The conformal transducer of a piezoelectric transducer system which yields results in PSI does not measure firing stress of the barrel steel over the chamber area. However, a strain gauge does.
Again, there is a correlation between CUP and PSI. However, as Ron Keysor states in his 1996 article in American Rifleman, Pressure Measurement, "shooters have been left to reconcile two pressure value systems that don't correlate in a sense we laymen can understand."