DAKOTA ULTRASONICS PVX-B Operation Manual

Contents

DAKOTA ULTRASONICS PVX-B Operation Manual Manual pdf 22 page image

20CHAPTER FOURPRINCIPALS OF ULTRASONIC MEASUREMENT4.1 Time versus thickness relationshipUltrasonic thickness measurements depend on measuring the length of time it takesfor sound to travel through the material being tested. The ratio of the thicknessversus the time is known as the sound velocity. In order to make accuratemeasurements, a sound velocity must be determined and entered into theinstrument.The accuracy of a thickness measurement therefore depends on having a consistentsound velocity. Some materials are not as consistent as others and accuracy will bemarginal. For example, some cast materials are very granular and porous and as aresult have inconsistent sound velocities.While there are many different ultrasonic techniques to measure thickness, which willbe discussed below, all of them rely on using the sound velocity to convert from timeto thickness.4.2 Suitability of materialsUltrasonic thickness measurements rely on passing a sound wave through thematerial being measured. Not all materials are good at transmitting sound.Ultrasonic thickness measurement is practical in a wide variety of materials includingmetals, plastics, and glass. Materials that are difficult include some cast materials,concrete, wood, fiberglass, and some rubber.4.3 Range of measurement and accuracyThe overall measurement capabilities, based on the wide variety of materials, isdetermined by the consistency of the material being measuredThe range of thickness that can be measured ultrasonically depends on the materialas well as the technique being used and the type of transducer. Thicknessmeasurements can be made from a minimum of 0.010 inch to 9.999” in steel.However, the maximum attainable thickness is much less for more attenuativematerials (materials that absorb sound).Accuracy, is determined by how consistent the sound velocity is through the soundpath being measured and, is a function of the overall thickness of the material. Forexample, the velocity in steel is typically within 0.5% while the velocity in cast ironcan vary by 4%.4.4 CouplantAll ultrasonic applications require some medium to couple the sound from thetransducer to the test piece. Typically a high viscosity liquid is used as the medium.The sound frequencies used in ultrasonic thickness measurement do not travelwww. GlobalTestSupply .comFind Quality Products Online at: sales@GlobalTestSupply.com