Jackstrat - I would have to say, in my opinion, while I definitely hear where you're coming from and fitting differential in there made me have to think for a minute, that deficit is the correct word. While the word differential most definitely has its place within this conversation about VPD, I'm going to venture out and say, it would refer to the difference between the temp and RH (as represented by the values on your metering) and the value that is presented to us as a result of this differential between these two factors is what would be referred to as a deficit. Labeled as such in leu of differential because it is what the ambient air LACKS (is deficient) before it becomes fully saturated with water vapor (100% RH). So finding out what the differential of temp vs RH is, is a step on the way to finding out what the deficit is. Just my opinion...anyone feel free to oppose it. I welcome healthy debates it is how true knowledge is attained and, ALAS, we learn shit....hope my ramblings helped.
peace and CBD grease