Essentially, there are only two highly accurate ways to measure water potential—with a tensiometer or with vapor pressure. The tensiometer works in the wet range of water potential—from 0 to about -0.2 MPa. Vapor pressure methods work in the dry range of water potential—from about -0.1 MPa to -300 MPa (0.1 MPa is 99.93% RH; -300 MPa is 11%). With recent advances in technology, a skilled user with excellent methods and the best equipment can finally measure the full water potential range in the lab.
The pressure plate doesn’t actually measure the water potential of a sample. Instead, it brings the sample to a specific water potential by applying pressure to the sample and allowing the excess water to flow out through a porous ceramic plate. When the sample comes to equilibrium, its water potential will be equivalent to the pressure applied.