Campbell CNR4 User manual

CNR4 CNR4
Net Radiometer
Issued: 29.7.13
Copyright © 2000-2013 Campbell Scientific, Inc.
Printed under licence by Campbell Scientific Ltd. CSL 874
USER MANUAL

Guarantee
This equipment is guaranteed against defects in materials and workmanship.
This guarantee applies for twelve months from date of delivery. We will
repair or replace products which prove to be defective during the guarantee
period provided they are returned to us prepaid. The guarantee will not apply
to:
• Equipment which has been modified or altered in any way without the
written permission of Campbell Scientific
• Batteries
• Any product which has been subjected to misuse, neglect, acts of God or
damage in transit.
Campbell Scientific will return guaranteed equipment by surface carrier
prepaid. Campbell Scientific will not reimburse the claimant for costs incurred
in removing and/or reinstalling equipment. This guarantee and the Company’s
obligation thereunder is in lieu of all other guarantees, expressed or implied,
including those of suitability and fitness for a particular purpose. Campbell
Scientific is not liable for consequential damage.
Please inform us before returning equipment and obtain a Repair Reference
Number whether the repair is under guarantee or not. Please state the faults as
clearly as possible, and if the product is out of the guarantee period it should
be accompanied by a purchase order. Quotations for repairs can be given on
request. It is the policy of Campbell Scientific to protect the health of its
employees and provide a safe working environment, in support of this policy a
“Declaration of Hazardous Material and Decontamination” form will be
issued for completion.
When returning equipment, the Repair Reference Number must be clearly
marked on the outside of the package. Complete the “Declaration of
Hazardous Material and Decontamination” form and ensure a completed copy
is returned with your goods. Please note your Repair may not be processed if
you do not include a copy of this form and Campbell Scientific Ltd reserves
the right to return goods at the customers’ expense.
Note that goods sent air freight are subject to Customs clearance fees which
Campbell Scientific will charge to customers. In many cases, these charges are
greater than the cost of the repair.
Campbell Scientific Ltd,
Campbell Park, 80 Hathern Road,
Shepshed, Loughborough, LE12 9GX, UK
Tel: [phone]
Fax: [phone]
Email: [email protected]
www.campbellsci.co.uk

PLEASE READ FIRST
About this manual
Please note that this manual was originally produced by Campbell Scientific Inc. primarily for the
North American market. Some spellings, weights and measures may reflect this origin.
Some useful conversion factors:
Area: 1 in2 (square inch) = 645 mm2
Length: 1 in. (inch) = 25.4 mm
1 ft (foot) = 304.8 mm
1 yard = 0.914 m
1 mile = 1.609 km
Mass: 1 oz. (ounce) = 28.35 g
1 lb (pound weight) = 0.454 kg
Pressure: 1 psi (lb/in2) = 68.95 mb
Volume: 1 UK pint = 568.3 ml
1 UK gallon = 4.546 litres
1 US gallon = 3.785 litres
In addition, while most of the information in the manual is correct for all countries, certain information
is specific to the North American market and so may not be applicable to European users.
Differences include the U.S standard external power supply details where some information (for
example the AC transformer input voltage) will not be applicable for British/European use. Please
note, however, that when a power supply adapter is ordered it will be suitable for use in your country.
Reference to some radio transmitters, digital cell phones and aerials may also not be applicable
according to your locality.
Some brackets, shields and enclosure options, including wiring, are not sold as standard items in the
European market; in some cases alternatives are offered. Details of the alternatives will be covered in
separate manuals.
Part numbers prefixed with a “#” symbol are special order parts for use with non-EU variants or for
special installations. Please quote the full part number with the # when ordering.
Recycling information
At the end of this product’s life it should not be put in commercial or domestic refuse
but sent for recycling. Any batteries contained within the product or used during the
products life should be removed from the product and also be sent to an appropriate
recycling facility.
Campbell Scientific Ltd can advise on the recycling of the equipment and in some cases
arrange collection and the correct disposal of it, although charges may apply for some
items or territories.
For further advice or support, please contact Campbell Scientific Ltd, or your local agent.
Campbell Scientific Ltd, Campbell Park, 80 Hathern Road, Shepshed, Loughborough, LE12 9GX, UK
Tel: [phone] Fax: [phone]
Email: [email protected]
www.campbellsci.co.uk

