To improve pressure difference evaluation of a fluid in a fluid system, especially in a fluid system having two or more fluids separated by membranes.

A device and a method are provided for correcting a measured pressure into an approximate value close to an actual pressure difference by using a calculation correction quantity. A system is pressurized so as to obtain parameters selected beforehand, and corresponding measured values of each converter are recorded in a data table to obtain a correction quantity. The correction quantity is applied, based on the recorded value in the data table close to an actual measured value in use. An effective use of this method is for a pressure difference between related fluids separated by a semi-permeable membrane, namely, TMP measurement.