Hi, i had the idea to buy a cheap LUX meter to calculate the aproximate DLI and PAR. I see people are doing that. Some guys alrady calibrated some meters from The apogee PRO devices. ChatGPT says he can calculate the data for me, it just needs the LEDs SPD data and i have it. My LED is Mars Hydro SP250W, perfect for 2x4space. The PAR meters are sooo expensive, i just cant afford them, so i thought this would be a solution, better that nothing at all.. Im wondering what would be the calculation error from LUX to PAR even if the Ai calculates it from the SPD data.. i thought if the error would be some 5-10% it not so important i could just adjust the height with lets say 10% reserve.. What do you think about this?
When I started growing in 2021, I tried Photone, which was called "Korona" at the time, and it could not provide a result even though I was using a diffuser and a blurple setting. I contacted the programmer (I've been a software engineer for 30+ years) and, after a few emails, decided it was best to go with an Apogee.
At the time, I wasn't convinced that a lux meter was a valid substitute for a PAR meter. I was wrong.
There are some situations where the accuracy of a PAR meter can be of value. If you've got the $$ and/or are slightly obsessive about getting the most out of your plants, I would say, yes, get a PAR meter.
On the other hand, using a lux meter + the conversion values in the document I've written, is in practical terms, as (in)accurate as using a PAR meter.
All instruments are "wrong". My Apogee is calibrated to 5%±. When I take a series of light readings on my grow, the results will never be the same because I cannot put the sensor in the same exact location as I did in my previous set of readings. Of course, if I'm taking one reading on the top of the reservoir, the location will be the same so the reading will be the same.
That's not a valid argument because it has no bearing on the real world.
If I lower the sensor of the meter, the PPFD changes by about 50µmol. That's > the inaccuracy of the meter
If I move the sensor from one side of a small bud to the other, the PPFD may change by 50 or 100 or 0µmol.
The best that I can hope for with my Apogee, is to get an indication of what the light levels are but it's wrong to think that those numbers are "correct". Instead,
they're a guide as to what to do to get the results I want from the plant.
My take - get an Apogee if you've got the resources. Second, in the attached paper I used a Uni-T lux meter. The sources for the conversion factors are as cited. The 0.015 factor has no cite because it's the factor that's was floating around when I first looked into this thee years ago.
The newer model of the lux meter has Bluetooth connectivity and I was hopeful that the meter would allow me to push a button on the meter and have it record the reading. Unfortunately, that's not the case. Instead, you can set a sample frequency and the data will be captured on the phone. I set mine to take a sample very two seconds and that worked out OK. I understand why Uni did it that way (it's cheaper than modifying the meter to allow it to capture by pushing) but, for me it was a disappointment.
Instead of that, I create a note in my iPhone, put the phone next to me, and put it into dictation mode and the read out the values.
Interestingly, Bugbee has been asked about using a lux meter and he pooh pooh's it. It seems that the left hand and the right hand aren't talking at Apogee because if you go to the iOS App Store, you can download "Clear Sky Calculator" which is used to take readings from the sun and generate a PPFD value so that you can check it against your PAR meter. That's exactly the same technique that the lux meter + conversion factor does except it is less accurate because of the inherent problems I using a tiny phone sensor to try to capture light that's not coming directly into the sensor (that's why a lux meter has a conical diffuser and you've got to kludge it on the I phone with a strip of paper).
