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Nowadays instrument transformers are essential in both automation and control systems. Instrument transformers are needed when physical quantities are used for a process control. The reason is that it is necessary to record a large number of quantities. In addition to the most common quantities as temperature or pressure, there are other parameters such as pressure, gas content or flow. For electronics to receive a readable signal, instrument transformers must convert the physical quantity to an electrical signal. To ensure flexibility, normalized signals have been determined to be read by many meters.

Here you can see the instrument transformers functioning:

The use of an analog normalized signal allows instrument transformers for any quantity to be connected to the same analog input of a digital display. The digital display should only be scaled, this means that digital display should indicate the value from instrument transformers as an electrical quantity corresponding to a physical quantity value. The next image shows the assignment of a 4-20mA output signal to a temperature range of  0 - 100 ºC:

Some instrument transformers have a flexible scale which allows the users to adjust the measurement range according to their needs. Apart form its flexible use, the normalized signal of 4-20 mA has other advantages such as: connection cables from one device to another can be very long, it is possible to add diverse units of analysis to the same circuit, so a current signal from the transducer can be used in different places. As even with the lowest measurement value current flows, users can use the 4 mA to feed sensors. Current below 4mA indicates that there is something wrong with the analysis units. In order to read the measurement value in situ, some instrument transformers have a display. Other transducer models have relays which make a contact when limit values are exceeded.