Vapor Pressure Deficit & What It Means To You!

Gazoo said:
Thought I would give a little update on my progress getting this dialed in

One thing I learned while reading up on all this is the relationship between the outside air and
the leaf temp. From what I have read we are looking for -2C (3.6F) for the best balance
BTW This can be adjusted by raising or lowering your lights.

I like it SINCE ONCE dialed in we have effectively balanced the LIGHT DISTANCE & VPD.
Makes a no brainier on Light distance ;)
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Oh great point Gazoo. I hadn’t considered that. I’ve always struggled with light distance. I’ll give it a shot as well.
You see how easy it is once you get the plants in zone? You may have to adjust here and there based on ambient climate. Today it’s raining in SoCal! I know I was excited and shocked too! The spike in humidity caused my dehu to kick in. I have it set as a backup in case the exhaust fails and RH climbs too high or there is too much RH for the fan to vent it all.
It’s a dance but the point is its a dance worth doing.
 
I hope everyone has their VPD dialed in this Thanksgiving! One more thing to be thankful for :).
 
Archiweedies said:
Oh great point Gazoo. I hadn’t considered that. I’ve always struggled with light distance. I’ll give it a shot as well.
You see how easy it is once you get the plants in zone? You may have to adjust here and there based on ambient climate. Today it’s raining in SoCal! I know I was excited and shocked too! The spike in humidity caused my dehu to kick in. I have it set as a backup in case the exhaust fails and RH climbs too high or there is too much RH for the fan to vent it all.
It’s a dance but the point is its a dance worth doing.
Click to expand...

Thing here is, this is the SET SPOT (light distance), do this first THEN check PAR

EDIT: If making adjustments for power (PAR) after setting distance we may need to move lights again ? so maybe its just
pointing us to the best distance and we should leave it there ?
 
Derbybud said:
Nice read archi. Just make sure you have plenty of air movement thru your plants and you're golden
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Thanks for checking it out DB!
When you say air movement, we’re talking above and below canopy air movement yea? I think I’m okay in that regard. I aim for leaves twitching a little here and there. Definitely not just blowing hard on them directly.
 
Hey Guys - Question on VPD as I’m having my first go with it and just started flower. According to charts, I’m in the green; however my initial reaction is it feels too humid. Though the plant seems to be healthy and growing VERY rapidly, several leaves are starting to show what appears to be calmag deficiencies.

While this could be a simple case of just add more calmag, my mix contains quite a bit already. I added more and it would be great if the fix is that easy, however some research says humidity can cause plants to show calmag deficiency even when they have adequate amounts available in soil or reservoirs... and I’m trying to understand if any of you have experienced that (specifically in green zone VPD environments)? I’ve always steered clear of overly humid rooms due to fears or mold/ mildew/ etc., but because so many amazing growers talk VPD I wanted to give it a shot. I’m finding even though I’m in the green on VPD, I’m uneasy and looking for a little input from the seasoned VPD guys. Appreciate any insight.

Air temp - 82*F
Leaf temp - 81.8*F
RH - 74%
CO2 - During day (1000-1250PPM)
 
When I look at the chart AW posted on page one, 82ºF (or 27.8ºC) green corresponds to 65-72% RH.

[edited after I got my reading glasses]
 
InTheShed said:
When I look at the chart AW posted on page one, 82ºF (or 27.8ºC) green corresponds to 57.5 - 65% RH. You'd have to be 95ºF for 74% to be in the green the way I'm reading it.
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This is the chart I’ve been using.
BCC3402C-A2B2-4F98-ACC9-8A2D4B96882C.jpeg
 
Is that cannabis-specific? The chart in the first post has different ranges for veg and the stages of flower.
 
Growca2268 said:
Hey Guys - Question on VPD as I’m having my first go with it and just started flower. According to charts, I’m in the green; however my initial reaction is it feels too humid. Though the plant seems to be healthy and growing VERY rapidly, several leaves are starting to show what appears to be calmag deficiencies.

