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Reef PAR / PPFD Underwater Correction Calculator

Quantum (PAR) sensors are calibrated by the manufacturer in air. The cosine head — the white diffuser that gives the sensor its 180° field of view — was designed for the refractive index of air (n ≈ 1.0). When you submerge it, water (n ≈ 1.33) against the diffuser lets more of the light that entered it scatter back out before reaching the detector, so the sensor reads low. Apogee calls this the immersion effect. For its full-spectrum SQ-500-series head (the MQ-500 meter) the correction is × 1.32 — a ~24 % under-reading; for the older SQ-110/120 head (MQ-200) it is × 1.08. The MQ-510 is Apogee's underwater meter: the correction is applied in firmware, so its display is already the underwater value.

reefcalcs editorialUpdated Reviewed for accuracy 4 sources cited(calculator)

PAR / PPFD underwater

Corrected PAR
238μmol/m²/s
Correction factor
× 1.32

Apogee immersion-effect factor for this in-air sensor used underwater.

Target band for this coral
200–350μmol/m²/s
GOOD

Apogee sensors calibrated in air under-read PAR underwater (the immersion effect). Apogee's published factors: × 1.32 for the MQ-500 (SQ-500-series head), × 1.08 for the MQ-200 (SQ-110/120 head). The MQ-510 applies its correction in firmware, so read it straight. Source: Apogee Instruments, "Underwater PAR Measurements."

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Apogee MQ-510 Underwater PAR Meter

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How this is calculated

Correction factor by sensor:
  MQ-510 (correction in firmware)    × 1.00
  MQ-500 (SQ-500 head, in-air cal.)  × 1.32
  MQ-200 (SQ-110 head, in-air cal.)  × 1.08
  Seneye PAR                          × 1.00 (already corrected)

Corrected PAR = raw_reading × factor

Coral target bands (μmol/m²/s):
  Softies (zoas, mushrooms)     50  – 100
  LPS (hammer, frogspawn)       100 – 200
  SPS — low light               150 – 250
  SPS — mid light               200 – 350
  SPS — high light              300 – 500
  Clams (Tridacna)              250 – 450

The 1.32 and 1.08 immersion factors are from Apogee Instruments' "Underwater PAR Measurements" page. Each is specific to one Apogee sensor head — Seneye uses a different optical path and reports water-corrected values natively. If your meter isn't Apogee or Seneye, check the manufacturer datasheet for an explicit underwater calibration; do not blindly apply 1.32 to a different brand, and never apply it to an MQ-510, which has already done it.

PAR target bands are from Joshi's "Coral Health" research and Reefco Aquariums' coral lighting guide. They're conservative — bleached, freshly-fragged, or recently-moved corals need 20–30 % less for acclimation. Move a coral up or down in 1″ steps over 1–2 weeks per step.

FAQ

Do I multiply an MQ-510 reading by 1.32?
No. Apogee applies the immersion-effect correction in the MQ-510's firmware, so the number on the display is already the underwater PAR. The 1.32 multiplier is for an in-air-calibrated MQ-500 used underwater; multiplying an MQ-510 reading by it overstates PAR by about a third.
Should I measure with all lights on or just one channel?
All channels on, full daytime intensity, at the coral's height. PAR is additive across spectrum — measuring blues-only and adding 30 % is wrong. Measure at the schedule you actually run.
How much does sensor angle matter?
A lot. Held perfectly horizontal under direct overhead light, you get peak PAR. Tilted 30° you can read 20 % low. Use a sensor holder or hold the wand level visually.
Is PUR more useful than PAR?
PUR (photosynthetically usable radiation) weights the 400–700 nm band by zooxanthellae absorption — peaks ~430 nm blue, troughs ~550 nm green. It's more biologically accurate, but no reefer-priced sensor measures it directly. PAR is what we have.

Where to buy

Affiliate links

Most current reef LED fixtures hit the PAR bands in this calculator at typical hanging distances. Verify with a meter at your specific install.

Kessil A360X Tuna BluePendant-style. Best 'shimmer' in the hobby. 24×24 in coverage.View on Amazon →EcoTech Radion XR15 Pro (G6)Best programmable channel control. 24×24 in SPS-grade.View on Amazon →AI Hydra 32HDStandard SPS-grade fixture. 24×24 in footprint at SPS PAR.View on Amazon →

Affiliate links. As an Amazon Associate, reefcalcs earns from qualifying purchases.

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  src="https://reefcalcs.com/embed/par-underwater/"
  width="100%"
  height="720"
  style="border: 2px solid #111412; max-width: 720px;"
  loading="lazy"
  title="reefcalcs Reef PAR / PPFD Underwater Correction Calculator"
></iframe>

Sources & references

  1. 1.
    Apogee Instruments — Underwater PAR Measurements (immersion-effect factors by sensor)
    https://www.apogeeinstruments.com/underwater-par-measurements/
  2. 2.
    Apogee Instruments — MQ-510 Full-Spectrum Underwater Quantum Meter (immersion correction in firmware)
    https://www.apogeeinstruments.com/mq-510-full-spectrum-underwater-quantum-meter/
  3. 3.
    Sanjay Joshi — PAR data from popular reef fixtures
    https://reefs.com/magazine/practical-aspects-of-photosynthetically-active-radiation-par-measurements-using-a-par-meter/
  4. 4.
    Reefco Aquariums — PAR levels for corals
    https://reefcoaquariums.com/blogs/news/lighting-coral-needs

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