A couple weeks ago we reported that Apogee had released a roughly estimated correction factor for measuring light underwater. Due to the the interaction of water and light and the immersion effect of the opaque sensor cover, it turns out that their new meter works perfectly for measuring PAR in air, but needs some slight adjustment when submerged in water.
The correction ends up being about a 7% increase for the old Apogee MQ-200 PAR Meter, and a 32% bump for the new MQ-500 PAR Meter. However if you’re interested in getting an even more accurate reading of Photosynthetically Active Radiation in your aquarium, Apogee has officially released an underwater PAR correction calculator.
As it turns out, characterizing the behavior of light through air, reflection from the surface of water and then through water is a little bit more involved than a simple percentage bump. The apogee underwater PAR Measurement calculator accounts for most of these variables and asks users to describe the distance of the light source from the sensor, and how much of that distance is through air and how much is through water.
The calculator itself is a simple spreadsheet which can be downloaded and saved for future use to correct the values displayed by Apogee’s PAR Meters. For a rough estimate of your PAR lighting intensity you can still use the simple percentage multiplier and get pretty close, but it’s nice to know that more accurate values are available to those who are willing to crunch just a few numbers. [Apogee]