An instrument may be defined as a device or a system which is designed to maintain a functional relationship between prescribed properties of physical variables and must include ways and means of communication to a human observer. The functional relationship remains valid only as long as the static calibration of the system remains same.
The main functional elements of a measuring system are:
Primary sensing element: The quantity under measurement makes its first contact with the primary sensor (sensing elment). The physical quantity to to be measured, in the first place is sensed and detected by an element which gives te output in a different analogous form which is then converted into electrical signal by a TRANSDUCER. This stage is known as the Detector Transducer Stage.
Variable conversion element: The o/p of the primary sensing element may be an electrical signal of any form. It may be a voltage, a frequency or some other electrical parameter. For the instrument to perform the desired function, it may be necessary to convert this output to a suitable form while preserving the information content of the original signal. Eg: an A/D can be used to convert an analog signal into a digital signal.Many instruments do no need a variable conversion element, while some may need more than one conversion element.
Variable manipulation element: The function of this element is to manipulate the signal preserving the original nature of the signal. Manipulation here means only a change in the numerical value of the signal. Eg: electronic amplifiers accepts a small voltage signal as input and produces an output signal which is also voltage but of greater magnitude. Vice versa, attenuators are used.
Signal conditioning element: The signal after being sensed cannot be directly transmitted to the next stage without removing the interferring sources, as otherwise highly distorted results may be obtained . Hence the signal has to undergo conditioning like amplification, attenuation, integration, differentiation, addition and subtraction for linear signals and modulation, detection, sampling. filtering, choping and clipping etc for non-linear signals.
Data transmission element: when the elements of a system are physically seperated, it becomes necessary to transmit data from one to another. The element that performs this function is called a Data Transmission element. eg: Telementry systems.
Data presentation element: The information about the quantity under measurement has to be conveyed to the personal handling the instrument ot the system for monitering, control or analysis purposes. This is done by a data presentation element. Visual display devices include analog and digital indicating instruments like ammeters, voltmeters etc. Recording instruments like magnetic tapes, high speed camera, T.V eqiptment, storage type C.R.T, printers etc. For control and analysis purpose micro-controllers and compters are used. This stage is known as the Terminating stage.