Published July 10, 2026 at 06:59
A pulp mill in northern Sweden has a pressure tank built in 2005 with a standard wall thickness of 10 mm. It contains hot pulp at 8 bar. Corrosion from the chloride-rich interior has gradually thinned the wall. The question the plant’s maintenance department must answer every six months: how much wall remains? If the answer is less than 6 mm, the tank must be taken out of service. Below 5 mm, it poses an acute risk — risk of rupture.
One way to find out: cut open the tank and measure the wall with a caliper. Cost: 800,000 SEK and three weeks of production stoppage. Another way: place an ultrasonic sensor on the outside, press the button, and read 7.3 mm on the display. Cost: 30 seconds and zero production stoppage. This is why the ultrasonic thickness gauge exists.
Pulse-echo measurement: how ultrasonic thickness measurement works
The instrument is based on the pulse-echo principle. Step by step:
- The piezoelectric crystal in the sensor sends a short ultrasonic pulse at 5 MHz (5 million cycles per second).
- The pulse travels through the coupling gel, into the wall, and across the wall’s thickness.
- At the inner surface of the wall (between the metal and the air/liquid on the inside), most of the pulse is reflected back.
- The reflected echo travels back through the wall, out into the gel, and is received by the same piezoelectric crystal.
- The instrument measures the time between transmission and reception: t.
- The wall thickness is calculated: d = c × t / 2, where c is the speed of sound in the material and /2 because the pulse travels there and back.
For steel, c ≈ 5920 m/s. A pulse returning after 2.45 μs means d = 5920 × 2.45 × 10⁻⁶ / 2 = 7.25 mm wall thickness.
Why 5 MHz is the standard frequency
Ultrasonic frequency is a compromise between three conflicting requirements:
- Higher frequency = better resolution: Shorter wavelengths detect thinner thicknesses. At 10 MHz, the minimum measurable thickness can be 0.5 mm; at 5 MHz, the limit is 1 mm; at 2 MHz, it is 2-3 mm.
- Higher frequency = more attenuation: Ultrasonic waves in thicker materials are attenuated exponentially with frequency. At 10 MHz, you can barely penetrate 100 mm of steel; at 5 MHz, you can handle 500 mm; at 2 MHz, over 1 meter.
- Higher frequency = more sensitive to surface roughness: At 10 MHz, 0.05 mm surface roughness affects the measurement value; at 5 MHz, the roughness must be 0.1 mm or more to be noticeable.
5 MHz is the sweet spot for general industrial measurement in steel with walls of 1-500 mm — covering 90% of corrosion monitoring needs. The 10 MHz sensor (optional) is for thin precision measurements on sheet metal or calibration work. Lower frequencies are for very thick walls or heavily attenuating materials (cast iron, cast steel).
Coupling gel: why it doesn’t work without gel
Ultrasonic waves travel through solid materials and liquids but NOT through air. At the transition from metal sensor → air → metal wall, ~99.99% of the pulse is reflected at the first air boundary — almost nothing reaches the wall.
The coupling gel (the 6 cl bottle included) is an acoustically matched gel-like substance that spreads between the sensor and the wall, eliminating the air gap. The gel’s acoustic impedance is close to that of the metal, so the pulse passes through the transition with only ~10% reflection.
Practical consequence: you cannot “just place the sensor against the wall.” You must apply 2-3 drops of gel, place the sensor, and rotate it 5-10° to press out any air bubbles. Then take the reading. After measurement: wipe off the gel.
Calibration against material type
The instrument measures TIME, not thickness directly. Thickness is calculated using the known speed of sound in the material:
- Carbon steel: 5920 m/s.
- Stainless steel (austenitic): 5660-5790 m/s.
- Aluminum: 6320 m/s.
- Copper: 4700 m/s.
- Brass: 4430 m/s.
- Cast iron: 4000-4600 m/s (varies more due to porosity).
- Plastic (PVC): 2340 m/s.
- Titanium: 6070 m/s.
If the instrument is calibrated for steel and you measure aluminum, you get a 7% measurement error (sound travels 7% faster in aluminum than in steel). For 10 mm of aluminum, a steel-calibrated TKG100 will thus show 9.3 mm — 7% below the true value.
Best practice: calibrate the instrument against a CALIBRATION PLATE of the same material as the measurement object. Calibration plates with known thickness (typically 5 mm) are included or can be purchased separately.
