The calibration constant for this one is 67. The lux reading divided by 67 equals the PAR. I took great care keeping the two sensors the same distance from the light. Did two readins at 26 and 45 micromols of PAR, and got 1750 and 3030 lux. I haven't decided which sensor to use for my PAR meter - most likely this one. When I measured the PAR with this at the substrate level in my tank, I got 22 micromols of PAR, in between the 20-25 readings I got with the PAR meter and my other sensor.
Now if anyone wants to make their own PAR meter, this version is as easy as possible to make, and costs:
$15 for the lux meter
$8.50 for the Weld On #16 cement
$2 for the electric cable
$1 for the little acrylic box
$26.50 plus tax and any shipping costs
And, it took me about 2 hours to make the sensor, at most.
Last edited by Hoppy; 06-30-2012 at 11:02 PM.