Great Mystery - temps rise/fall for no reason!

There has to be a reason you say? A couple days ago I would have agreed, but listen to this:

I got my new AC reflector and put it up over plants at 3 weeks of flower. I was elated because the first day, at half the distance from tops as before, temps were ranging from 70-72. With temps like that I can handle more watts, so I ordered a new ballast and bulb. 5:30pm I look at it, 72. Just before lights off at 7, I look again - 86. Nothing had changed. Three days later it is still rising and falling. Awhile ago it was 70 (and this is 7 inches under the light), 20 min later it is 79.

Before posting theories let me explain what I have done.

1. ruled out swings in ambient temp, it has not varied more than two degrees and zero degrees during some of the dramatic shifts

2. ruled out changes in air intake, all fine no change

3. ruled out changes in air exhaust - yes there are some turns but no change from when it was 70 to much higher

4. ruled out thermometer problem - I'm using two and keeping them right at the level of the tops at both ends of the reflector. I even switched their positions and they both read the same

The issue is not the temps being too high (except for the 86), it is that if they will hold 72 so much of the time, why, with no variables changing, could temps possibly be climbing as much as 10 degrees in a few minutes? This is frankly, the weirdest damn thing I have ever seen.

Can a priest do an exorcism blindfolded?

look how an a/c unit works and there you will find your answer

jesus of Cannabis said:

look how an a/c unit works and there you will find your answer

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AC means air cooled in reflector-speak

My theory is this, you are using a large HID bulb, 600-1000 watts and you have your thermometers in the beam of light. This will send your temps screwy. All thermometers and thermostats say to keep out of direct sunlight, that many watts is pretty close to direct sunlight and will adversely effect your temp readings. To keep it accurate keep them out of the light and keep enough air circulating around the room that they get a good feel for the temp, even out of the light path.

jesus of Cannabis said:

look how an a/c unit works and there you will find your answer

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Air conditioned reflector?!? I think it was pretty obvious what he meant.

TruenoAE86coupe said:

My theory is this, you are using a large HID bulb, 600-1000 watts and you have your thermometers in the beam of light. This will send your temps screwy. All thermometers and thermostats say to keep out of direct sunlight, that many watts is pretty close to direct sunlight and will adversely effect your temp readings. To keep it accurate keep them out of the light and keep enough air circulating around the room that they get a good feel for the temp, even out of the light path.

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All good points, but I'm thinking this: why only now? I've been putting them in that location for a long time, trying to get an accurate reading of the temps the foliage tops that I want in the beam of the light are living with. I always thought putting a thermometer any where else was just a false sense of security since it would be measuring temps lower than the max the plant is experiencing. But, the accuracy of that aside, what changed because I didn't see fluctuations like that before.

The hood!!! its obviously much better than what you had before (and its half as far away), is concentrating the light more on the areas of the thermometers.
I have run my thermometer outside of the light path for years, works fine for me, and i still have the same temp requirements. I have always found a fan blowing between the canopy and light fixture helps even out that temp a lot. And while learning a new hood i hand check it while learning usable hieghts. I think u are fine.

TruenoAE86coupe said:

The hood!!! its obviously much better than what you had before (and its half as far away), is concentrating the light more on the areas of the thermometers.
I have run my thermometer outside of the light path for years, works fine for me, and i still have the same temp requirements. I have always found a fan blowing between the canopy and light fixture helps even out that temp a lot. And while learning a new hood i hand check it while learning usable hieghts. I think u are fine.

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I'm not sure I follow. How does a change from old hood to new hood, explain rapid fluctuations with the new hood?

It still seems to me we should be looking for the max temp the plants have to bear, and that's the tops under the light, so why measure anywhere else?

Illegal Smile said:

I'm not sure I follow. How does a change from old hood to new hood, explain rapid fluctuations with the new hood?

It still seems to me we should be looking for the max temp the plants have to bear, and that's the tops under the light, so why measure anywhere else?

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There is no temp fluctuations, it is the thermostats getting confused i believe. You never stated that they were in the light path, i know this from personal experience, so i asked. You changed something, therefore you can expect results, in this case your light has increased enough to start confusing your thermometers. Seriously look at any package for any thermometer it clearly states to keep out of direct sunlight, because of this exact reason.

If you have proper air flow and distance between the light and the plants you will have a constant temp across the whole room, so where the thermometer is doesn't matter.

You clearly don't like my theory here, but you stated you have 2 thermometers, so why not leave one where it currently sits and move one out of the light like i suggested, (this is the one people argue with me on most believe it or not) and then compare, will give you a good control.
Then let us know what happens, but i am sure this is your issue.

TruenoAE86coupe said:

There is no temp fluctuations, it is the thermostats getting confused i believe. You never stated that they were in the light path, i know this from personal experience, so i asked. You changed something, therefore you can expect results, in this case your light has increased enough to start confusing your thermometers. Seriously look at any package for any thermometer it clearly states to keep out of direct sunlight, because of this exact reason.

If you have proper air flow and distance between the light and the plants you will have a constant temp across the whole room, so where the thermometer is doesn't matter.

You clearly don't like my theory here, but you stated you have 2 thermometers, so why not leave one where it currently sits and move one out of the light like i suggested, (this is the one people argue with me on most believe it or not) and then compare, will give you a good control.
Then let us know what happens, but i am sure this is your issue.

