Detectors of various instruments can be classified as calorimetric, thermomechanical, thermoelectric and photoelectric.
(I) Calorimetric Sensors:
In the calorimetric instrument, the radiant energy is incident on a high conductivity metal coated with a non selective black point of high absorptance. The radiant energy is converted into heat that can be measured by a variety of means.
II) Thermomechanical sensors:
In the instruments based on the thermomechanical principle, the radiant flux is measured through bending of a bimetallic strip. The metal strips with different thermal expansion properties are rigidly held together, one end is fastened and the other is free to move. One stripe is coated with a highly absorbent black paint and the other given a highly reflective coat. The blackened strip is exposed to solar radiation and the other is shielded from it. The two strips are insulated from each other to prevent heat flow from one to another. The unequal temperature and unequal coefficient of thermal expansio cause bending of the plates into a curve. The distortion is transmitted optically or mechanicall to an indicator.
III) Thermoelectric sensors;
A thermoelecctric device consists of two dissimilar metalic wires with their ends connected.An electromotive force(emf) is developed when the two junctions are at different temperatures. The emf developed is proportional to the temperatue difference and depends on material of the two metals.
E proportional to change in temperature and material
One unction is exposed to sunlight and other is shielded. Since emf by one thermocouple is low in practice many thermocouples are joined ( used in such an arrangement is called thermocoil.) The hot junction is coated with black paint and cold with white to shield from solar radiation. The cold junctions are kept as constant temperature.
IV) Photoelectric sensors:
Among the photoelectric devices, photovoltaic instruments are the most numerous in the field of solar radiation measurement. A photovoltaic device is made of a semiconducting material such as silicon. There si a formation of pn junction between p-type and ntype material of a semiconductor as shown in figure.
When radiation at an energy level capable of ionising atoms falls on pn junction an electric current is produced from the continous movement of excess elctrons and holes.
A major disadvantage of silicon cell device is their spectral response which is strong only in the red and near infrared portions of the solar spectrum. However, their advantages are lower cost and faster response times for instantaneous measurements.
(I) Calorimetric Sensors:
In the calorimetric instrument, the radiant energy is incident on a high conductivity metal coated with a non selective black point of high absorptance. The radiant energy is converted into heat that can be measured by a variety of means.
- The heat can be carried away by a flowing fluid whose change of enthalpy is measured. The change of enthalpy is an indication of the incident radiant flux.
- The heat gives rise to a change in the enthalpy of the absorbing metal(sensor). Again this increase in enthalpy (or increase in temperature) can be measured easily.
- In the modern cavity type instruments the temerature difference across a transducer is maiintained constant by additional elecrical heating required between shielded and exposed phases. The irradiance is then proportional to the difference in classify heating in two phases.
II) Thermomechanical sensors:
In the instruments based on the thermomechanical principle, the radiant flux is measured through bending of a bimetallic strip. The metal strips with different thermal expansion properties are rigidly held together, one end is fastened and the other is free to move. One stripe is coated with a highly absorbent black paint and the other given a highly reflective coat. The blackened strip is exposed to solar radiation and the other is shielded from it. The two strips are insulated from each other to prevent heat flow from one to another. The unequal temperature and unequal coefficient of thermal expansio cause bending of the plates into a curve. The distortion is transmitted optically or mechanicall to an indicator.
III) Thermoelectric sensors;
A thermoelecctric device consists of two dissimilar metalic wires with their ends connected.An electromotive force(emf) is developed when the two junctions are at different temperatures. The emf developed is proportional to the temperatue difference and depends on material of the two metals.
E proportional to change in temperature and material
One unction is exposed to sunlight and other is shielded. Since emf by one thermocouple is low in practice many thermocouples are joined ( used in such an arrangement is called thermocoil.) The hot junction is coated with black paint and cold with white to shield from solar radiation. The cold junctions are kept as constant temperature.
IV) Photoelectric sensors:
Among the photoelectric devices, photovoltaic instruments are the most numerous in the field of solar radiation measurement. A photovoltaic device is made of a semiconducting material such as silicon. There si a formation of pn junction between p-type and ntype material of a semiconductor as shown in figure.
When radiation at an energy level capable of ionising atoms falls on pn junction an electric current is produced from the continous movement of excess elctrons and holes.
A major disadvantage of silicon cell device is their spectral response which is strong only in the red and near infrared portions of the solar spectrum. However, their advantages are lower cost and faster response times for instantaneous measurements.
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