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

A single PAR reading taken under the middle of a fixture is the number most often quoted, and it describes almost nothing about what a plant at the end of the tank receives. On a 90 cm tank lit by one pendant, the distribution 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 twenty repeatable positions corresponding to plant groups, 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 at full output 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. This is a standing requirement for new surveys. The original twenty-position survey predates it: fixture height is known for that survey as a whole, but complete per-position depth was not preserved.

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 twenty repeatable 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 layoutTwenty underwater PPFD measurements mapped against the planted layout.CompletedSolar RGB mounting height testRaising the fixture was tried, PAR at the ends decreased, and the fixture was returned to its original height.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