CALLIBRATION OF DIRECT BEAM RADIATION MEASURING INSTURUMENT

The direct beam radiation measurement is done with the help of pyrheliometer. Its calibration is done mainly by following two processes.I) Intercomparision II) Calibration with standard instrument.

I) Intercomparision:
In thiis method, one pyrheliometer is taken as reference among the numbers of field instruments. The pyrheliometer which is taken as reference is calibrated against standard one and the calibration factor is determined for all field instruments.

II)Calibration with standard instrument:
An instrument whose calculated performance has been determined with a high degree of precision is called an absolute instrument. Abbot water-flow is taken as absoulute pyrheliometer. Number of pyrheliometers are calibrated against the absolute one are standard pyrheliometer. Generally field instruments are used for daily measurement of radiation. Field instruments are callibrated against standard pyrheliometer. Calibration is done by operating the standard and field instrument simultaneously in a clearday then calibration factor is ratio of measurement from field to that of measurement from the standard pyrheliometer.

CALLIBRATION OF PYRANOMETER

There are no absolute pyranometers like pyrheliometers. All pyranometer are calibrated instruments.There are number of methods of calibrating pyranometers using the sun or laboratory sources. Two of them are as follows.

I)Calibration against a reference standard pyrheliometer:
A perfectly clear day should be choosen to conduct this calibration. The test pyranometer should be fitted with a shading disk to temporarily eliminate the direct component. The disk should be large enough to shade the glass dome at all zenith angles. The test should begin by keeping the pyrheliometer pointed directly at the sun.At the same time, the shading disk should alternately shade and expose the pyranometer for about 10 minutes. The pyranometer calibration factor K is obtained as
 
         K= The ratio of (pyranometer reading average over exposed period- average over the shaded period) and In Cos thitaz
 where In is the direct normal irradiance calculated from the pyrheliometer reading and thitaz is Zenith angle.

II) Calibration  against a reference pyranometer:
In this method, the test and reference pyranometer are both placed in a horizontal position and operated simultaneously in a clear day. The calibration factor is a ratio of the measurement from the test  pyranometer to the measurement from the reference pyranometer.

HOW PYRANOMETER MESURES SOLAR ENERGY ARRIVING EARTH

A simple sketch for half shaded pyranometer is shown in figure. When the shaded part is removed, all the radiation ( direct + diffuse), that is, global fall on it. Inner part of the band is coated with black to remove the multiple reflection. The sensor kept inside the instrument convert radiant energy into heat energy and instrument is able to measure the radiant flux. When shade is placed, the direct radiation coming from sun will be blocked and only diffuse part of the radiation will fall on the instrument. Thus diffuse part of radiation can be measured by it.

CLASSIFICATION OF EARTH'S ATMOSPHERE IN ACCORDANCE WITH HTHE VERTICAL TEMPERATURE PROFILE

Vertical temperature profile is important to understand the actual concentration of atmospheric constituents and atmospheric pressure at pa particular height from the sea level in the earths's atmosphere. The vertical temperature profile for the standard atmosphere is depicted in the figure.

On the basis of vertical temperature profile,, the atomsphere has four layers. These are troposphere, stratosphere, mesosphere and thermosphere. The top of each sphere are respectively called tropopause, stratopause, mesopause and thermopause as shown in the figure above.

Troposphere:

It is the bottom layer where temperature decreases with increase in altitude with average value of lapse rate as 6.6 K/km. The temperature decrease continues to an average height of approximately 12 Km. However, the thickness of the troposphere is not same everywhere. It reaches to a height of 16 km in the tropic region, but in polar region it is about 9 Km. Most of the weather activities, for example cloud, rain, frost, fog, strom etc occur in troposphere. This layer contains nitrogen, oxygen, carbondioxide, dust and water vapour.

