Infrared thermometers (IR)

Infrared thermometers (IR pyrometers) are non-contact instruments for measuring the surface temperature of bodies, based on detecting the thermal radiation emitted naturally by every body at a temperature above absolute zero, according to the Stefan-Boltzmann law and Planck’s law. The infrared radiation (wavelength 0.7-100 µm) is picked up by a sensor (pyrosensor, microbolometer, quantum sensor), focused by optics, processed by the electronics and shown as a temperature on the display. They are widely used in industry, predictive maintenance, building inspection, food, safety and process control, thanks to the speed of measurement, the absence of contact and the ability to measure inaccessible points.

Physical principle

Every body at a temperature T emits thermal radiation with a spectrum described by Planck’s law, with a peak at a wavelength inversely proportional to T (Wien’s law: λ_max·T = 2898 µm·K). The total intensity is proportional to T⁴ (Stefan-Boltzmann law: W = ε·σ·T⁴, where ε is the emissivity of the material and σ the Stefan-Boltzmann constant). The pyrometer measures the radiation intensity in a spectral band defined by the optics and the sensor, and calculates T from the known ε of the material. Emissivity varies with the material (black bodies ε≈1; polished metals ε=0.1-0.3; organic materials and plastic ε=0.9-0.95) and must be set correctly in the instrument to obtain accurate measurements.

Features of the instruments

Portable pyrometers cover typical ranges of -30 °C to +500 °C (general models) up to +2000 °C and beyond (models for metallurgy). Typical accuracy is ±1 °C or ±1% of the reading. Resolution is 0.1 °C. The working distance and the size of the target are characterized by the D:S ratio (distance-to-spot ratio, typically 10:1 up to 50:1 on high-quality models). A laser pointer (visible as a dot or a circle) makes it easier to target the measuring point. Advanced functions include adjustable emissivity, max/min/avg, alarm thresholds, a data logger and Bluetooth/USB output. Models with dual optics (telescopic and standard) cover different applications.

Spectral bands

Pyrometers are distinguished by spectral band: short wavelength (1-2 µm, for high temperatures and metals, insensitive to emissivity in dual-color mode); medium wavelength (3-5 µm, for many industrial applications, excellent for glass and ceramic materials); long wavelength (8-14 µm, for low and medium temperatures, excellent for HVAC and organic materials). The choice of band depends on the typical temperature (choose the band containing the material’s emission peak) and on the material (some materials are transparent in certain IR bands).

Applications

The applications are highly varied: electrical maintenance (checking for hot spots on MV/LV switchboards, motors and substations); mechanical maintenance (bearings, brakes, shafts, fans); building inspection (heat loss, thermal bridges, integrity of insulation); the food industry and HACCP (product temperature control, less accurate than contact probes but far faster); the paper industry (dryers); plastics (film blowing lines, thermoforming); metallurgy (furnaces, rolling, forging, casting control); medical applications (body screening, medical monitors); and fire safety.

Emissivity limitations

The main limitation of IR pyrometry is its dependence on the emissivity of the material: polished metal surfaces (ε≈0.1) give very uncertain measurements unless ε is known exactly. To overcome this limitation the following are used: dual-color or ratio pyrometers (simultaneous measurement in two spectral bands, the ratio being independent of emissivity to a first approximation); the application of matt black paint or black adhesive tape (ε≈0.95) as a patch of known emissivity at the measuring point; indirect measurement on parts of the system with known emissivity (internal caps, cavities, observation ports); and calibration against a reference contact thermometer.

Standards and calibration

IR pyrometers are characterized metrologically to VDI/VDE 3511 and ASTM E2847. Calibration is carried out with reference black bodies at known temperatures, with ISO 17025 certificates, at no fewer than 3-5 points of the measuring range. The typical interval is annual, or according to intensity of use and customer requirements.