ASME PTC 2-2001 pdf download.DEFINITIONS AND VALUES.
2.1 INTRODUCTION The Codes provide test procedures which yield results of the highest level of accuracy consistent with the best engineering knowledge and practice, taking into account the cost of tests and monetary value of efficiency to industry. Performance of equip- ment is determined in part by measurements of physical quantities. A measurement consists of sens- ing a physical variable and translating this result into data that is either indicated or recorded. Analog data are indicated by the position of a pointer on a dial or by a point or line on a chart. Digital data are indicated by a visual display of numbers or by a numerical printout. Devices used to make measurements are called instruments but many de- vices called instruments must be used with additional components to measure certain physical variables and quantities. A millivoltmeter, for example, can measure voltage but a thermocouple must be used in conjunction with the voltmeter to obtain a temper- ature measurement.
2.3 UNCERTAINTY OF MEASURING SYSTEMS Measurement of a physical quantity never continu- ously gives a result which is correct in an absolute sense. The numerical value determined nearly always differs by some amount from the true value, and the extent of the deviation (called error) depends upon the type of measurement system used. Code writers and test engineers must demonstrate that the test measurements used will provide results suffi- ciently accurate to accomplish the purposes of the test. The accuracy obtainable for a given measurement is dependent upon the following three components: (a) the characteristics of the measured quantity, (b) the accuracy of the observation, and (C) the measurement system used. (1) The intrinsic accuracy of the measurement system. (2) The in-situ conditions of its use. ltem (C)(1) is generally well treated by most engi- neers and data concerning measurement system components is given in the Instruments and Appara- tus Supplements. ltem (C)(2) is often responsible for gross errors of measurement. Specific analysis is necessary for each application and installation.
2.6 OBSERVATION ACCURACY Accuracy of observation depends primarily upon the following two factors: (a) acidental mistakes, e.g, misreading of scales, parallax, incorrect log entries, failure to perform some required manipulation, etc; and (b) personal characteristics, e.g. ability to interpo- late between graduations, bias in observation (ten- dency to read high or low), speed of observation. These types of errors may be minimized (but never completely eliminated) by selection and training of test personnel, by selection of scales with easily-read graduations, and by other human-factor engineering. Modern technology permits the design of instrument systems that will give digital printout, and the use of these should be encouraged to eliminate observation error when their inherent accuracy and in situ condi- tions permit. However, digital systems may contain programming mistakes, and these systems must be debugged thoroughly.ASME PTC 2 pdf download.