Air temperature, plant temperature...

[foolededucator]

Another thing I've been struggling with is what exactly people mean by 75°f/25°c...

Note that this is more of an itch I have to scratch, it's not an actual problem but I'm just curious =)

I have a small grow cabinet, 90cm tall, 80 cm wide, 45 deep (3 feet by 2 feet 5 inches by 1 foot 5 inches). I have a PVC tube construction that doubles as ducting and support for the lights, and a cardboard box goes over it as a cover.

Lighting is 1 progrow 260 LED (180w), 2 25w cfls. Venting sucks air (no filter) through the PVC tubing (two ducts, 4 intakes with little paper cup hoods) and into a plastic box with two computer fans mounted in parallel.

I have two thermometers, a wall mounted in/out/humidity that can also stand on a surface (little support leg) which measures air temperature obviously, and an IR thermometer, which measures the temperature of objects with significant radiating mass.

At floor level, air temperature is about 22°c in the day, 20°c at night (real day/night, not photoperiod which is noon to midnight).

At plant level (free standing thermometer on top of a pot) i get between 23°c and 26°c.

Where I initially mounted it, right next to the LED's vents, I get 33°c, sometimes more. That initially scared me into not using the CFLs.

Finally, my nerd toy IR thermometer tells me the medium is usually 19°c, and that the plants themselves are also around that range, but it varies considerably from 17°c to about 22°c.

So as I understand it, in terms of air temperature next to the plants, my venting is doing enough.

But isn't that temperature gradient a bit steep? What about the big difference between the plants and the air?

I'm really having trouble finding information about this, most resources just say "temperature" without going into more detail. I found two threads here involving IR thermometers which raised similar issues but didn't really get an answer.

Cheers

 

[SeaMaiden]

I'm trying to figure out what your question is, exactly, and I'm just not able to. So I'm just going to throw this one out--vapor pressure deficit.

Or are you asking about root zone temps vs canopy temps?

 

[headband 707]

If I understand your question right you can't keep the temp right on your grow and you want to know how? If that is your question then you might want to look into a fan that moves your air.. headband 707

 

[Mister_D]

Fooled - Generally speaking on the weed forums growers are referring to the air temperature around the tops of the plants directly under the lights. There are very few people that are using IR meters to measure actual leaf temps, though more of us probably should...

 

[foolededucator]

First off thanks everyone for responding =)

To clarify, my assumption is that air temperature can be 25° under many scenarios... It's just one metric for a pretty complex system of variables.

Based on that figure, I'm OK. At canopy level it's consistently 25-26°. I'm trying to get some perspective on the other measurements also affect things, since air temperature is a proxy the heat of the stuff surrounding the air, the thermal inputs and outputs to the system, blah blah blah...

What it really comes down to is that I don't understand what the dynamics are clearly enough, since the single point air temperature measurement is a just one summary value... I have the tools to get a higher resolution view of things, but I don't yet have the mental model with which to make sense of those additional measurements.

So yes, I'm asking about root zone temps vs. canopy temps, if the difference between the plant matter and the air that I measured is small or large, whether the air temperature gradient I have due to the design of the lamps and stratification in the chamber itself is significant, or any other information.

VPD also seems very relevant, but I'm not sure how to connect it in my head since I'm still trying to make sense of what I have in front of me... Based on what I've just read it should be a function of RH/air temp only, correct?

FWIW, here are some of the more informative older threads I found on the subject, but I was hoping for a bit more =)

https://www.icmag.com/ic/showthread.php?t=97127

https://www.icmag.com/ic/showthread.php?t=71067

https://www.icmag.com/ic/showthread.php?t=48175

Cheers

 

[SeaMaiden]

VPD is essentially the plant's circulation system. If VPD shuts down, the plant cannot move water or nutrients through its tissues effectively or efficiently. VPD is the interplay between ambient temperatures (air) and relative humidity (again, air). If you want to be really precise about VPD values and parameters, then you'll want to know leaf temperature and add that into the calculations.