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Contents
PDF viewers note: These page numbers refer to the printed version of this document.
Use the Adobe Acrobat® bookmarks tab for links to specific sections.
1. General Description.....................................................1
2. Sensor Specifications.................................................2
2.1 CNR4 Specifications ................................................................................ 3
2.2 Pyranometer Specifications ...................................................................... 4
2.3 Pyrgeometer Specifications ...................................................................... 5
2.4 Optional CNF4 Heater/Ventilator ............................................................ 6
2.4.1 CNF4 Specifications ....................................................................... 6
3. Installation....................................................................6
4. Using the Optional CNF4 Heater/Ventilator Unit.......8
5. Using the CNR4 in the Four Separate
Components Mode ....................................................9
5.1 Measuring Short-wave Solar Radiation with Pyranometer ...................... 9
5.2 Measuring Long-wave Far Infrared Radiation with Pyrgeometer ........... 9
5.3 Measuring CNR4 Temperature with Thermistor ................................... 10
5.4 Calculation of Albedo ............................................................................ 12
5.5 Calculation of Net Short-wave Radiation .............................................. 13
5.6 Calculation of Net Long-wave Radiation ............................................... 13
5.7 Calculation of Net (Total) Radiation ...................................................... 13
6. Wiring .........................................................................14
7. Datalogger Programming..........................................16
7.1 Sensor Sensitivity ................................................................................... 17
7.2 Example Programs ................................................................................. 17
7.2.1 Example 1, CR1000 Program Using Differential Measurements ...... 17
7.2.2 Example 2, CR3000 Program Using Differential Measurements ...... 21
7.2.3 Example 3, CR5000 Program Using Differential Measurements ...... 24
8. Troubleshooting ........................................................28
8.1 Testing the Pyranometer ........................................................................ 28
8.2 Testing the Pyrgeometer ........................................................................ 28
8.3 Testing the Thermistor ........................................................................... 29
8.4 Testing the Pt-100 .................................................................................. 29

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9. Maintenance and Recalibration................................29
9.1 Cleaning Windows and Domes .............................................................. 29
9.2 Recalibration .......................................................................................... 29
9.3 Replacing the Drying Cartridge .............................................................. 30
9.4 Replacement Parts .................................................................................. 30
Appendices
A. CNR4 Performance and Measurements under
Different Conditions.............................................A-1
B. CNF4 Heater/Ventilator...........................................B-1
B.1 General Information ............................................................................. B-1
B.2 Attaching the Optional CNF4 Heater/Ventilator Unit to CNR4 .......... B-2
B.3 Wiring .................................................................................................. B-6
B.4 Example B, CR3000 Datalogger Program with Heater/Ventilator
Control ............................................................................................. B-7
B.5 CNF4 Heater/Ventilator Maintenance ............................................... B-10
B.5.1 Testing the Heater .................................................................... B-10
B.5.2 Testing the Ventilator ............................................................... B-10
B.5.3 Replacing the Filter for the Ventilator ..................................... B-10
C. CR3000 Program for Measuring Pt-100
Temperature Sensor.............................................C-1
Figures
2-1. The CNR4 net radiometer with cables and mounting rod, top view ....... 2
2-2. The CNR4 net radiometer with CNF 4 heater/ventilator unit, top view . 3
3-1. Attaching the mounting rod to the CNR4 body ...................................... 7
3-2. Attaching the CNR4 onto the mounting rod (010759) using vertical
pole or horizontal crossarm .................................................................. 8
6-1. The CNR4 sensor with SOLAR and TEMP cables ............................... 14
6-2. The marks on the end of the CNR4: S for SOLAR cable, and T for
TEMP cable ....................................................................................... 14
6-3. Labels on the pigtail end of the SOLAR cable...................................... 15
6-4. Labels on the pigtail end of the TEMP cable ........................................ 15
9-1. Replacing the Drying Cartridge ............................................................ 30
A-1. Different measurement conditions and signals .................................. A-2
A-2. Partly cloudy day for the upward facing pyrgeometer ....................... A-2
A-3. Clear day for the downward facing pyrgeometer ............................... A-3
B-1. CNF4 Package Contents ..................................................................... B-2
B-2. Attaching the CNF4 to CNR4 using pan-head screws and washers ... B-3
B-3. Making sure the cables are clear from the edges ................................ B-4
B-4. CNF4 solar shield and four flat-head screws ..................................... B-4
B-5. Attaching the solar shield to CNF4 using four flat-head screws ........ B-5
B-6. Affixing the sensor label to CNF4...................................................... B-5
B-7. Connecting the CNF4 power control cable and the mounting rod ..... B-5

