Hey Hoppy, how do you calibrate your PAR meter? Thanks.
I use a Quantum PAR meter, or one of my PAR meters that I have very carefully calibrated several times. I set the two sensors side by side under a 50-50, 36 watt PC AH Supply light that is rigidly mounted above a stand I made for a 10 gallon tank. If my PAR meter reading isn't within 5% of the other meter, I adjust the spacing between the photodiode and diffuser, for the version I am making now, or for the older versions I adjusted the number of white diffuser filters I used, until I get the one being calibrated to be within about 2 digits at about 40 PAR. Since there is no way to adjust the slope of the PAR vs Quantum PAR, I only calibrate at one PAR reading.