chap 2c- pyrometry.pdf
Post on 08-Nov-2014
30 Views
Preview:
DESCRIPTION
TRANSCRIPT
RADIATION METHODS (PYROMETRY)
× Measures temperature by detecting thermal radiation from an object without touching the object.
× Consists of an optic system which accumulates visible infrared energy and focuses the energy to the detector.
× The detector will convert the energy to electricity, where the electric energy is proportionate to temperature.
× Radiation emission:
× where, T is the temperature of the object in Kelvin.
Classes of Pyrometry × For temperatures above 650oC, the heat radiations emitted
from the body are of sufficient intensity to be used for measuring the temperature
× Instruments that employ radiation principles fall into three general classes:
1. Total radiation pyrometer × sensitive to all the radiation that enters the instrument
2. Selective (or partial) radiation pyrometers × Sensitive to radiation of a particular wavelength
3. Infrared (IR) pyrometer × the IR pyrometers employ the infrared portion of the spectrum by using
a thermal detector to measure the temperature on the surface of the body
Total Radiation Pyrometer × The total radiation pyrometer receives a controlled sample of the
total radiation of a hot body and focuses it on to a temperature sensitive transducer.
× The term 'total radiation' includes both visible (light) and invisible (infrared) radiations.
× Wave lengths of light in the visible range is from 0.3 to 0.72 µm & infrared radiations is from 0.72 to 1000 µm.
× Require special optical materials for focusing - ordinary glass is unsatisfactory
� It consists of blackened tube T open at one end to receive the radiations from the object whose temperature is to be measured
� The other end of the tube has a sighting aperture in which an adjustable eyepiece is usually fitted.
� The detector disc is usually of blackened platinum sheet/foil and is connected to thermocouple or thermopile junctions or to a resistance thermometer bridge circuit
� Leads from the detector are led out of the casing to a meter for measuring the thermoelectric emf or the variation the electric resistance of the platinum foil.
Selective Radiation Pyrometer
× The classical form of optical pyrometer is the disappearing filament optical pyrometer
× It is most accurate of all radiation pyrometers - ± 50C in the range of 850-12000C
× However, its use is limited to temperature, greater than about 7000C, since it requires visual brightness match by a human operator.
× This instrument used to realize the International Practical Temperature Scale above 10640C.
� In the disappearing filament instrument shown an image of the target is superimposed on the heated filament
� The tungsten lamp (stable) is previously calibrated, so that when the current through the filament is known, the brightness temperature of the filament is also known.
� A red filter that passes only a narrow band of wavelengths around 0.65 pm is placed between the observer eye and the tungsten lamp and the target image
� The observer controls the lamp current until the filament disappears in the superimposed target image.
Thermopile × Thermopile is normally used to detect infrared energy
where it normally consists of a few small thermocouples.
× Thermopiles are used to provide an output in response to temperature as part of a temperature measuring device, such as the infrared thermometer.
× These small thermocouples are connected serially to increase sensitivity of the pyrometer where the effect of serial connection will increase the sensitivity of sensor as much as n times.
Physical Construction of Thermopile
Examples of Pyrometer Applications
Measurement of exhaust gas temperature in the engine
× The above diagram shows a typical exhaust gas temperature control using a thermocouple. A DC amplifier is used to amplify the thermocouple output to a suitable value to be connected to a “matching network”.
Temperature measurement in a hot water tank.
× A platinum resistance thermometer is placed in a liquid filled tank. The resistance change will be changed to voltage through a “bridge network”. A DC amplifier produces enough voltage for the recorder to operate. A stirrer is used to keep the temperature uniform
Telemetry In Temperature Measurement
× Sometimes temperature measurement from a distance is needed and this is when the telemetry comes in. Voltage from a thermocouple is used to modulate the frequency to high wave radio frequency. This radio wave is transmitted and received by the radio frequency receiver where a demodulator is used to get the initial temperature reading
top related