Stratosphere:

It is the layer of the atmosphere which lies above the tropopause. On this layer the temperature at first remains nearly constant to a height of about 20 Km. It begins a sharp increase that continues until the stratospause is reached at a height of about 48 km. Ozone occurs chiefly in this layer. In addition thhin layer of aerosol is also observed  to persist for long period of time within certain altitude rane of stratosphere. It is also called Ozonosphere.

Mesosphere:

It is the layer of atmosphere above the stratopause in which temperature decreases with altitude wntil at the mesopause to about 80 km above the surface. Temperature of this layer decreases upto about -100*C. It has very less ozone. Mesosphere has very strong winds blow eastward in winter and westward in summer. The upper boundary of this layer is Mesopause.

Thermosphere:

Extending upward from the mesopause and having no well defined upper limit in the atmosphere is the thermosphere.In the extremely rarified air of this outermost layer, temperature again increases as a result of the absorption of very short wave of solar radiation by the atoms of oxygen and nitrogen. Dute to which temperature increases sharply and reaches more than 1000*C.

Ozone is mostly found in stratosphere. A maximum in O3 is achieved near 30 Km. The radiative energy budget is controled by shortwave heating  due to Ozone absorption in the 9.6Km. Ozone thus determines the thermal structure of the middle atmosphere.

PYRHELIOMETER AND PYRANOMETER

PYRHELIOMETER
A special type of pyrheliometer is used to measure direct solar flux at normal incidence. it is a telescope type of instrument with narrow opening called aperture. This instrument faces the sun and follows its motion. The interest in establishing the value of the solar constant has been the main force behind development of this instrument. The instruments called water-flow, water-stir and silver disk pyrheliometers are based in the calorimetric principle.

A) Abbot water-flow and water-stir pyrheliometers:

• Inlet and outlet temperatures are measured at D1 and D2
• Accurate measurement of rate of water flow and temperature difference in D1 and D2 gives the measure of solar flux.
• The principle of calorimetry is used.
• Solar irradiance is calculated in true heat units from physical parameters of the instrument. For this reason they are called absolute pyrheliometers.

B) Abbot Silver-Disk pyrheliometers:

Sensor is silver disk coated with black paint

Temperature rises after the exposure is callibrated with the absolute one.

    Field Instruments:

The field instruments ( direct solar flux measuring devices) are calibrated on the basis of absolute pyrheliometer as reference. The field instruments are:

• Eppley pyrheliometer
• Kipp and Zonon pyrheliometer

Eppley Pyrheliometer

It is popularly called (NIP) Normal incidence pyrheliometer.

It works in the principle of thermopiles.

It has quartz window.

Kipp and Zonon pyrheliometer

It is also called actinometer

It uses Moll thermopiles

It measures long wave and short wave radiations, so long wave cut off filters are used.

Pyrheliometric Scales

• Angstrom scale is used as standard scale (A*S 1905)
• Smithsonian scale is used as revised scale(SS 1913)
• In 1956, Davos IRC ( International Radiation Commision ne Scale (IPS 1956)
• IPS 1956 = 1.015 (AS 1905)
•                = 0.98 (SS1913)
• WRR ( World Radiometric reference)
•               WRR= 1.022 (IPS 1956)

PYRANOMETERS

Global solar irradiance is measured by radiometer with hemispherical fields of view called pyranometers.Sensing elements of most common pyranometers are based on thermoelectric, thermo-mechanical or photovoltaic principles. Unlike the canonical absorbers, some of the pyranometers are flat sur flat surfaces. In routine meterological measurements, pyranometer are always placed in horizontal position.

Cosine Effect:

The sensing element of pyranometer is invariably coated with some form of highly absorbent black paint. Although it is easier to achieve high absorptance in the spectal sense, it is difficult to achieve the same in the directional sense ie. the absorpatance varies with the angle of incidence. Absorptance remain almost constant until the incident angle exceeds about 70*. Beyond this point absorptance increases considerably  and then drops rapidly when angle of incidence approaches 90* . This angular dependence of absorptance  is called the cosine effect.

RADIATION SENSORS

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.

• 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.