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Nono, I don't dislike your theory at all. In fact, it is the best I have heard. I'm just probing to see if it can explain all the facts.

I'm willing to stipulate for the moment, that regular thermometers from walmart can't measure temps under the lights for the reasons you say. But I still have two problems with the theory:

1. I still don't see where it explains the fluctuations. The meter was under the lights when it read 72, and under the lights in the same position when it measured 82. What changed?

2. I don't see how we can just say don't measure tempos under the light. It seems to me that is exactly where we need to measure them. If regular thermometers can't do it reliably, then don't we need other instruments?

Thanks for your input!

Can't say i can explain it any further, just experience has taught me.

2. I don't see how we can just say don't measure tempos under the light. It seems to me that is exactly where we need to measure them. If regular thermometers can't do it reliably, then don't we need other instruments?

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IMO a grow room the temp in any area should not be more than a 3 degree (F) hotter or cooler than any other area, including directly under the light. Temp is measured by the air, inside and out, so just like outside the sun can make it feel much much warmer than it really is, but the air temp is still low. I live in Colorado, if it is 40 degrees out and the sun is beating down and there is no wind it can be really nice out, cover the sky with clouds, it is still 40 out but freezing ass cold out.
Same idea in your grow room, the air temp is the important part.

If I leave my thermometer in the light I feel like the plastic is what's heating up after a period of time, not the actual air temperature of the room. I stick it under a little bit of shade but still right by the plants and it reads a lot more consistently.
The fan oscillates on them every few seconds blowing cool air on and around them so I think it's ok.

TruenoAE86coupe said:

Can't say i can explain it any further, just experience has taught me.



IMO a grow room the temp in any area should not be more than a 3 degree (F) hotter or cooler than any other area, including directly under the light. Temp is measured by the air, inside and out, so just like outside the sun can make it feel much much warmer than it really is, but the air temp is still low. I live in Colorado, if it is 40 degrees out and the sun is beating down and there is no wind it can be really nice out, cover the sky with clouds, it is still 40 out but freezing ass cold out.
Same idea in your grow room, the air temp is the important part.

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I don't think so. Temperatures should fall as distance from the light increases at approximately the same rate that foot-candles decreases. My temps are at least 10 degrees cooler on top of the res as under the lights. And yes, there is a difference between air temp and radiant heat. But since we know radiant heat can burn plants, if that's what the thermometers are measuring, it's important. If the highest temperatures (whether from radiance or not) is ok, then all the lower temps elsewhere are ok too. But I think many growers are under-estimating the heat environment in their growspace by at least 5 degrees.

FWIW - none of this gets to the question of what causes short-term fluctuations.

lets see, whats the new thing responsible for cooling your lights that might be fluctuating . . .hmm you installed a new air cooled reflector and maybe your air cooled reflector sucks, maybe your AC that you installed to your new light sucks and is turning on then off and it takes a while to get started hmm

your plants do like to get cooled and heated all the time, and a small change in ambiet air temp in your Hood will be a big change to your room

Samwell Seed Well said:

lets see, whats the new thing responsible for cooling your lights that might be fluctuating . . .hmm you installed a new air cooled reflector and maybe your air cooled reflector sucks, maybe your AC that you installed to your new light sucks and is turning on then off and it takes a while to get started hmm

your plants do like to get cooled and heated all the time, and a small change in ambiet air temp in your Hood will be a big change to your room

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AC = Air cooled not air conditioning, there is no air conditioning involved

I've concluded that thermometer from walmart and such are inadequate to measure anything other than ambient temp in the shade. So, growers are measuring that only. We can't ignore radiant heat under the lights, where the plant tops are. Plants have more tolerance for radiant heat, but we can't just ignore it and wait for them to burn as a sign it is too high. We need a way to measure radiant heat under the lights accurately, I'm trying the device in my other thread on the subject.

No you worry too much, they are plants, if you can keep good air flow and good light distance, you will learn your set up and not have to worry about silly things like "radiant heat". When you first set up i guess this is an alright idea, but all temps are based on air temp, from the weather report to every thermostat in every house out there.

Your fancy new temp gun is making you way over think things...

cowboylogic said:

Your fancy new temp gun is making you way over think things...

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WTF?!?!? Posting from above?!?! I was under the impression that CL had "moved on"........ Now i see the infamous name again?!? what gives?

TruenoAE86coupe said:

No you worry too much, they are plants, if you can keep good air flow and good light distance, you will learn your set up and not have to worry about silly things like "radiant heat". When you first set up i guess this is an alright idea, but all temps are based on air temp, from the weather report to every thermostat in every house out there.

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You're wrong and I'm right. Sorry. And I'll bet I was growing weed before you were born.

AHAHAHA if you think so man!!!
I am not wrong, you are the one coming in with noob questions...... Not me.
You use walmart themometers and you didn't think you were going to have issues? ever heard of a thermostat?!?!
Seriously we may not agree, but neither of us is wrong, you just worry too much!!!
I have set up many many grow rooms, hell just built a new one last night. I have hung dozens of hid lights and have never had more than a few degrees directly below a hood. I am also probably running more watts than you right now, cooled and uncooled hoods both and can still keep all temps within reason.
Good luck but you are putting too much energy into a simple issue we have all dealt with at one point or another.

But it was a good enough thread to bring opinions from the dead...... wtf?