My LST offset is almost always -2 but has been -4 at times. My understanding is that LST should be lower than ambient due to evaporative cooling of the leaf surface.
What are the impacts on the plant if LST is > ambient?
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Whatever the leaf surface temperature is, that is what VPD the plant experiences. If the LST is higher than ambient temperature, the plant is experiencing a higher VPD than the measured air VPD.
Leaf temps can be higher than ambient when irradiative heating from lights or the sun exceeds the cooling caused by transpiration.
I’ve seen formulas somewhere (maybe even on the Pulse site somewhere , I’m not sure) where non plant leaf temps are taken into account also to calculate the vpd. What are your thoughts on this peet, should this be taken into account?
leaf temps definitely should be taken into account, and the way that plant temps and leaf temps work together is as follows:
- You calculate the saturation vapor pressure in the air, lets call it “aSPV”
- You calculate the saturation vapor pressure in the leaf, lets call it “lSPV”
- You calculate leaf VPD: Leaf VPD = lSVP – (aSVP x RH/100)
SVP formula: SVP = 610.78 x e^(T / (T +237.3) x 17.2694)
- T is temperature in Celsius
- You use the temperature for the air for air SVP and for the leaf for leaf SVP
- e is a mathematical constant called Euler’s Number, approximately equal to 2.71828.
- The result, SVP, is in pascals (divide by 1000 to get kPa)
It’s all outlined here: The Ultimate Vapor Pressure Deficit (VPD) Guide - Pulse Grow
Sorry, I wasn’t very clear. What I meant to ask is your opinion on whether or not one should take into account non plant items in the tent, such as the actual pot, walls, any objects that may be in there, etc. I’ve read somewhere where this is to be considered.
I wouldn’t worry about the temperature of other objects apart from using them in calculating the air VPD when measuring with an IR thermometer