Introduction to Pyrometers
Pyrometer, an instrument for measuring temperature.
Although the term pyrometer is generally considered to apply to
instruments that measure high temperatures only, some pyrometers are
designed to measure low temperatures. They measure the
tempereatue of the surface of objects.
It is the most accurate method for temperature measurement under
severe conditions and it is based on non-intruisive
(indirect) temperature techniques.
The
word pyrometer comes from the Greek word
for fire, "πυρ" (pyro), and meter,
meaning to measure.[6] The amount of thermal energy
or heat leaving a body by radiation
and the wavelength of that radiation are
functions of the temperature of the body. This
dependence on temperature of the
characteristics of radiation is used as
the basis of temperature measurement in these
instruments.
In a
Pyrometer, temperature is measured by sensing the heat radiated from
a hot body through a fixed lens that focuses the heat energy on to a
thermopile; this is a noncontact device. Furnace temperatures, for
instance, are normally meas-ured through
a small hole in the furnace wall. The distance from the source to
thepyrometer can be fixed and the
radiation should fill the field of view of the
sensor.A very Basic Design of a radiation thermometer
(pyrometer) is shown. [1]
An ideal blackbody is one that at all temperatures will absorb
all radiation falling on it without
reflecting any whatever in the direction of
incidence. The absorptive power of the surface, being the proportion
of incident radiation absorbed, will be unity. Most surfaces
do not absorb all incident radiation but reflect a portion of it.
That is, they have an absorptive power of less than unity.
A blackbody is also a perfect radiator. It will radiate more
radiation than a body with an absorptive power of less than unity.
The emissivepower is called the “emissivity”
of a surface. The emissivity is the ratio of the radiation emitted
at a given temperature compared to the radiation from a perfect
blackbody at the same temperature.
Stefan-boltzmann law states that “The
total power of radiant flux of all wavelengths R emitted into the
frontal hemisphere by a unit area of a perfectly black body is
proportional to the fourth power of the temperature Kelvin:
R
= ∂T4
Where ∂ is the Stefan-Boltzmann constant, having an accepted
value of 5.67032 x 10-8 W .m-2K-4,
and T is the temperature Kelvin.
This law is very important, as most total radiation pyrometers are
based upon it. If receiving element at a
temperature
T1
iarranged so that
radiation from a source at temperature
T2
falls upon it. then it will receive heat at
the rate of
∂T24
and emit it at a rate
∂T14
,It will therefore gain heat at the rate
∂(T14-T24).
If the temperature of the receiver is small in comparison with that
of the source then T14 may be neglected and
the radiant energy will be directly proportional to the fourth power
of the radiator.
Besides the stefan-boltzmann law,
Prevost thery of exchange and
Wiens law principles are also involved
in the working of pyrometers.
Since the energy radiated by an object is a function of
its absolute temperature this is a suitable property
for the non-contact and non-intrusive measurement
of temperature. Instruments for temperature measurement
by radiation are called radiation thermometers. The terms pyrometer
or radiation pyrometer were formerly used.[7]
There are four principal techniques for
the measurement of temperature by the
radiation from a hot body
1.
Total radiation,
2.
Pyroelectric.
3.
Photo-electric
4.
Optical Pyrometers
Instruments using the first three of these techniques are normally
constructed in the same general physical form. The figure 1 shows
the general format of one of these instruments. It consists of
a cylindrical metal body made of aluminum alloy, brass, or plastic.
One end of the body carries a lens, which, depending on the
wavelength range required, consists of germanium, zinc sulfide,
quartz glass, and sapphire.
The opposite end carries the electrical terminations for connecting
the sensing head to its signal conditioning module. A diagrammatic
sketch of the construction of the instrument is shown in Fig 3.
Infrared energy from a target area on the object whose temperature
is to be measured is focused by the lens onto the surface of the
detector. This energy is converted to an electrical signal which may
be amplified by a head amplifier on the circuit board. Power
is supplied to the instrument and the output
transmitted down a cable which is connected
to terminals in the termination box. In
instruments working in the near-infrared region where the lens is
transparent to visible light a telescope can be provided, built into
the instrument, so that it can be focused and aligned by looking
through the lens.
An important advantage of pyrometers, especially when used to
measure high temperatures, is that the
instrument measuring head can be mounted remote
from the hot zone in an area cool enough not to exceed the
working temperature of the semiconductor electronics
Typically about 50-75oC. However where the instrument has
to be near the hot region,such as
attached wall of furnace, or where it is needed to be of rugged
construction,it can be housed in an
air-cooled or water-cooled housing.
