AdrienneGC said:

I would think that the surface temperature should be taken at the leaf. The VPD is the difference between the room conditions and leaf conditions. The bud is probably somewhere in between these conditions (is my guess). Here's a Calculator where adjusting temperature and/or humidity calculates the VPD.

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Oh, Wow! An HVAC designer. So COOL. Pun intended :-). The bud should be at a higher temperature because the leaf stomata serve to regulate transpiration and thus lower the leaf temperature by the isothermal evaporation of water vapor.

jguit said:

@Blastfact, that's odd.. My cheap IR thermometer is pretty rock solid, doesnt swing whatsoever. If something changes in your environment (open door, etc), it'll swing. Otherwise, it's pretty solid but yea, I agree.. Chasing VPD is a bit of a waste.

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Your cheap IR Thermo isn't doing you any good. My Fluke that cost $200, 20 Plus years ago which is now $500 to replace is rated at >100 C +/- 1% / <100 C +/- 1 C. This new $30 Inkbird IR Thermo is spec'ed at 2% / 2 C. And most if not all the cheap China junk IR Thermos are 3% to 3 C and reality when you test them there more like 5% to 5 C. So seeing most wont buy a $500 IT Thermo or PAR Meter your learning to read the plants and rock on. And since raising the lights a bit ago the twisted leaves are starting to flatten back out. I just raised them 2" more to 26". So as you made point of observation is so important. And knowing how to react. All the tech in the world at the consumer level isn't worth a hill of beans when your working with lab grade charts.

Totally agree, for the most part but cheap inaccurate tools can still provide a useful metric if you know what you're looking for. For instance, if my ambient temps are 83ish, rh is 60ish and my cheapo ir +/- 5% accuracy gun is reading 78ish, there's a 99% chance that my vegging plants are looking pretty nice. Necessary? Not at all but still useful.

I like to take notes when plants look good/bad and inaccurate tools or not, you'll notice trends. YMMV

Today I pulled out my trusty $12 IR thermometer and took some readings over my plants. The air temp is 73-75 below the canopy and 79-80 just above it.

I measured buds and leaves in various spots from center to corners, at top and within canopy. Readings were repeatable. The leaves and buds were all 76.1 or 76.2 if they were at the top of the canopy and getting direct light. This was true right under the center of the light and all the way to a corner. Two very tall colas sticking several inches up (but at the edge of the light) measured 80F.

Pointing at leaves 6 inches below the canopy top all read around 71F.

RH is typically 49-50%. Not sure this tells me anything. My light is too close, at 12-14 inches and PAR around 1300 in the center. But the leaves beneath don't have a temperature difference compared to top leaves throughout the tent.