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CNR4 Net Radiometer

1. General Description

The CNR4 is a four component net radiometer that measures the energy

balance between incoming and outgoing radiation.

The CNR4 net radiometer consists of a pyranometer pair, one facing upward,

the other facing downward, and a pyrgeometer pair in a similar configuration.

The pyranometer pair measures short-wave solar radiation, and the

pyrgeometer pair measures long-wave far infrared radiation. The upper long-

wave detector of CNR4 has a meniscus dome. This ensures that water droplets

roll off easily and improves the field of view to nearly 180°, compared with a

150° for a flat window. All four sensors are integrated directly into the

instrument body, instead of separate modules mounted onto the housing. Each

sensor is calibrated individually for optimal accuracy.

Two temperature sensors, a thermistor and a Pt-100, are integrated with the

CNR4 body. The temperature sensor is used to provide information to correct

the infrared readings for the temperature of the instrument housing. Care has

been taken to place the long-wave sensors close to each other and close to the

temperature sensors. This assures that the temperatures of the measurement

surfaces are the same and accurately known. This improves the quality of the

long-wave measurements. Campbell Scientific adds a completion resistor in

the pig tail end of the thermistor cable, so that it is easily interfaced with our

dataloggers for half-bridge measurement.

The CNR4 design is very light in weight and has an integrated solar shield that

reduces thermal effects on both the short-wave and the long-wave

measurements. The cables are made from Santoprene® jacket, which is

intended for outdoor use, and is resistant to a variety of pollutants and UV-

radiation. The mounting rod can be unscrewed for transport.

An optional ventilation unit with a heater, CNF4, is designed as an extension of

the solar shield and can be fitted new to the CNR4 or retrofitted later. The

heater/ventilation unit is compact and provides efficient air-flow over the

domes and windows to minimize the formation of dew and to reduce the

frequency of cleaning. The integrated heater can be used to melt frost.

The CNR4 specifications when used with CNF4 comply with the WMO

classification of Good Quality.

The CNR4 design is such that both the upward facing and the downward-

facing instruments measure the energy that is received from the whole

hemisphere (180° field of view). The output is expressed in W/m

. The total

spectral range that is measured is roughly from 0.3 to 42 μm. This spectral

range covers both the short-wave solar radiation, 0.3 to 2.8 μm, and the long-

wave far infrared radiation, 4.5 to 42 μm. The gap between these two produces

negligible errors.

Unlike the CNR1, four probes in the CNR4 have different sensitivity values.

This makes each measurement from four sensors more accurate than when they

are made to have the same sensitivity value with shunt and series resistors.

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