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Pyrometers

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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]

1.  Blackbody radiation & Stephen-Boltzmann Law

 

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.

2.  Types 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

3.1 Brief Process Description

 

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.

2.1.1    Function Of lens

 

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.

2.2                   Total Radiation Pyrometers

3.2.1       Working 

 

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 (0.3-20Ám) 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.

        3.2.2       Calibration

 

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 = /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.

 

2.3                   Pyro-Electric technique thermometers

2.3.1       Working

 

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.

2.4                   Photo-electric radiation pyrometers

 

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.

2.4.1    Working

 

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]

2.5 Optical Pyrometers (Disappearing Filament)

 

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.

2.5.1    Working & Construction

 

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]

3.  Applications of pyromters

 

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]

References

 

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)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
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