pyrometer

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A temperature-measuring device, originally an instrument that measures temperatures beyond the range of thermometers, but now in addition a device that measures thermal radiation in any temperature range. This article discusses radiation pyrometers; for other temperature-measuring devices See also Bolometer.

The illustration shows a very simple type of radiation pyrometer. Part of the thermal radiation emitted by a hot object is intercepted by a lens and focused onto a thermopile. The resultant heating of the thermopile causes it to generate an electrical signal (proportional to the thermal radiation) which can be displayed on a recorder.

Elementary radiation pyrometer. (After D. M. Considine and S. D. Ross, Process Instruments and Controls Handbook, 2d ed., McGraw-Hill, 1974)

Unfortunately, the thermal radiation emitted by the object depends not only on its temperature but also on its surface characteristics. The radiation existing inside hot, opaque objects is so-called blackbody radiation, which is a unique function of temperature and wavelength and is the same for all opaque materials. However, such radiation, when it attempts to escape from the object, is partly reflected at the surface. In order to use the output of the pyrometer as a measure of target temperature, the effect of the surface characteristics must be eliminated. A cavity can be formed in an opaque material and the pyrometer sighted on a small opening extending from the cavity to the surface. The opening has no surface reflection, since the surface has been eliminated. Such a source is called a blackbody source, and is said to have an emittance of 1.00. By attaching thermocouples to the black-body source, a curve of pyrometer output voltage versus blackbody temperature can be constructed. See also Blackbody; Heat radiation.

Pyrometers can be classified generally into types requiring that the field of view be filled, such as narrow-band and total-radiation pyrometers; and types not requiring that the field of view be filled, such as optical and ratio pyrometers. The latter depend upon making some sort of comparison between two or more signals.

The optical pyrometer should more strictly be called the disappearing-filament pyrometer. In operation, an image of the target is focused in the plane of a wire that can be heated electrically. A rheostat is used to adjust the current through the wire until the wire blends into the image of the target (equal brightness condition), and the temperature is then read from a calibrated dial on the rheostat.

The ratio, or “two-color,” pyrometer makes measurements in two wavelength regions and electronically takes the ratio of these measurements. If the emittance is the same for both wavelengths, the emittance cancels out of the result, and the true temperature of the target is obtained. This so-called gray-body assumption is sufficiently valid in some cases so that the “color temperature” measured by a ratio pyrometer is close to the true temperature.

An instrument for measuring temperature by the change of electrical resistance within a thermocouple. It is a milli-voltometer calibrated in degrees of temperature.

An instrument for measuring high temperatures.

Instrument for measuring the intensity of heat when this is beyond the range of the mercury thermometer.

An optical pyrometer

A sailor checking the temperature of a ventilation system

A pyrometer is a non-contacting device that intercepts and measures thermal radiation, a process known as pyrometry. This device can be used to determine the temperature of an object's surface.

The word pyrometer comes from the Greek word for fire, "πυρ" (pyro), 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).

Contents 1 Principle of operation 2 Applications 2.1 Smelter Industry 2.2 Over-the-bath Pyrometer 2.3 Tuyère Pyrometer 2.4 Steam boilers 3 External links 4 See also

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.

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 were the traditional devices used for this purpose, 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.

(1) Display.(2) Optical.(3) Fibre optic cable and Periscope. (4) Pyrometer tuyère adapter having:i. Bustle pipe connection. ii. Tuyère clamp iii. Clamp washer iv. Clamp stud c/w and fastening hardware v. Gasket vi. Noranda Tuyère Silencer vii. valve seat viii. ball (5) Pneumatic Cylinder: i. Smart Cylinder Assembly with Internal proximity switch ii. Guard Plate Assembly iii. Temporary Flange Cover Plate used to cover periscope entry hole on tuyère adapter when no cylinder is installed on the tuyère. (6) Operator station panel (7) Pyrometer light station (8) Limit switches (9) 4 conductor cab tire (10) Ball Valve (11) Periscope Air pressure switch. (12) Bustle Pipe Air pressure switch. (13) Airline filter/regulator (14) Directional control valve, Sub-plate, silencer and speed control mufflers. (15) 2" nom. low pressure air hose, 40m length

Steam boilers

A steam boiler may be fitted with a pyrometer to measure the steam temperature in the superheater.

External links

• The tuyère pyrometer patent
• Infrared and radiation pyrometers

See also

Wikimedia Commons has media related to: Pyrometers

• Infrared thermometer
• Thermal radiation
• Thin filament pyrometry

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