ASME PTC2:2001 pdf free download

ASME PTC2:2001 pdf free download DEFINITIONS AND VALUES
2.1 INTRODUCTION
The Codes provide test procedures which yieldresults of the highest level of accuracy consistentwith the best engineering knowledge and practice,taking into account the cost of tests and monetaryvalue of efficiency to industry. Performance of equip-ment is determined in part by measurements ofphysical quantities. A measurement consists of sens-ing a physical variable and translating this resultinto data that is either indicated or recorded. Analogdata are indicated by the position of a pointer ona dial or by a point or line on a chart. Digital dataare indicated by a visual display of numbers orby a numerical printout. Devices used to makemeasurements are called instruments but many de-vices called instruments must be used with additionalcomponents to measure certain physical variablesand quantities. A millivoltmeter,’for example,canmeasure voltage but a thermocouple must be usedin conjunction with the voltmeter to obtain a temper-ature measurement.
2.2 MEASUREMENT sYSTEM
ln order to make a measurement of a physicalquantity it must first be sensed, and the informationabout the energy change due to sensing must betransmitted to a component that communicates thedata.The requirements for measurement are met bythe system shown in Fig. 2.2.
The primary element is that part of the measure-ment system ‘that first senses’the variable to bemeasured.The energy change produced by the sens-ing must be transmitted to an information-communi-cating unit where it may be used directly or changed(transduced) to some other form to indicate or recorddata. The measurement system may be very simpleor very complex but the three functions appearingin Fig.2.2 are required to make a physical measure-ment. The measurement system may be a singlecomponent such as a liquid-in-glass thermometer where the sensing is done by the bulb, the transmit-ting by the liquid column,and the data display bythe scale. On the other hand,the measurementsystem may be multi-component such as flow mea-surement with orifices where the primary element(the orifice) causes fluid acceleration to produce apressure differential which is transmitted via tubingto a manometer where the data are displayed onthe scale.
2.3 UNCERTAINTY OF MEASURING sYSTEMSMeasurement of a physical quantity never continu-ously gives a result which is correct in an absolutesense.The numerical value determined nearly alwaysdiffers by some amount from the true value,andthe extent of the deviation (called error) dependsupon the type of measurement system used.’Codewriters and test engineers must demonstrate that thetest measurements used will provide results suffi-ciently accurate to accomplish the purposes ofthe test.
The accuracy obtainable for a given measurementis 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 measurementsystem.
(2)The in-situ conditions of its use.
ltem (c)(1) is generally well treated by most engi-neers and data concerning measurement systemcomponents is given in the instruments and Appara-tus Supplements. ltem (c)(2) is often responsibie forgross errors of measurement. Specific analysis isnecessary for each application and installation.ASME’PTC 19.1,iest Uncertainty，defines accu-racy as the closeness of agreement between a mea-sured value and the true value; error as the differencebetween the true value and the measured value;and uncertainty as a numerical estimate of the error.
2.4 INTRINSIC ACCURACY
The intrinsic or inherent accuracy of a measure-ment system depends upon
(a) materials,
(b) construction,and
(c) physical condition at time of use.
while the inherent accuracy of the first two itemscan be estimated from published data concerning anindividual measuring system component, the actualaccuracy at the time of use must be determined bycalibration. For this reason,all significant and pri-mary measuring system components must be cali-brated or checked before and after PTC tests toestablish the effect of their physical condition oninherent accuracy, unless there is no cause for theircalibration to change.
2.5 IN SITU CONDITIONS
The accuracy of a measurement system as useddepends upon its ability to
(a) sense the variable to be measured,(b) transmit energy change,
(c) apply or transduce energy,and(d)display data.
The last two items are essentially independent ofuse unless environmental conditions such as vibra-tion, temperature, humidity, etc., are of such magni-tude as to prevent normal operation. lf care is takento ensure proper environmental conditions, then thecommunication function of accuracy is simply theintrinsic accuracy of the components.