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Questions [Solved] VPD Leaf or VPD air?

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Topic starter

Greetings growers! I'm using the VPD calculator to guide my temps and humidity. (Link: removed link ). Apparently for seedlings you should aim for a VPD of .8.  But there are two different VPD's in the calculator:  VPD Leaf and VPD Air. Which one is supposed to be .8?

Thanks!

 

I look forward to the day when a chicken can cross the road without having its motives questioned.

Topic Tags
VPD
5 Answers
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Google: Pulse Grow VPD Chart Maker and Calculator

(We can't post the links).

I think it is ok for me to put it this way: pulsegrow dot com/blogs/learn/vpd 

To get this Google Doc for yourself, you will then scroll down until you see this:

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Click on Blue Box and this comes up:

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Fill it out and you have a link to that form anytime you like.

Look for this chart:

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Then once you plug in your data, there is a tab below, that say VPD Chart Maker (see circle and arrow on my screen grab).

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That will open up a page like this:

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Be sure to fill in the stage of growth (Cone, Veg, Flower = blue circle above) and Leaf to air temperature differential (red oval). This mean Leaf Temp minus Air Temp.

For instance, if the air is 75 and the leaf is leaf is 73, you would put "-2" (minus 2). If the air was 73 and the leaf was 75 then you would put "2" (no minus). When air and leaf are same temp then put "0" (zero).

Note when you put in your values the chart shifts left or right.

I hope this is clear.

 

“The happiness of your life depends on the quality of your thoughts.” ~ Marcus Aurielius

1

Leaf is what matters most because of transpiration. 

“The happiness of your life depends on the quality of your thoughts.” ~ Marcus Aurielius

Thank you....in that case, I guess I've been doing it wrong. I've been aiming for .8 on the VPD Air. Currently, the humidity is 73%, the temp is 74.6F (23.7C), the VPD Air is .8 and the VPD Leaf is .67. So I need to bring the VPD Leaf up to .8?

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This is the Infrared Thermometer Gun I've been using for about 4 years.

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“The happiness of your life depends on the quality of your thoughts.” ~ Marcus Aurielius

Curious what emissivity you use for cannabis leaves?

@Sevans

Google this PubMed article:

image

It's not cannabis specific, but still interesting.

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Oddly enough, I just got this email from Pulse, about the importance of VPD at night. Here's the gist of what they say:

 

Seven key points about VPD at night:

  1. VPD is mostly about transpiration
  2. Plants transpire significantly less at night
  3. Use night period for preventing disease by keeping humidity and temps lower
  4. Try to minimize VPD fluctuations (less than 0.4kPa)
  5. VPD still matters for respiration at night
  6. Daytime VPD is about 10x more important than nighttime VPD
  7. Optimal VPD can increase yields as much as CO2 enrichment

To sum it all up:

Although VPD at night isn't nearly as important as your day VPD,
it's still an important factor to consider for vigorous plant growth and increased yields

“The happiness of your life depends on the quality of your thoughts.” ~ Marcus Aurielius

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Topic starter

Thanks very much , I'll give it a try. (How do you measure leaf temperature?)

I look forward to the day when a chicken can cross the road without having its motives questioned.

infrared temperature gun, $50-$70, go shoot your plants. 

I went ahead and bought the Pulse One. I followed the instructions for the leaf offset, which involved taking temperature points on various leaves as well as inorganic objects in the tent. And then I calibrated by AC Infinity to the temperature and humidity readings of the Pulse One. I've got my VPD where I want it during the day, assuming the Pulse meter is accurate. I find it disconcerting that there are such discrepancies between my various meters.

Keep in mind, most “good” humidity sensors (including the pulse one) have a tolerance range of +/- 2%, if all your sensors are within a 4% band they area all within the tolerance.

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