I) PRIMARY SENSING ELEMENT
This is the element that first receives energy from the measured medium and procedures an output depending on some way on the measured quantity (measured).
The output is some physical variable e.g. displacement or voltage. An instrument always extract some energy from the measured medium. The measured quantity is always disturbed by the act of measurement, which makes a perfect theoretically impossible. Good instruments are designed to minimize this loading effect.
II) VARIABLE CONVERSION ELEMENT
If may be necessary to convert the output signal of the primary sensing element to another more suitable variable while preserving the information content of the original signal. This element performs this function.
III) VARIABLE MANIPULATION ELEMENT
An instrument may require that a signal represented by some physical variable be manipulated by some way.
By manipulation we mean specifically a change in numerical value according to some definite rule but a preservation of the physical nature of the variable. This element performs such a function.
IV) DATA-TRANSMISSION ELEMENT
When functional elements of an instrument are actually physically separated, it becomes necessary to transmit the data from one to another. This element performs this function.
V) DATA PRESENTATION ELEMENT
If the information about the measured quantity is to be communicated to a human being for monitoring, control, or analysis purposes, it must be put into a form recognizable by one of the human senses. This element performs this ‘translation’ function.
VI) DATA STORAGE/PLAYBACK ELEMENT
Some applications require a distinct data storage/playback which can easily recreate the stored data upon command.