Summoning the VPD Lords 🙌 - Leaf Temp Issues

[abzem]

A wise man once told me "Speak to the VPD Lord, and you will have the answers." So, I've traveled a long way and need your help.

I've read threads and a boatload of write-ups online about VPD and I'm struggling to keep my leaf temp down and therefore cannot find my Goldilocks ;) @Aqua Man

I'm in week 3 of flower.

I'm growing Auto's in 3 Gal Fabric pots in Ocean Forest + 20% pearlite with Nature's Living Soil.
VIPARSPECTRA XS1500 Full spectrum LED 28 inches from canopy & the dimmer at 75% strength. Only getting 33,000lux/462ppfd. (so I don't think I'm frying them.)
Watering ph of 6.8/7 about every 2/3 days depending when the pot feels nice and light.
Plenty of airflow on and below the canopy + good ventilation.

I've tried every parameter from 75F/55%RH to 82F/65%RH and the leaf temp always stays the same as the ambient temp or even 1 or 2 degrees above making my VPD too high.

Any advice on how to lower my leaf temps??

Thank you!

 

[AdrienneGC]

A wise man once told me "Speak to the VPD Lord, and you will have the answers." So, I've traveled a long way and need your help.

I've read threads and a boatload of write-ups online about VPD and I'm struggling to keep my leaf temp down and therefore cannot find my Goldilocks ;) @Aqua Man

I'm in week 3 of flower.

I'm growing Auto's in 3 Gal Fabric pots in Ocean Forest + 20% pearlite with Nature's Living Soil.
VIPARSPECTRA XS1500 Full spectrum LED 28 inches from canopy & the dimmer at 75% strength. Only getting 33,000lux/462ppfd. (so I don't think I'm frying them.)
Watering ph of 6.8/7 about every 2/3 days depending when the pot feels nice and light.
Plenty of airflow on and below the canopy + good ventilation.

I've tried every parameter from 75F/55%RH to 82F/65%RH and the leaf temp always stays the same as the ambient temp or even 1 or 2 degrees above making my VPD too high.

Any advice on how to lower my leaf temps??

Thank you!

I am not sure if this will help you but here is a VPD Calculator . Good luck!

 

[growsince79]

75% at 28"= 33000lux is hard to believe.

 

[Jungle1]

I've read threads and a boatload of write-ups online about VPD and I'm struggling to keep my leaf temp down and therefore cannot find my Goldilocks ;) @Aqua Man

I'm in week 3 of flower.

I'm growing Auto's in 3 Gal Fabric pots in Ocean Forest + 20% pearlite with Nature's Living Soil.
VIPARSPECTRA XS1500 Full spectrum LED 28 inches from canopy & the dimmer at 75% strength. Only getting 33,000lux/462ppfd. (so I don't think I'm frying them.)
Watering ph of 6.8/7 about every 2/3 days depending when the pot feels nice and light.
Plenty of airflow on and below the canopy + good ventilation.

I've tried every parameter from 75F/55%RH to 82F/65%RH and the leaf temp always stays the same as the ambient temp or even 1 or 2 degrees above making my VPD too high.

Any advice on how to lower my leaf temps??

Thank you!

To begin with, I'm not sure 33,000 lux is anywhere near 462ppfd, but whatever. I don't think that's your issue.

How are you measuring leaf temp? It'd kind of hard to believe that leaf temp is more than air temp (with only 462ppfd). I've got more light than that, and my leaf temps are 2F below air temp. Of course, I'm RDWC with a chiller set to 68F, so maybe that's got something to do with it.

I just went through the "how can I set things up so VPD is where I want it?", and found two important things:

• It's not possible to do this without automated controls that control both temp & humidity.
• It's not possible to do this looking at individual readings.

What I did was instrument some data collection (temp, rh), and plot a moving average of VPD.... see below. I honestly don't know how you can do much about VPD without some averaging values... I'm trying to keep between 1.2 and 1.5 kPa, and I'm mostly there (only at sunrise/sunset is there any issue).