Campbell SR20-T2 User manual

Revision: 05/2020
Copyright © 2018– 2020
Campbell Scientific, Inc.

Table of contents
1. Introduction 1
2. QuickStart 1
3. Siting 4
4. Mounting procedure 5
5. Wiring 7
6. Analog programming 11
7. Integrated thermistor 12
8. Maintenance and troubleshooting 12
Appendix A. Importing Short Cut code into CRBasic Editor 14
Appendix B. SR20-T2 example programs 15
Table of Contents - i

1. Introduction
The SR20-T2, manufactured by Hukseflux Thermal Sensors, is an ISO 9060 spectrally flat Class A
(secondary standard) pyranometer that measures solar short-wave radiation in a full hemisphere
of the sky. It has a built-in case temperature sensor and embedded heater for removing dew and
light rain. It connects directly to a Campbell Scientific data logger and is designed for
applications that require high measurement accuracy in demanding applications such as
scientific meteorological observation networks and utility scale solar-energy-power production
sites.
2. QuickStart
A video that describes data logger programming using Short Cut is available at:
www.campbellsci.com/videos/cr1000x-datalogger-getting-started-program-part-3. Short Cut is
an easy way to program your data logger to measure the sensor and assign data logger wiring
terminals. Short Cut is available as a download on www.campbellsci.com. It is included in
installations of LoggerNet, PC200W, PC400, or RTDAQ.
NOTE:
Short Cut uses the two-wire configuration to measure the integrated thermocouple.
The following procedure also shows using Short Cut to program the sensor.
1. Open Short Cut and click Create New Program.
2. Double-click the data logger model.
SR20-T2 ISO Spectrally Flat Class A (Secondary Standard) Pyranometer 1

3. In the Available Sensors and Devices box, type SR20 or locate the sensor in the Sensors >
Meteorological >Solar Radiation folder. Double-click the SR20-T2 ISO Secondary Standard
Pyranometer. Type the Sensitivity and temperature coefficient values listed on the
calibration sheet. These values are unique for each sensor.
SR20-T2 ISO Spectrally Flat Class A (Secondary Standard) Pyranometer 2

4. Click the Wiring tab to see how the sensor is to be wired to the data logger. Click OK after
wiring the sensor.
5. Repeat steps three and four for other sensors.
6. In Output Setup, type the scan rate, meaningful table names, and Data Output Storage
Interval.
SR20-T2 ISO Spectrally Flat Class A (Secondary Standard) Pyranometer 3

7. Select the measurement and its associated output option.
8. Click Finish and save the program. Send the program to the data logger if the data logger
is connected to the computer.
9. If the sensor is connected to the data logger, check the output of the sensor in the data
logger support software data display in LoggerNet, PC400, RTDAQ, or PC200W to make
sure it is making reasonable measurements.
3. Siting
The solar radiation sensor is usually installed horizontally, but can also be installed at any angle
including an inverted position. Site the sensor to allow easy access for maintenance while ideally
avoiding any obstructions or reflections above the plane of the sensing element. It is important
to mount the sensor such that a shadow or a reflection will not be cast on it at any time. If this is
not possible, try to choose a site where any obstruction over the azimuth range between earliest
sunrise and latest sunset has an elevation not exceeding 5°. Diffuse solar radiation is less
influenced by obstructions near the horizon. The sensor should be mounted with the cable
pointing towards the nearest magnetic pole. For example, in the northern hemisphere, point the
cable toward the North Pole.
SR20-T2 ISO Spectrally Flat Class A (Secondary Standard) Pyranometer 4

4. Mounting procedure
Required tools:
lDiopter
lSolar compass
l8 mm (5/16-inch) open-end wrench for U-bolt nuts
1. On a level surface, level the solar radiation sensor using the leveling feet on the sensor.
Alternatively, remove the sensor leveling feet to allow it to be mounted directly to the
mounting bracket.
2. Secure the solar radiation sensor to the mounting bracket. The blue dots in the following
figure indicate the mounting holes used for this pyranometer.
3. Using a diopter in combination with a solar compass, install and orient the crossarm on the
tripod or the mast. If installing the mounting bracket on a vertical pole, ensure the pole is
truly vertical.
4. Secure the mounting bracket to the crossarm or vertical pole using the hardware included
with the mounting bracket. The CM255 uses one U-bolt, nuts, flat washers, and lock
washers to mount the bracket, as shown in the following figure.
SR20-T2 ISO Spectrally Flat Class A (Secondary Standard) Pyranometer 5

