Pyrgeometer

A pyrgeometer consists of the following major components:

• Athermopile sensor which is sensitive to radiation in a broad range from 200 nm to 100 μm
• A silicon dome or window with a solar blind filter coating. It has a transmittance between 4.5 μm and 50 μm that eliminates solar shortwave radiation.
• Atemperature sensor to measure the body temperature of the instrument.
• A sun shield to minimize heating of the instrument due to solar radiation.

The atmosphere and the pyrgeometer (in effect its sensor surface) exchange long wave IR radiation. This results in a net radiation balance according to:

${\displaystyle E_{\mathrm {net} }=E_{\mathrm {in} }-E_{\mathrm {out} }}$ 

Where (inSI units):

• E_net = net radiation at sensor surface [W/m²]
• E_in = long-wave radiation received from the atmosphere [W/m²]
• E_out = long-wave radiation emitted by the sensor surface [W/m²]

The pyrgeometer's thermopile detects the net radiation balance between the incoming and outgoing long wave radiation flux and converts it to a voltage according to the equation below.

${\displaystyle E_{\mathrm {net} }={\frac {U_{\mathrm {emf} }}{S}}}$ 

Where (in SI units):

• E_net =net radiation at sensor surface [W/m²]
• U_emf = thermopile output voltage [V]
• S = sensitivity/calibration factor of instrument [V/W/m²]

The value for S is determined during calibration of the instrument. The calibration is performed at the production factory with a reference instrument traceable to a regional calibration center.^[1]

To derive the absolute downward long wave flux, the temperature of the pyrgeometer has to be taken into account. It is measured using a temperature sensor inside the instrument, near the cold junctions of the thermopile. The pyrgeometer is considered to approximate a black body. Due to this it emits long wave radiation according to:

${\displaystyle E_{\mathrm {out} }=\sigma T^{4}}$ 

Where (in SI units):

• E_out = long-wave radiation emitted by the earth surface [W/m²]
• σ = Stefan–Boltzmann constant [W/(m²·K⁴)]
• T = Absolute temperature of pyrgeometer detector [K]

From the calculations above the incoming long wave radiation can be derived. This is usually done by rearranging the equations above to yield the so-called pyrgeometer equation by Albrecht and Cox.

${\displaystyle E_{\mathrm {in} }={\frac {U_{\mathrm {emf} }}{S}}+\sigma T^{4}}$ 

Where all the variables have the same meaning as before.

As a result, the detected voltage and instrument temperature yield the total global long wave downward radiation.

Pyrgeometers are frequently used in meteorology, climatology studies. The atmospheric long-wave downward radiation is of interest for research into long term climate changes.

The signals are generally detected using a data logging system, capable of taking high resolution samples in the millivolt range.

• Pyranometer
• Radiometer