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There is another way in which instruments or measurement systems may be calssified. This classification is based  upon the functions they perform. The three main functions are:

1. Indicating function:
Instruments and systems use different kinds of methods for supplying information concerning the variable quantity under measurement. Most of the time this information is obtained as a deflection of a pointer of a measuring instrument. In this way, the instrument performs a function which is commonly known as indicating function. For example, the deflection of pointer if a speedometer indicated the speed of the automobile at that moment. A pressure gauge is u8sed for indicating pressure.

2. Recording function:
In many cases the instrument makes a written record, usually on paper pf the value of the quantity under measurement against time or against some other variable.  Thus the instrument performs a recording function. For example, a potentiometric type of recorder used for monitoring temparature records the instantaneous values of temparatures on a strip chart recorder.

3. Controlling function:
This is one of the most important functions especially in the foeld of industrial control processes. In this case, the information is used by the instrument or the system to control the original measured quantity.