External measuring transducers normally supply an output current of 0 to 20 mA that is directly proportional to the physical quantity being measured – the temperature, for example.

If the output current of the measuring transducer is directly proportional to the measured quantity only in certain ranges, linearization can be arranged, provided that the measured data input is set accordingly. Furthermore, for certain applications it may be necessary to limit the range being monitored or to monitor certain parts of the range with a higher or lower sensitivity.

By setting the value pair MEASI: IDC 1 and MEASI: IDC,lin 1, the user specifies which input current IDC will correspond to the current that is monitored by the Limit Value Monitoring function, i.e., IDC,lin. (These two setting parameters refer to value pair number 1; setting parameters for value pairs 2 to 20 are available, too.)

The resulting points, called “interpolation points”, are connected by straight lines in an IDC‑IDC,lin diagram. In order to implement a simple characteristic, it is sufficient to specify two interpolation points, which are also used as limiting values (see Fig. 3-28). Up to 20 interpolation points are available to implement a complex characteristic.

When setting the characteristic the user must remember that only a rising/rising or falling/falling curve sense is allowed (no peak or vee-shapes). If the setting differs, the signal SFMON: Invalid scaling IDC will be generated.

Beyond the linearization described above, the user has the option of scaling the linearized values. Thereby negative values, for example, can be displayed as well and are available for further processing by protection functions.