Pyrometers and Non-Contact Temperature Measurement: Industrial Optical Thermometry

Every body with a temperature above absolute zero emits infrared radiation proportional to its temperature. The pyrometer captures this radiation with optics, focuses it on a detector and, knowing the material emissivity, calculates the temperature. It does not require contact, so it is ideal for very hot, moving or corrosive surfaces.

1. Emissivity: key to accuracy

Emissivity (ε) is the ratio between the radiation emitted by the material and that of an ideal black body (ε=1) at the same temperature. Each material has its own emissivity depending on composition, roughness, temperature and viewing angle.

• The lower the emissivity, the harder it is to measure: polished metal surfaces (e.g. aluminum, bright steel) reflect the surroundings and have low emissivity, producing errors.
• Most pyrometers allow adjusting emissivity. It is recommended to calibrate it with a contact measurement or reference material, or use a two-color pyrometer when emissivity is uncertain.

2. Types of pyrometers

• Single color (brightness) pyrometer: Measures radiation in a spectral band. Simple and economical, but accuracy depends on correct emissivity setting.
• Two-color / ratio pyrometer: Measures radiation in two bands and calculates the ratio. Less sensitive to emissivity, partial target size and attenuation by smoke or dust. Ideal for metallurgy and vapor environments.
• Infrared thermography (cameras): Provide complete thermal maps, useful for diagnostics, although with lower point accuracy.

3. Distance to spot ratio (D:S)

The D:S ratio indicates the maximum distance at which the pyrometer can measure a target of a given size. For example, a 100:1 ratio allows measuring at 10 m a 10 cm target. It is essential to ensure the field of view is fully covered by the target; if the target is smaller than the field of view, surrounding surfaces are also measured.

4. Selection criteria

• Temperature range: Must cover the process (from -50 °C to over 3000 °C depending on model).
• Operating wavelength: For high temperatures short bands are used; for glass and plastic, specific bands that avoid material transparency; for metallurgy, near-infrared bands.
• D:S ratio and field of view: Define the distance and minimum target size.
• Response time: For fast processes or moving parts a fast response (milliseconds) is required.
• Output and communication: 4-20 mA, thermocouple or Ethernet/Modbus to integrate into control.
• Environmental protection: Housings with air or water cooling, air purge and protection windows for furnaces.

5. Installation and maintenance

The target must be in the field of view and unobstructed. In furnaces, a sighting tube with clean air purge is often installed to protect the optics from dust and vapors. The window or lens must be kept clean and, if there is attenuation by dirt, two-color ratio pyrometers tolerate partial signal loss better.

Conclusion

Non-contact temperature measurement with pyrometers is the ideal solution for high-temperature processes and inaccessible or moving targets. The key to accurate measurement is the correct management of emissivity, D:S ratio and wavelength, along with proper installation and maintenance.

MARPATECH SAC represents in Peru, Argentina and Colombia the non-contact thermometry systems of Ametek Land, including pyrometers and cameras for furnaces, boilers, metallurgy and high-temperature process monitoring.