PYROMETER

WHAT IS PYROMETER?

A pyrometer is a type of thermometer used to measure high

temperatures. Various forms of pyrometers have historically existed. In

the modern usage, it is a non-contacting device that intercepts and

measures thermal radiation, a process known as pyrometry. The

thermal radiation 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/INVENTOR

The potter Josiah Wedgwood invented the first pyrometer to measure

the temperature in his kilns, which first compared the color of clay fired

at known temperatures, but was eventually upgraded to measuring

the shrinkage of pieces of clay, which depended on the heat of the

kiln. Later examples used the expansion of a metal bar.

Modern 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 emissivity's and the wavelengths where the

measurements are taken. Two-color ratio pyrometers cannot

measure whether a material’s emissivity is wavelength

dependent.

PRINCIPLE OF OPERATION

A modern pyrometer has an optical system and a 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, theconstant 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 thermocouples and resistance temperature detectors

(RTDs).

There 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.

WORKING OF A PYROMETER

A 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).

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

OPTICAL PYROMETER

The 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.

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 microns (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 °. 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

Simple assembling of the device enables easy use of it.

Provides a very high accuracy with +/-5 degree Celsius.

There is no need of any direct body contact between the opticalpyrometer and the object. Thus, it can be used in a wide variety ofapplications.

As long as the size of the object, whose temperature is to measured fitswith the size of the optical pyrometer, the distance between both ofthem is not at all a problem. Thus, the device can be used for remotesensing.

This device can not only be used to measure the temperature, but canalso be used to see the heat produced by the object/source. Thus,optical pyrometers can be used to measure and view wavelengths lessthan or equal to 0.65 microns. But, a Radiation Pyrometer can be usedfor high heat applications and can measure wavelengths between0.70 microns to 20 microns.

DISADVANTAGES

As the measurement is based on the light intensity, the device can

be used only in applications with a minimum temperature of 700

degree Celsius.

The device is not useful for obtaining continuous values of

temperatures at small intervals.

RADIATION PYROMETER

As discussed earlier, an Optical Pyrometer can be not only be used

for temperature measurement, but also can be used to see the heat

that is measured. The observer is actually able to calculate the

infrared wavelength of the heat produced and also see the heat patterns by the object. But the amount of heat that the device can

sense is limited to 0.65 microns. This is why the radiation pyrometer is

more useful, as it can be used to measure all temperatures of

wavelengths between 0.70 microns and 20 microns.

The wavelengths measured by the device are known to be pure radiation wavelengths, that is, the common range for radioactive heat. This device is used in places where physical contact temperature sensors likeThermocouple, RTD, and Thermistors would fail because of the high temperature of the source.

The main theory behind a radiation pyrometer is that the temperatureis measured through the naturally emitted heat radiation by the body.This heat is known to be a function of its temperature. According to theapplication of the device, the way in which the heat is measured canbe summarized into two:

Total Radiation Pyrometer – In this method, the total heat emittedfrom the hot source is measured at all wavelengths.

Selective Radiation Pyrometer – In this method, the heat radiatedfrom the hot source is measured at a given wavelength.

WORKING OF A RADIATION PTROMETER

the radiation pyrometer has an optical system, including a lens, a

mirror and an adjustable eye piece. The heat energy emitted from

the hot body is passed on to the optical lens, which collects it and is

focused on to the detector with the help of the mirror and eye piece arrangement. The detector may either be a thermistor or

photomultiplier tubes. Though the latter is known for faster detection

of fast moving objects, the former may be used for small scale

applications. Thus, the heat energy is converted to its corresponding

electrical signal by the detector and is sent to the output

temperature display device.

ADVANTAGES

The device can be used to measure very high temperatures without

direct contact with the hot source (Molten metal).

The biggest advantage is that the optical lens can be adjusted to

measure temperature of objects that are even 1/15 inch indiameter and that too kept at a long s=distance from the

measuring device.

The sight path of the device is maintained by the construction of

the instrument components, such as the lens and curved mirrors.

APPLICATION

Pyrometers 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 melt and degrade.

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.

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.