Basics of Pyrometers

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Contentso What is Pyrometer?

o Theory of Thermal Emission

o Important parameters for accurate thermal emission

• Temperature range

• Emissivity

• The field of view(FOV)

• Spectral Responsivity

o Pyrometer types

• Fiber optic pyrometers

• Single color pyrometers

• Ratio type Pyrometers

• Multi wavelength Pyrometers

o Advantages

o Applications

What is Pyrometer

o A pyrometer is a non-contacting device.

o Pyrometer intercepts and measures thermal radiation. This device can be used to determine the temperature of an object's surface without contact to the surface.

Theory of Thermal Emission

o The Thermal emission is based on the Black Body concept.

o Black Body is an object that absorbs all electromagnetic radiation

that falls onto it.

o No radiation passes through it and none is reflected.

o Electromagnetic radiation just depends on the temperature of

black body.

o The total emittance from a black body including all wavelengths is

directly proportional to the Fourth power of its temperature. This

temperature is called “Brightness Temperature, T”

Important parameters for accurate temperature measuremento Temperature Range:

• Select right temperature span.

o Emissivity:• Emissivity is defined as the ratio of

the energy radiated from a material's surface to that radiated from a blackbody (a perfect emitter) at the same temperature and wavelength and under the same viewing conditions.

• The emissivity of a surface depends not only on the material but also on the nature of the surface.

o The field of view (FOV):

• The FOV allows easy calculation of the minimum target size for a working

distance.

• A convenient measure is the distance to target ratio, e.g 100 :1 indicating

a minimum target of 10mm at a 1000mm measuring distance.

• To avoid errors that could arise from adjustments made during

operation, it is advisable to use an object size slightly larger than the spot

size.

• When the object does not fill the spot size completely, measuring errors

occur.

o Spectral Responsivity:• The spectral responsivity range

should be selected so that the emissivity of the surface is as high as possible to maximize the characteristic radiation from the measurement object and minimize the reflected ambient radiation. With metals the emissivity rises in the short wavelength band.

• Materials like glass, plastic etc should be measured in special wavelength so that, the material is impenetrable at this wavelength band.

• Temperature of non metals can be accurately measured in longer wavelength band of 8…14 μm.

Pyrometers typeso Fiber Optic Pyrometers

A fiber optics pyrometer consists of Optical head, Fiber optic cable (single or multi fiber) and Electronic unit. The special construction helps the pyrometer useful for• High ambient temperature with no need of cooling system

(Ambient temperature up to 250°C).• Strong electromagnetic field.• Limited space for installation.• Small spot size.

o Single Color Pyrometers Operate over a narrow range of Wavelengths Basically used for glass at 5.14µm. Metals can also be measured as

their rate of emissivity is high only in a narrow band. Spectral response of a particular device depends on the type of the

detector used. Works on selected wavelength range with the help of filters.

o Ratio radiation Pyrometers(two color Pyrometers): Measures the radiated energy of an object between two narrow

wavelength bands and then calculates the ratio of the two energies.

This ratio is the function of the temperature of the object.

Even if the object does not fully cover the spot the output signal will

not change.

Temperature measurement is independent of emissivity, so the

errors caused by the emissivity variation, surface finish, and energy

absorbing materials between the pyrometer and the target can be

minimized or removed.

Used for measuring high temperature.

o Multi wavelength Pyrometers Accurate temperature can be gained only with the knowledge

of surface emissivity.

Multi wavelength pyrometer overcome many well known

difficulties of pyrometry.

The instrument consists of Software which collect data from

numerous similar applications.

Each measurement is analyzed and then temperature is

calculated.

Advantages of Non-Contact Pyrometers

• Ability to monitor temperature in situations where the object is

inaccessible or moving.

• Where the object may be contaminated or damaged by a contact

sensor.

• Where contact is not possible due to extremely high temperatures,

or where the object is electrically active.

• An Infrared Thermometer can be mounted remotely from the hot

target, enabling it to operate for long periods with minimal

maintenance.

• Very fast response time in tune of msec.

Applicationso Glass Industries

o Nuclear Industries

o Smelter Industry

o Pharmaceutical Industries

o Cement Industries

o Power Industries

o Non-Ferrous Industries

o Chemical and Fertilizer Industries

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