Tent temp vs leaf temp help please

You are figuring out the VPD for where you are taking the numbers from. Are your plants above the light or below it?

As far as the "One said it can interfere with readings having it under the light. " that was referring to if you put it directly under the light as in just a few inches below it afaik

If your plants are like mine they are below the light. The temp/RH you want is from the leaf and top of the canopy* or basically what the plants are actually living in, not up above the light.

*Some people will move the thermometer around the tent to get several readings to average out the temps/rh for their air vpd calculation.

What you have done essentially is instead of correct the environment you moved the measuring devices to get the readings you wanted/needed

ALPHA.GanjaGuy said:

You are figuring out the VPD for where you are taking the numbers from. Are your plants above the light or below it?

As far as the "One said it can interfere with readings having it under the light. " that was referring to if you put it directly under the light as in just a few inches below it afaik

If your plants are like mine they are below the light. The temp/RH you want is from the leaf and top of the canopy* or basically what the plants are actually living in, not up above the light.

*Some people will move the thermometer around the tent to get several readings to average out the temps/rh for their air vpd calculation.

What you have done essentially is instead of correct the environment you moved the measuring devices to get the readings you wanted/needed

Click to expand...

So thing is, the hygrometer is telling me things are in the late veg/early flower range but if the hygro meter was where it last was, I would be ok with 92F room temp, 80F leaf temp and 52rh%.

The grid changes fast when turning the dial on room temp and leaf temp, hence my need to ask why my readings are so off. The grid diverts two different ways quick.

I figured you take an average. I had it in canopy for a while and can say it is 90F around there.

That is because it is battered by lights, holding your hand under the light has heat energy from light; that you aren’t trying to plug into vpd.

Correct me if I am wrong, we are actually having a very relevant conversation.

I essentially still have this issue but chalked it to crammed space with micro climates, making this much harder. What would you do?

I get ranges from 65F to 75F but no matter what, cannot get leaf temps higher or space evened out.

Edit: It says generally leaf temps are 5F cooler than room temp, not canopy.

I thought so it can fully take into account and function in the atmospheric rh and temps and behavior in accordance to what cannabis plants generally need to photosynthisize.

Perhaps including averages as this is like whack a mole.

So thing is, the hygrometer is telling me things are in the late veg/early flower range but if the hygro meter was where it last was, I would be ok with 92F room temp, 80F leaf temp and 52rh%.
-Maybe I am not following what you are doing. Are you talking about using a smart controller? If so hang the sensors where they tell you for best results.

The grid changes fast when turning the dial on room temp and leaf temp, hence my need to ask why my readings are so off. The grid diverts two different ways quick.
-What grid?

I figured you take an average. I had it in canopy for a while and can say it is 90F around there.
-That would be the temp of your canopy then. You don't want to use some random temp you found in the tent because the math works better, that will surely have you struggling in time.

That is because it is battered by lights, holding your hand under the light has heat energy from light; that you aren’t trying to plug into vpd.
-yes, your light does contribute to your temperature.

Correct me if I am wrong, we are actually having a very relevant conversation.
-I agree

I essentially still have this issue but chalked it to crammed space with micro climates, making this much harder. What would you do?
-This is possible, what is the room/tent like? How many plants? Tent size? Light?

I get ranges from 65F to 75F but no matter what, cannot get leaf temps higher or space evened out.
-Confused again, I thought your canopy was at 90 degrees? What is the leaf temp?

Edit: It says generally leaf temps are 5F cooler than room temp, not canopy.
-Yes but typically the room temp is being taken at the level of the canopy afaik.


I may be wrong but that is how I have always understood it.

For example, Pulse Grow states;

"How Do You Calculate VPD?
To calculate Air VPD you just need two things: temperature & humidity, but there are a couple of steps.
However, since the inside of a plant is water, the plants feel a different VPD than just the normal Air VPD. If you want to figure out what’s going on with the plant, you need to consider the cooling effect of evaporating water as it exits the leaves’ stomata. To calculate this, you need to know the difference between the air temperature at canopy level, and the temperature of the leaves. An IR thermometer is useful for this."

