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Radiative temperature measurements:

Radiative temperature measurements The sensor for thermal radiation need not be in contact with the surface to be measured , making this method attractive for a wide variety of applications . A distinct advantage to measuring temperature by detecting thermal radiation . The basic operation of a radiation thermometer is predicated upon some knowledge of the radiation characteristics of the surface whose temperature is being measured, relative to the calibration of the thermometer .

Slide 2:

Radiation refers to the emission of electromagnetic waves from the surface of an object. This radiation has characteristics of both waves and particles , which leads to a description of the radiation as being composed of photons . The photons generally travel in straight lines from points of emission to another surface, where they are absorbed, reflected, or transmitted . The thermal radiation emitted from an object is related to its temperature and has wavelengths ranging from approximately 107 to 103 m . It is necessary to understand two key aspects of radiative heat transfer in relation to temperature measurements.

Slide 3:

First, the radiation emitted by an object is proportional to the fourth power of its temperature . In the ideal case, this may be expressed as: Where Eb is the flux of energy radiating from an ideal surface, or the blackbody emissive power. The emissive power of a body is the energy emitted per unit area and per unit time.

Slide 4:

Second, the emissive power is a direct measure of the total radiation emitted by an ob ject . However, energy is emitted by an ideal radiator over a range of wavelengths, and at any given temperature the distribution of the energy emitted as a function of wavelength is unique.

Planck distribution of blackbody emissive power as a function of wavelength.:

Planck distribution of blackbody emissive power as a function of wavelength.

Slide 6:

Consider an electrical heating element , as can be found in an electric oven. With no electric current flow through the element, it appears almost black, its room temperature color . With a current flow, the element temperature rises and it appears to change color to a dull red , and perhaps to a reddish orange . If its temperature continued to increase, eventually it would appear white . This change in color signifies a shift in the maximum intensity of the emitted radiation to shorter wavelengths, out of the infrared and into the visible. There is also an increase in total emitted energy. The Planck distribution provides a basis for the measurement of temperature through color comparison .

Radiation Detectors:

Radiation Detectors Radiative energy flux can be detected in a sensor by two basic techniques . The first technique involves a thermal detector in which absorbed radiative energy elevates the detector temperature. A second basic type of detector relies on the interaction of a photon with an electron , resulting in an electric current.

Slide 8:

A radiometer , measures a source temperature by measuring the voltage output from a thermopile detector . The increase in temperature of the thermopile is a direct indication of the temperature of the radiation source . One application of this principle is in the measurement of total solar radiation incident upon a surface .

Slide 9:

Fig shows a schematic of a pyranometer , used to measure global solar irradiance. It would have a hemispherical field of view, and measures both the direct or beam radiation, and diffuse radiation. The diffuse and beam components of radiation can be separated by shading the pyranometer from the direct solar radiation, thereby measuring the diffuse component.

Infrared (IR) thermopile sensors:

Infrared (IR) thermopile sensors IR manufactured using micromachining and advanced semiconductor processing methods. The hot junction of the thermopile is placed under the IR filter and the cold junctions under the IR mask . Manufacturing methods allow hundreds of junctions to be created in a micro-device . These sensors form the basis for many practical applications, ranging from timpanic thermometers to automatic climate control systems for automotive applications.

Slide 11:

Pyrometry Optical pyrometry identifies the temperature of a surface by its color , or more precisely the color of the radiation it emits . The major advantage of an optical pyrometer lies in its ability to measure high temperatures remotely . For example, it could be used to measure the temperature of a furnace without having any sensor in the furnace itself. For many applications this provides a safe and economical means of measuring high temperatures.

Slide 13:

A standard lamp is calibrated so that the current flow through its filament is controlled and calibrated in terms of the filament temperature. Comparison is made optically between the color of this filament and the surface of the object whose temperature is being measured. The comparator can be the human eye . Uncertainties in the measurement may be reduced by appropriately filtering the incoming light. Corrections must be applied for surface emissivity associated with the measured radiation; uncertainties vary with the skill of the user, and generally are on the order of 5°C . Replacing the human eye with a different detector extends the range of useful temperature measurement and reduces the random uncertainty .

Slide 14:

Optical Fiber Thermometers The optical fiber thermometer is based on the creation of an ideal radiator that is optically coupled to a fiber-optic transmission system. In this method temperature sensor is a thin, single-crystal aluminum oxide (sapphire) fiber ; a metallic coating on the tip of the fiber forms a blackbody radiating cavity, which radiates directly along the sapphire crystal fiber . The single-crystal sapphire fiber is necessary because of the high-temperature operation of the thermometer . The operating range of this thermometer is 300 to 1900C. Signal transmission is accomplished using standard, low-temperature fiber optics. A specific wavelength band of the transmitted radiation is detected and measured, and these raw data are reduced to yield the temperature of the blackbody sensor.

Slide 16:

Explain the working of mercury in glass thermometer? list out its advantages and disadvantages. Explain working of Pressure thermometer and resistance thermometer Explain the construction and working of Bimetallic strip and Thermocouple . Differentiate Thermocouple and RTD. Describe with neat sketch a)Radiation pyrometers b)Optical pyrometers Describe the working principle of platinum resistance thermometer in detail . Discuss the operation of temperature measurement using thermistors . Explain the operation of thermocouple for measurement of temperature . Classify the temperature measuring instruments and indicate approximate range of each category Explain with the help of neat sketch the principle and construction of elastic transducer