5. For pyranometers mounted horizontally, ensure the mounting bracket is horizontal in two
dimensions. For pyranometers mounted at an angle, set the mounting bracket angle to the
desired angle prior to tightening the mounting hardware.
SR20-T2 ISO Spectrally Flat Class A (Secondary Standard) Pyranometer 6

6. Verify mounting hardware is firmly tightened, and that the mounting bracket is at the
desired angle.
5. Wiring
Table 5-1 (p. 7) provides the 2-wire half bridge wiring and Table 5-2 (p. 8) provides the 4-wire
half bridge wiring. FIGURE 5-1 (p. 10) provides the schematic for the 2-wire configuration of the
integrated thermistor and FIGURE 5-2 (p. 10) provides the schematic for the 4-wire configuration
of the integrated thermistor.
Table 5-1: Pin-out, wire color, function, and data logger connection for 2-wire half bridge
Wire
color
Pin out Function Data logger connection
-PT -PW Differential Single-ended
White 7 7 Pyranometer
signal high
Uconfigured for
differential input1,DIFF H
(differential high, analog-
voltage input)
Uconfigured for single-
ended analog input1,SE
(single-ended, analog-
voltage input)
Green 5 9 Pyranometer
signal reference
Uconfigured for
differential input1,
DIFF L (differential low,
analog-voltage input)
⏚(analog ground)
Brown 4 4
Heater (polarity
does not matter,
1.5W at 12VDC)
Switched heater power:
SW,SW12,SW12V,
SW Battery (switched 12 V)
Constant heater power:
12V
Switched heater power:
SW,SW12,SW12V,
SW Battery (switched 12 V)
Constant heater power:
12V
Yellow 1 1 Heater ground G G
Red 2 2 Thermistor signal
Uconfigured for single-
ended analog input1,SE
(single-ended, analog-
voltage input)
Uconfigured for single-
ended analog input1,SE
(single-ended, analog-
voltage input)
Blue 6 6 Thermistor
reference ⏚(analog ground) ⏚(analog ground)
SR20-T2 ISO Spectrally Flat Class A (Secondary Standard) Pyranometer 7

Table 5-1: Pin-out, wire color, function, and data logger connection for 2-wire half bridge
Wire
color
Pin out Function Data logger connection
-PT -PW Differential Single-ended
Orange NC NC Thermistor
voltage excitation
Uconfigured for voltage
excitation1,EX,VX
(voltage excitation)
Uconfigured for voltage
excitation1,EX,VX
(voltage excitation)
Purple 3 3 Thermistor signal
Uconfigured for single-
ended analog input1,SE
(single-ended, analog-
voltage input)
Uconfigured for single-
ended analog input1,SE
(single-ended, analog-
voltage input)
Gray 8 8 Thermistor
reference ⏚(analog ground) ⏚(analog ground)
Black 9 5 Ground G G
Clear NC 9 Shield ⏚(analog ground) ⏚(analog ground)
1U channels are automatically configured by the measurement instruction.
Table 5-2: Pin-out, wire color, function, and data logger connection for 4-wire half bridge
Wire
color
Pin out Function Data logger connection
-PT -PW Differential Single-ended
White 7 7 Pyranometer
Signal High
Uconfigured for
differential input1,DIFF H
(differential high, analog-
voltage input)
Uconfigured for single-
ended analog input1,SE
(single-ended, analog-
voltage input)
Green 5 9 Pyranometer
Signal Reference
Uconfigured for
differential input1,DIFF L
(differential low, analog-
voltage input)
⏚(analog ground)
Brown 4 4
Heater (polarity
does not matter,
1.5 W at 12 VDC)
Switched heater power:
SW,SW12,SW12V,SW
Battery (switched 12 V)
Constant heater power:
12V
Switched heater power:
SW,SW12,SW12V,SW
Battery (switched 12 V)
Constant heater power:
12V
SR20-T2 ISO Spectrally Flat Class A (Secondary Standard) Pyranometer 8