The root zone is a different mess, however you've got more leeway there, too. Most important to keep in mind is that the root zone must be in the 60-65*F range (IIRC) for nutrient uptake to occur. If the root zone gets too hot, you have dead plants. I've never tested root zone temps of plants in pots killed by heat, but it's happened to me more often than I'd like to mention.

As for the one summary value, I get what you're saying, but I'm not sure what other parameters you really need to be monitoring. Air O2/CO2 content? Light intensity, PAR, PUR? (photosynthetically available radiation, photosynthetically usable radiation, respectively) I keep it more simple than that, I monitor air temps and relative humidity. I keep air circulating, so my assumption is that it's fairly well mixed in terms of both those parameters.

 

[foolededucator]

This is exactly what I was looking! I completely failed to realize how VPD was the energy source for vascular plants' circulation. Anyway, I now have a lot to go on for making sense of this stuff, and plenty of reading ahead of me, hopefully by the end of which I will be less confused.

As for what I actually need to monitor, well, i'm assuming what's good enough for everyone else is good enough for me, so technically nothing more especially at the scale I'm working on... However, I did have the instruments to take those measurements.

The reason I posted is that measurements are an easy way of getting facts (modulu instrument/methodological errors, etc), and if these facts surprise me (they did) then obviously i've stumbled onto something I don't understand. Known unknowns are easier to ask about on the internet than unknown unknowns, and this sort of roundabout approach to learning really seems to work well for me, so I thought I might try the hivemind ^_^

Thanks again

 

[SeaMaiden]

Logging is something I was trained to do when working at a large public aquarium. I am an advocate, including for purely organic grows, for logging data. Chasing numbers is a different matter entirely.

Logging data is a good method for resolving problems when, not if, they arise.

Autodidactism is also a good method for resolving problems, and learning.

I totally want one of those point & shoot thermometers.

 

[rives]

SeaMaiden, your comment on VPD and plant vascular circulation triggered both a memory and a question. I have always just assumed that most of the plant's circulatory system functioned as a result of capillary action.

I recall one time several decades ago when I was using a flood & drain system, I had just harvested the plant a couple of minutes before the pump turned on and flooded the tray. The stump, which was about 3"-4" tall, started flowing water out of the cut at a very impressive rate. At the time, I was amazed and attributed it to capillary action (waaay before being able to research anything on the internet!). I don't see how this could have been a function of VPD. What is your understanding of how this all ties together?

 

[foolededucator]

If I understand correctly water is moving through capillary action and is eventually transpired, so a positive VPD keeps this wick effect going by allowing transpiration and, capillary action drives it... I don't think transpiration can explain what was going on. Very cool either way, nature's water related machinery is much more elegant than ours.

 

[SeaMaiden]

SeaMaiden, your comment on VPD and plant vascular circulation triggered both a memory and a question. I have always just assumed that most of the plant's circulatory system functioned as a result of capillary action.

I recall one time several decades ago when I was using a flood & drain system, I had just harvested the plant a couple of minutes before the pump turned on and flooded the tray. The stump, which was about 3"-4" tall, started flowing water out of the cut at a very impressive rate. At the time, I was amazed and attributed it to capillary action (waaay before being able to research anything on the internet!). I don't see how this could have been a function of VPD. What is your understanding of how this all ties together?

I wish I could say something meaningful and enlightening on this, but I can only say I've seen something similar when we cut down an old pine tree. It leaked for months.

 

[rives]

Foolededucator pretty well covers what, at least to me, seems like the "logical" explanation - VPD drives transpiration while capillary action insures that the leaves have water available to transpire. I know that I was startled by the volume of water that was being moved by the stump - it wasn't just weeping, it looked like a water hose that had been oriented vertically and turned on. Mother nature is an amazing old gal.

 

[jammie]

Logging is something I was trained to do when working at a large public aquarium. I am an advocate, including for purely organic grows, for logging data. Chasing numbers is a different matter entirely.

