Metrology and Metrological Terminology

• Metrology is derived from two Greek word, one is metro which means measurement and

other is logy which means science. Metrology is basically the science of measurement.

• Metrology is field of knowledge concerned with measurement and includes both theoretical

and practical problems with reference to measurement.

• Metrology is the name given to the science of pure measurement.

• Engineering Metrology is restricted to measurements of length & angle.
Metrology is mainly Concerned with:• Establishing the units of measurement, reproducing these units in the form of standards and
ensuring the uniformity of measurements.• Developing methods of measurement.
Analysing the accuracy of methods of measurement, researching into the causes of measuring errors and eliminating them.
Type of Metrology
Metrology is separated into following categories with different levels of complexity and accuracy:

1. Scientific Metrology

2. Industrial Metrology

3. Legal Metrology

4. Fundamental Metrology

Scientific Metrology deals with the organization and development of measurement standards

and with their maintenance.Industrial Metrology has to ensure the adequate functioning of measuring instruments used
in industry as well as in production and testing processes.

Legal Metrology is concerned with the accuracy of measurements where these have

influence on the transparency of economical transactions, and health and safety, e.g., the
volume and quality of petrol purchased or the weight and quality of prepackaged flour. It
seeks to protect public against inaccuracy in trade.Fundamental Metrology may be described as scientific metrology, supplemented by those
parts of legal and industrial metrology that require scientific competence. It signifies the
highest level of accuracy in the field of metrology.

METROLOGICAL TERMINOLOGIES

 

Accuracy is the closeness of agreement between a test result and the accepted reference value.

Precision is the closeness of agreement between independent test results obtained under stipulate

conditions.

Repeatability conditions are where independent test results are obtained with the same method on

identical test items in the same laboratory by the same operator using the same equipment within
short intervals of time.

Reproducibility conditions are where test results are obtained with the same method on identical

test items in different laboratories with different operators using different equipment.

Correction is the value which, added algebraically to the uncorrected result of a measurement,

compensates for an assumed systematic error.

Drift is a slow change of a metrological characteristic of a measuring instrument.

Magnification In order to measure small difference in dimensions, the movement of the measuring

tip in contact with work must be magnified and, therefore, the output signal from a measuring

instrument is to be magnified many times to make it more readable. In a measuring instrument,
magnification may be either mechanical, electrical, electronic, optical, pneumatic principle or a
combination of these.

Reference, accepted value serves as an agreeed-on reference for comparison, and which is derived

as theoretical or established value, based on scientific principles or experimental work.

Range is the capacity within which an instrument is capable of measuring.

Readability refers to the ease with which the readings of a measuring instrument can be read. If the

graduation lines are very finely spaced, the scale will be more readable by using a microscope, but

the readability will be poor with the naked eye.

Response time is the time which passes after a sudden change of the measured quantity until the

instrument gives an indication different.

Resolution is the smallest change of the measured quantity which changes the indication of a

measuring instrument. Resolution describes the degree to which a change can be detected.

Sensitivity of the instrument denotes the smallest change in the value of the measured variable to

which the instrument responds. Sensitivity describes the smallest absolute amount of change that

can be detected by a measurement, often expressed in terms of millivolts, microhms

Stability refers to the ability of a measuring instrument to constantly maintain its metrological

characteristics with time.

Standardization is a process of formulating and applying rules for orderly approach to a specific

activity for the benefit and with the cooperation of all concerned in particular. This is done for the

promotion of overall economy, taking due account of functional conditions and safety requirements.

standardisation is the process of developing and implementing technical standards. By using

standardisation, groups can easily communicate through the set guidelines, in order to maintain

focus. The method is made to facilitate processes and tasks

Testing is a technical investigation, e.g., as to whether a product fulfils its specified performance.

Trueness is the closeness of agreement between the average value obtained from a large series of

test results and an accepted reference value . The measure of trueness is usually expressed in terms

of bias

Verification is an investigation that shows that specified requirements are fulfilled.

Dead zone it is the range within which variable can vary without being detected.
Tolerance it is the range of inaccuracy which can be tolerated in measurements.
Accuracy
Precision