For those interested in how to calculate VPD for your home-built projects.
HOW TO CALCULATE AIR VPD:
Figure out the SVP
SVP = 610.78 x e(T / (T +238.3) x 17.2694))
e is a mathematical constant.
T is in degrees Celsius
The result, SVP, is measured in pascals (divide by 1000 to get kPa)
Calculate the VPD
SVP x (1 – RH/100) = VPD
HOW TO CALCULATE SURFACE VPD:
Figure out the air SVP (ASVP)
Same formula as getting the SVP for air VPD
Figure out the surface SVP (SSVP)
It is the same formula as ASVP, but you use the surface temperature in your calculation (typically 1-3 °C or 2-5 °F cooler). This is where that IR thermometer comes in handy.
Surface VPD = SSVP – (ASVP x RH/100)