Process variable transmitter with self-learning loop diagnostics

A method of performing diagnostics in a two-wire process variable transmitter of an industrial process, comprising:

• sensing a process variable of a process fluid of the industrial process;
• providing an output on a two-wire process control loop which is related to the sensed process variable;
• determining the loop current flowing through the two-wire process control loop when the loop current is related to the sensed process variable;
• coupling terminal voltage measurement circuitry to terminals of the two-wire process variable transmitter, the terminal voltage measurement circuitry configured to measure a terminal voltage across terminals which are configured to couple to the two-wire process control loop;
• during normal operation of the two-wire process control loop during which the loop current is related to the sensed process variable, determining coefficients of a polynomial equation which relates the determined loop current to the measured terminal voltage ;
• and
• performing diagnostics on the two-wire process control loop based upon the determined coefficients.