Application
Measurement of wall thickness and concentricity of small diameter metal and plastic tubing.

Background
Small metal and plastic tubing with a diameter less than 3 mm or 0.125 in. is used in a wide variety of industrial and medical applications. Because the tube walls are so thin, manufacturing tolerances for thickness and concentricity are typically very tight, and proper wall thickness is especially important for safety-critical products as well as tubing used in medical environments. Ultrasonic gaging offers a fast and nondestructive alternative to the common process of cutting tubing samples for optical measurement of thickness.

Equipment
The recommend instrument for small diameter tubing measurements is the Model 38DL PLUS thickness gage. In order to efficiently couple sound energy into small diameter test pieces, it is necessary to focus the sound beam into a very small spot. In the case of diameters under approximately 3 mm (0.125 in.), this is usually done with a 20 MHz focused immersion transducer that focuses the sound beam through a column of water. The transducer is commonly held in a v-notch bubbler that also positions the tube sample in the center of the beam. The RBS-1 Recirculating Bubbler System is a convenient tank, pump, and bubbler mounting fixture for benchtop immersion measurements.

Procedure
The optimum transducer and instrument setup for a given tubing product are selected after initial testing of product samples. Using a transducer such as a 20 MHz M316-SU F=.75" and an appropriate fixturing device such as a bubbler, the 38DL PLUS gage is set up for a Mode 3 (metal) or Mode 2 (plastic) measurement. This setup involves adjustment of gain and blanking parameters for optimum echo detection. The minimum measurable wall thickness is typically 0.1 mm (0.004 in.) for diameters as small as 1.5 mm (0.060 in.). If a v-notch bubbler is used, the tubing can be rotated to quickly obtain thickness measurements all around the circumference as a concentricity check, and slid along to measure thickness at different points along the length. Since the process is nondestructive, long sections of tubing do not need to be cut for measurement.

The examples below show typical measurements of small metal and plastic tubing with a 38DL PLUS gage.


Steel tubing, 1.5 mm (0.060 in.) diameter, 0.15 mm (0.006 in.) wall thickness

Plastic tubing, 2 mm (0.080 in.) diameter, 0.37 mm (0.014 in.) wall thickness