The primary function of an instrument is to measure a process variable. While doing so, it performs several functions such as Transmitting,Signaling,Registering,Indicating,Recording.
Transmitting – Instrument sends the information concerning the measured quantity over some distance.
Transmitters are used in process control loop.
Signaling – Instruments indicates the general value or range of values of its measured quantity.
Registering – Instrument indicates by numbers or some other symbol of discrete increments, the value of the quantity being measured.
Indicating – An indicating instrument indicates the instantaneous value of the variable being measured at that time. Such an instrument normally uses a calibrated scale and pointer.
Recording – Instruments makes a written record of the value of the measured quantity against some other variable or against time.