Forewords

In the field of industrial production, the wall thickness of small-diameter copper tube is a key parameter that directly affects its performance and service life. Ultrasonic thickness gauge as an efficient and accurate measurement tool, has been widely used in small diameter copper tube wall thickness detection. This paper describes in detail the use of ultrasonic thickness gauge to measure the wall thickness of small diameter copper tube.

The Measurement Principle

Ultrasonic thickness gauges measure the wall thickness of small diameter copper tubes based on the principle of reflection of ultrasonic pulses. When the probe emits ultrasonic pulses at a specific frequency and angle into the copper pipe, the pulses are reflected as they reach the inner wall (the interface between the copper pipe material and the air). The probe then receives the reflected echo. Since the velocity of ultrasound propagation in a given material (e.g., copper) is known and stable, the wall thickness can be calculated using the formula: Thickness = (Material Sound Velocity × Time) / 2, where the time interval between the transmission and reception of the pulses is precisely measured. For small diameter copper tubes, the curvature of the tube may affect the propagation path of the ultrasound. Therefore, the probe must be vertically tight against the tube wall to ensure vertical propagation and minimize measurement errors caused by angular deviations.

The Preparation Before Measurement

Selection of measurement instruments

According to the conditions of tested workpiece, we need an ultrasonic wall thickness tester with high precision and small diameter probe.For example, the PM5 Gen2 Ultrasonic Thickness Gauge manufactured by YUSHI INSTRUMENTS utilizes the delay line probe and holder designed to match the curvature of small diameter tubes to ensure optimal ultrasonic coupling and propagation. Check that the instrument is working properly, that the battery is fully charged, and that the readings are clear.

Preparation of the tested workpiece

Surface Cleaning: Remove oil, rust, coatings, or welding slag from the tube surface using sandpaper, wire brushes, or cleaning agents. For coated tubes, strip the coating from the measurement area to expose the metal substrate.

Edge Treatment: If measuring near the tube end, trim the edge to eliminate burrs or chamfers, ensuring stable probe placement.

Couplant Selection: Use a couplant (e.g., glycerin, machine oil, or petroleum jelly) with moderate viscosity and good fluidity to eliminate air gaps between the probe and tube. Glycerin is recommended for small-diameter tubes due to its excellent wetting properties.

Measurement Procedure

(1) Instrument calibration

Calibrate the instrument using a standard reference block of known thickness and material. Apply couplant to the probe, place it on the measurement block, and adjust the parameters (e.g., gain, speed of sound) until the displayed thickness matches the reference value. Ensure that the probe remains stable during calibration.

(2) Selection of measurement points

Select multiple points evenly distributed along the pipe. For straight pipe sections, select at least 3 points per cross-section (e.g. every 60° or 90°). For curved sections or complex geometries, the number of points should be increased. Avoid defects such as welds, scratches or dents.

(3) Probe placement and measurement

Apply couplant to the probe so that it is perpendicular to the catheter axis and press gently to ensure complete contact. Hold the probe steady until the reading stabilizes, then record the value. Clean the probe after each measurement.

(4) Repeat measurement and averaging

Take at least 3 measurements at each point and calculate the average. Maintain consistent probe position and pressure. Investigate outliers due to surface irregularities or improper coupling.