Classification of Instrument Measuring Instrument types

In this article we will learn about the classification of instrument measuring instrument types. From this article we will also know classification of voltmter and ammeter.

Classification of Instrument Measuring Instrument types

The result of comparison between unknown quantity and a predefined standard, expressed as a numeric value.

Classification of Instrument

Broadly these may be classified as 

• Absolute Instrument

These Instruments give the magnitude of the quantity being measured in terms of deflection of the instrument and their constant. No comparison is done with other instrument. Ex Tangent Galvanometer.

• Secondary Instrument

These instruments either are calibrated by comparison with an absolute instrument or already calibrated instrument. These instruments are direct reading instrument. Analog voltmeter / Ammeter, Digital voltmeter /Ammeter etc. It is also provide help to understand title which is classification of instrument measuring instrument types.  

There are two other classification are also available :

Deflection type instrument

In this type of instrument deflection provide basis for measurement .

Null type instrument

In this type of instrument Null/Zero indication leads to determination of the magnitude of quantity. Ex. Potentiometer

Analog Instrument

Signals vary in continuity and there is infinite no. of values in a given range.

Digital Instrument

Here signal vary in discrete steps and finite no. of values in a given range.

Type of Instrument

(Based on secondary instrument)

Indicating Instrument

Here value is measured directly by means of scale and pointers. Ex Ammeter, voltmeter etc.

Recording Instrument

Variation of an electrical quantity is recorded continuously. Ex recording voltmeter in substation.

Integrating Instrument

• This instrument measures total amount of a quantity for a particular time.
• Ex. Ampere hour meter, half hour meter.
• Some Important terms of Instrument

Calibration

It is a process of determination of correct value of each scale reading on a meter by comparison with a standard device.

Accuracy

It is the characteristics of instrument, which shows the closeness of measured quantity ith true value. 

Precision

It is the closeness with which individual measurement are departed whether we take nos. of reading.

Repeatability

It is ability of instrument to give particular same value for same input.

Drift

Undesirable change in input and output relationship.

Resolution

Ability of instrument to detect smallest increment in input with certainty.

Classification of Voltmeter and Ammeter

After describe above title which is classification of instrument measuring instrument types . We will study about classification of voltmeter and ammeter.

1. Moving coil Instrument

• PMMC ( Permanent Magnet Type)
• Dynamometer Type

2. Moving Iron-instrument

• Attraction Type Iron-Instrument
• Repulsion Type Iron-Instrument

3. Thermal Instrument

• Hot wire Type Instrument
• Thermocouple Type Instrument

4. Electronic Instrument
5. Induction Instrument
6. Rectifier Instrument

• PMMC type instrument can be used only for direct current .
• Induction type can be used only for AC
• Permanent Magnet Moving Coil

Working principle of  PMMC Instrument

The basic principle of this device is interaction of magnetic fields. Here current carrying conductor experiences force by an external magnetic field. It is also decribe the above title which is classification of instrument measuring instrument types.

Construction of  PMMC Instrument

It consist of a permanent magnet with a moving coil wound on a cylinder is suspended with the help of spring which provide controlling torque. Edy current damping provides a damping. A pointer and scale arrangement is used. It is also describe title which is classification of instrument measuring instrument types.

Advantage of  PMMC Instrument

• Lower power consumption
• Uniform scale
• Range can be extended by shunt & multipliers.
• High torque /weight relation

Disadvantage of  PMMC Instrument

• A.C quantity can be measured
• Maintanance of spring required as for maintaining controlling torque.
• With passing time, field strength reduces.

Error in PMMC Instruments

• Friction error
• Temperature error due to I²R loss in coil.
• Error due to weaking of permanent magnet
• Thermoelectric error

Hence these are classification of instrument measuring instrument type . From above article we have learned about the classification of voltmeter and ammeter