Is the lighting in PFC version 1.0 insufficient?


#1

Hi All,

I have built a custom PFC based loosely on the 1.0 design. I’ve been having trouble getting robust plant growth and I am trying to get to the bottom of it. My PFC uses the same lighting as the original PFC 1.0 design and I notice that PFC 2.0 uses different lighting. Was the version 1 lighting insufficient? Or is there another reason for the new lighting design? I’d love to hear from current PFC 1.0 users on their experiences with growing.

FYI, my system is aquaponic and I am getting the major nutrients from a 90 gallon aquarium. So nutrient issues are a possibility as well. I supplement with micronutrients, but have not been adding any N or P to the system.

Heat is also an issue and I am not surprised to see a chiller in the PFC 2.0 design. I have an exhaust fan that runs when the temp hits 30C and turns off at 28C. That’s about as cool as I can get without stripping all the humidity out of the box and running the exhaust fan constantly. I know this is a little warm for some greens.

Thanks for any feedback you can offer.


Minimalist Growing
#2

I haven’t built a PFC, but here’s an old thread with some folks discussing PAR values for the PFC 1.0 LEDs:

If you want to measure the actual PAR your lights deliver to your plants, here are some links for good quantum meters:

For technical background on measuring PAR, you could check out Chapter 1 of the Plant Growth Chamber Handbook.

For technical background on how much PAR plants need, you could read about Cornell’s work on Daily Light Integrals (DLI).

[edit: I dug through more of Cornell’s publications and found this one on Greenhouse Lighting. They recommend 8-12 mol/m2/day for seedlings, 15-17 mol/m2/day for hydroponic lettuce, and 20-30 mol/m2/day for tomatoes.]


#3

I’ve been wanting to understand PAR and light levels better anyway, so I did a bit of math to follow up on Cornell’s DLI recommendations.

Unit conversions

  1. I’m assuming a quantum meter that measures PAR flux as Photosynthetic Photon Flux Density (PPFD) using units of micro-moles per square meter per second (µmol/m2/s)
  2. Cornell specifies daily light integrals of PAR in moles per square meter per day (mol/m2/d)
  3. 1 mole is 1,000,000 micro-moles
  4. There are 60*60 = 3600 seconds in an hour
  5. A 12 hour light cycle would provide 3600 * 12 = 43200 seconds of light per day
  6. A 16 hour light cycle would provide 3600 * 16 = 57600 seconds of light per day
  7. An 18 hour light cycle would provide 3600 * 18 = 64800 seconds of light per day

Lettuce: 12 hour light cycle – target DLI of 15 mol/m2/d

For a 12 hour light cycle, here’s how I calculate the target PAR measurement to satisfy Cornell’s minimum recommendation of 15 mol/m2/d for lettuce:

  1. We need 15 mol/m2 of PAR, so that’s equal to 15 * 1,000,000 = 15,000,000 µmol/m2
  2. The lights will be on for 12 hours during that day, so that’s 43200 seconds
  3. We need a PAR flux of 15,000,000 µmol/m2 / 43200 seconds which reduces to about 347 µmol/m2/s for the reading we want to see on the quantum meter

Lettuce: 16 hour light cycle – target DLI of 15 mol/m2/d

A 16 hour light cycle gives 4 more hours to hit the target DLI, so the PAR flux can be lower. The target measurement would go down to:
15,000,000 µmol/m2 / 57600 seconds = 260 µmol/m2/s on the quantum meter

But, lettuce is long day facultative. We don’t want it to bolt, so running the lights for more than 14 hours a day might not be a good idea.

Tomatoes: 18 hour light cycle – target DLI of 20 mol/m2/d

Since tomatoes are day-neutral, we can run the lights for longer. For a light cycle of 18 hours on and 6 hours off, the target PAR flux on the quantum meter would be:
20,000,000 µmol/m2 / 64800 seconds = 309 µmol/m2/s

Conclusions

In the the other thread I linked to earlier, @serein calculated a PAR of 48 µmol/m2/s. According to the Cornell Lettuce Handbook, that’s almost enough for germinating seeds, but it wouldn’t be adequate for vegetative growth. [edit: I’m not sure if that was for the PFC 1.0 LEDs though, he might have been using other lamps.]