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Tables
5-1. Resistance values versus CNR4's thermistor temperature in °C ........... 11
5-2. Resistance values versus CNR4's Pt-100 temperature in °C ................. 12
6-1. Datalogger Connections for Differential Measurement ........................ 16
6-2. Datalogger Connections for Single-ended Measurement ..................... 16
A-1. Typical output signals of CNR4 under different meteorological
conditions. Explanation can be found in the text ............................ A-1
B-1. CR1000 and CR3000 Datalogger Connections for Differential
Measurement with Heater/Ventilator Control ................................. B-6
C-1. Datalogger Connections for Differential Measurement with Pt-100 . C-1

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1
CNR4 Net Radiometer
1. General Description
The CNR4 is a four component net radiometer that measures the energy balance
between the incoming short-wave and long-wave radiation versus the surface-
reflected short-wave and outgoing long-wave 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 the short-wave solar radiation, and the pyrgeometer
pair measures the 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.
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 is designed as an extension of the solar
shield and can be fitted new to the CNR4 or retro-fitted later. The
heater/ventilation unit CNF4 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 Net Radiometer, CNR4, is intended for the analysis of the radiation balance
of short-wave and long-wave radiation. The most common application is the
measurement of net (total) radiation at the earth's surface.
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/m2. 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, the four probes in the CNR4 have different sensitivity values.
This makes the measurements more accurate than when they are made to have the
same sensitivity value with shunt and series resistors.

CNR4 Net Radiometer
2
The design of CNR4 is such that the short-wave radiation and the long-wave
radiation are measured separately. The short-wave radiation is measured by two
pyranometers, one for measuring incoming short-wave radiation from the sky, and
the other, which faces downward, for measuring the reflected short-wave
radiation. From these two pyranometers, albedo, the ratio of the reflected to the
incoming radiation, can also be determined. The long-wave radiation is measured
by two pyrgeometers, one for measuring the long-wave radiation from the sky, the
other from the surface.
2. Sensor Specifications
The CNR4 consists of two pyranometers, for measuring short-wave radiation, and
of two pyrgeometers for measuring long-wave radiation. Two temperature
sensors are available as standard, a thermistor and a Pt-100. The optional
heater/ventilator unit CNF4 is available. See Appendix B for more information on
the CNF4.
The properties of the CNR4 are mainly determined by the properties of the
individual probes. Generally the accuracy of the CNR4 will be higher than that of
competitive net-radiometers, because the solar radiation measurement performed
by the pyranometer is accurate, and offers a traceable calibration. Also the
optionally integrated heater/ventilator unit improves the accuracy. Due to the fact
that the net short-wave radiation can be very intense, 1000 W/m2compared to a
typical -100 W/m2net long-wave radiation, the accuracy of the short-wave
radiation measurement is critical. Wind corrections, as applied by less accurate
competitive instruments are not necessary. The robust materials used imply that
the CNR4 will not suffer damages inflicted by birds. Figure 2-1 and Figure 2-2
show the CNR4 with and without the CNF4 heater/ventilator. From a spectral
point of view, the pyranometer and pyrgeometer are complementary, and together
they cover the full spectral range.
Figure 2-1. The CNR4 net radiometer with cables and mounting rod, top view.

