Campbell CNR4 Manuál s instrukcemi stáhnout pdf (Strana 33)

CNR4 Net Radiometer

8.1 Testing the Pyranometer

As a first test, check the sensor impedance. It should have a nominal value as

indicated in the specifications. Zero, or infinite resistance, indicates a failure in

hardware connection.

Before starting the second test measurement, let the pyranometer rest for at

least five minutes to let it regain its thermal equilibrium. For testing, set a

voltmeter to its most sensitive range setting. Darken the sensor. The signal

should read zero; this response can take up to one minute. Small deviations

from zero are possible; this is caused by the thermal effects, such as touching

the pyranometer with your hand. This thermal effect can be demonstrated by

deliberately heating the pyranometer with your hand. If the zero offset is

within specifications, proceed with the third test.

In the third test, the sensor should be exposed to light. The signal should be a

positive reading. Set the voltmeter range in such a way that the expected full-

scale output of the pyranometer is within the full-scale input range of the

voltmeter. The range can be estimated on theoretical considerations. When the

maximum expected radiation is 1500 W/m

, which is roughly equal to normal

outdoor daylight conditions, and the sensitivity of the pyranometer is 15 μV per

W/m

, the expected output range of the pyranometer is equal to 22500 μV, or

22.5 mV. One can calculate the radiation intensity by dividing the

pyranometer output as measured by the voltmeter (for example, 22.5 mV) by

the sensor sensitivity (15 μV/W/m

). If no faults are found up to this point,

your pyranometer is probably operating correctly.

8.2 Testing the Pyrgeometer

It is assumed that the zero offset is no more than a few watts per square meter

(see second test in Section 8.1, Testing the Pyranometer).

The CNR4 body and the ambient air should be at the same temperature. Let

the pyrgeometer rest for at least five minutes to regain its thermal equilibrium.

Set the voltmeter to its most sensitive range. To test if the pyrgeometer is

working properly, put your hand in front of the pyrgeometer. The thermal

radiation from your hand will cause the pyrgeometer to generate a positive

voltage when the surface temperature of your hand is higher than the

pyrgeometer temperature. The pyrgeometer will generate a negative voltage if

the hand is colder. The signal is proportional to the temperature difference (see

the rule of thumb in Section 7.1.6, Calculation of Net Long-wave Radiation).

The radiation emitted by the hand can be calculated by dividing the

pyrgeometer output by the sensor’s sensitivity value, and subsequently

correcting for the temperature, according to Equation 7-2. If there are still no

faults found, your pyrgeometer is probably operating correctly.

8.3 Testing the Thermistor

Using a multimeter, measure the resistance between the black and white wires

of the thermistor, and compare the value with the resistance values listed in

TABLE 7-1. The resistance should be around 10 k Ω at 25 °C, and the cable

resistance should add about 0.026 Ω per each foot of cable. When in doubt, the

Pt-100 resistance (temperature) can be checked as well for reference.

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