Thickness Measurement on Flat Metals
X-Ray & Laser Technologies

X-ray thickness gauges measure the attenuation of an X-ray beam passing through the strip. The amount of absorption is directly related to the mass per unit area (thickness × density) of the material.

A typical configuration uses an X-ray source above the strip and a detector below (or vice versa). The system is calibrated with known samples of the same alloy, and then provides real-time thickness readings at the measurement point.

X-ray thickness gauges typically achieve ±0.1% to ±0.5% of the measured thickness, depending on the alloy and thickness range. For a 1 mm strip, this means ±1 µm to ±5 µm.

Measurement range depends on the X-ray energy: low-energy systems for thin foils (down to 0.01 mm), high-energy systems for heavy plate (up to 20 mm or more).

X-ray gauges can measure steel, stainless steel, aluminium, copper, brass, zinc, and most alloys. The measurement is alloy-sensitive, so calibration must be done for each alloy family.

Limitations: very light elements (e.g., pure aluminium, plastics) require low-energy X-rays; very thick or very dense materials require high-energy systems. Also, X-ray gauges need safety shielding and regulatory compliance.

Laser thickness gauges use two laser triangulation sensors, one above and one below the strip. Each sensor measures the distance to its respective strip surface. The thickness is calculated as:

Thickness = Baseline − (Distance_top + Distance_bottom)

The baseline is the known distance between the two sensors, calibrated with a master sample. This method is independent of strip vertical position as long as both sensors remain in range.

Laser thickness gauges achieve ±1 µm to ±5 µm accuracy for thin strip (0.1–3 mm), with repeatability better than ±0.5 µm. Range depends on sensor selection; typical range is 0.05 mm to 10 mm.

Laser gauges measure the distance to the surface, so they are sensitive to:

• Surface condition: oil, emulsion, scale, or heavy oxidation can cause scattering or false readings.
• Strip flutter: if the strip moves vertically beyond the sensor range, measurement is lost.
• Alloy sensitivity: not alloy-sensitive (good), but requires known surface emissivity for some materials.
• Thickness limit: very thick or very rough surfaces may exceed the sensor's depth of field.

Application: Hot or cold rolling of steel and aluminium through a series of stands. Thickness is reduced progressively; the gauge is typically placed after the last stand (exit) and sometimes between stands (interstand).

Recommended technology: X-ray — because of high speed, possible coolant/oil, and the need for through-thickness measurement independent of surface condition. X-ray gauges are the standard for tandem mills worldwide.

Key requirements: Fast response (1–5 ms), high accuracy (±0.1%), automatic alloy compensation, and integration with Automatic Gauge Control (AGC).

Application: Light reduction (0.5–2%) after annealing or galvanizing to improve surface finish, flatness, and mechanical properties. Thickness measurement ensures the target reduction is achieved without over-rolling.

Recommended technology: X-ray for steel and most metals; laser can be used for thin, cold, clean strip where X-ray is not desired.

Key requirements: Very high accuracy (±0.1% or better), stable measurement despite strip flutter, and fast feedback to the skin-pass control.

Application: Removal of scale and oxides from hot-rolled steel using acid. Thickness measurement before and after pickling monitors material removal and ensures final gauge.

Recommended technology: X-ray — because the strip is wet and may have residual acid, laser triangulation would be unreliable. X-ray is unaffected by surface liquid.

Key requirements: Robust housing (acid-resistant), air purging, and alloy compensation for different steel grades.

Application: Hot-dip or electro-galvanizing applies a zinc coating. Thickness gauges measure the steel substrate thickness (before coating) and sometimes the total thickness (steel + zinc).

Recommended technology: X-ray for substrate thickness; X-ray fluorescence (XRF) or laser for coating weight/thickness. Laser can measure total thickness if the surface is dry and stable.

Key requirements: Compensation for zinc coating in the X-ray path (multi-layer calibration), high accuracy for thin substrates, and fast response for coating weight control.

Application: Electrolytic tinning, chroming, or other electro-deposited coatings on steel or aluminium. Thickness measurement ensures both substrate and coating are within specification.

Recommended technology: X-ray for substrate; XRF or electrolytic methods for coating. Laser is rarely used due to the wet environment.

Key requirements: Very thin substrate measurement (down to 0.1 mm), high accuracy, and multi-layer calibration to separate coating from substrate.

Application: Final processing including tension levelling, inspection, and oiling. Thickness measurement confirms final gauge before shipment.

Recommended technology: X-ray or laser — depending on surface condition and speed. Laser is often preferred for dry, clean, cold strip.

Key requirements: High accuracy, fast response, and integration with quality logs and cut-to-length systems.

Application: Rewinding coils after processing. Thickness measurement ensures the coil is wound with the correct gauge and that no thickness deviations occur during recoiling.

Recommended technology: Laser — because the strip is cold, dry, and clean. Laser gauges are compact and can be mounted close to the recoiler.

Key requirements: Compact sensor head, fast response, and ability to handle strip flutter during winding.

Application: Electrolytic tinning of steel for packaging (tinplate). Thickness measurement of the steel substrate and tin coating is critical for can-making performance.

Recommended technology: X-ray for substrate thickness; XRF or coulometric methods for tin coating weight. Laser is not suitable for wet tinplating lines.

Key requirements: Very high accuracy for thin steel (0.1–0.5 mm), multi-layer calibration, and fast response for coating control.

Application: Cold rolling, annealing, pickling, and finishing of stainless steel (200, 300, 400 series). Thickness measurement is essential due to tight tolerances and high material value.

Recommended technology: X-ray for most stainless lines (especially wet or high-speed); laser for dry, cold, clean strip in finishing or recoiling.

Key requirements: Alloy compensation for different stainless grades (ferritic vs austenitic), high accuracy, and corrosion-resistant sensor housings.

Application: Cold rolling, annealing, pickling, and finishing of copper and brass strip. Thickness tolerances are very tight, especially for electronic and connector applications.

Recommended technology: X-ray — copper and brass are dense, and X-ray provides excellent accuracy. Laser can be used for clean, dry, cold strip with high reflectivity considerations.

Key requirements: Alloy compensation (copper, brass, bronze), high accuracy (±0.1%), and resistance to surface oxidation and oil films.

Application: Slitting wide coil into narrower strips. Thickness measurement ensures each strip meets gauge tolerance after slitting, and monitors for thickness variations across the width.

Recommended technology: Laser — slitting lines are typically dry and cold. Laser gauges can be mounted after the slitter head, one per strip or scanning across the width.

Key requirements: Multi-strip capability (one gauge per strip or a scanning gauge), fast response, and compact design for tight spaces.

Application: Cutting flat sheets from coil to precise lengths. Thickness measurement confirms the gauge before cutting and ensures sheet thickness uniformity.

Recommended technology: Laser — because the strip is cold, dry, and clean. Laser gauges are compact and can be mounted before the shear.

Key requirements: High accuracy, fast response, and integration with length measurement and sorting systems.