ALPHA.GanjaGuy said:

So thing is, the hygrometer is telling me things are in the late veg/early flower range but if the hygro meter was where it last was, I would be ok with 92F room temp, 80F leaf temp and 52rh%.
-Maybe I am not following what you are doing. Are you talking about using a smart controller? If so hang the sensors where they tell you for best results.

The grid changes fast when turning the dial on room temp and leaf temp, hence my need to ask why my readings are so off. The grid diverts two different ways quick.
-What grid?

I figured you take an average. I had it in canopy for a while and can say it is 90F around there.
-That would be the temp of your canopy then. You don't want to use some random temp you found in the tent because the math works better, that will surely have you struggling in time.

That is because it is battered by lights, holding your hand under the light has heat energy from light; that you aren’t trying to plug into vpd.
-yes, your light does contribute to your temperature.

Correct me if I am wrong, we are actually having a very relevant conversation.
-I agree

I essentially still have this issue but chalked it to crammed space with micro climates, making this much harder. What would you do?
-This is possible, what is the room/tent like? How many plants? Tent size? Light?

I get ranges from 65F to 75F but no matter what, cannot get leaf temps higher or space evened out.
-Confused again, I thought your canopy was at 90 degrees? What is the leaf temp?

Edit: It says generally leaf temps are 5F cooler than room temp, not canopy.
-Yes but typically the room temp is being taken at the level of the canopy afaik.


I may be wrong but that is how I have always understood it.

Click to expand...

2x2, one auto plant, 5’ tall tent.

Fecida cr1200



It is packed, so I figured I would have micro climates everywhere.


The side lighting led strip actually helped even out the temps from above, mid and below canopy.



I do not auto feed or auto dial anything in. I do all by hand.

When I say the grid moves around, I mean vpdchartdotcom, where it says you should be moves dramatically in two different directions.

This is a scrog (more of just packing in buds) so I expected some type of issue/struggle unless I got lucky.

The air temp around canopy was 90F but the laser gun reads 70-75F on leaf temp, no matter how hot I get the tent.

That sounds like your infrared thermometer is broken or bad batteries or needs to be calibrated. I don't see how air temp could be 90 but leaf temp is 15-20 degrees lower.

If leaf temps are 75 I see RH should be 35-40% from what I can see

pic below shows what I get with the numbers you gave 90 at canopy, 75 leaf temp, 52% rh

ALPHA.GanjaGuy said:

That sounds like your infrared thermometer is broken or bad batteries or needs to be calibrated. I don't see how air temp could be 90 but leaf temp is 15-20 degrees lower.

If leaf temps are 75 I see RH should be 35-40% from what I can see

pic below shows what I get with the numbers you gave 90 at canopy, 75 leaf temp, 52% rhView attachment 5267948

Click to expand...

75 is nice to see but it struggles to hit 75F, more like 71F and below. Only 75F in a few places. 73F is most common and then below as you measure leaves below the first layer across canopy. It is lower in temps across first layer too.

The laser is not broken, my skin temp is 95.5F which sounds about right, lets not have the arguement about why skin temp is cooler haha just kidding, we can if you want but I am sure you been known that.


Edit: My tent temp reads steady 83-85F, canopy is about 85-90F. I can cool it down a lot if needed with winter temps.

The space is inhibited so rh cannot go below 40 on the dehuey and cannot get too hot either but plants wont like that either. Avoiding too much RH and too cold temps.

I think locking in vpd will be out of the question for your current grow do to overcrowding and micro-climates.

I would suggest trying to keep temps in the 80's and RH as low as you can to help ward off any chances of pm or rot as the girls continue to grow.

I know the feeling, I currently have an overstuffed 4x4 with similar issue..

ALPHA.GanjaGuy said:

I think locking in vpd will be out of the question for your current grow do to overcrowding and micro-climates.

I would suggest trying to keep temps in the 80's and RH as low as you can to help ward off any chances of pm or rot as the girls continue to grow.

I know the feeling, I currently have an overstuffed 4x4 with similar issue..

Click to expand...

Thanks for helping me understand it better though and others reading along, I had a feeling I just had to do what ever to survive the run lol.

If it were somewhere that was not inhibited, could easily hit the continuous button and strip the room of any water just about haha.

I did it once but then got health issues and was like DAMN this thing is real. Trippy emptying it out to think we are like fishes in a air-like sea haha.