems fill in.Planned

References

Measured PAR distribution · Apogee MQ-510 · Experiment 001 · PAR mapping · PAR Mapping Across a 90P: full method and raw data · Experimental Methodology · Apogee Instruments: MQ-510 specifications, immersion correction and calibration uncertainty