Methods of Measurement

There are numerous ways in which a quantity can be measured.

The method is selected according to the accuracy required for the measurement.

Table of Contents

The following are the different methods for measuring a physical variable.

Direct Method
Indirect Method
Comparative Method
Substitution Method
Null Method
Fundamental Method

Direct Method

In the direct method measurement, the value of a quantity is obtained directly by comparing the unknown with the standard.

It involves, no mathematical calculations to arrive at the results.

Examples:

Measurement of Length of cloth with Scale
Measurement of Resistance by Ohms Meter
Measurement of Length using Vernier Caliper
Measurement of Voltage using Voltmeter

Indirect Method

When direct measurement is not possible, the indirect measurement method is used.

Direct Measurement is measuring exactly the same quantity you want to measure, while Indirect measurement is measuring other quantity and required value is determined by mathematical relationship.

Examples:

Measurement of Volume by measuring Mass and Density

(Volume=Mass/Density)

Measurement of Speed by measuring Distance and Time

(Speed=Distance/Time)

Measurement of Temperature by using RTD or Thermocouple

Output of ‘RTD’ and ‘Thermocouple’ is in ‘Ohm’ and ‘millivolt’ respectively. This is converted into temperature by respective standard conversion tables.

Measurement of DC Current by using shunt(Resistance)

(Ohms Law V=IR i.e. I=V/R) I is current, V is Voltage, R is Resistance

Comparative Method

In the comparative measurement method, the quantity to be measured is compared with the known value of the same quantity.

Examples:

Comparing of Sound Level meter with reference sound level meter
Comparing of Lux meter with reference Lux Meter
Comparing of Voltmeter with a reference Voltmeter

Substitution Method

The substitution measurement method involves the replacement of the value of the quantity to be measured with the known value of the same quantity.

In this method, the quantity to be measured is measured by direct comparison on an indicating device by replacing the measuring quantity with some other known quantity that produces the same effect on the indicating device.

Example:

Calibration of weights by Substitution method

Null Method

In the null measurement method, the quantity to be measured is compared with a known quantity, and the difference between these two is made zero i.e. Indicator Indicates Zero reading.

Null measurement techniques achieve greater accuracy.

Example:

Measuring unknown resistance using Wheatstone bridge

Fundamental Method

In the fundamental measurement method, the measurement is based on the measurement of the base quantities used to define the quantity.

The quantity to be measured is directly measured and linked with the definition of that quantity.

Examples:

Measurement of the temperature by fixed point apparatus
Measurement of voltage by Josephson Junction
Measurement of time by cesium atomic clock
Measurement of length by using Iodine Stabilize Helium-Neon laser

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