Air VPD or Leaf VPD? Which one?

[TomH]

I saw a VPD calculator posted here recently that displays both air and leaf VPD, but it seems that there is a significant difference between the two (in my tent anyway).

When the plants are really happy looking, I see a 6 to 7 degree difference between leaf and air. In the calculator, it appears that if I’m concerned about air temp, my VPD is dangerously high, but the leaf VPD is pretty low (for late veg anyway).

So, which one is actually important, and why do we have such difference in calculation for a healthy growing environment?

I tend to think it’s actually leaf VPD, because that seems to be the one that is “in range”, when the plants look best. But why would we even care about air VPD in that case?

I have also noticed that the sadder the plants look, the closer to equal are the two temperatures. When it looked like I had a lockout, there was only a difference of a degree or two, but perky plants the difference is significant.

Just curious.

 

[steamroller]

These are the charts I use.
They use difference between leaf and room/air temp to calculate.
I don't understand how you could calculate VPD without the difference but my same link does a calculation without leaf temp they call room VPD and gives the option to add leaf temp and give that reading as leaf VPD.
There are 6 separate charts that go by leaf difference.
I like this one myself?

Vapor Pressure Deficit VPD Calculator - Dimlux Lighting - The Best Grow Lights

Vapor Pressure DeficitCalculator Calculate both Room VPD and Leaf VPD automatically Just enter the value below to generate the VPD values for your room and your plants. You’ll need to know the room’s temperature, relative humidity, and the plant’s temperature. We recommend calibrating your...

www.dimluxlighting.com

​

 

[TomH]

These are the charts I use.
They use difference between leaf and room/air temp to calculate.
I don't understand how you could calculate VPD without the difference but my same link does a calculation without leaf temp they call room VPD and gives the option to add leaf temp and give that reading as leaf VPD.
There are 6 separate charts that go by leaf difference.
I like this one myself?

Vapor Pressure Deficit VPD Calculator - Dimlux Lighting - The Best Grow Lights

Vapor Pressure DeficitCalculator Calculate both Room VPD and Leaf VPD automatically Just enter the value below to generate the VPD values for your room and your plants. You’ll need to know the room’s temperature, relative humidity, and the plant’s temperature. We recommend calibrating your...

www.dimluxlighting.com

​

This was the one I was using. My air temp was in the upper 70s (varying slightly between 75 and 77), the leaf temp was 68 consistently.

I have a digital weather station pickup in my tent for calculating the air VPD, I have an IR thermometer set to surface reading to get the leaf VPD (using RH from the weather station).

Its my presumption that this is the proper method for the calculation, but I’m open to recommendation if I need to get data differently.

 

[steamroller]

I have seen RH read differently with different tools.
I have 2 or more ways to register room temp and RH and use an InkBird IR gun for leaf temp.
The reason for multiple ways to check is the difference from one to another.
Looking for middle ground usually.

 

[Aqua Man]

Leaf VPD ONLY.

VPD (Vapour Pressure Deficit) Explained by Aqua Man

WHAT IS VPD: VPD stands for Vapor Pressure Deficit. All gases have vapor pressures, but when we’re growing, we’re interested in water vapor. Vapor pressure deficit is the difference between the pressure of water vapor in 100% saturated air at a given temperature (basically a leaf’s vapor...

budbuilders.org

 

[freezeland]

Leaf VPD ONLY.

VPD (Vapour Pressure Deficit) Explained by Aqua Man

WHAT IS VPD: VPD stands for Vapor Pressure Deficit. All gases have vapor pressures, but when we’re growing, we’re interested in water vapor. Vapor pressure deficit is the difference between the pressure of water vapor in 100% saturated air at a given temperature (basically a leaf’s vapor...

budbuilders.org

^^^^^^^^^^
That

 

[moe.red]

Just to be clear there is only 1 VPD. What the plant sees. That’s at the leaf. The other is just temp.

That said there are a lot of leaves and microclimates exist. Typically the lower leaves are cooler.

Interesting you have a 7* drop tho. That’s pretty atypical. What lights? Are you in a tent?

 

[TomH]

Just to be clear there is only 1 VPD. What the plant sees. That’s at the leaf. The other is just temp.

That said there are a lot of leaves and microclimates exist. Typically the lower leaves are cooler.

Interesting you have a 7* drop tho. That’s pretty atypical. What lights? Are you in a tent?

Its a 4 bar style LED, but not a big name brand. It was sold as a 300 watt draw light. I’ll need to look at the label as I don’t recall the MFG.

 

[Aqua Man]

LED can have as much as 10f lower leaf temp than air temp. This is from high transpiration rates and evaporative cooling

 

[TomH]

@moe.red,

It’s apparently only 240 watts, based on the model number.

As of this pic, air temperature was 82, leaf temperature was 75.
RH was 60%

 

[moe.red]

LED can have as much as 10f lower leaf temp than air temp. This is from high transpiration rates and evaporative cooling

Yup yup but you can only have high transpiration if VPD is right. Rh too low and stomata close. Too high and there is no room in the air for more water vapor.

It’s a balancing act for sure.