Introduction_to_Measurements - Introduction to Mechanical...

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Introduction to Mechanical Engineering Measurements Author: John M. Cimbala, Penn State University Latest revision, 14 January 2008 Two Main Purposes of Measurements Engineering experimentation - This is where we new information , and is generally done when developing a new product . Some example questions which may be asked by the engineer are: How hot does it get? How fast does it go? Operational systems - This is where we monitor and control processes, generally on existing equipment rather than in the design of new products. For example, consider the heating and/or air conditioning control system in a room. The system measures the temperature, and then controls the heating or cooling equipment. Dimensions and Units Primary (or Basic) Dimensions There are seven primary dimensions (also called basic dimensions ). All other dimensions can be formed by combinations of these. The primary dimensions are: mass , length , time , temperature , current , amount of light , and amount of matter . Unit Systems Unit systems were invented so that numbers could be assigned to the dimensions . o There are three primary unit systems in use today: ± the International System of Units (SI units, from Le Systeme International d’Unites , more commonly simply called the metric system of units ) ± the English Engineering System of Units (commonly called English system of units ) ± the British Gravitational System of Units (BG) o The latter two are similar, except for the choice of primary mass unit and use of the degree symbol. The two dominant unit systems in use in the world today are the English system and the metric system . [The BG system is no longer popular, and I do not recommend that you use it.] The table below shows each of the primary dimensions, along with their symbols and units in the SI, English, and BG unit systems: Primary dimension Symbol SI unit English unit BG unit mass m kg (kilogram) lbm (pound-mass) slug length L m (meter) ft (foot) ft (foot) time t s (second) s (second) s (second) thermodynamic temperature T K (kelvin) R (rankine) o R ( o Rankine) current I (or i) A (ampere) A (ampere) A (ampere) amount of light (luminous intensity) C (or I) cd (candela) cd (candela) cd (candela) amount of matter N mol (mole) mol (mole) mol (mole) All other dimensions and units can be derived as combinations of these seven . These are called secondary dimensions , with their corresponding secondary units . A few (units) examples are given in the table below: Secondary dimension Symbol SI unit English unit BG unit force F N (newton = kg m/s 2 ) lbf (pound-force) lbf (pound-force) acceleration a m/s 2 ft/s 2 ft/s 2 pressure P (sometimes p ) Pa (pascal = N/m 2 ) lbf/in 2 (psi) lbf/ft 2 (psf) energy E J (joule = N m) ft lbf (foot-pound) ft lbf (foot-pound) power W ± (sometimes P ) W (watt = J/s) ft lbf/s ft lbf/s
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Example Given: An engineer is measuring surface tension.
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This note was uploaded on 07/23/2008 for the course ME 345 taught by Professor Staff during the Spring '08 term at Penn State.

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Introduction_to_Measurements - Introduction to Mechanical...

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