Classification of Instrumentation System

Classification of Instrumentation System


Classification of Instrumentation System
Instrumentation system


Null and Deflection type Instruments:


Deflection type instruments:


The instruments in which deflection of the instrument forms the basis of measuremenl are called as deflection type instruments.

The example of deflection type instruments is the permanent magnet moving coil (PMMC) ammeter.

Deflection type Instruments
PMMC AMMETER


The deflection of the coil is proportional to the current that is to be measured.

Torque Td, that acts on the moving coil is proportional to current.The opposing effect is produced by a spring, having torque Tc which is proportional to the deflection.

When the spring is balanced Tc=Td.

The value of the current depends on the deflection.Thus,the value of the measured quantity depends upon the calibration of the instrument.

Null type Instruments


The instrument in which, a null or zero indication will lead to the determination of the magnitude of the measured quantity such instruments are called null type instruments.

The null condition depends on other known conditions.

The DC potentiometer is an example of null type potentiometer.

Fig. shows the D.C.potentiometer.

Null type Instruments
DC potentiometer


The unknown emf is to be measured.The slide wire of the potentiometer is calibrated in terms of emf with the help of a standard emf source.

The null detector is a galvanometer whose deflection is proportional to the unbalance emf.When the difference between the two sliding contacts is zero, no current flows through the galvanometer and it will show zero deflection.

Thus,the unknown emf is calibrated directly on scale.

 Analog and Digital Instruments:


Analog Instruments:


The signals which continuously vary and takes infinite values for a given range are called as analog signals.The instrument which produces analog signals is called as an analog instrument.

In analog system the function continuously varies.

Examples of analog instruments are deflection type instruments like PMMC ammeter, voltmeter, null deflection instruments like DC potentiometer, wheatstones meter bridge etc.

Digital Instruments:


The signals which vary in discrete steps and take a finite different values in the given range are called digital signals.

The instrument which produces digital signals is called as digital instrument.

Examples of digital instruments are logic analysers,microprocessor based instruments, computers etc.

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