The Planted Tank Forum - View Single Post - Assembling a DIY PAR Meter
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post #3 of (permalink) Old 09-26-2013, 12:48 AM Thread Starter
Planted Tank Guru
Join Date: Dec 2005
Location: Sacramento, CA
Posts: 21,015
I would trust my PAR meters to be accurate to +/-10%, but not much better, while the Apogee Quantum PAR meter is claimed to be accurate to +/-4%. But, it is very hard to measure PAR for an aquarium and do it well enough to trust that you are within +/-10%, no matter what PAR meter you use. Small errors in how you make the measurements make a noticeable difference in PAR readings. Those can be errors caused by light reflecting off your arm, by light reflecting off a close white wall, by small errors in how you point the PAR sensor, by errors in placement of the sensor, and by the fact that some types of lighting take up to 15 minutes to stabilize. (My PC light starts at about 50% of its maximum PAR, builds up to maximum reading in about 5 minutes, the drops back a bit over the next 5-10 minutes. If I take a reading right after turning the light on, then take other readings as I adjust things like height of the light, then go back to the first reading, I will get different readings, and by more than 10% different too.)

I think we get 90% of the benefit of a PAR meter with only +/-10% accuracy compared to 100% of the benefit, with +/-4%. And we pay a lot to get that added accuracy.

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