Hand-held ultrasonic thickness gages are an accurate, repeatable tool for nondestructively measuring wall thickness from one side of a part in both manufacturing and service environments. They can be used on metals, plastics, composites, ceramics, glass, and rubber at thicknesses from approximately 0.1 mm (0. 004”) up to very thick sections of hundreds of millimeters or many inches. They work by measuring the time it takes for a pulse of high frequency sound waves to travel through the test piece. A small probe known as an ultrasonic transducer that generates and receives sound waves is placed on the surface of the part and acoustically coupled with a drop of liquid. The sound pulse travels from the contact surface to the inside wall and bounces back as an echo that is detected by the gage. Using that measured time interval and a Programmed speed of sound for the test material, the gage then calculates and displays its thickness. To obtain the best performance from an ultrasonic thickness gage, you should follow these simple guidelines: 1. BE SURE YOU HAVE THE RIGHT EQUIPMENT FOR THE JOB There are a wide variety of ultrasonic gages and transducers commercially available, with different capabilities. Any ultrasonic measurement system – a gage, a transducer, and a gage setup – will be able to measure across a specific range of thicknesses in specific materials as specified by its manufacturer, and unlike a caliper or micrometer the range will vary with different materials. Additionally, measurement accuracy will vary between different instruments and setups. Always consult the manufacturer’s range and material specifications when selecting a gage and transducer to Insure that it meets your needs. 2. CALIBRATE SOUND VELOCITY For accurate readings, ultrasonic gages must always be programmed with the speed of sound in the test material. Most gages provide default velocity settings as a starting point, but for best accuracy you should always perform a velocity calibration using a sample of the actual material to be measured whose thickness is known. This is usually a very simple procedure that is described in the gage’s operating manual. 3. CHECK YOUR SETUP It is good practice to verify that your instrument is properly calibrated by measuring one or more reference samples of the part to be measured before taking data, especially in cases where a gage has been programmed with multiple setups for different transducers and materials. This normally takes only a few seconds. 4. APPLY COUPLANT Because sound energy at the frequencies used for ultrasonic gaging does not travel well through air, it is necessary to use a small drop of coupling liquid between the transducer and the test piece. Glycerin and propylene glycol are commonly used, but other common liquids such as water and oil often work well. Gel couplants are normally recommended when testing on rough surfaces, and special high temperature couplants are available if the test piece is hot. 5. HOLD THE TRANSDUCER STRAIGHT For best results the transducer should be pressed firmly onto the test piece, and in the case of curved surfaces it should be held straight and centered. Avoid rocking or tilting the transducer, which can affect accuracy. Springloaded holders are available to assist with transducer alignment when testing tubing and other sharply curved test pieces. FOR FURTHER INFORMATION Olympus NDT provides a comprehensive collection of technical papers and application notes on both the theory of ultrasonic thickness gaging and on approaches for specific tests on its web site at www.olympus-ims.com/en/applications. Olympus NDT also offers a tutorial which provides a basic introduction to the theory and practice of ultrasonic thickness gaging, for both newcomers and more experienced users who would like a review. It covers basic ultrasonic theory, how ultrasonic gages Work and how they are used, and discusses a number of specific gage applications. Please see www.olympus-ims.com/en/ndt-theory/thickness-gage/.
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