ULTRASONIC THICKNESS MEASUREMENT PROCEDURE
1.0 SCOPE
This procedure intends to use for thickness measurement by using ultrasonic thickness gauge.
2.0 REFERENCE
2.1 ASTM E797: Standard Practice for Measuring Thickness by Manual Ultrasonic Pulse-Echo contact Method, latest code adopted edition and addenda
2.2 ASME, Boiler and Pressure Vessel Code Section V, latest code adopted edition and addenda
3.0 EQUIPMENT
3.1 Main Instrument
The instrument shall be direct thickness readout or direct thickness readout with A-scan display.
3.2 Search units
Search units may contain either single or dual transducer elements and shall be straight beam contact type. For thickness expect to be less than 3 mm, use probe diameter less than 6 mm with frequency at least 5 MHz.
3.3 Couplant
A couplant having well wetting properties (water, emulsifiable oil etc.) shall be used. The couplant must be compatible with the temperature of the specimen.
4.0 MEASURING PROCEDURE
4.1 Calibration Frequency
4.1.1 Calibration should be checked prior to examination.
4.1.2 Calibration should be checked each time there is a change of operators, search units, new batteries, etc. to ensure that the system is not changing.
4.2 Calibration Block (Step Wedge)
4.2.1 The reference standard material should be similar in velocity as production material.
4.2.2 The calibration block (step wedge) shall be measured its thickness by micrometer that can be traceable.
4.3 Step of Calibration
4.3.1 Adjust sound velocity for production material and do zero setting by using the calibration block (step wedge), adjust real thickness of the block to tune the equipment. The calibration shall be done on at least 2, preferably 3, calibration thickness covering the thickness range to be measured.
4.3.2 Retest by reading the thickness from equipment compare to the real thickness. The reading shall be within 0.05 mm. of thickness of calibration block c. Any equipment that fails to meet the requirement shall be readjusted and recalibrated. If not, the equipment shall not be put into service.
4.4 Step of Measuring
4.4.1 Remove the paint for conventional equipment.
4.4.2 Apply couplant to the test block or material at the spot to be measured.
4.4.3 Place the tip of the transducer to the surface of material to be measured. Use moderate to firm pressure and keep the transducer as flat as possible on the material surface.
4.4.4 Slightly move the probe until obtain a minimum value, read the material thickness on the display for digit display type. For A-scan display type, move the correspond gates to correct echoes.
Note: 1. Align the a sound barrier transverse to pipe axis
2. Beware that in the case of no thickness display or loss of echo, it could be the indication of extremely loss of thickness.
3. Compensation of paint thickness for certain type of equipment in combination of corrected probe.
5.0 REPORT
Test conditions, thickness reading and location map shall be reported.