Min-capture mode for corrosion detection
A corroding wall is rarely thinned evenly — it has pitting corrosion (local pits). A 10 mm wall may have an average thickness of 8.5 mm but individual pits down to 4.2 mm. It is the pits that are dangerous.
The TKG100’s min-capture mode works like this: the operator places the sensor against the wall, activates min-capture, and slowly drags the sensor over a surface (typically a 100 × 100 mm square). The instrument updates the display continuously but REMEMBERS the lowest value it has seen so far. After a 30-60 second sweep over the surface, the display shows the lowest measured thickness in that zone.
This is standard technique in corrosion monitoring: you sweep the sensor over the entire pressurized vessel area, find points with the minimum value, and document these as “critical points” for the next inspection.
Who uses the TKG100 daily
Pressurized vessel inspectors: Reactors, steam tanks, compressor tanks, boilers in the pulp industry. ASME and EU Directive 2014/68/EU requirements for periodic wall measurement.
Pipeline integrity technicians: Oil, gas, district heating, industrial process pipes. NDT inspection according to ASME B31.4/B31.8.
Ship inspectors: Hull thickness on commercial vessels. IACS requirements for 5-year interval inspections. Ultrasonic measurement at 200-500 points per ship.
Offshore platform inspectors: Pipes, support legs, critical components. Marine corrosion is accelerated, requiring more frequent inspection intervals.
Petrochemical plants: Refineries, gas power plants. High temperatures and aggressive media make corrosion monitoring critical.
Facility maintenance: District heating connection pipes, sub-centers, boilers in large households. Lower inspection frequency but the same technique.
Alternative NDT methods
X-ray (RT): Provides images of the wall’s internal structure, finding cracks and weld defects. Disadvantage: requires access from both sides, radiation risk, takes hours per image, tens of thousands of SEK per inspection job.
Magnetic particle (MT): Finds surface-breaking cracks. Disadvantage: only works on magnetic materials, does not measure thickness.
Penetrant test (PT): Surface-breaking cracks on non-magnetic materials. Does not measure thickness.
Ultrasonic (UT — TKG100 belongs here): Measures thickness directly. Fastest and cheapest per measurement point. The standard method for corrosion monitoring.
What the instrument cannot do
- Measure through multiple layers: If a pipe has insulation or cladding, it must be removed before measurement. The TKG100 measures the first echo — which comes from the insulation-metal boundary, not from the innermost surface of the wall.
- See cracks: If a crack is parallel to the wall (delamination crack), the measurement value will be incorrect. The crack reflects the pulse earlier than the innermost surface of the wall, so the thickness is shown as too small. However, this is not entirely wrong — a delamination is practically a thinning.
- Measure through angled or curved surfaces with a standard sensor: For curved pipes, an angle-adjusted sensor must be used. The TKG100 5 MHz standard sensor requires a flat surface.
- Provide absolute accuracy below 0.05 mm: Basic accuracy is ±0.1 mm with good surface preparation, ±0.2-0.3 mm in practice.
High temperatures and special sensors
The standard sensor works at room temperature. The TKG100 also supports high-temperature sensors (as an option) for measurement on workpieces at 150-500 °C — critical for in-service measurement on pressurized vessels in operation without stopping.
Other optional sensors:
- Twin-crystal sensors for measurement on heavily corroded or rough surfaces.
- Small-footprint sensors for tight zones.
- Angle sensors (5-70°) for measurement on angled surfaces or for crack localization.
What you get for the money
Ultrasonic thickness gauge TKG100, 5 MHz standard sensor, measurement range 1-510 mm in steel (0.5-510 mm with 10 MHz option), dot-matrix display with backlight for reading in sunlight, min-capture mode for corrosion detection, calibration capability against any sound speed for different materials, robust portable construction, powered by 2 AA batteries, 6 cl coupling gel included, 5 MHz sensor included, hard carrying case. 33,852 SEK.
The standard instrument for corrosion monitoring in process industry, shipping, offshore, petrochemicals, and facility maintenance. Alternative NDT methods (X-ray, magnetic particle) are either 5-10× more expensive per inspection or do not measure thickness at all. The TKG100 costs 33,852 SEK once and serves thousands of measurement points per year over a lifespan of 10+ years.
