How do you calibrate an IR thermometer for measuring leaf temp?
Check YouTube. Briefly, change emissivity on the thermo to that of water and then do a 2 point calibration using an ice bath and then boiling water, with temp for the latter corrected to altitude.
Re. VPD - the phrase "chasing" is pejorative and I've noticed that many growers who use that term seem not to be that familiar with VPD.
There are optimal values for VPD but what does optimal mean? To my way of thinking, it means transpiration rates will be such that you don't have to change your normal watering practices and that you can run standard strength nutrients.
VPD is nothing more than one number to describe a range of temperature and RH values and VPD is not restrictive-it's simply a way of telling you what the "feels like" temperature is to your plants. It's a handy way to determine if your plant is going to be transpiring a lot, if VPD is higher than optimal, and, if it's lower than optimal, that there might be transpiration and mold/fungus issues. It's a measurement that a grower uses to understand what's happening in the grow environment.
Unfortunately, growers have glommed on to idea that VPD is an end itself. I'll admit that I labored under that perception when I started growing. It took a while to understand that it isn't the goal, rather it is just a signpost along the way. Cannabis will do fine, even under very high VPD, as long as the grower compensates for unusual conditions.
If your VPD is 2.x, for example, you're going to need to make sure that your plants get enough water and that your nutrients are diluted enough to avoid nutrient imbalance. Within reasonable limits, a VPD of X, in and of itself, won't kill a plant. I wouldn't grow a plant at VPD of 2.x but it won't kill the plant if VPD hits that value intermittently,
as long as you tend to your plants.
Again, VPD is nothing more than one number to describe a range of temperature and RH combinations. Take a look at a VPD chart and you've got to have a really harsh environment to get to unusually high or low values.
As an analogy, humans rarely die by over heating but, even in moderate temperatures, we're dead in a few days without water.
