The measurement of a given quantity is essentially an act or the result of comparison between the quantity (whose magnitude is unknown) and a predefined standard. Since two quantities are compared, the result is expressed in numerical values. In fact, measurement is the process by which one can convert physical parameters to meaningful numbers. The measuring process is one in which the property of an object or system under consideration is compared to an accepted standard unit, a standard defined for that particular property. The number of times the unit standard fits into the quantity being measured is the numerical measure. The numerical measure is meaningless unless followed by a unit used, since it (unit) identifies the characteristic or property measured. In order that the results of the measurement are meaningful, there are two basic requirements:

• the standard used for Comparison purposes must be accurately defined and should be commonly accepted, and
• the apparatus used and the method adopted must be provable.

The methods of measurements may be broadly classified into two categories:

1. Direct Methods and
2. Indirect Methods.

Direct Methods
In these methods, the. unknown quantity (also called the measurand) is directly compared against standard. The result is expressed as a numerical number and a unit. The standard, in fact, is a physical embodiment of a unit. Direct methods are quite common for the measurement of physical quantities like length, mass and time. Suppose we want to measure the length of a bar. The unit of length is metre. A bar is so many times long because that many units on our standard have the same length as the bar.

Indirect Methods
Measurement by direct methods is not always possible, feasible and practicable. These methods in most of the cases, are inaccurate because they involve human factors. They are also less sensitive. Hence direct methods are not preferred and are rarely used. In engineering applications Measurement Systems are used which require need of indirect methods for measurement purposes.

Measurements involve the use of instruments as a physical means of determining quantities or variables. The instrument serves as an extension of human faculties and enables the man to determine the value of unknown quantity or variable which his unaided human faculties cannot measure, a measuring instrument exists to provide information about the physical value of some variables being measured. Because of the modular nature of the elements within it, it is common to refer the measuring instrument as a measurement system.