The function of the lens is to concentrate the radiation from the
source of radiations onto the surface of the
sensor.This also has the great advantage that the reading of
the instrument is greatly independent of the distance from the
source of radiation. As long as the source of radiations is large
enough to fully fill the sensor area with its image. The material of
construction of the lens depends upon the
wavelegth of the radiation source. In other words, it depends
upon the temperature range that is being measured. At lower
temperatures, the material suitable with longer wavelength is used,
while at higher temperature, a material is chosen with which
radiation of shorter wavelength are measurable. It is due to the
fact that the wavelengths of radiations vary inversely with the
total radiations energy.
Here the radiation emitted by the radiant body or fluid whose
temperature is to be measured is focused on a thermal receiving
surface. This receiving element may have a variety of forms. It may
be a resistance element. it is normally
in the form of a very thin strip of blackened platinum, or a
thermocouple or thermopile, the change in temperature of this
surface is measured.
Usually in a radiation thermopile a large number of
thermocouples in the form of strips are connected in
series and arranged side by side,
or radially in a circular manner to make a wheel.
So that all the hot junctions, which are
blackened to increase the energy-absorbing ability, fall within a
very small target area. The thermoelectric characteristics of
the thermopile (thermocouple) are very stable because the
thermocouples are rarely connected directly to the furnace and hence
are not present at a temperature of more than a few hundred degrees.
Thus a thermopile has an advantage over other detectors. They also
give the same response to incoming radiations in the range
(0.3-20µm) irrespective of wavelength within this range.
Among the disadvantages is the fact that the speed of response of
these thermopiles is usually very slow. The speed of response of
these thermopiles can be accomplished by decreasing the temperature
difference between the junctions i.e the
cold junction temperature is increased. But that results in a
decrease in accuracy i.e lesser
emf and hence less resultant output.[3]
Other alternates for thermopiles that can be used include
thermistors and pyroelectric detectors.
The advanatage that can be obtained with
thermistors is the fact that they are small in size, hence have less
response time. But they too have a disadvantage of
non linearity. But that can be overcome
with provision to linearize the radiant energy signal.
The calibration of total radiation pyrometers is done with black
body radiation. The output temperature T4.
When the pyrometer is used to measure the temperature of a fluid or
a hot body, the emmisivity has to be
known. If the emmisivity is not
correctly known, then the temperature that is measured will not be
corrected and some degree of error will be present. The extent of
error is calculated as follow; the output thermometer temperature is
directly proportional to T4 and is given as
E = KЄT4
Here K is a constant.
By Differentiating, we get
DT/T = dЄ/4Є
Hence a 10 percent error in the value of
emmisivity will result in 2.5 percent error in the
temperature of the radiant object that is measured.
Pyroelectric
detectors for thermal radiations are a relatively new form of
pyrmometers. The construction
material is usually ceramics are materials whose molecules have a
permanent electric dipole because of the position of the electrons
in molecules. Normally these molecules lie in a random “mish-mash”
manner all across the bulk of the material hence there is no net
electrification as a whole. Also, at ambient temperatures the
location or orientation of these molecules is more or less fixed.
If the temperature is raised above some level characteristic to the
particular material, the molecules start to rotate freely. The
temperature at which this start to happen
is called the Curie temperature.
If a piece of pyroelectric material is
placed between two electrodes at ambient temperature (fig 5), then
the molecular dipoles are almost fixed throughout the structure.
When the temperature of the radiant object is increased, then the
temperature of the pyrolectric material
increases above the is curie temperature and an electric potential
is applied (fig 6), then the molecules of the ceramic will align
themselves and an electric field will be generated in the ceramic.
If the temperature of the ceramic material is increased, then the
molecualar dipoles will now
rotate/oscillate at a higher angle. Thus greater the temperature
of the radiant object, greater will be the angle of oscillation of
the molecular dipole.
When the pyroelectric surface is used as
detector in a pyrometer, when the radiations from the source are
absorbed by the pyroelectric material,
its surface temperature increases .In the beginning the charge on
the electrodes would be leaked away through the external electrical
circuit and hence the measured voltage between the electrodes would
be zero. When the pyroelectric
surface heats up a voltage is detected between the two electrodes.
As the temperature is further increased, further voltage is
increased. Through this voltage value we can measure the
temperature. The physical construction of a
pyroelectric pyrometer is similar to the total radiation
thermometer.
In order to obtain a constant flux of radiations, or in other words
constant temperature signal, we require chopping by using an
oscillating shutter.
In terms of construction they are similar to a total radiation
pyrometer except for the fact that it requires a
shutter. This shutter is placed in front
of the detector. Figure 7 shows the construction of a
pyroelectric thermometer and the
location of the shutter is also identified.
Although the measurement obtained with an optical thermometer shows
a greater temperature error than a total radiation thermometer. This
is because the emissivity error for a given temperature and a
known emissivity is proportional to the
wavelength of the radiation used to make the
measurement. The optical pyrometers still have some
disadvatages, the fact that it can only
be used for point measurement rather than
continuos measurement too, and the speed of response is low
and hence are not very suitable for control purposes.
