Net radiometer

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.

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.

Working Principle

The net radiometer is based on a thermopile sensor whose warm joints are in thermal contact with the receiver while the upper cool joints are in thermal contact with the lower receiver. The temperature difference between the two receivers is proportional to the net irradiation. The temperature difference between hot and cold junction is converted into a voltage by Seebeck effect. The two receivers are made from a portion of spherical coated Teflon®. The particular form of the two receivers provides a response in accordance with the cosine. The Teflon® coating, as well as allowing outdoor installation for long periods without risk of damage, can have a constant spectral response from ultraviolet (200 nm) up to far infrared (100 μm).

Installing and mounting the net radiometer for total irradiance measurements

To allow cleaning the two receiving surfaces regularly, LP NET 07 should be mounted in easily reachable places. The surfaces can be washed with plain water or pure ETHIL alcohol. Mount the instrument so that no shadow will be cast on it at any time of day and of the seasons, from obstructions such as buildings, trees, or any other obstacle. In the NORTHERN hemisphere, the net radiometer is normally oriented towards SOUTH, while it should be oriented NORTHWARD, in the SOUTHERN hemisphere. The instrument should be mounted at a height of at least 1.5 m above the ground. Please note that the flow on the lower receiver is representative of a circular area with a radius of 10 times the height. When installing the net-radiometer avoid, wherever possible, to touch the surfaces of the receiving net-radiometer.

Terminology

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.

The surface of the upper receiver measures the direct solar radiation plus the diffuse one and the radiation at longer wavelengths emitted from the sky (clouds), while the lower receiving area measures the solar radiation reflected from the ground (albedo) and the radiation length wavelengths emitted from the earth. The instrument is designed and constructed to be used outdoors in any weather conditions. Besides its use in meteorology to measure energy balance, it can be used indoors for the measurement of radiant temperature (ISO 7726).

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

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

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

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.
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See also

References
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