CNR4-L 4-Component Net Radiometer

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1. If a CNR4-L is connected to a data logger by cable for a distance of 60 to 70 m, does the CNR4 analog signal need to be amplified?

   Yes. The signal from this sensor is not great enough to compensate for its dissipation because of the cable length.

2. Which net radiation sensors can be mounted using the 26120 mounting kit?

   As of June 2013, all of our current and retired net radiation sensors can be mounted using this kit. These include:

   • Current sensors: CNR4-L, NR-LITE2-L, and NR01-L
   • Retired sensors: CNR1, CNR1-L, CNR2-L, NR-LITE-L, and Q7.1-L
3. How many 4WPB100 modules are needed to measure the internal PRT in an NR01-L or CNR4-L 4-way net radiometer?

   Only one 4WPB100 is needed to measure the internal PRT in the radiometer.

4. Where should a net radiometer be installed?

   Mount the net radiometer so that no shadow will be cast on it at any time of day from obstructions such as trees, buildings, the mast, or the structure on which it is mounted.

   Campbell Scientific recommends installing a net radiometer in an open area, away from the main weather station structure on a separate vertical mast. If it is necessary to install this sensor on the main tall tower (30 ft or taller), the sensor should be installed at the top of the tower. In the northern hemisphere, the sensor should be facing south. In the southern hemisphere, the sensor should be facing north. If the tower uses a solar power system (that is, solar panels), ensure that the solar panels are installed away from the main tower.

5. When a 4-way net radiometer is used, which data loggers require the use of a 4WPB100 to measure the internal PRT?

   The CR1000 requires a 4WPB100 to measure the internal PRT. (Data loggers such as the CR3000 and CR5000 have the necessary PRT bridge module built in to measure the PRT.) Note that the CNR4-L also includes an internal thermistor, which can be directly measured by the CR1000. Because of this, when using a CR1000, Campbell Scientific typically recommends monitoring the internal temperature of the CNR4-L using its internal thermistor instead of the PRT. 

6. What are the different cable length choices available for Campbell Scientific sensors?

   Most Campbell Scientific sensors are available as an –L, which indicates a user-specified cable length. If a sensor is listed as an –LX model (where “X” is some other character), that sensor’s cable has a user-specified length, but it terminates with a specific connector for a unique system:

   • An –LC model has a user-specified cable length for connection to an ET107, CS110, or retired Metdata1.
   • An –LQ model has a user-specified cable length for connection to a RAWS-P weather station.

   If a sensor does not have an –L or other –LX designation after the main model number, the sensor has a set cable length. The cable length is listed at the end of the Description field in the product’s Ordering information. For example, the 034B-ET model has a description of “Met One Wind Set for ET Station, 67 inch Cable.” Products with a set cable length terminate, as a default, with pigtails.

   If a cable terminates with a special connector for a unique system, the end of the model number designates which system. For example, the 034B-ET model designates the sensor as a 034B for an ET107 system.

   • –ET models terminate with the connector for an ET107 weather station.
   • –ETM models terminate with the connector for an ET107 weather station, but they also include a special system mounting, which is often convenient when purchasing a replacement part.
   • –QD models terminate with the connector for a RAWS-F Quick Deployment Station.
   • –PW models terminate with the connector for a PWENC or pre-wired system.
7. Where are the available cable termination options listed for a particular sensor?

   Not every sensor has different cable termination options. The options available for a particular sensor can be checked by looking in two places in the Ordering information area of the sensor product page:

   • Model number
   • Cable Termination Options list

   If a sensor is offered in an –ET, –ETM, –LC, –LQ, or –QD version, that option’s availability is reflected in the sensor model number. For example, the 034B is offered as the 034B-ET, 034B-ETM, 034B-LC, 034B-LQ, and 034B-QD.

   All of the other cable termination options, if available, are listed on the Ordering information area of the sensor product page under “Cable Termination Options.” For example, the 034B-L Wind Set is offered with the –CWS, –PT, and –PW options, as shown in the Ordering information area of the 034B-L product page.

   Note: As newer products are added to our inventory, typically, we will list multiple cable termination options under a single sensor model rather than creating multiple model numbers. For example, the HC2S3-L has a –C cable termination option for connecting it to a CS110 instead of offering an HC2S3-LC model. 

8. What are some common issues related to using thermopile instruments?

   Because of the loss of IR radiation, nearly all thermopile instruments typically have a negative offset. This offset is most easily visible at night-time, when a small negative value is read instead of zero. This same offset is present during the daytime, but it is not as visible because of the large solar signal.

   Another common issue involves leveling an instrument. Leveling a thermopile instrument can cause errors in the direct beam component because the cosine response is not correct. These errors are more notable when the sun is close to the horizon because the angle is so shallow.

9. What are the different cable termination options available for Campbell Scientific sensors?

   Many Campbell Scientific sensors are available with different cable termination options. These options include the following:

   • The –PT (–PT w/Tinned Wires) option is the default option and does not display on the product line as the other options do. The cable terminates in pigtails that connect directly to a data logger.
   • In the –C (–C w/ET/CS110 Connector) option, the cable terminates in a connector that attaches to a CS110 Electric Field Meter or an ET-series weather station.
   • In the –CWS (–CWS w/CWS900 Connector) option, the cable terminates in a connector that attaches to a CWS900-series interface. Connection to a CWS900-series interface allows the sensor to be used in a wireless sensor network.
   • In the –PW (–PW w/Pre-Wire Connector) option, the cable terminates in a connector that attaches to a prewired enclosure.
   • In the –RQ (–RQ w/RAWS Connector) option, the cable terminates in a connector that attaches to a RAWS-P Permanent Remote Automated Weather Station.

   Note: The availability of cable termination options varies by sensor. For example, sensors may have none, two, or several options to choose from. If a desired option is not listed for a specific sensor, contact Campbell Scientific for assistance.

10. Using a CNR4-L, how should albedo data be handled at night when the reflected short-wave radiation is zero and the calculation ends up dividing by zero?

    Technically, because albedo is the fraction of the sun’s radiation reflected from a surface, albedo cannot be quantified at night.

    When calculating albedo, it is important to remember that when radiation readings are very low, there is a significantly large error associated with the ratio. For example, as the sun drops to a lower position on the horizon, the ratio of reflected and incoming radiation becomes somewhat meaningless.

    Albedo can be calculated from the simultaneous incoming and reflected pyranometer readings, with the average stored. Both of these pyranometer signals should be in a differential input mode.

    • At night-time, the upper pyranometer faces a cold sky, which causes the domes to cool and the readings to be negative. (For example, there may be readings of -2 to -7 W/m², depending on the sky conditions.)
    • In contrast, the lower pyranometer faces the ground, which could be warmer or cooler. The readings are unlikely to be positive, and may even be -2 W/m².

    In principle, two negative values could result in a night-time albedo of 0.5.

    Because of the input resolution, noise, and offsets of the data logger used, it is highly unlikely that any individual pyranometer reading is exactly zero. Depending upon the configuration used, any positive irradiation values recorded by the data logger as less than 2 W/m² may, actually, be less than zero.

    If an upper pyranometer reading, a lower pyranometer reading, or both is/are less than 2 W/m², the albedo value should be described as “undefined” or “invalid.”

    Another approach is to just not calculate albedo when the flux values get small. For example, set a cut-off point for the minimum flux value that will be used in albedo calculations.

    If solar position is being calculated, one other approach is to use a solar position calculation, such as 1° above the horizon.