Pyrometers
To measure temperature of a very hot body
Where thermometers cannot brought into contact or
Where hot bodies are moving
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
For practical radiating bodies
We have to consider absorbtion, reflectivity, transmissibility, emissivity
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
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
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
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