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CNR1 Net Radiometer

For the CG3 only E = V/C + 5.67 ⋅ 10

-8

∗ T

(4.2)

In this equation C is the sensitivity of the sensor. Please bear in mind that T is

in Kelvin, and not in Celsius or Fahrenheit.

4.3 Measuring the CNR1’s Temperature with the Pt-100

The Pt-100 temperature sensor is located in the CNR1 body. It will not

measure the exact temperature of the CG3 unless the whole instrument is in

thermal equilibrium. Errors, however, are minimized in the design by making

solid metal connections between the sensors and the temperature sensor. When

the sun is shining, the largest expected deviation between real sensor

temperature and Pt-100 reading is 2 degrees. This results in a worst-case error

for the CG3 of 10 Watts per square meter.

The Pt-100 will not give a good indication of ambient air temperature; at 1000

Watts per square meter Solar radiation, and no wind, the instrument

temperature will rise approximately 12 degrees above ambient temperature.

The offsets of both pyranometers and pyrgeometers might be larger than 10

Watts per square meter if large temperature gradients are forced on the

instrument (larger than 5 K/hr). This happens, for example, when rain hits the

instrument. The occurrence of this can be detected using the Pt-100 readout. It

can be used as a tool for quality assurance of your data.

The 4WPB100 module is used to interface the Pt-100 to the datalogger, and is

included with the CNR1 sensor purchased from CSI. The 4WPB100

configures the Pt-100 as a 4-wire half bridge circuit that requires one excitation

and two differential channels on the datalogger (Section 5).

4.4 Calculation of the Albedo for Solar Radiation

The albedo is the ratio of incoming and reflected Solar radiation. It is a figure

somewhere between 0 and 1. Typical values are 0.9 for snow, and 0.3 for

grassland. To determine albedo, the measured values of the two CM3s can be

used. The CG3s are not involved, as they do not measure Solar radiation. Do

not use measured values when solar elevation is lower than 10 degrees above

the horizon. Errors in measurement at these elevations are likely and thus

yielding unreliable results. This is due to deviations in the directional response

of the CM3s.

Albedo = (E lower CM3) / (E upper CM3) (4.3)

In the above formula, E is calculated according to Equation 4.1.

Albedo will always be smaller than 1. Checking this can be used as a tool for

quality assurance of your data. If you know the approximate albedo at your

site, the calculation of albedo can also serve as a tool for quality control of your

measured data at this specific site.

4.5 Calculation of the Net Solar Radiation (Net Short-Wave)

Net Solar radiation is equal to the incoming solar radiation minus the reflected

solar radiation.

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