While this could be a simple case of just add more calmag, my mix contains quite a bit already. I added more and it would be great if the fix is that easy, however some research says humidity can cause plants to show calmag deficiency even when they have adequate amounts available in soil or reservoirs... and I’m trying to understand if any of you have experienced that (specifically in green zone VPD environments)? I’ve always steered clear of overly humid rooms due to fears or mold/ mildew/ etc., but because so many amazing growers talk VPD I wanted to give it a shot. I’m finding even though I’m in the green on VPD, I’m uneasy and looking for a little input from the seasoned VPD guys. Appreciate any insight.

Air temp - 82*F
Leaf temp - 81.8*F
RH - 74%
CO2 - During day (1000-1250PPM)
Click to expand...

Are you using the "1 deg C (1.8 F) cooler than Air Temp Chart" ?
vpd_1degree.gif


OR THIS one for zero differance

VPD_ZERO.png
 
Thanks guys. Ok so just looked at chart on first page, pretty much the same. Going by chart on first page I’m in the correct zone for this stage of early flower growth:

27.7C, 74% RH = .966 kPa

I guess a different (and maybe better) way for me to ask the question is if I’m in the sweet spot for VPD, do I need to worry about the normal pitfalls of a humid grow room?
 
Growca2268 said:
Thanks guys. Ok so just looked at chart on first page, pretty much the same. Going by chart on first page I’m in the correct zone for this stage of early flower growth:

27.7C, 74% RH = .966 kPa

I guess a different (and maybe better) way for me to ask the question is if I’m in the sweet spot for VPD, do I need to worry about the normal pitfalls of a humid grow room?
Click to expand...

Using the - 1c chart that falls in the Early Veg

Using the ZERO Different chart and it looks like
Veg stage VPD
 
So my math or understanding/ reading of the charts must be off (probably both). Can you please walk me through what you’re calculating and referencing back to? Appreciate the assistance.

Edit: Missed your previous post with the two charts (I was basing my kPa against the -1C chart), but clearly I’m looking at it wrong?
 
Vpd is the difference in pressure between the air inside the leaf stomata and the ambient air. Vpd changes based on leaf temp assumed to be 100%rh vs room temp/ humidity. If your room is 80/65% the vpd would be different for leaves at 70° and leaves at 80°
 
Growca2268 said:
So my math or understanding/ reading of the charts must be off (probably both). I’m also not familiar with - 1C. Can you please walk me through what you’re calculating and referencing back to? Appreciate the assistance.
Click to expand...

Sure from you post i understood your Tent AIR TEMP was 1 deg Higher than your Leaf Temp. This would
mean you need to use a chart that has taken that into account. Only Charts I have been able to find are
always in C so a - 1c chart (leaf is minus one deg C from air temp) Since the difference we are measuring is
in F its not perfect but closer. I cant seem to get any difference at 24 inches below the HPS so I use the ZERO Chart
 
Oooook, were on the same page now. Thanks man. Yeah given the minimal *F temp delta the best way to measure is definitely on the Zero Difference chart. Verdict = my humidity is too high, I need to drop it to 68% and I’ll be money.

That’s why you make the big bucks, Gazoo!
 
Growca2268 said:
Oooook, were on the same page now. Thanks man. Yeah given the minimal *F temp delta the best way to measure is definitely on the Zero Difference chart. Verdict = my humidity is too high, I need to drop it to 68% and I’ll be money.

That’s why you make the big bucks, Gazoo!
Click to expand...
Shit he did all the heavy lifting too thanks Gazoo!
Glad you got it worked out.

On the topic of the higher humidity, bacteria can still thrive in warm humid environments and although your plants maybe transpiring like crazy there still a risk.
Without an actual VPd meter, we have to use the charts available to us or do the math the old fashioned way.
One thing I can’t be certain of in your case is that you are running CO2 in presumably a sealed environment. Naturally heat and humidity will be higher. You are able to get away with much higher temps when your running CO2. That could make the chart you use a little different, I can’t be sure.
Given your environmental temps and leaf temps it sounds like your running LED’s or ducted hoods.
If you have an air conditioner, you may have to lower temps to the 78F range so you can run a slightly lower humidity.
Humidity really is the factor we need to control after all.
 
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