3 Tips for Accurate Small-Diameter Tubing Measurements

By Sarah Williams - 23 April, 2019

Small-diameter metal and plastic tubing is used in a wide variety of industrial and medical applications. Some of these tubes can be smaller than 0.125 inches (3 mm) in diameter, with very thin walls. When their applications are safety-critical, they need to meet tight manufacturing tolerances for wall thickness and concentricity. In the past, manufacturers would have to cut the samples to visually inspect the tube or measure the wall thickness using calipers. Today, there’s a fast and nondestructive alternative for quality control inspections of small-diameter tubing, ultrasonic (UT) thickness gaging.

Recommended UT Thickness Gage, Equipment, and Inspection Setup

For tubing with a diameter of less than 0.125 in. (3 mm), we usually recommend using a 20 MHz focused immersion transducer. To efficiently transmit sound energy into a small-diameter part, it’s necessary to focus the sound into a narrow beam. Focused immersion transducers use a contoured acoustic lens along with a column of water to focus the sound beam for increased sensitivity. The RBS-1 bubbler is a desktop immersion-tank and probe fixture that creates a steady column of water and helps keep the test piece centered in the sound beam. To create a consistent, low-flow water column, the immersion tank must be used with one of the bubbler nozzles, which should be matched to the application. A B103 V-notch bubbler simplifies tubing concentricity checks, since it enables you to rotate the tubing to quickly measure the thickness around the entire circumference and slide the tubing over the probe to measure the thickness along the length of the part.

B103 V-notch bubbler with the RBS-1-15 recirculating bubbler system

We recommend using an Olympus 38DL PLUS^® or 45MG ultrasonic thickness gage (using the 45MG-SE single element and 45MG-WF waveform software options) along with a 20 MHz M316-SU immersion transducer with a 0.75 in. (19 mm) focus for this application.

Small-diameter tubing inspections can be easy when using the proper equipment and correctly calibrated instruments, but measurement errors may still occur. Here are 3 simple tips to help you avoid common issues.

Use the IF Blank function to avoid the interface echo trailing edge

When measuring steel tubing with an immersion setup, it’s important to account for the water-to-steel interface echo. The acoustic impedance of water and steel are sufficiently different that they create an impedance mismatch, which results in a large interface echo. Thin-walled tubing creates back wall echoes that are very close together. In most cases, the trailing edge of the interface echo interferes with the first returning back wall echo. To avoid this interference, use the IF Blank function, which conceals the trailing edge of the interface echo, and choose a pair of clean successive echoes that are further out in time and interference free.

The trailing edge of the large interface echo interfering with the first back wall echo

The IF Blank setting can be adjusted to select the strongest pair of signals. Although the selected pair of signals changes as the IF Blank is adjusted, the time of flight between successive back wall echoes remains the same.

Varying IF Blank settings results in the same thickness measurement

Watch for varying back wall signal amplitude when using a focused immersion transducer

Because the water path length in the B103 bubbler plus the metal wall thickness of the part is significantly shorter than the transducer’s focal length, the largest echo amplitude occurs around the third or fourth multiple of the back wall echo, where the focus occurs. This effect is common when measuring thin metal, and it should not affect accuracy if the measured echoes are clean and undistorted, as seen here. To take an accurate measurement, use the IF Blank function to select two successive, distinct back wall echoes.

3^rd and 4^th multiples are the highest amplitude due to the focal length

Don’t set the M3 Blank out too far

To avoid a false measurement, decrease the M3 Blank value until the gage records a measurement using successive back wall echoes, correctly representing the part thickness.

Detection between the 3^rd and 5^th back wall echoes

With the proper techniques, measuring wall thickness of thin-walled tubing is fast and simple with the 38DL PLUS and 45 MG ultrasonic thickness gages. For information on other thickness gaging applications, check out the links below.