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VPD numbers, is a VPD of 1.00 the same across all temps?

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VPD numbers, is a VPD of 1.00 the same across all temps?

TomH 8 Replies 6,427 Views
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TomH

TomH

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#1
Looking at a VPD chart, I can find temp/RH combos to achieve a VPD of 1.00 from 61 degrees through 90 degrees (F), each temp would have a different RH associated with it.
My question is:
Does of VPD of 1.00 with a temp of 61 degrees F and RH of about 33% equal a VPD of 1.00 when the temp is 90 degrees F and RH of about 72.5%??
If not, what is the actual point of making a VPD chart as a spectrum rather than just one best temp/humidity calculation?
Sorry if this is a very basic concept, I can’t quite get this one down!
 
#2
VPD tells you if your temperature and humidity levels are well matched. Separately, temperature is a driver of growth rate. All else equal, if you have two plants with VPDs of 1.0 and one of them is at 70F and the other is at 85F, the plant at 85F should grow faster.
 
#3
Yeah the VPD does change a lot based on temps. That doesn't mean you should be running at 92F though, if that's what you're getting at.

Warm air is like a sponge, but the hotter the bigger the sponge. If the sponge is too dry, the plant transpires too much. If the sponge is soaked, the plant can't transpire.

"Vapor Pressure Deficit or VPD is very simply the measurement of how much drying power the air has. This effect’s the plant directly. The greater this force, the faster the plants dry out. ... Vapor Pressure Deficit is a numerical representation of this force. We can control VPD mainly by controlling temperature and humidity." reference: https://www.globalgarden.co/knowledge/vapor-pressure-deficit-vpd-explained/
 
#4
www.thcfarmer.com

VPD (vapor pressure deficit)

Ok i was gonna sit down and write out a whole article but i have been lazy so instead instead I'm going to copy paste because im lazy. This is not my work so let start. You may find this very long but its worth the read. Some may want to skip ahead. I have added here and there to what i feel is...
www.thcfarmer.com www.thcfarmer.com
 
#5
Now to add to this… ideal PLANT TEMPS (not air temps) for photosynthesis is around 75-77F with 72ish to 80f generally being acceptable.

I CANNOT STRESS ENOUGH HOW IMPORTANT PLANTS TEMPS ARE. A cheap IR thermometer that measure surface temp is a huge benefit to have as a tool in the grow room.

EG.


in the first link i explain how VPD works. But basically you want to get your plant temps in range by adjusting room temps, under LED you may need 80+f and under HID you may need only 75f to achieve ideal plant temps.

Then you adjust humidity to reach the ideal VPD or KPA.

If you have questions after reading feel free to tag me.

There are several things that influence plant temps but lets start there.
 
#6
ArtfulCodger said:
VPD tells you if your temperature and humidity levels are well matched. Separately, temperature is a driver of growth rate. All else equal, if you have two plants with VPDs of 1.0 and one of them is at 70F and the other is at 85F, the plant at 85F should grow faster.
Click to expand...
To a point once plants temps (not air temps) pass about 80f its common to see it negatively affect growth and health
 
#7
effexxess said:
Yeah the VPD does change a lot based on temps. That doesn't mean you should be running at 92F though, if that's what you're getting at.

Warm air is like a sponge, but the hotter the bigger the sponge. If the sponge is too dry, the plant transpires too much. If the sponge is soaked, the plant can't transpire.

"Vapor Pressure Deficit or VPD is very simply the measurement of how much drying power the air has. This effect’s the plant directly. The greater this force, the faster the plants dry out. ... Vapor Pressure Deficit is a numerical representation of this force. We can control VPD mainly by controlling temperature and humidity." reference: https://www.globalgarden.co/knowledge/vapor-pressure-deficit-vpd-explained/
Click to expand...
Kinda but not really… the higher the VPD the more water is pulled from the roots. the dryer the air the higher the VPD. This is how plants uptake water and nutrients. Ideal ranges at stages of growth are to manage this flow so that it’s adequate enough for water and nutrient uptake but not so high to create root stress (transpiration stress)

RH is relative humidity… meaning relative to temperature . 1 cup of water in the air can be high or low humidity depending on the temperature. at higher temps the humidity will be lower and at cooler temps it will be higher. Same amount if water in the air but very different RH.

this is because warmer air holds more moisture and %RH is a % of the saturation point for a given temperature in which the air will not hold anymore water
 
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#8
Here's a really easy VPD Calculator. You can play around with different room and leaf temperatures and relative humidity.

Vpd
 
#9
Aqua Man said:
www.thcfarmer.com

VPD (vapor pressure deficit)

Ok i was gonna sit down and write out a whole article but i have been lazy so instead instead I'm going to copy paste because im lazy. This is not my work so let start. You may find this very long but its worth the read. Some may want to skip ahead. I have added here and there to what i feel is...
www.thcfarmer.com www.thcfarmer.com
Click to expand...
I’m growing in a semi sealed room have a CO2 generator. What is the correct temperature? If I keep the room about 83° the VDP goes way off cannot get it within the correct .8 that they say it should be in flower should I be worried about the VPD or should I just keep the temperature on the higher side as all the information out there says 80 to 85 is the correct temperature for CO2 to be used
I am grown and I living soil type grow

Please help
 
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