The word pyrometer originated from the Greek word, fire, " (pyro), and meter, meaning to measure. Pyrometry is the science of measuring temperature greater than 1500 degree Celsius and therefore Pyrometer is the instrument that is used to generally measure high temperatures. They usually measure the temperature of the surface of the object. The surface of the object emits radiation which is used as a measurement for pyrometers. Due to the high temperature conditions, the measurement uses an indirect temperature technique.
Instruments for temperature measurement by radiation are called radiation thermometers. The four principal techniques for the measurement of temperature by radiation are; 1. Total radiation 2. Pyroelectric thermometers 3. Optical disappearing filament thermometers 4. Photoelectric radiation pyrometers 1. Total Radiation Assembly and Operational Principal Using this type of instrument, the radiation emitted by the body in which the temperature is required is directed on a thermal receiving element.
The receiving element comes in a variety of forms which would include a thermocouple or a thermopile or a resistance element which usually comes in a form of a thin strip of blackened platinum. In a radiation thermopile, a large number of thermocouple in the form of fine strips are arranged circularly to make a wheel so that all the hot junctions will fall within a very small target area. These strips are blackened to increase the energy absorbing ability. The hot junctions are rarely above a few hundred degrees Celsius and that makes it very stable.
The thermocouple are also not exposed to the contaminated surrounding of the furnace. Advantage and Disadvantage. Having the thermopile as a detector produces a measurable e. m. f which acts as an advantage. As mentioned above, the hot junctions of the thermopile makes it a stable instrument of measurement. However, the thermopile has a slow speed of response. Increasing the rate of response will be a substitution to a lower e. m. f produced. Application. The total radiation pyrometer is the most commonly used pyrometer in the industry.
These pyrometers are often used in furnaces. Conditions in the furnace may be disrupted by smoke, flame or furnace gasses. In the instance these conditions are present, the total radiation pyrometer will generally indicate the temperature in between temperature of the furnace atmosphere and the temperature if the atmosphere was clear. 2. Pyroelectric Thermometers Assembly and Operational principal Pyroelectricity is the ability of certain materials to generate a temporary voltage when they are heated or cooled.
Pyroelectric materials which are mainly ceramics, are materials whose molecules have a permanent electric dipole due to the location of the electrons in the molecules. There are no net electrification because the molecules in the material lie in random orientation. At ambient temperatures the orientation of these molecules is fixed. If the temperature rises above a certain level, these molecules start to rotate freely. Curie temperature is described as the temperature these occurrence starts to take place. The figures above shows the movement of the molecules as temperature varies.
In the use of a pyroelectric surface as a detector, the radiation will increase the surface temperature. Initially, the charge on the electrodes would be leaked away and the measured voltage between the electrodes would be zero. As the temperature increases, and the surface heats up, there will be voltage detected between the two electrodes. the temperature is measured through this voltage value. Application Pyroelectric thermometers are used in voltage responsivities 3. Optical disappearing filament thermometers Assembly and Operational principal
Optical Pyrometers work on the basic principle of using the human eye to match the brightness of the hot object to the brightness of a calibrated lamp filament inside the instrument. It is a simple and accurate mean for measuring temperatures in the range 600 "C to 3000 "C. They provide an effective way of making spot measurements and for calibration of total radiation thermometers. In construction an optical radiation thermometer is similar to a telescope. However, a tungsten filament lamp is placed at the focus of the objective lens.
When the current through the filament or the temperature is too high it will be bright and the filament can be seen. if the current is too low or temperature is low the filament will still appear in the image. The current through the lamp filament is adjusted until the filament disappears in the image. The temperature of the filament is known from its electrical resistance. Temperature readout is achieved either by a meter measuring the current through the filament or by temperature calibrations on the control resistor regulating the current through the lamp. Application. 4. Photoelectric radiation thermometers
Assembly and Operational principal Photo-electric pyrometers are used in places where the radiations of the measured object are of shorter wavelength or at very high temperatures such as ultraviolet radiation. They are structurally similar to the total radiation thermometer. The one major difference in construction is the use of photodiode as the detector rather than thermopile. In the photoelectric effect, electrons are emitted from matter (metals and non-metallic solids, liquids or gases). A photodiode is usually a semiconductor diode and it could be made of germanium or silicon.
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. Energy is obtained by collision of the photons and electrons. The energy of photons is inversely proportional to the wavelength. Therefore 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. At high temperatures, voltage reading would be higher. the temperature is recorded indirectly by measuring voltage.