Chapter 2
Non-Contact Temperature
Measurement
• Pyrometer
– any non-contacting device that intercepts and
measures thermal radiation.
• Pyro is Greek for Fire
• Meter is Greek for Measure
• Thus Pyrometer measures fire.
– (actually temperature!)
• Pyrometer
– works on basis of Stephan-Boltzman Law
• E = ε ρ T4
E = object’s radiant incidance
(W / m2)
ε = emissivity of object
(no units 0 < ε < 1)
ρ = Stephan-Boltzman Constant
5.672 x 10-12 W/(cm2 deg4)
T = temperature in Kelvin
• We assume objects temperature is directly proportional
to the radiant incidence of the object.
• Thermocouples and Resistance Temperature Detectors
(RTDs) need direct contact. Pyrometers don’t.
• Emissivity is amount of energy emitted by object.
Blackbody has ε = 1.
• http://www.brannan.co.uk/thermometers/invention.html
• The coldest that water can get without
freezing is when it contains its maximum
salt content. Salt water can get as low as
0 degrees Fahrenheit.
• Putting salt on roads won’t work below 0
degrees Fahrenheit.
• Thermometers were
marked at 12 degree
increments.
• 96 is divisible by 12
Measuring Temperature
• Thermometer
– Typically based on thermal expansion
• Bimetallic Strip
• Liquid in glass
• 2 reference points
– Ice point (freezing point)
– Steam point (Boiling point at 1 atm
pressure.)
Temperature Scales
• Fahrenheit
– Ice point = 32OF
– Steam point = 212OF
– Divide into 180 equal steps
212o
100o
180
steps
100
steps
32o
0o
• Celsius
– Ice point =
– Steam point = 100OC
– Divide into 100 equal steps
0OC
Temperature Scales
• Kelvin
– Based on concept of ‘absolute zero.’
– Coldest temperature possible = -273OC
– 0K = -273OC
• Note it is NOT OK!
• Rankin
– 0R = -460OF
• Coldest temp possible
• measured in Fahrenheit.
Conversions
• Between OC and K
– TK = TC + 273
– TC = TK - 273
• Between OC and OF
– Must correct for step size and starting point
– TF = (9/5)TC + 32
– TC = (5/9)(TF - 32)
Example
• Convert 70oF to Celcius.
– TC = (5/9)(70 - 32) = 21oC
• Convert 21oC to Fahrenheit
– TF = (9/5)21 + 32 = 70oF
• Convert 21oC to Kelvin
– TK = 21 + 273 = 294 K
• Convert 294 K to Celcius
– TC = 294 – 273 = 21oC
• E = ε ρ T4
E = object’s radiant incidance
(W / m2)
ε = emissivity of object
(no units 0 < ε < 1)
ρ = Stephan-Boltzman Constant
5.672 x 10-12 W/(cm2 deg4)
T = temperature in Kelvin
• Emissivity and absolute temperature influence
radiation (or energy or radiant incidance).
• Object at 200o C has 16 times greater energy
than one at 100o C, since 24 = 16
• Emissivity – measure of objects ability to emit or
absorb radiant energy.
• 0<ε<1
• No units
• ε = 0 is perfect reflector. Surface doesn’t emit nor
absorb radiant energy. Not good for IR temperature
sensing.
• ε = 1 is perfect blackbody. Absorbs all energy.
Very good for temperature sensing.
• Greybodies – not quite a blackbody, but very close.
• Other factors which influence surface
emissivity
– Surface texture (degree of roughness or
oxidation)
– Surface temperature
– Wavelength of emitted energy
• Don’t use IR pyrometer for surfaces with low
emissivity value less than 0.5.
– Won’t get accurate results.
• You can enhance a surface with low emissivity
with the following:
– Texture the surface by sanding or sandblasting
– Oxidize the surface
– Anodize the surface
• Electrolytic process to oxidize the surface.
– Paint the surface with dull, highly-absorbent coating.
• Non-contact IR sensors have advantages
over contact sensors
– Can mount the sensor away from the heat
source
– Can sense the temperature of a moving
object
– Sensor won’t sink or draw energy from the
object
– Can view object through window in
contaminated or dangerous environment.
Nongray Bodies
• False readings if emissivity < 1.0
• Set compensator depending on emissivity of
object.