[edit # 2: I fixed a couple math errors for the lettuce.]


#4

Thanks for the detailed response.

I had previously run across the thread on Par Values, but the discussion seemed to indicated some fudge factor was involved to account for the light source which was why I was wondering about real-world results. In addition, the lux values measured by the sensor change dramatically with distance changes so that added to the confusion.

I appreciate you breaking it down and providing additional resources. I also had no idea about the necessity of the day night cycle for certain plants (tech guy not plant guy). Thanks!

I’m going to do some more research and dig through the links you’ve referenced. I’ll also try posting in the grow category and see if anyone else wants to share results.


#5

Great. I’m curious to hear how it goes.

The fudge factor you mentioned is because lux and lumens measure a wider spectrum of light wavelengths that correspond to human vision. I think it’s possible to translate from lux or lumens to PAR if you have detailed knowledge of the light source’s output spectrum and the math chops to make sense of how much of that is PAR. An accurate conversion would involve techniques kinda like the DSP used for audio bandpass filters. I’m working to understand that better, but I’m not there yet.


#6

I purchased the same lights for a lettuce growing tent. The tent is 2’ x 4’. I have 3 lights. It can be sufficient.

What do you mean by “robust” plant growth? For the purpose of my response I am going to assume that your plants are kind of leggy and the stems are weak and not capable of supporting the full weight of the leaves.

I had a similar issue. I made two changes and I think both contribute to more “fibrous” leaves and stems.

  • Light distance from plant - I had my lights 3 feet above the plants. It took me a couple weeks of poor growth that caused me to do a google search. This distance was way too far away. The light intensity is very dependent on the distance. And then I noticed in the Food Computer V1 design that the light height can be adjusted. So I went and got some light weight chain from Home Depot. Good growth occurred for me at 6-8 inches from the plant. At this height you now have to consider size of growing surface you can support. I am using the VersaGrow 10 pot drip system and it just so happens that it does not take up the full 2’ x 4’. 3 lights would not be enough to support the full space.
  • Fan blowing on leaves - I also found this in a Google search: Plants rely on more features of growing outside than I originally thought. Wind blows across plants. The plants react by growing more robust stems. So I added a fan to grow across the leaves. I used a cheap desktop fan and it wore out after a couple months. The plants now have the same size leaves as without the fan, but now the stalks are kind of soft and they are not as straight. This leads me to believe that the wind also play a part in shaping the plant.

I have since removed one of the lights. I am going to try to start some plants for outdoors before the last frost date.

I also now have the problem that when I plant new lettuce seeds, its way too far from the light height required by the mature lettuce.

I am working toward switching to a Deep Water Culture system similar to the Food Computers. And then I plan on rotating the plants through 3 zones to better support seedlings and adults. The zones will also make it so that I don’t have to continuously adjust the light height. The end goal is to be able to continuously have a salad meal 3-4 times a week.


#7

Thanks Jeremy!

We must have run across some of the same web sources. I installed a 12 cm circulating fan in mine after reading something similar. I also installed a circulating pump in the tray after noticing that plants nearest to the water inlet were greener and maturing faster.

I’ve reduced my “daylight” hours from 18 hours to 13 hours since reading some of @wsnook’s references and I am finding my lettuce is doing much better.

I’ve also tried to reduce the lighting distance by using longer carriage bolts in the light mounting and raising the water level in the tray. Chains might be next.

Below is a current pic of my rig with my second planting (variety of seeds). The large basil on the left is the leftover from my first crop. I kept it in to keep some continuity with the nutrient removal. All the others are from seeds planted at the same time. The cilantro plants are small and spindly, but one of them (far right and nearest the aquarium inlet) has already gone to seed. There are some small pepper seedlings and parsley that are starting to grow. I think my My next “crop” will be monoculture so I can focus on what works. I really wanted fresh cilantro - still trying to figure that one out.

The biggest problem for me is still heat. I am looking for a cooling solution. That solution might be to remove the front shield, but it would require less-blinding grow lights. I currently have two layers of film tint on the acrylic window…