The Uni-T Bluetooth is the model to buy. I have no particular attachment to Uni-T - it was a popular meter when I started looking into this two years ago and, with the addition of Bluetooth, it's gone from a good solution to an excellent solution, in my opinion.
A lux meter reads light in the spectrum that humans see. PAR is a very different spectrum, however. What's the connection between lux and PPFD (PPFD is the amount of PAR expressed in µmols/sq meter/second for which I use "µmols") , you ask?
Well, you've got to do a bit of math to get the answer because even though different grow lights might put out the same amount of lux, they may not be putting out the same amount of PAR. Howzat? Check out the document that I've attached.
That document gives some idea of the differences in PAR that are generated by different lights but, in most situations, go with the 0.015 factor. What's a "factor"? Something that you multiple something else with. In this case, you multiply the reading from your lux meter by 0.015. If your canopy is getting 30000 lux, multiply that by 0.015 and that light is generating approximately 450µmols.
If you've got a light that's listed in the table, use that conversion factor if you want. The difference between 0.015 and 0.016 at 90k lux is 900µmols vs 960µmols (that's the range where you want your light levels to be, BTW). That doesn't sound like a lot but some plants will do fine at 900 but not at 960. On the other hand, there's about a 3% increase in yield according to research. But, if you can't be sure which number is accurate, how do you know what light level to use?
Get your plants to a high light level and watch your plants. If they show signs of stress (tacking/canoeing or rotating away from the light), back off by ½ of your increase, wait a few days, and try again.
That's why being perfectly accurate is not of any
practical value — how plants handle high levels of light will vary from grow to grow and even plant to plant within a grow. Even more, the amount of light falling on the top of the canopy is significantly higher than the amount of light falling on the plants that are just a few inches lower. Yup, all of the work to buy an "accurate" meter and the best it's going to do is get us near our target. I
t's up to the grower to fine tune the light settings.
How much light should you try to get for your plants? Cannabis has a light saturation point of 800-1000µmols in an ambient CO2 assuming that light is the limiting factor. The latter phrase means that if you've mucked up you're watering or your VPD is off, your plant is not healthy enough to be able to process all of those photons. On the other hand, if your grow is set up well (refer to the "9 parameters" graphic). You'll be able to get to the light saturation point and your grow will reward you with a
lot of weed.
