Pyrometers

Pyrometers

To measure temperature of a very hot body

Where thermometers cannot brought into contact or

Where hot bodies are moving

Pyrometers

Measurements done by measuring energy radiated by a hot body or by comparison of colour

Wave length of thermal radiation-0.01 to 100 µ m

Black body radiation

Sefan- Boltzmann’s law

Wb = σ T4

b/w two bodies

Wb = σ (T14 – T2 4)

Emissive power of blackbody is

Wby = C1 λ -5 / e (C2

/ λ T)

where λ – wavelength in cm

C1 – I radiation const

- 3.74 X10 9 erg/s/sq.cm/0.01µcm

C2 – 1.438 cm K

T- absolute temp of black body

As temperature of emitting source changes colour changes

Ie, for lower temp the colour is dull

For practical radiating bodies

We have to consider absorbtion, reflectivity, transmissibility, emissivity

Emissivity

ε = 0.365√(ρ/λ)

ρ - resitivity

Here detector temp > casting temp

Detector losses its temp. in all ways

If detector temp. finally settles to T2 & casting temp is at T0 and target temp is T1

Ko (T2- T0 ) = K1 T1 4 = heat input

Value of T0 - important for detectors

V o α (T2- T0 ) α T1 4

V= Kv T n

n b/w 3.5 to 4.5

Kv- Experimental value

Comman detectors in pyrometers

Thermopile-

photocell

Metal bolometer

thermister

Thermopile

Larger o/p

Lower response time

Adaptable for industrial applications

Bolometer

Fast in response

Good sensitivity

costly

Thermister

Not used due to

Poor precision

Difficult to provide compensation

Low response time

Photocell

Fast

Large sensitivity

But limited spectral sensitivity

Lens – for focusing targets

Uses selective materials to prevents absorption by them

1. Thallium iodide – more suitable for high temp with shorter emission

2. fused silica -High temp with shorter wavelength – b/w 250 to 1900 C

3. Pyrex

Sources of errors in pyrometers

Interleaving media has to be clear

Carbon dioxide, water vapour etc absorb selective bans of wavelength

Emittance errors

Sensitive to obstruction

Advantages

Abitity to measure high temp

No need for contact

Fast response speed

High o/p

Moderate cost

disadvantages

Non linear scale

Emmisivity of target affect measurement

Errors due to interleaving media

Application

Can be used where life of thermocouple limits

Optical pyrometer

Optical pyrometer

Optical pyrometer

Range b/w 700 to 3000 oC

More accurate.

Lower limit determined by human eye

Human eye compares the radiation

Optical pyrometer- workingdisappearing filament type

Radiation received by objective lens- focus on filament- viewed through a microscope system- filament appears in background.

red cadmium filters makes the radiation appear momchromatic

working

working

At Bt = Bf – milli ammeter gives a current prop ional to temperature

tungsten is used

advantages

Flexible

Portable

Convient to use

Light weight

Can monitor moveable objects

Non contact type

accurate

disadvantages

Expensive

Human error

Not useful for measuring temp of clear gas

At high temp. filament erodes frequently

Can also use double control pyrometers

Radiation pyrometer disappering filament type

Thank u