Classification of Types of Measuring Instruments

The electrical and electronic measuring instruments can be broadly classified into following two types.

• Absolute instruments and
• Secondary instruments

Absolute Instruments

These are the instruments which give the value
and magnitude of the quantity being measured in terms of the physical constants of the measuring instrument. These are accurate instruments and no calibration is needed. Absolute instrument are not simple to use and time consuming. Hence these are not used practically for carrying out general measurements. These instruments are used for the purpose of making some standard measurements and for calibrating secondary instruments.

Examples of Absolute Instruments

1. Pendulum
2. Tangent galvanometer
3. Rayleigh current meter
4. Absolute electro-meter and
5. Lorentz force method etc.

Secondary instruments

Secondary instruments will directly show the value of the measuring quantity by its indication or deflection. They require calibration and are calibrated either by using an absolute instrument or by using another calibrated secondary instrument. These instruments are used widely for measurements in industries and laboratories.

Examples:
Ammeter, Voltmeter, Wattmeter, and Energy meter etc.

Classification of Secondary Instruments

There are many basis for classifying the secondary instruments. The below displays some of the categories of classifying secondary instruments

1. Classification based on the nature of their operation

Indicating type

These instruments indicates the value of the quantity by the movement of a needle or pointer

Example: Ammeter

Recording type

These instruments record the values of an electrical quantity with respect to time. It reads the continuous values of the quantity and outputs these reading on a chart or dial.

For example, a seismograph continuously measures the intensity, direction and duration of movement of the earth surface and plots on a paper. It is a recording type of instrument.

Integrating type instruments

The integrating instruments record and integrates the magnitude of the particular quantity over a given period of time. These will not show any indication of the rate change or variation of the quantity.

Example: Energy Meter

2. Classification based on the method of measurement.

Direct deflection instruments

These instruments display the value of an unknown quantity directly with a deflection or indication. These are simple inexpensive and easy to use. These are reasonably accurate

Indirect or comparison type of instruments

These instruments give the value of the measuring quantity in terms of a standard. These are accurate than direct deflection type instruments.
Examples: Potentiometers

3. Classification based on the nature of Signal.

Analog Instruments

The analogue instruments indicate the magnitude of the quantity under measurement with the help of movement of a pointer or a needle on a scale. The errors to human can be higher when taking the values which are fractions of the scale.

Digital Instruments

The digital instruments indicate the magnitude of the quantity being measured in digital format directly in numbers commonly with the help of a seven segment display. Taking readings from digital meters are easy eliminates the error due to human.