This is where photo-electric pyrometers are used, in places where
the radiations of the measured object are of shorter wavelength
i.e at very high temperatures. They are
also very similar in construction to radiation pyrometers and are
hence often classified as its type. The one major difference in
construction though is the use of photodiode as the detector rather
than thermopile.
A photodiode is usually a semiconductor diode, it could be made of
germanium or silicon since both are good semiconducting elements and
the diode is constructed in such a manner that the incident
radiations can reach the junction region of the semiconductor. If
germanium is used, the diode will be a
simple P-N junction, but if silicon is used it could be a P-N or
P-I-N junction. A voltage in reverse is applied across the
diode i.e., in non-conduction direction. In these conditions
the current carriers, i.e., electrons in the semiconductor
do not have sufficient energy to cross
the energy barrier of the
junction.However, when incident radiations are
directed towards them, some electrons gain enough energy
to cross the junction. They will obtain this energy by
collision with photons. The energy of photons is inversely
proportional to the wavelength.thus as
the radiant energy impacted upon the surface of the photoelectric
diode increase, more electrons cross the barrier and hence more
voltage reading will be obtained. This will obviously happen at
higher source temperature, thus the temperature is measured
indirectly by measuring the voltage reading. [2]
Optical radiation thermometers are a simple in construction and they
are accurate for temperature measurement between 600
oC to 3000
oC. Because they require the
eye and the decision making of the viewer (operator), thus they are
not a suitable device for recording or control purposes. But
nevetheless, they are very effective for
point measurements and for calibration of total radiation
thermometers.
In terms of construction, they are similar to a telescope. Here a
tungsten filament lamp is placed at the focus of the objective lens.
Figure 9 below show the construction of an optical radiation
thermometer. To use the instrument the point where the
temperature is required to be known is viewed through the pyrometer.
The current passing through the filament of the lamp is adjusted in
such a way that the filament disappears in the image. Figure 8
shown below represent the manner in which the filament appears in
the eyepiece against the background of the radiant object whose
temperature is being measured.In (a) the
current through the filament(i.e the
temperature) is too high and it looks bright against the light
coming from the radiant object, at (c) the current is too low
and the filament still appears in the image thus
meanig that the temperature of the
filament is lower than that of the radiant object while at (b)
the filament is at the same temperature
as the radiant object indicated by the fact that the filament
has dissappeared from the image. [2]
The temperature as well as the resistance of the filament is known.
Thus the temperature of the radiant object is also the same since
they are the same; this is one of the main drawbacks of this
instrument, the fact that the measured temperature is
dependant on the viewers’
judgement about when the filament has
disappeared from the image. [5]
Typical applications of different types of pyrometers are given
below:
Optical Pyrometers
are generally used in the process industry for occasional
measurement. They have high precision and are hence used as a
reference insrument with which other
pyrometers are compared. The accuracy and
precison of other pyrometers are measured by comparing with
it. They are also used for temperature measurement of non-black
bodies. Their temperature range is high, they are the most commonly
used high temperature measuing devices
used in the laboratory. One of the drawbacks is the fact that they
can only be used by experienced personnel. But they are being
gradually replaced by the modern photoelectric pyrometers.
Pyro-electric and photoelectric pyrometers
are used in the industry mainly as a reference instrument to
determine the true temperature of an onject
having unknown emmisivity. Photoelectric
instruments are very precise and are thus replacing the above
mentioned optical type pyrometers. While the
pyro elecric thermometers still have
relatively limited applications.
Total radiation pyrometers
used with quartz or glass lense are most
commonly used pyrometers in the industry, one of the main reasons
behind that is the fact that they can give
continuos measurement and can also be used for bodies
that are not perfect black bodies or non-black bodies. These
pyrometers are often used in electric chamber furnaces, glass tank
furnaces and other industrial areas.[4]
1.
BIBLIOGRAPHY Boyes, W. (2002). Instrumentation Reference Book
Third Edition (Vol. 1). Burlington: Elsevier Science
Publications.
2.Dunn,
W. C. (2005). Fundamentals of Industrial instrumentation and
process control (Vol. 1). Chicago: The McGraw Hill Publications.
3.Michalski,
L., Eckersdorf, K., Kucharski, J., & McGhee, J. (2001).
Temperature Measurement Second Edition. West Sussex: John Wiley
& Son Ltd.
4.Sirohi,
R. S., & Krishna, H. C. (1991). Mechanical Measurements Third
Edition. New Delhi: New Age International Limited Publisher.
5.Sutko,
D. A., & Faulk, D. J. (1996). Industrial Instrumentation.
MidWest State University: Delmar Thompson Learning.
6.
www.en.wikipedia.org/wiki/Pyrometer
(Retrieved on November 09, 2010)
7.
http://science.howstuffworks.com/dictionary/physics-terms/pyrometer-info.htm
(Retrieved on November 09, 2010)
