Net radiometer

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4-component net radiometer showing the instrument's main components: 2 pyranometers (with domes, one visible at right facing up and the second at right facing down obscured by the white radiation shield above it) and 2 pyrgeometers (flat windows, again one visible (facing up) and one obscured (facing down)). Dimensions: diameter of the pyranometer dome is 20 mm. Photo shows model NR01.

A net radiometer is a type of actinometer used to measure net radiation (NR) at the Earth's surface for meteorological applications. The name net radiometer reflects the fact that it measures the difference between downward/incoming and upward/outgoing radiation from Earth. It is most commonly used in the field of ecophysiology.

Terminology[edit]

Although there are many types of net radiometers, the 4-component design at present is most popular for scientific applications.

A 4-component net radiometer serves to measure 4 separate components of surface radiation balance: SWin direct incoming short wave radiation, SWout or reflected short wave radiation, LWin diffused long-wave radiation from the sky and LWout long-wave radiation emitted by the ground surface. In net radiometers, shortwave radiation is measured with pyranometers which measure incoming shortwave radiation and reflected shortwave radiation (albedo), and longwave radiation is measured with pyrgeometers. The working range of pyranometers is 300 to 2800 nm wavelength and that of pyrgeometers is 4500 to 100000 nm wavelength.

Cross section of a 4-component net radiometer showing the instrument's main components: (1) SWin solar radiation sensor or pyranometer, (2) LWin far infrared radiation sensor or pyrgeometer, (3) radiation shield, (4) leveling assembly for x and y axis, block plus bolts for x-axis adjustment (5) leveling assembly for x and y axis, horizontal rod, (6) connection body, containing Pt100 temperature sensor, heater and hole for users own temperature sensor (add cable gland M8) , (7) LWout far infrared radiation sensor or pyrgeometer, (8) leveling assembly for x and y axis, bolts for y-axis adjustment, (9) SWout solar radiation sensor or pyranometer.

Calculations[edit]

NOTE: the following formulas have T in kelvins. Add 273.16 to convert to temperature in degrees Celsius.

U is the voltage output of a sensor, E is radiation at the sensor surface, up = upfacing instrument, down = downfacing instrument, SW = shortwave or solar radiation, LW = longwave or far infrared (FIR) radiation, in = incoming, out = outgoing, T = temperature, NR = net radiation.

SWin = Upyrano,up / Epyrano,up

SWout = Upyrano,down / Epyrano,down

LWin = (Upyrgeo,up / Epyrgeo,up) + 5.67×10-8 Tpyrgeo4

LWout = (Upyrgeo,down / Epyrgeo,down) + 5.67×10-8 Tpyrgeo4

NOTE: in the LWnet the instrument temperature is cancelled:

LWnet = (Upyrgeo,up / Epyrgeo,up) - (Upyrgeo,down / Epyrgeo,down)

SWnet = (Upyrano,up / Epyrano,up) - (Upyrano,down / Epyrano,down)

NR = SWnet + LWnet

Special parameters that can be deduced:

SWalbedo = SWin / SWout

Tsurface = (LWout / 5.67×10-8)1/4

Tsky = (LWin / 5.67×10-8)1/4

The SWalbedo and the Tsurface must be estimated from other sources, and the NR can be calculated using these plus the SWin and LWin measurements.

SWalbedo typically is assumed to be a constant, typically taken from local satellite observations; Tsurface can often be calculated from air temperature or ground temperature measurements.

Usage[edit]

Net radiometers are frequently used in meteorology, climatology, solar energy studies and building physics. They can be seen in many meteorological stations—typically installed horizontally.

Standardisation[edit]

Net-radiometers are not standardised.

Example of a domeless net radiometer. The sensor contains two black-surface sensors (second one not visible) and has a single output signal representing the total net radiation. This instrument is typically used for lower accuracy net radiation measurement.

References[edit]

Specifications, drawings and pictures courtesy of Hukseflux Thermal Sensors, www.Hukseflux.com

www.kippzonen.com

See also[edit]