Table 5-2: Pin-out, wire color, function, and data logger connection for 4-wire half bridge
Wire
color
Pin out Function Data logger connection
-PT -PW Differential Single-ended
Yellow 1 1 Heater Ground G G
Blue 2 2 Thermistor Signal
Uconfigured for
differential input1,DIFF H
(differential high, analog-
voltage input)
Uconfigured for
differential input1,DIFF H
(differential high, analog-
voltage input)
Red 6 6 Thermistor
Reference
Uconfigured for
differential input1,DIFF L
(differential low, analog-
voltage input)
Uconfigured for
differential input1,DIFF L
(differential low, analog-
voltage input)
Purple NC NC Thermistor Signal
Uconfigured for
differential input1,DIFF H
(differential high, analog-
voltage input)
Uconfigured for
differential input1,DIFF H
(differential high, analog-
voltage input)
Orange 3 3 Thermistor Signal
Uconfigured for
differential input1, 2,
DIFF L (differential low,
analog-voltage input)2
Uconfigured for
differential input1, 2,
DIFF L (differential low,
analog-voltage input)2
Gray 8 8 Voltage Excitation
Uconfigured for voltage
excitation1,EX,VX
(voltage excitation)
Uconfigured for voltage
excitation1,EX,VX
(voltage excitation)
Black 9 5 Ground G G
Clear NC 9 Shield ⏚(analog ground) ⏚(analog ground)
1U channels are automatically configured by the measurement instruction.
2Jumper to ground with a user-supplied wire.
SR20-T2 ISO Spectrally Flat Class A (Secondary Standard) Pyranometer 9

FIGURE 5-1. Schematic of the integrated thermistor (2-wire configuration)
FIGURE 5-2. Schematic of the integrated thermistor (4-wire configuration)
SR20-T2 ISO Spectrally Flat Class A (Secondary Standard) Pyranometer 10

6. Analog programming
The pyranometer outputs a low level voltage that is measured using either the VoltDiff()
CRBasic instruction or VoltSE() CRBasic instruction.
CAUTION:
Nearby AC power lines, electric pumps, or motors can be a source of electrical noise. If the
sensor or data logger is located in an electrically noisy environment, the measurement should
be made with the 60 or 50 Hz rejection integration option as shown in the example programs.
If measurement time is not critical, the autorange option can be used in the VoltDiff() or
VoltSE() instruction; the autorange adds a few milliseconds to the measurement time.
Otherwise, select the input range as follows:
1. Estimate the maximum expected input voltage by multiplying the maximum expected
irradiance (in W × m–2) by the calibration factor (in μV / W × m–2). Divide the answer by
1000 to give the maximum in millivolt units.
2. Select the smallest input range that is greater than the maximum expected input voltage.
If electromagnetic radiation can be a problem, use an fN1 of 50 or 60 Hz. Select 60 Hz Noise
Rejection for North America and areas using 60 Hz AC voltage. Select 50 Hz Noise Rejection for
most of the Eastern Hemisphere and areas that operate at 50 Hz. The multiplier converts the
millivolt reading to engineering units. Table 6-1 (p. 11) provides the calculations required for the
various units. The offset will normally be fixed at zero (see SR20-T2 example programs (p. 15)).
Table 6-1: Multipliers required for flux density and total fluxes
Units Multiplier Output processing
W × m–2 M Average
MJ × m–2 M × t × 0.000001 Totalize
kJ × m–2 M × t × 0.001 Totalize
cal × cm–2 M × t × 0.0239 × 0.001 Totalize
cal × cm–2 × min–1 M × 1.434 × 0.001 Average
W × hr × m–2 M × t / 3600 Totalize
M = 1000/c, where c is the sensor output in μV / W × m–2
t = data logger program execution interval in seconds
SR20-T2 ISO Spectrally Flat Class A (Secondary Standard) Pyranometer 11

