Mechanical Engineering and Mechatronics Handbook by D. Kumar

Author:D. Kumar
Language: eng
Format: mobi, epub
Publisher: Mercury Learning and Information LLC
Published: 2022-10-15T00:00:00+00:00

7.1. MEASUREMENT AND INSTRUMENT

The word measurement is used to tell us the length, the weight, the temperature, the color, or the change in one of these physical entities of a material. Measurement is the result of an opinion formed by one more observer about the relative size or intensity of some physical quantity. The opinion is formed by the observer after comparing the object with a quantity of the same kind chosen as a unit, called standard. The result of measurement is expressed by a number representing the ratio of the unknown quantity to the adopted standard. This number gives the value of the measured quantity. For example, 10 cm length of an object implies that the object is ten times as large as 1 cm; the unit employed in expressing length.

The measurement standard is the physical embodiment of the unit of measurement. This places a sizable responsibility on the observer, he may be an engineer or a technician, to be certain that the standard used by him is accurately known and commonly accepted. Further, the procedure and apparatus employed for obtaining the comparison must be provable, that is, accuracy can be reproduced anywhere in the world. This is essential so that measurements obtained by him can be accepted with confidence. For consistency and quantitative comparison of physical parameters, certain standards of mass, length, time, temperature, and electrical quantities have been established. These standards are internationally accepted and well-preserved under controlled environmental conditions.

The physical quantity or the characteristic condition which is the object of measurement in an instrumentation system is variously termed as measurand, measurement variable, instrumentation variable, and process variable. The measurand may be a fundamental quantity (length, mass, and time), a derived quantity (speed, velocity, acceleration, power, etc.), or a quality (pressure, temperature, etc.).

The human senses cannot provide exact quantitative information about the knowledge of events occurring in our environments. The stringent requirements of precise and accurate measurements in the technological fields have, therefore, led to the development of mechanical aids called instruments. Scientific instruments allow humans to observe and measure aspects of the physical universe beyond the range and precision of the unaided human senses. Instruments are the essential extensions of human sensing and perception without which scientific exploration of nature would be impossible. The instrument would sense a physical parameter (pressure, temperature, velocity, etc.), process and translate it into a format and range which can be interpreted by the observer.

The man-made instruments are not only accurate and sensitive in their response but also retain their characteristics for extended periods of time. Instruments may be quite simple, such as a liquid-in-glass thermometer, or extremely complex such as the device to sense the physiological reactions of a man during space flight.

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