Download 3. Contactless Temperature Measurement 3.1. Whole

3. Contactless Temperature Measurement
3.1. Whole-spectrum (wide-band) pyrometer
Objectives
1. Test the functionality of contactless temperature meter (pyrometer) using a blackbody
device Omega BB2-A. Examine the influence of wrong setting of emissivity (check if
the emissivity corresponds to the supplier-specified correct value 0.95). To prevent
injury from hot device and to avoid contamination of the blackbody surface DO NOT
TOUCH the blackbody.
2. In all calculations, use temperature T in Kelvin (= t + 273.15)!
3. Examine the influence of translucent materials (plastic foil, glass) in optical path.
Explain the changes of measured temperature values.
4. Measure the surface temperature of human skin using contact thermometer and using
pyrometer with emissivity set to ε = 1. Calculate the real emissivity of human skin,
enter this value into the pyrometer settings and repeat the measurement to check the
calculation.
Introduction
The presented contactless thermometer works like whole-spectrum IR radiation pyrometer
and the temperature is calculated according to the Stefan-Boltzmann law:
I = ε σ ( T 4 - TA4 )
where I
(3.1)
is intensity of IR radiation [W/m2],
ε
is emissivity of the object [-],
σ
is Stefan-Boltzmann constant 5.6703 .10-8 W m-2 K-4,
T
is object temperature [K],
TA
is ambient temperature [K].
Whole-spectrum pyrometers are more sensitive then the narrow-band pyrometers, but their
results are very dependent on the correct setting of the emissivity of measured object. The
device measures the intensity of IR radiation I using an IR sensor and the ambient temperature
TA using a built-in temperature sensor. Emissivity ε is set by user. Formula (3.1) is used by
the device to calculate the measured temperature T.
Black body standard Omega BB-2A
Max. temp.
Emissivity
343 °C
0,95 (theoretical value specified by supplier)
3.2. Optical pyrometer (vanishing filament pyrometer)
Objectives
Measure the temperature of glowing light bulb filament and determine the spectral
transmissivity of the presented material (plastic foil) and the spectral transmissivity of the
built-in gray filter used for scale-switching.
Instructions for Temperature Measurement
The optical pyrometer (unlike the whole-spectrum wide-band pyrometer) measures the
intensity of radiation on a single wavelength (in a narrow-band), typically in human-visible
red light band). It is less sensitive than wide-band pyrometer, but it is also much less sensitive
to wrong setting of the correct value of emissivity ε. The vanishing filament pyrometers are
used in metallurgy to inspect temperature of molten iron in the furnace.
Fig. 1 Optical pyrometer
a)
b)
Fig. 2 View through the pyrometer with comparison filament a) too cold and b) too hot
Set the bulb supply voltage with the variable transformer to approx. 30V. Aim the
instrument’s axis to the bulb filament. Push the white button above display and control the
current in the comparison filament with the built-in adjustable resistor. (The current control
“knob” is at the circumference of the device front panel). Compare the brightness of the two
filaments and regulate the current until the brightness of the two filaments is the same (the
comparison filament is not discernible on the bulb filament background). Read out the
temperature on the scale when balance is achieved. Change the current slowly, since the
comparison filament reacts with some delay to the current change. On the display, read the
temperature value (this is “black temperature” T0).
The real (“gray”) objects always have emissivity – i.e. the ability to emit energy –
lower than the absolute black body. Therefore their temperature must be higher to achieve the
same spectral radiance. The real temperature TS must be computed from the black temperature
indicated by the pyrometer and from known emissivity of the object. The bulb filament is
made of tungsten (wolfram) and has emissivity of ε = 0.428.
Instructions for the spectral transmissivity measurement
After you have measured and calculated the real temperature TS in previous case put a
sample of the translucent material between the bulb and the pyrometer and measure the new
temperature. By inserting the material into the optical path, a real situation is modeled when
dust, smoke, etc. is present between the pyrometer and measured object. For the pyrometer it
looks like a sudden drop of temperature of the measured object. From the newly measured
temperature T0’ (and from the knowledge of real temperature TS calculated in the previous
measurement) the transmissivity of material can be calculated. In a similar way, you can also
measure the transmissivity of the built-in gray filter (normally used for switching of
instrument sensitivity). It is engaged/disengaged by the small lever on top of the pyrometer
and the measurement procedure is the same as in previous cases.
These equations apply:
1
1 λ
=
+ ln(ε λτ λ ) ,
TS T0 c 2
(3.2)
αλ = 1 −τ λ ,
where TS [K] is the real temperature of the measured object,
T0 [K] is the temperature displayed by the pyrometer (black temperature),
λ is wavelength [m], the center of the band-pass of the red filter, i.e. 650nm,
c2 is a physical constant, c2 = 1.44.10-2 [m.K],
ελ is the spectral emissivity of the object, for tungsten ελ = 0.428,
τλ is the spectral transmissivity (for clean air, consider τλ = 1.0),
αλ is the spectral absorption.