Definitions

# pyrometer

[pahy-rom-i-ter]

Instrument for measuring relatively high temperatures, as in furnaces. Most pyrometers work by measuring radiation from the body whose temperature is to be measured (radiation devices have the advantage of not having to touch the material being measured). Optical pyrometers measure the temperature of glowing bodies by comparing them visually with an incandescent filament of known temperature whose temperature can be adjusted. In resistance pyrometers, a fine wire is put in contact with the object; the instrument converts the change in electrical resistance caused by heat to a reading of the temperature of the object.

Pyrometer is any non-contacting device that intercepts and measures thermal radiation. This measure is used to determine temperature, often of the object's surface.

The word pyrometer comes from the Greek word for fire, "πυρ", and meter, meaning to measure. Pyrometer was originally coined to denote a device capable of measuring temperatures of objects above incandescence (i.e. objects bright to the human eye).

## Principle of operation

A pyrometer has an optical system and detector. The optical system focuses the thermal radiation onto the detector. The output signal of the detector (Temperature T) is related to the thermal radiation or irradiance j* of the target object through the Stefan–Boltzmann law, the constant of proportionality σ, called the Stefan-Boltzmann constant and the emissivity ε of the object.

j^{star} = varepsilonsigma T^{4} This output is used to infer the object's temperature. Thus, there is no need for direct contact between the pyrometer and the object, as there is with thermocouple and Resistance temperature detector (RTDs).

## Applications

Pyrometer are suited especially to the measurement of moving objects or any surfaces that can not be reached or can not be touched.

### Smelter Industry

Temperature is a fundamental parameter in metallurgical furnace operations. Reliable and continuous measurement of the melt temperature is essential for effective control of the operation. Smelting rates can be maximized, slag can be produced at the optimum temperature, fuel consumption is minimized and refractory life may also be lengthened. Thermocouples used to be the traditional device, but they are unsuitable for continuous measurement because they rapidly dissolve.

### Over-the-bath Pyrometer

Continuous pyrometric measurement from above the bath surface is still employed, but is known to give poor results because of emissivity variations, interference by gases and particulate matter in the intervening atmosphere, and dust accumulation on the optics.

### Tuyère Pyrometer

The Tuyère Pyrometer is an optical instrument for temperature measurement through the tuyeres which are normally used for feeding air or reactants into the bath of the furnace.