Infrared pyrometer for hot metal and furnace processes

01

Handle emissivity by choosing the wavelength

Fluke Process Instruments defines emissivity as a surface's ability to emit infrared energy, from 0 (shiny mirror) to 1.0 (blackbody), noting that "Most organic, painted, or oxidized surfaces have emissivity values close to 0.95." Its metals table gives steel at 0.35 polished and 0.8–0.9 oxidised or cold-rolled at 1.0 µm, molten steel 0.35 at 1.0 µm and 0.25–0.4 at 1.6 µm, and marks polished aluminium, polished brass, polished copper, chromium, gold and polished zinc as not recommended at 8–14 µm. It advises finding the emissivity for the instrument's spectral range, shielding the target from reflections of hotter surroundings, and using shorter-wavelength instruments for hotter objects where possible. [1]

Micro-Epsilon's infrared basics give Wien's displacement law, λmax · T = 2,898 µm·K, and the radiation constant c₂ = 1.44 × 10⁻² K·m, and show reading error for a 10% emissivity error falling as wavelength shortens from 8–14 µm to 1.1 µm. It recommends window materials by wavelength, such as Al₂O₃ for 1–4 µm up to 1,800 °C. [3]

Editorial calculation: a billet at 1,200 °C (1,473 K) peaks at 2,898 / 1,473 ≈ 2.0 µm. Using the Wien approximation, a 10% emissivity error shifts the reading by about λ T² / c₂ × 0.1: roughly 15 K at 1 µm, 24 K at 1.6 µm, but about 150 K at 10 µm. [3] This is why short-wavelength pyrometers are preferred on hot metal with uncertain emissivity.

Give the supplier the material and surface state (oxidised, scaled, bright, molten), temperature range, target size and mounting distance, line-of-sight conditions (smoke, steam, dust, windows), required response time and accuracy, ambient temperature at the sensor and cooling or purge available, output signal, and mounting and sighting needs.

02

Decide between one-colour and ratio pyrometers

Fluke Process Instruments lists cases where a one-colour pyrometer reads incorrectly: targets too small to fill the field of view, dust, smoke or steam in the line of sight, dirty process windows, and emissivity that changes with alloy or surface. A ratio (two-colour) pyrometer uses two detectors at two wavelengths seeing the same target and calibrates temperature from the ratio of their signals. [2]

In Fluke's example, a 1,500 °C blackbody gives 500 and 1,000 units at the two wavelengths, a ratio of 2; with 90% signal loss both fall to 50 and 100 units and the ratio and reading are unchanged. It states each two-colour instrument has an attenuation limit that can be as high as 95%. Because some targets such as molten metal have different emissivity at the two wavelengths, ratio pyrometers have an E-slope adjustment, set against a reference such as a disposable thermocouple. [2]

Micro-Epsilon defines the optics by the D:S ratio, measuring distance to spot diameter, and states a measurement is correct only if the object is larger than the spot; a target filling only part of the spot is read as an average with the cooler background. [3] Editorial calculation: at a D:S of 100:1 and 2 m distance the spot is 20 mm, so a one-colour pyrometer needs a target wider than that, while a ratio pyrometer can tolerate partial fill within its attenuation limit.

03

Match wavelength to the temperature range: one maker's model table

AMETEK Land's SPOT+ pyrometer page tabulates how wavelength tracks measuring range across one family. Single-wavelength (mono) models run from the 1.0 µm M100 at 500–1,800 °C (extendable to 2,500 °C) and the 1.6 µm M160 at 250–1,600 °C, through the 2.3 µm M210 at 50–1,100 °C and the 3.9 µm M390 at 150–1,800 °C, to the 3–5 µm M3-5 at 0–500 °C. Two-wavelength (ratio and mono) models are the R100 at 1.0 and 1.2 µm for 400–1,800 °C (extendable to 3,500 °C), the R160 at 1.0 and 1.5 µm for 250–1,600 °C, and the R210 at 2.1 and 2.4 µm for 125–1,100 °C; fibre-optic versions of the M100, M160, R100 and R160 keep the same ranges. Application models cover aluminium (200–900 °C), galvanneal strip (200–1,000 °C), molten metal (600–1,800 °C) and tube metal (300–1,800 °C). The page lists EtherNet/IP, Profinet, Modbus TCP and REST API connectivity, and purge air quality class 3 for particles, water and oil. [4]

Editorial calculation: by Wien's law (above), 1.0 µm is the peak for a body near 2,900 K, so short wavelengths sit on the steep side of the curve for hot metal and give smaller errors from emissivity changes, while the 3–5 µm band suits targets below 500 °C. For hot steel with dust or steam, a 1.0/1.2 µm ratio model matches the two-colour case described above; ask the supplier for the wavelength, D:S and response time of the exact model quoted. [4] [3] [2]

04

Two-colour, fibre-optic and wide-range pyrometers for metal processing

Listings include two-colour, fibre-optic and wide-range pyrometers for metal processing and general-purpose fixed pyrometers sold with process instruments; ask each for spectral range, D:S, attenuation limit for ratio models, response time and calibration certificate.

05

Optics and window, detectors, signal processing, sighting, and housing with cooling and purge

The detector row separates the two types: a one-colour unit reads one wavelength and needs the emissivity set, while a ratio unit uses two detectors on the same target and an E-slope setting for unequal spectral emissivity. [2]

06

Temperature range, emissivity, D:S, spectral band, accuracy and output for a pyrometer order

Match the spectral row to the range: AMETEK Land pairs 1.0 µm with 500–1,800 °C and 3–5 µm with 0–500 °C in one model family, and a shorter wavelength cuts the reading error from an emissivity error on hot metal. [4] [3]

07

Send the target, environment and output data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Target: [material; surface; temperature range; size]
Geometry: [distance; required spot; moving or static]
Sight path: [smoke; steam; dust; window]
Type: [one-colour or ratio; wavelength]
Sensor site: [ambient temperature; cooling; purge]
Output: [signal; interface; response time]
Documents: [calibration certificate]

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