Questions [Completed] VPD Leaf or VPD air?

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:

Click on Blue Box and this comes up:

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

Look for this chart:

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).

That will open up a page like this:

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.

Leaf is what matters most because of transpiration. 

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

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:

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