I. Contact Measurement Method: Suitable for static high-precision inspection
This method obtains data directly through physical contact, suitable for laboratory or sampling inspection scenarios, and offers high accuracy.
1. Wall Thickness Micrometer Measurement
Use a dedicated wall thickness micrometer (accuracy up to 0.001mm) to take 8 evenly distributed points at each end and the middle section of the steel pipe for measurement.
Repeat the measurement 3 times at each point and take the average value to reduce error. Particularly suitable for precision steel pipes with high accuracy requirements (deviation ≤ ±0.03mm).
Note: Ensure the probe is perpendicular to the inner and outer walls during measurement to avoid tilting, which could lead to a lower reading.
2. Caliper-Assisted Measurement (Preliminary Judgment)
Vernier calipers can be used for rapid on-site inspection, but the accuracy is lower (typically ±0.02mm). It is recommended to take the minimum value in at least four directions at the pipe end as a reference.
II. Non-Contact Measurement Method: Suitable for dynamic or special working conditions inspection
This method does not require direct contact and is suitable for high-temperature, coated, or continuous production line inspections.
1. Ultrasonic Thickness Gauge (Most Commonly Used): Calculates wall thickness using the time difference of ultrasonic wave propagation within the material. A coupling agent (such as glycerin or machine oil) needs to be applied to ensure signal transmission.
Before measurement, the sound velocity must be calibrated using a standard test block of the same material as the steel pipe (approximately 5900 m/s for carbon steel and 5850 m/s for stainless steel).
Suitable for batch testing, scenarios where samples cannot be damaged, or where the inner wall is difficult to access. Accuracy can reach ±0.02 mm.
2. Laser Thickness Gauge: Irradiates the inner and outer surfaces of the steel pipe with two parallel laser beams, and calculates the displacement difference using an optical sensor to obtain the wall thickness.
Advantages include no mechanical wear, suitable for online inspection on hot-rolled/cold-drawn production lines (speed ≤60 m/min), especially suitable for thin-walled steel pipes (wall thickness <3 mm).
3. Electromagnetic Ultrasonic Thickness Gauge: Requires no coupling agent. Ultrasonic waves are excited through electromagnetic induction, allowing online inspection of hot steel pipes in high-temperature environments (≤600℃).
Suitable for steel pipes with anti-corrosion coatings, measurement can be performed without peeling off the coating, but accuracy is affected by the material's magnetic permeability (carbon steel ±0.08mm).
III. Specialized Methods for Special Scenarios
1. X-ray Imaging Method: Uses X-rays or gamma rays to penetrate the steel pipe, calculating the wall thickness based on image grayscale differences. It can visually display internal corrosion pits or uneven wall thickness.
Complies with GB/T 19293 standard, suitable for corrosion detection of in-service pipelines, accuracy ±0.1mm.
2. Eddy Current Thickness Measurement Method: Utilizes electromagnetic induction to detect changes in pipe wall conductivity, indirectly reflecting wall thickness differences. Primarily used for non-ferrous metal steel pipes (such as copper and aluminum pipes), accuracy ±0.05mm.
IV. Measurement Precautions and Error Control
To ensure accurate measurement results, the following points should be noted:
1. Ambient Temperature Control: Measurements should be performed in an environment of 20±2℃. If the on-site temperature deviation is large, correction should be made according to the coefficient of thermal expansion (e.g., for carbon steel, for every 1℃ deviation, the correction value = actual wall thickness × 11.5 × 10⁻⁶ × temperature difference).
2. Surface Treatment: Remove oil stains and oxide scale, and grind to a surface roughness Ra≤1.6μm to avoid affecting probe contact or signal reflection.
3. Ellipticity Correction: If the ellipticity of the steel pipe is >1%, the number of measuring points should be increased to 6 directions, and the average value should be taken as the final wall thickness.
4. Avoid Defect Areas: During measurement, avoid areas such as welds, scratches, and indentations to prevent data distortion.
