To identify the final period TU and final gain PU of the process, the control engineer running the test temporarily disables the PID block and replaces it with an on/off relay that forces the process variable to oscillate.
To identify the ultimate period TU and ultimate gain PU of the process, the control engineer running the test temporarily disables the PID block and replaces it with an on/off relay that forces the process variable to oscillate.  The engineer then measures TU directly from the square-wave control effort and computes the value of PU according to   PU = (4 · b) / (î�° · a)  where a is the amplitude of the controller’s square wave and b is the amplitude of the process variable’s resulti