• If emissivity of object is 0.5, set compensator to
0.5
• Errors are in series.
– If object is 0.5 and measuring through glass with 0.4,
then total emissivity is 0.5 * 0.4 = 0.2
– Table 2-1 on page 37 shows some emissivity values.
Wavelength (microns)
Ultraviolet ranges from 0.2 µm to 0.5 µm
Infrared ranges from 0.7 µm to 1.0 µm
Environment may attenuate signal.
Wavelength (microns)
Ultraviolet ranges from 0.2 µm to 0.5 µm
Infrared ranges from 0.7 µm to 1.0 µm
Planck’s Law
W=
C1
C2
5 (e T  1)
µm4/m2
K
in Watts µm/m2
C1 = 3.74 x 108 W
C2 = 1.44 x 104 µm
T = temperature in Kelvin
Rayleigh-Jeans Law
predicted infinite energy as wavelength decreased –
called ultraviolet catastrophe
Planck was able to add exponential term
to cause function to go to zero at λ = 0.
As λ approaches ∞, denominator approaches hc /λkT and we get
back the Rayleigh-Jeans Law again.
• As temperature increases, the amplitude
of curve increases and the area under the
curve increases. Area is the energy of the
object.
• Wavelength at peak energy shifts to the
shorter wavelength end of the scale as
temperature increases.
Peak shifts to the left as temperature increases.
Area under curve is energy. Higher frequency has
higher energy.
Rayleigh-Jeans Law
Ultraviolet catastrophe!
Ultraviolet Infrared
400 nm
700 nm
Wien’s Displacement Law
there is an inverse relationship between
the wavelength of the peak of the emission
of a black body and its temperature.
where b = 2.89 x 103 µm K
and T is temperature in Kelvin
λ will be in µm
• Find peak wavelength of object at 2617o C.
• Convert temperature to Kelvin
K = oC + 273 = 2617 + 273 = 2690 K
λ = 2.89 x 103 µm K / 2690 K = 1.07 µm
Conservation of Energy
• ER + EA + ET = 1
ER = Radiated Energy
EA = Absorbed Energy
ET = Transmitted Energy
• If ε = 0.80, then EA = 80%
so ER + ET = 1 – 0.80 = 0.20 = 20%
• For blackbody, ε = 1.0, so ER + ET = 0
so ER = ET = 0
• For blackbody, all energy is either reflected or
transmitted. No energy is absorbed.
How IR Sensor Works
•
•
•
•
Precision Optics – focus energy onto dector
IR Detector – create signal proportional to IR
Sensor Housing - shielding
Support Electronics – amplify signal
Single Color Pyrometer
• Narrow Band pyrometers
– Looks only at narrow sector of IR (2.2 microns)
– Need to amplify signal
– Expensive
• Wide Band pyrometers
– Look at wide band of 8 to 14 microns.
– Requires only low gain amplifiers
– Not dependent on distance
• Low Cost are very wide band
– 0.7 to 20 microns with no amplifier
– Distance sensitive because of atmospheric absorption
Two-color Pyrometer
• Two-color Pyrometer
– Measures ratio of two IR bands so also called
Ratio Pyrometer
– Can eliminate effect of emissivity
– Target does not need to fill entire field of view.
– With single color pyrometer, if target covers only half
the field of view, you only get half the energy
– With two color pyrometer, both IR band energies are
cut in half, but ratio is the same.
– If cloud of dust is between object and sensor, reduction
in energy occurs in both bands, but ratio will remain the
same.
Sensor Placement
• Sensor should be at right angle to target
• Reduces effect of reflected energy.
• Don’t position sensor at more than 45o
angle.
• Make target size twice as large as the
desired spot size to reduce errors from
background radiation
Response Time
• Thermocouples and RTD contact sensors
require one time constant
• IR requires only 240 msec
Sources of Error
• Dust, gases, suspended particles, water
vapor
• Non-cooled IR sensor will show lower
temperature than a cooled IR sensor.
• Viewing through glass will reduce energy
received, so will have lower measured
temperature.
• If measured from shiny surfaces with low
emissivity
Calibrating Pyrometers
• Use Blackbodies
– Know emissivity of object at wavelength of IR
– Know temperature of object
• NIST can’t certify blackbodies since the
age of the blackbody affects the
emissivity.