7. Integrated thermistor
The data logger program needs the resistance of the thermistor to calculate temperature. CRBasic
instructions used are the BrHalf() (2-wire configuration) or BrHalf4W (4-wire
configuration). The BrHalf4W() instruction returns a resistance measurement, but the
BrHalf() instruction requires the following expression to convert the value to resistance:
Rs = Rf*(Vx/(1-Vx))
Where,
Rf = thermistor resistance in ohms (for example, for a 10 kohm thermistor, Rf is 10000)
Vx = value returned by the BrHalf() instruction
Both the 2-wire and 4-wire configurations use the Steinhart-Hart equation to convert resistance
to temperature. The Steinhart-Hart equation for converting resistance to degree Celsius is as
follows:
Temperature = 1/[A + B x LN(resistance) + C x (LN(resistance))^3] - 273.15
Where A, B, and C are coefficients for the Steinhart-Hart equation.
The coefficients for the Steinhart-Hart equation are specific to the thermistor contained in your
pyranometer. A calibration certificate that lists these coefficients is shipped with each
pyranometer. In CRBasic, the Steinhart-Hart equation is entered as a mathematical expression
(SR20-T2 example programs (p. 15)).
8. Maintenance and
troubleshooting
The SR20-T2 has no service items requiring scheduled replacement. There is no accessible
desiccant cartridge to maintain. Use pure alcohol or distilled water and a lint-free cloth to clean
the dome, removing smears and deposits. Local conditions and application dictate cleaning
interval. Sophisticated research applications require daily cleaning. For typical PV applications,
clean once per week, bi-monthly, or monthly. The SR20-T2 should be recalibrated following
industry standard best practices such as ASTM G167, ISO 9846, ASTM E824 or ASTM G207 by an
accredited lab. The recommended recalibration interval is two years. Contact Campbell Scientific
for more information.
SR20-T2 ISO Spectrally Flat Class A (Secondary Standard) Pyranometer 12

Unexpected results typically occur because of improper wiring or programming, electromagnetic
radiation, or damaged cables. Ensure that the data logger program includes the correct
parameters for the measurement instructions. Check for the presence of strong sources of
electromagnetic radiation and use the 50 or 60 Hz integration option in the data logger program
if electromagnetic radiation can be a problem. Check the cable for damage and ensure that it is
properly connected to the data logger.
SR20-T2 ISO Spectrally Flat Class A (Secondary Standard) Pyranometer 13

Appendix A. Importing Short
Cut code into CRBasic Editor
Short Cut creates a .DEF file that contains wiring information and a program file that can be
imported into the CRBasic Editor. By default, these files reside in the C:\campbellsci\SCWin
folder.
Import Short Cut program file and wiring information into CRBasic Editor:
1. Create the Short Cut program. After saving the Short Cut program, click the Advanced tab
then the CRBasic Editor button. A program file with a generic name will open in CRBasic.
Provide a meaningful name and save the CRBasic program. This program can now be
edited for additional refinement.
NOTE:
Once the file is edited with CRBasic Editor, Short Cut can no longer be used to edit the
program it created.
2. To add the Short Cut wiring information into the new CRBasic program, open the .DEF file
located in the C:\campbellsci\SCWin folder, and copy the wiring information, which is at
the beginning of the .DEF file.
3. Go into the CRBasic program and paste the wiring information into it.
4. In the CRBasic program, highlight the wiring information, right-click, and select Comment
Block. This adds an apostrophe (') to the beginning of each of the highlighted lines, which
instructs the data logger compiler to ignore those lines when compiling. The Comment
Block feature is demonstrated at about 5:10 in the CRBasic | Features video .
SR20-T2 ISO Spectrally Flat Class A (Secondary Standard) Pyranometer 14

Appendix B. SR20-T2 example
programs
Table B-1: Wiring for example program
Wire color CR1000X, 2-wire configuration CR1000X, 4-wire configuration
White 1H 1H
Green 1L 1L
Brown SW12-1 SW12-1
Yellow G G
Red SE3 2L
Blue ⏚(analog ground) 2H
Orange VX1 1L1
Purple SE3 1H
Gray ⏚(analog ground) VX1
Black G G
Clear ⏚(analog ground) ⏚(analog ground)
1Jumper to ground with a user-supplied wire.
CRBasic Example 1: CR1000X 2-wire configuration for measuring the SR20-T2
'CR1000X Series Data Logger
'Hukseflux SR20-T2 Pyranometer
'SR20 is a ISO 9060 Secondary Standard pyranometer
'T2 uses a 10k Ohm thermistor
SequentialMode
'Measurement function
'S(T) = So * (a*T^2 + b*T + c)
'S(T) = Sensitivity (10^-6 V/(W/m^2)) at instrument body temperature, T
'So = Sensitivity at 20 DegC
'a,b,c are temperature coefficients from 2nd order polynomial fit
'Calibration Constants for Irradiance and Temperature
SR20-T2 ISO Spectrally Flat Class A (Secondary Standard) Pyranometer 15

