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.