31-08-2013, 04:58 PM
PYROMETERS
PYROMETERS.pptx (Size: 311.22 KB / Downloads: 93)
What is a Pyrometer?
Pyrometer, an instrument for measuring temperature.
As the name itself signifies, they are high temperature measuring devices
They measure the tempereatue of the surface of objects.
Does not involve any physical contact
Basic pyrometer diagram
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.
Here the 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
Black-Body
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.
A perfect Black Body is the one that has an absorptive power of one
Bodies that donot absorb any incident radiations at all or only some portion of it have an absorptive power of less than unity.
Stefan-Boltzmann Law
It 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”
Mathematically given by,
R = ∂T4
This law has great deal of importance since most, if not all pyrometers are based primarily on this principle.
Total Radiation Pyrometers
Here the radiation emitted by the radiant body or fluid whose temperature is to be measured is focused on a thermal receiving surface.
The receiving element may be a resistance element, usually it is a blackened platinum, thermopile or a thermopile
A thermopile is a set of thermocouples connected in series or side by side to form a wheel
Thermopile thermal detecting elements are most commonly used in total radiation detectors.
Working
If it is placed between a pair of electrodes, at ambient temperature the molecules will be fixed.(shown in first figure)
By applying voltage across electrodes and increasing temperature beyond curie temperature, molecules start to align in a certain orientation and electric field is generated as a result.(shown in fig 2)
As the ceramic temperature is further increased, the free rotation also increases.
What is a photo-diode?
A photodiode is usually a semiconductor diode, it could be made of germanium or silicon.
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