ASME B89.4.23-2020 pdf download
ASME B89.4.23-2020 pdf download.X-Ray Computed Tomography (CT) Performance Evaluation.
The accuracyspecifications are the MPEs associated with the measurementofthe specific dimensional and geometric characteristics of the test objects. These values are specified by the CT system manufacturer, and they represent the largestCT measurement errors permitted when a CT system is tested per the ASME B89.4.23 testprotocol. A CT system that generates measurement errors greater than the MPE values is not in conformance with this Standard.datacollectionspeeds, ambienttemperatureconditions, thenumberofradiographs inascan,X- ray source voltages and currents, operator training and skill requirements, and other metrological factors that are required to be satisfied for the system to function properly; these are described in section 5. 2.3 Objectives of the Test Protocol Testing is designed to verify the metrological specifications of the CT system. The test protocol described in this Standard has several objectives: to provide a thorough yet efficient test, to limit time and money spent on testing, and to assure users of the capabilities of their CT system.This Standard meets these objectives in several ways.nonconformanceallsurfacepoints:thesetofpointsextractedfromasurfaceofahemisphereaccordingtoaregulargridthathasspacingnot to exceed the voxel size.
No extracted surface points from this setshall be excluded or refined when all surface points are used to evaluate a measurand. NOTE: This type of surface point extraction is denoted by the subscript “all” in the symbol for an MPE specification, e.g., P size.sph.all:CT,MPE ; see paras. 6.1 through 6.3. base material: the part ofthe test object that holds the spheres used as metrological geometric elements. Base material shall be in the same material class as the spheres, or it shall be a material that is low-density and low-CTE. Use ofa low- densityand low-CTE base material shall be disclosed by the manufacturer in the specification sheetfor the CT system in accordance with Mandatory Appendix II. fixturingmaterial:anymaterialotherthanthetestobjectorobstructingbodythatisusedtoholdthetestobjectinposition during a CT scan. It may be any density or CTE as long as it is not in the X-ray material path. low-CTE material: material with a coefficient of thermal expansion (CTE) less than 2 × 10 −6 /°C. low-density material: material with a density less than 2 g/cm 3 . maximum penetration length, L P,max : the maximum allowed penetration length for a specific material class (plastic, aluminum, steel), as stated by the CT system manufacturer. obstructing body: an uncalibrated and arbitrarily shaped material in the test object that is used to provide increased penetrationlength. Itshallbeinthesame materialclass as thespheres usedas metrologicalgeometricelements. Asphere under measurement cannot be its own obstructing body. NOTE: An obstructingbodymaycomprise spheres otherthan thoseundermeasurement, base material, and/orfixturingmaterial, orit may be a separate object.NOTE: The sphericity of a test object may pertain to a portion of a sphere, e.g., a hemisphere. voxel:aunitinathree-dimensional(3D) arraythatdiscretizes areconstructedworkzoneusingtheprinciple ofcomputed tomography. voxel size: the size of a magnified pixel for a CT scan, which is calculated as pixel size divided by magnification. NOTES: (1) Magnification is the X-ray source-to-detector distance divided by the X-ray source-to-object distance. TheratedoperatingconditionsarestatedbytheCTsystemmanufacturerandrepresenttheconditionsforwhichtheCT system has MPE values. Although a CT system may be capable of obtaining measurements outside its rated operating conditions, the user cannot be sure ofthe accuracy ofsuch measurements. The rated conditions make up the applicable domainwheremeasurementscanbeaccuratelyobtained. Consequently,whenselectingaCTsystem,usersshouldreview all the rated operatingconditions andespeciallythe rated material classes and theirmaximum penetration lengths. Ifthe largestpenetration length ofa workpiece, in anyradiograph, exceeds the maximum penetration length (stated bythe CT system manufacturer) for thatmaterial class, then thatmeasurementis outside the rated operating conditions ofthe CT system. Sincematerialeffects areamajorsourceoferrorinCTmeasurements,theusershouldensurethattheaccuracyof the CT system is sufficient at the largest penetration length ofany workpiece measurement anticipated and should also testthesystematthiscondition.Theworkzoneis theboundaryofthephysicalspaceintheCTsystemwithinwhichameasurementcanbeperformed; it has an MPE rating. The CT system manufacturer shall specify the work zone as a rated condition. Work-zone size and shape are generally constrained by the maximum reconstructed volume from a CT scan. For each rated material and scanning mode, the rated work zone shall be unambiguously defined.