Logging data is a good method for resolving problems when, not if, they arise.

Autodidactism is also a good method for resolving problems, and learning.

I totally want one of those point & shoot thermometers.

tis the seasons for sales- i've seen very good infrared thermomters for as low as $40. i got mine a couple years back at home depot. use it for everything- checking grill/oven temps, looking for air leaks around windows. ask santa, perfect stocking stuffer!

 

[foolededucator]

i got the cheapest well reviewed one from deal extreme, it $20 including shipping

 

[SeaMaiden]

It is the season for sales, and for hints from a wife, mother, daughter and sister to her family members.

Yes, I would totally geek out on something like an IR thermometer. I can see myself using it as often as I use my binoculars.

 

[trem0lo]

I use an IR thermometer for cooking. It's great for knowing how hot oil is for frying, but can also be useful to shine onto plants. Most of the time it's not much more than a few degrees off from the canopy temp.

 

[BasementGrower]

im just wondering.. whats the lowest Lights Off Air Temp that will not slow down growth or budding.. im havnig a small issue.. now that its getting down to around 0 degrees outside. the basement is getting down to 50.. so.. the lung room gets down to 60 even.. at the coldest part of the day.. it will stay around 60 F .. then wen lights are on .. it pops up to around 70. sometimes a little more up to 75..

but the thing is.. im using passive intake threw the cracks under the doors and in the walls and cielings in the lung room to get fresh air in there.. wen the lights are out.. if i stop all the exhuast fans.. and turn them off. my temps stay around 63-66 .. with lights off.. but i dont think this is a good. idea.. whats the best heater to use in a grow room that will not burn down my house if i leave and anything happens.

 

[jammie]

hey there BG- like you, i also grow do a winter grow in an unfinished furnace room (10x10x8) in my basement. i find its easier to manage cold winter temps than hot summer ones. i've found that root zone temps are more important for growth than plant/leaf temps. i do dwc and if water temps drop below 65 growth slows dramatically. if you doing soil, a transient air temp drop to the low 60's/upper 50's shouldnt be an issue. i use a small ceramic cube heater on low that timed to come on an hour after lights out to maintain temps above 60 degrees.

 

[SeaMaiden]

im just wondering.. whats the lowest Lights Off Air Temp that will not slow down growth or budding.. im havnig a small issue.. now that its getting down to around 0 degrees outside. the basement is getting down to 50.. so.. the lung room gets down to 60 even.. at the coldest part of the day.. it will stay around 60 F .. then wen lights are on .. it pops up to around 70. sometimes a little more up to 75..

but the thing is.. im using passive intake threw the cracks under the doors and in the walls and cielings in the lung room to get fresh air in there.. wen the lights are out.. if i stop all the exhuast fans.. and turn them off. my temps stay around 63-66 .. with lights off.. but i dont think this is a good. idea.. whats the best heater to use in a grow room that will not burn down my house if i leave and anything happens.

I do a couple of things, but once I switched to open hoods keeping things warm overnight became a hell of a lot easier. I still need a temp-controlled fan controller (keep losing the damn bookmark!), but putting my exhaust on a timer helped as well.

Insulation, insulation, insulation. And, I also made something that I will still be using in my veg-table-box-thing--an Electric Blanket Sandwich. It's because I'm too cheap to buy seedling mats for $30 that cover something like 3'sq when I can get an electric blanket for the same price and be able to cover the area of a twin bed. Just fold it up and put it in a few contractor's bags, or use some 6mil or thicker plastic, taped up well.

 

[jammie]

I use an IR thermometer for cooking. It's great for knowing how hot oil is for frying, but can also be useful to shine onto plants. Most of the time it's not much more than a few degrees off from the canopy temp.

that's the reason to check leaf temps. if the temp is more than 4-5 degrees higher than ambient temp, it indicates that the leaf isn't transpiring and allows you to be proactive and either lower temps or increase humidity (see also vapor pressure deficits)