PYROMETER

PyrometerA 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).

HISTORY OF PYROMETERModern pyrometers became available when the first disappearing filament pyrometer was built by L. Holborn and F. Kurlbaum in 1901. This device superimposed a thin, heated filament over the object to be measured and relied on the operator’s eye to detect when the filament vanished. The object temperature was then read from a scale on the pyrometer.The temperature returned by the vanishing filament pyrometer and others of its kind, called Brightness Pyrometers, is dependent on the emissivity of the object. With greater use of brightness pyrometers, it became obvious that problems existed with relying on knowledge of the value of emissivity. Emissivity was found to change, often drastically, with surface roughness, bulk and surface composition, and even the temperature itself. To get around these difficulties, the ratio or two-color pyrometer was developed. They rely on the fact that Planck's Law, which relates temperature to the intensity of radiation emitted at individual wavelengths, can be solved for temperature if Planck’s statement of the intensities at two different wavelengths is divided. This solution assumes that the emissivity is the same at both wavelengths  and cancels out in the division. This is known as the gray body assumption. Ratio pyrometers are essentially two brightness pyrometers in a single instrument. The operational principles of the ratio pyrometers were developed in the 1920s and 1930s, and they were commercially available in 1939.

As the ratio pyrometer came into popular use, it was determined that many materials, of which metals are an example, do not have the same emissivity at two wavelengths . For these materials, the emissivity does not cancel out and the temperature measurement is in error. The amount of error depends on the emissiveties and the wavelengths where the measurements are taken. Two-color ratio pyrometers cannot measure whether a material’s emissivity is wavelength dependent.

PRINCIPLE OF OPERATIONA 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 proportionalityσ, 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 withthermocouple and Resistance temperature detector 

WORKING OF A PYROMETERThere are two basic kinds of pyrometers: optical pyrometers, where you look at a heat source through a mini-telescope and make a manual measurement, and electronic, digital pyrometers that measure completely automatically. Some devices described as pyrometers actually have to be touching the hot object they're measuring. Strictly speaking, instruments like this are really just high-temperature thermometers based on thermocouples. Since they don't measure temperature at a distance, they're not really pyrometers at all.

Pyrometer, or radiation thermometer, is a non-contact instrument that detects an object's surface temperature by measuring the temperature of the electromagnetic radiation (infrared or visible) emitted from the object.

The wavelength of thermal radiation ranges from 0.1 to 100 µm (4 ~ 4,000 µin), i.e., from the deep ultraviolet (UV) across the visible spectrum to the middle of the infrared region (IR).

Typical Broadband Pyrometer

WORKING

The Electromagnetic Radiation Spectrum

Pyrometers are essentially photodetectors which are capable of absorbing energy, or measuring the EM wave intensity, at a particular wavelength or within a certain range of wavelengths.

TYPES OF PYROMETERS

The following are some of the most commonly and widely used pyrometers:

• OPTICAL PYROMETER

• RADIATION PYROMETER

• DIGITAL PYROMETER

• INFRARED PYROMETER

OPTICAL PYROMETERThe Optical Pyrometer is a highly-developed and well accepted noncontact temperature measurement device with a long and varied past from its origins more than 100 years ago. In spite of the fact that more modern, automatic devices have nearly displaced it, several makers still produce and sell profitable quantities each year.In general, opticals, as they are often called, can be described as fitting into two seperate types, according to the two USA companies that produce them. However, there are actually several different types that vary in compexity and cost. A quick review of the descriptions below will provide some of the differences and a check of the web sites of the two companies will yield even more information. We suspect that there are other makers overseas and we are looking to find more details about them and their web presence.

•They range between 700 to 3000 c

•More accurate

•Lower limit determined by human eye

•Human eye compares the radiation

WORKING OF A OPTICAL PYROMETER

Optical Pyrometers work on the basic principle of using the human eye to match the brightness of the hot object to the brightness of a calibrated lamp filament inside the instrument. The optical system contains filters that restrict the wavelength-sensitivity of the devices to a narrow wavelength band around 0.65 to 0.66 micons (the red region of the visible spectrum).Other filters reduce the intensity so that one instrument can have a relatively wide temperature range capability. Needless to say, by restricting the wavelength response of the device to the red region of the visible, it can only be used to measure objects that are hot enough to be incandescent, or glowing. This limits the lower end of the temperature measurement range of these devices to about 700 °C.Some experimental devices have been built using light amplifiers to extend the range downwards, but the devices become quite cumbersome, fragile and expensive. Modern radiation thermometers provide the capability to measure within and below the range of the optical pyrometer with equal or better measurement precision plus faster time response, precise emissivity correction capability, better calibration stability, enhanced ruggedness and relatively modest cost.

ADVANTAGES

• FLEXIBLE

• PORTABLE

• CONVIENT TO USE

• LIGHT WEIGHT

• CAN MONITOR MOVEABLE OBJECTS

• NON CONTACT TYPE

• ACCURATE

DISADVANTAGES

• EXPENSIVE

• HUMAN ERRORS

• NOT USEFUL FOR MEASURING TEMP OF CLEAR GAS

• AT HIGH TEMP. FILAMENT ERODES FREQUENTLY

RADIATION PYROMETER

COMMON DETECTORS IN PYROMETERS

• THERMOPILE

• PHOTOCELL

• METAL BOLOMER

• THERMISTER

THERMOPILE • LARGER O/P

• LOWER RESPONSE TIME

• ADAPTABLE FOR INDUSTRIAL APPLICATION

BOLOMETER

• FAST IN RESPONSE

• GOOD SENSITIVITY

• COSTLY

THERMISTERNOT USED DUE TO :

• POOR PRECISION

• DIFFICULT TO PROVIDE COMPENSATION

• LOW RESPONSE TIME

PHOTOCELL

•FAST

• LARGE SENSITIVITY

• LIMITED SPECTRAL SENSITIVITY

ADVANTAGES

•ABILITY TO MEASURE HIGH TEMP

• NO NEED FOR CONTAC

• FAST RESPONSE SPEED

• HIGH O/P

• MODERATE COST

DISADVANTAGES

• NON LINEAR SCALE

• EMMISIVITY OF TARGET AFFECT MEASUREMENT

• ERRORS DUE TO INTERLEAVING MEDIA

APPLICATIONPyrometers 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 Salt bath furnaces operate at temperatures up to 1300 °C and are used for heat treatment. At very high working temperatures with intense heat transfer between the molten salt and the steel being treated, precision is maintained by measuring the temperature of the molten salt. Most errors are caused by slag on the surface which is cooler than the salt bath.

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.

Tuyère Pyrometer

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

Hot Air Balloons A hot air balloon is equipped with a pyrometer for measuring the temperature at the top of the envelope in order to prevent overheating of the fabric.

REFERENCES

• ^ a b c L. Michalski et al, “Temperature Measurement, Second Edition.(Wiley, 2001), pp. 162-208.

• ^ a b c d C. Mercer, Optical metrology for fluids, combustion, and solids.(Kluwer Academic, 2003), pp. 297-305.

• ^ a b D. Ng and G. Fralick, ‘’Use of a multiwavelength pyrometer in several elevated temperature aerospace applications’’, Rev. Sci. Instrum. 72, 1522 (2001)

• ^ a b D. Olinger, J. Gray, R. Felice, ‘’Successful Pyrometry in Investment Casting’’, Proceedings of the 55th Annual Technical Conference on Investment Casting (Investment Casting Institute, 2007)

• ^ L. Michalski et al, “Temperature Measurement, Second Edition.(Wiley, 2001), pp. 403-404.