Primary Sensing Element: primary sensing element of system is that which first receives energy from the measured medium and produces an output depending in some way on the value of measured quantity.
Variable Conversion Element: A variable conversion element merely converts the output signal of the primary sensing element into a more suitable variable or condition useful to the function of the instruments.
Variable Manipulation Element: It manipulates the signal represented by some physical variable, to perform the intended task of an instrument. In the manipulation process, the physical nature of the signal is preserved.
Data Transmission Element: It transmits the data from one element to other element.
Data presentation Element: It performs the translation function, such as the simple indication of a pointer moving over a scale or recording of a pen moving over a chart.
The Primary Element/Transducer: The input receives the quantity whose value is to be measured and is converted into its proportional incremental electrical signal such as voltage, current, resistance change, inductance or even capacitance. Thus, the changed variable contains the information of the measured variable. Such a functional element or device is called a transducer.
The Secondary Element/Signal Processing Unit :The output of the transducer is provided to the input of the signal processing unit. This unit amplifies the weak transducer output and is filtered and modified to a form that is acceptable by the output unit. Thus this unit may have devices like: amplifiers, filters, analog to digital converters, and so on.
The Final Element/Output Unit: The output from the signal processing unit is fed to the input of the output unit. The output unit measures the signal and indicates the value to the reader. The indication may be either through: an indicating instrument, a CRO, digital computer, and so on