Methods of Measurement

METHODS OF MEASUREMENT

(i) Direct Methods

(ii) Indirect Methods

(i) Direct Methods: In these methods, the unknown quantity (called the measurand) is directly compared against a standard.

(ii) Indirect Method: Measurements by direct methods are not always possible, feasible and practicable. In engineering applications measurement systems are used which require need of indirect method for measurement purposes.

Instruments and Measurement Systems

Measurement involves the use of instruments as a physical means of determining quantities or variables. Because of modular nature of the elements within it, it is common to refer the measuring instrument as a Measurement System.

Evolution of Instruments

(i) Mechanical

(ii) Electrical

(iii) Electronic Instruments.

(i) Mechanical: These instruments are very reliable for static and stable conditions. But their disadvantage is that they are unable to respond rapidly to measurements of dynamic and transient conditions.

(ii) Electrical: It is faster than mechanical, indicating the output are rapid than mechanical methods. But it depends on the mechanical movement of the meters. The response is 0.5 to 24 seconds.

(iii) Electronic: It is more reliable than other system. It uses semiconductor devices and weak signal can also be detected.

Classification of Instruments

i. Absolute Instruments.

ii. Secondary Instruments.

Absolute: These instruments give the magnitude if the quantity under measurement terms of physical constants of the instrument.

Secondary: These instruments are calibrated by the comparison with absolute instruments which have already been calibrated. Further it is classified as

a. Deflection Type Instruments

b. Null Type Instruments.

Functions of instrument and measuring system can be classified into three. They are:

i. Indicating function.

ii. Recording function.

iii. Controlling function.

Application of measurement systems are:

i. Monitoring of process and operation.

ii. Control of processes and operation.

iii. Experimental engineering analysis.

1. Types of Instrumentation System

Intelligent Instrumentation (data has been refined for the purpose of presentation)

Dumb Instrumentation (data must be processed by the observer)

2. Elements of Generalized Measurement System

i. Primary sensing element.

ii. Variable conversion element.

iii. Data presentation element.

Primary Sensing Element: The quantity under measurement makes its first contact with the primary sensing element of a measurement system.

Variable Conversion Element: It converts the output of the primary sensing element into suitable form to preserve the information content of the original signal.

Data Presentation Element: The information about the quantity under measurement has to be conveyed to the personnel handling the instrument or the system for monitoring, control or analysis purpose.