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Figure 2-2. The CNR4 net radiometer with CNF 4 heater/ventilator unit, top view.
2.1 CNR4 Specifications
Sensor sensitivities: Four probes have unique sensitivity
values. Please refer to the calibration
sheets or label on the bottom of the
sensor for the sensitivity values.
Operating temperature: -40 to +80°C (-40 to 176°F)
Operating humidity: 0 to 100 % RH
Bubble level sensitivity: < 0.5°
Sensor type: Thermopile
Receiver paint: Carbon Black
Desiccant: Silica gel (replaceable)
Housing material: Anodized aluminium body
Shock/vibration: IEC 721-3-2-2m2
CE: Complies with EC guideline
89/336/EEC 73/23/EEC
Environmental protection: IP 67
Requirements for data acquisition
Radiation components: 4 differential or 4 single-ended
analogue channels
Thermistor: 1 voltage excitation and 1 single-
ended analogue channel
Pt-100 temperature: 1 current excitation and 1 differential
analogue channel.
Cable length: User defined
Weight
Sensor: 0.85 kg (1.89 lbs) without cables
Heater/Ventilator, CNF4
(optional):
0.50 kg (1.11 lbs) without cables
Mounting rod: 34.7 cm (13.67 ”) length
1.6 cm (0.63 ”) diameter
CNR4 Package includes: CNR4 sensor
Mounting rod (1 ea.)
Solar cable (labelled SOLAR)
Temperature cable (labelled TEMP)
Drying cartridge (2 ea.)

CNR4 Net Radiometer
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WRR Traceable Calibration
Certificate for pyranometers
WRR Traceable Calibration
Certificate for pyrgeometers
Extra Calibration Sticker (to be put on
CNF4, if used)
CNF4 Package includes: CNF4 Heater/Ventilator
CNF4 cable
2.2 Pyranometer Specifications
* indicates ISO specifications.
Spectral range: 305 to 2800 nm (50% points)
Sensitivity: 10 to 20 µV/W/m2
Response time*: < 18 seconds (95% response)
Non-linearity*: < 1% (0-1000 W m-2 irradiance)
Non-stability*: < 1%
Temperature dependence of
sensitivity*:
< 4% (-10° to +40°C)
Tilt response*: < 1% at any angle with 1000 W/m2
Directional error*: < 20 W/m2at angle up to 80° with
1000 W/m2
Zero offset due to 0 to -200 W/m2
IR net irradiance*:
< 15 W/m2
Zero offset due to temperature
change*:
< 3 W/m2 (5K/hr temperature change)
< 1 W/m2 (with CNF4 installed)
Operating temperature: -40°C to +80°C
Field of view
Upper detector:
Lower detector:
180°
150° (due to lower solar shield to
prevent illumination at low zenith
angles)
Maximum solar irradiance: 2000 W/m2
Expected accuracy for daily totals: ±10 %

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5
Typical signal output for
atmospheric application:
0 to 15 mV
Impedance: 20 to 200 Ω, typically 50Ω
Detector: Copper-constantan multi junction
thermopile
Level accuracy: 1 degree
Irradiance: 0 to 2000 W/m2
Spectral selectivity: < 3% (330-1500 nm spectral interval)
Uncertainty in daily total: < 5% (95% confidence level)
Instrument calibration: Indoors. Side by side against reference
CMP3 pyranometer according to ISO
9847:1992 annex A.3.1
2.3 Pyrgeometer Specifications
Spectral range: 4.5 μm to 42 μm (50% points)
Sensitivity: 5 to 15 μV/W/m2
Impedance: 20 Ωto 200 Ω(typically 50)
Response time: < 18 seconds (95% response)
Non-linearity: < 1% (-250 to +250 W/m2 irradiance)
Temperature dependence of
sensitivity:
< 4% (-10° to +40°C)
Tilt error: < 1% (deviation when tilted at any
angle off horizontal)
Zero offset due to temperature
change:
±4 W/m2 (5K/hr temperature change)
Field of view
Upper
Lower
180 degrees
150 degrees
Net-irradiance: -250 to +250 W/m2
Non-stability: < 1% (sensitivity change per year)
Window heating offset: < 6 W/m2 (1000 W/m2 solar
irradiance)
Uncertainty in daily total: < 10% (95% confidence level) indoor
calibration
Typical signal output for
atmospheric application:
±5 mV
Temperature sensors
Thermistor: 10k Ω
Pt-100: DIN class A
Instrument calibration: Indoors, side by side against reference
CG(R) 3 pyrgeometer. On request
outdoors, side by side against
reference CG(R) 4 pyrgeometer

