I've been reading all the uv threads this week and still undecided how to proceed. There is a thread where a guy burned up his whole crop from switching to a Pure UV and ran it full time (lemme see if I can find that thread again and I will link to it). I'd start out with like 5 minutes per hour and work your way up from there. That Pure UV is serious business.
@Randomblame has posted a ton of useful information in most of the lighting threads I have read. Reading about the uW/cm2 makes my eyes glaze over and I start looking for someone with a setup like mine to see what worked for him/her.
So I have 23226 cm2 x 250 uW = 5.8 watts. What the heck does that tell me?
EDIT: found that thread on Pure UV burning plants
https://www.rollitup.org/t/uv-burn.948061/
I can only say from my experience last run with a 39w/3ft Arcadia D3 Desert bulb(12%/30% UVB/A, 7000°K/CRI 90) and a 4x 2' area.
Last veg week I started with 2 + 2h (2h in the morning, 2h in the evening) and extended weekly at 2h, so that it ran from the 8th week for the full 12h's and went on and off together with the main light.
The distance was about 30cm/12", which according to Arcadia is about 150-180μW/cm²(with alu reflector).
At no time the girls showed signs of stress but they grew a little bit more compact due to the increased blue content.
If I could, I would have gone to 250-300μW/cm², but then the area the bulb covered would have been too small.
The PureUV is a lot more powerful and compared to the Arcardia you will get 440μW's at 12"/30cm. At 24"/60cm it's 170μW which is about the same a reptile bulb delivers at 12".
According to this numbers it need's a height of around 16-18" to get the desired 250-300μW/cm².
Benefit, the pureUV bulb covers more than twice as much area as a reptile bulb when set to the same intensity.
Below is a chart showing the UVB content on a mid summer day in the northern hemisphere and near the equator. As you can see the intensity can be up to 350-450μW for a few hours. 250-300μW indoors should be save. I will again start with only a few hours(2+2h) and increase on a weekly base until I see first signs of stress.
I'll split it to 2h in the morning and 2h in the evening because the plants have time to recover between the on period. It is important to watch the plants very well! Burned tips and leaf margins are first signs that it is aready too much.
With a distance between 16 and 24 " one bulb covers a 30 - 40" wide area. You could also hang them vertically, but with less distance. The 440/170μW were measured with aluminum reflector.
I do not know in which distance to the other the two PureUV ran in the thread linked by you. If they were closer than 1m away from each other, there are already some overlaps. At only 50cm to each other even a hotspot.
Slowly approach to the maximum and watch the upper leafes well, then it should be easy to find the sweet spot.
There is also an other aspect. Such a pureUV bulb still emits more usable UVB light than a reptile bulb even after 1000h of usage.
