chap01 - PROBLEM 1.1 FIND Explain the hierarchy of...

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PROBLEM 1.1 FIND: Explain the hierarchy of standards. Explain the term standard . Cite example. SOLUTION The term standard refers to an object or instrument, a method or a procedure that provides a value of an acceptable accuracy for comparison. A primary standard defines the value of the unit to which it is associated. Secondary standards , while based on the primary standard, are more readily accessible and amenable for use in a calibration. There is a hierarchy of secondary standards: A transfer standard might be maintained by a national standards lab (such as NIST in the United States) to calibrate industrial “laboratory standards”. It is costly and time-consuming to certify a laboratory standard, so they are treated carefully and not used too regularly. A laboratory standard would be maintained by a company to be used to certify a more common in-house reference called the working standard . A working standard would be calibrated against the laboratory standard. The working standard is used on a more regular basis to calibrate everyday measurement devices or products being manufactured. Working standards are more the norm for most of us. A working standard is simply the value or instrument that we assume is correct in checking the output operation of another instrument. Example: A government lab maintains the primary standard for pressure. It calibrates a an instrument called a “deadweight tester” (see C9 discussion) for high pressure calibrations. These form its transfer standard for high pressure. A company that makes pressure transducers needs an in-house standard to certify their products. They purchase two deadweight testers. They send one tester to the national lab to be calibrated; this becomes their laboratory standard. On return, they use it to calibrate the other; this becomes their working standard. They test their manufactured transducers using the working standard – usually at one or two points over the transducer range to assure that it is working. Because the working standard is being used regularly, it can go out of calibration. Periodically, they check the working standard calibration against the laboratory standard. See ASME PTC 19.2 Pressure Measurements for a further discussion. A test standard defines a specific procedure that is to be followed.
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PROBLEM 1.2 FIND: Why calibrate? What does calibrated mean? SOLUTION: The purpose of a calibration is to evaluate and document the accuracy of a measuring device. A calibration should be performed whenever the accuracy level of a measured value must be ascertained. An instrument that has been calibrated provides the engineer a basis for interpreting the device‟s output indication. It provides assurance in the measurement. Besides this purpose, a calibration assures the engineer that the device is working as expected.
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This note was uploaded on 10/31/2009 for the course MECH 420 taught by Professor M.s during the Spring '09 term at American University of Beirut.

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chap01 - PROBLEM 1.1 FIND Explain the hierarchy of...

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