CNR4 Net Radiometer
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2.4 Optional CNF4 Heater/Ventilator
The purpose of the heater/ventilator is to prevent dew deposition on the
pyrgeometer and pyrgeometer window, thus enhancing the measurement accuracy
and reliability. Using the heater/ventilator will have negligible effect on the
pyranometer reading.
Generally, the errors caused by the heater/ventilator will be small relative to the
errors that would have been caused by water deposition.
2.4.1 CNF4 Specifications
Heater
Power consumption:
10 W @ 12 Vdc (15 Ω)
Ventilator
Power consumption:
Supply voltage:
5 W @ 12 Vdc
8 to 13.5 Vdc
Weight: 0.5 kg (1.11 lbs)
Operating temperature: -40 to +80°C
3. Installation
For measurement of the net radiation, it is most important that the instrument is
located in a place that is representative of the entire area that one wishes to study.
When installed on a mast, the preferred orientation should be such that no shadow
is cast on the net radiometer at any time during the day. In the Northern
Hemisphere this implies that the net radiometer should be mounted south of the
mast.
It is suggested that the CNR4 is mounted at a height of at least 1.5 metres above
the surface to avoid shading effects of the instruments on the soil and to promote
spatial averaging of the measurement. If the instrument is hmetres above the
surface, 99% of the input of the lower sensors comes from a circular area with a
radius of 10h. Shadows or surface disturbances with radius
< 0.1hwill affect the measurement by less than 1%.
It is recommended that the CNR4 be mounted to a separate vertical pipe at least
25 feet from any other mounting structures. The mounting bracket (010759) is
used to mount the CNR4 directly to a vertical pipe, or to a CM20x series Sensor
Crossarm. Mount the sensor as follows:
1. First, attach the mounting rod to the CNR4, as shown in Figure 3-1.
2. Attach mounting bracket to the vertical mounting pipe or CM20x series
Sensor Crossarm, using the U-bolts provided as shown in Figure 3-2.

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3. Insert the mounting rod of the CNR4 sensor into the mounting block of
the mounting bracket, making sure the sensor points to the direction of
the arrows marked as “SENSOR” on top of the bracket (see Figure 3-2).
Perform a coarse levelling of the sensor using the bubble level on the
top of the CNR4, and tighten the four screws on top of the mounting
bracket to properly secure the mounting rod so that it does not rotate.
Do not attempt to rotate the instrument using the sensor heads, or
you may damage the sensors; use the mounting rod only.
4. Perform the fine levelling using the two spring-loaded levelling screws:
one on the front and the other on the back of the bracket.
Figure 3-1. Attaching the mounting rod to the CNR4 body.
NOTE

CNR4 Net Radiometer
8
Figure 3-2. Attaching the CNR4 onto the mounting rod
(010759) using vertical pole or horizontal crossarm.
For installation in buildings or in solar energy applications, one will often have to
mount the CNR4 parallel to the surface that is being studied. This may be in a
tilted or a vertical position. The sensitivity of the radiometers will be affected, but
only in a minor way. This is specified as the so-called tilt effect. From the
specifications one can see that the tilt effect (this is a change in sensitivity)
remains within 1 %. See specifications in Section 2 for more details.
4. Using the Optional CNF4 Heater/Ventilator Unit
The optional heater/ventilator unit for CNR4 is available from the manufacturer.
You can purchase the CNF4 heater/ventilator from Campbell Scientific, Inc. with
the custom cable length. Please refer to the Appendix B for details on the CNF4
heater/ventilator, including the assembling instructions and sample programs to
control the CNF4 unit.

User Manual
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5. Using the CNR4 in the Four Separate Components
Mode
In the four separate components mode configuration (measuring two short-wave
radiation signals, two long-wave signals), all signals are measured separately.
Calculation of net-radiation and albedo can be done on-line by the datalogger, or
off-line by the user during post-processing, using the stored raw data.
The two pyranometers will measure the short-wave radiation, both incoming and
reflected. The two pyrgeometers will measure the long-wave radiation. For
proper analysis of the pyrgeometer measurement results, they must be temperature
corrected using the temperature measurement performed by the onboard
thermistor or Pt-100 sensor.
The following paragraphs describe how one should treat the instrument, and how
different parameters like net short-wave radiation, net long-wave radiation, soil
temperature, sky temperature, and net (total) radiation can be calculated.
5.1 Measuring Short-wave Solar Radiation with Pyranometer
The pyranometer generates an mV signal that is simply proportional to the
incoming short-wave radiation. The conversion factor between voltage, V, and
W/m2of solar irradiance E, is the so-called calibration constant C (or sensitivity).
For each pyranometer
E = V/C (5-1)
Measuring with a pyranometer can be done by connecting two pyranometer wires
to a datalogger. Incidental light results in a positive signal. The pyranometer
mounting plate and ambient air should be at the same temperature, as much as
possible. Conversion of the voltage to irradiance can be done according to
equation 5-1, and this is done inside the datalogger program.
With the upward-facing pyranometer the so-called global (solar) downwelling
radiation is measured. The downward-facing pyranometer measures the reflected
upwelling solar radiation. When calculating the net radiation, the upwelling
radiation must be subtracted from the downwelling radiation. See Section 5.5.
5.2 Measuring Long-wave Far Infrared Radiation with
Pyrgeometer
When using the pyrgeometer, you should realize that the signal that is generated
by the pyrgeometer represents the exchange of long-wave far infrared (thermal)
radiation between the pyrgeometer and the object that it is facing. This implies
that the pyrgeometer will generate a positive voltage output, V, when it faces an
object that is hotter than its own sensor housing, and that it will give a negative
voltage signal when it faces an object that is colder. This means that for
estimating the far infrared radiation that is generated by the object that is faced by
the pyrgeometer, usually the sky or the soil, you will have to take the pyrgeometer
temperature, T, into account. This is why the temperature sensors are
incorporated in the CNR4's body near the pyrgeometer sensing element, and has,
therefore, the same temperature as the pyrgeometer sensor surface. The
calculation of the long-wave far infrared irradiance, E, is done according to the
following equation:

CNR4 Net Radiometer
10
For the pyrgeometer only
E = V/C + 5.67*10-8*T4 (5-2)
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.
The downward-facing pyrgeometer measures the far infrared radiation that is
emitted by the ground. The upward-facing pyrgeometer measures the far infrared
radiation from the sky. As the sky is typically colder than the instrument, one can
expect negative voltage signals from the upward-facing pyrgeometer. The
Equation 5-2 is used to calculate the far infrared irradiance of the sky and of the
ground.
5.3 Measuring CNR4 Temperature with Thermistor
The CNR4 has two temperature sensors built inside: thermistor and Pt-100. They
both have the identical accuracy. We recommend using the thermistor with
Campbell Scientific, Inc. dataloggers. The thermistor has a larger resistance (10
kΩ@ 25°C) than Pt-100 sensor (100 Ω@ 0°C), and the change in resistance with
respect to temperature, in absolute terms, is greater. Therefore, the cable
resistance can be neglected, and the thermistor can easily be measured using half-
bridge measurement instruction on Campbell Scientific, Inc. dataloggers. This
makes it simpler to program, uses less number of measurement channels, and offer
better compatibility across the family of our dataloggers.
Table 5-1 shows the thermistor resistance values as a function of temperature.
Relatively small errors occur when the CNR4 is not in thermal equilibrium. This
happens for example when the heater is on, or when the sun is shining. When the
heater and ventilator are on, the largest expected deviation between the real sensor
temperature and the thermistor reading is 1 degree. This results in a worst case
error for the pyrgeometer of 5 W/m2. When the sun is shining, the largest
expected deviation between the real sensor temperature and the thermistor reading
is again 1 degree. This results in a worst case error for the pyrgeometer of 5
W/m2.
The thermistor will not give a good indication of ambient air temperature; at 1000
W/m2solar radiation, and no wind, the instrument temperature will rise
approximately 5 degrees above the ambient temperature.
The offsets of both the pyranometers and the pyrgeometers might be larger than
5W/m2if 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 thermistor readout, and can be used for data
filtering.
The thermistor measurement can be done by the datalogger, using the half bridge
measurement method that requires one voltage excitation and one single-ended
analogue channel.
A 1K precision resistor is incorporated into the free end of the
temperature cable to complete the thermistor bridge. Do not shorten
or extend the cable without replacing/moving this resistor at the end
of the cable.
NOTE