CRBasic Example 1: CR1000X 2-wire configuration for measuring the SR20-T2
Const SR20T2_SENS_So = 16.12 'uV/W*m^-2 @ 20 DegC
Const SR20_A = -7.3905E-6 'Deg C^-2
Const SR20_B = -0.3711E-4 'Deg C^-1
Const SR20_C = 1.0037
'Resistor built into CSI cable (RES 0.1% 10K 1/8W 5PPM)
Const Rf = 10000
Public SR20T2_mV
Public SR20T2_IRR 'using SR20T2_SENS_So (sensitivity at 20 DegC)
Public SR20T2_IRR_TC 'using temperature characterization result S(T)
Public SR20T2_BodyTemp
Public SR20T2_SENS_TC
Dim Rs
Dim V1_Vx
'Thermistor Constants for Steinhart-Hart Linearization
'Constants for YSI 44031 type 10K thermistor
Const A = 1.0295*10^-3
Const B = 2.391*10^-4
Const C = 1.568*10^-7
Units SR20T2_IRR = W/m^2
Units SR20T2_IRR_TC = W/m^2
Units SR20T2_BodyTemp = DegC
Units SR20T2_SENS_TC = uV/(W/m^2)
DataTable (OneMin,1,-1)
DataInterval (0,1,Min,10)
Average (1,SR20T2_IRR,IEEE4,False)
Average (1,SR20T2_IRR_TC,IEEE4,False)
Maximum (1,SR20T2_IRR_TC,IEEE4,False,False)
Minimum (1,SR20T2_IRR_TC,IEEE4,False,False)
StdDev (1,SR20T2_IRR_TC,IEEE4,False)
Average (1,SR20T2_BodyTemp,IEEE4,False)
EndTable
DataTable (SR20_T2_MetaData,1,-1)
DataInterval (0,1,Hr,10)
Average (1,SR20T2_SENS_TC,IEEE4,False)
Maximum (1,SR20T2_SENS_TC,IEEE4,False,False)
Minimum (1,SR20T2_SENS_TC,IEEE4,False,False)
StdDev (1,SR20T2_SENS_TC,IEEE4,False)
EndTable
BeginProg
Scan (1,Sec,0,0)
VoltDiff(SR20T2_mV,1,mV200,1,True ,0,60,1.0,0)
SR20T2_IRR = SR20T2_mV * (1000/SR20T2_SENS_So)
BrHalf (V1_Vx,1,mV5000,3,Vx1,1,2500,True ,0,60,1.0,0)
Rs = Rf*(V1_Vx/(1-V1_Vx))
SR20T2_BodyTemp=(A+(B*(LN(Rs))+C*(LN(Rs))^3))^-1 - 273.15
SR20T2_SENS_TC = SR20T2_SENS_So * (SR20_A*SR20T2_BodyTemp^2 + _
SR20_B * SR20T2_BodyTemp + SR20_C)
SR20-T2 ISO Spectrally Flat Class A (Secondary Standard) Pyranometer 16

CRBasic Example 1: CR1000X 2-wire configuration for measuring the SR20-T2
SR20T2_IRR_TC = SR20T2_mV * (1000/SR20T2_SENS_TC)
'SW12V HEATER CONTROL FUNCTION
If TimeIsBetween (0,7,24, hr) Then
SW12 (SW12_1,1,0)
Else
SW12 (SW12_1,0,0)
EndIf
CallTable (OneMin)
CallTable (SR20_T2_MetaData)
NextScan
EndProg
CRBasic Example 2: CR1000X 4-wire configuration for measuring the SR20-T2
'CR1000X Series Data Logger
'Hukseflux SR20-T2 Pyranometer
'SR20 is a ISO 9060 Secondary Standard pyranometer
'T2 uses a 10k Ohm thermistor
'4-wire temperature measurement example
SequentialMode
'Measurement function
'S(T) = So * (a*T^2 + b*T + c)
'S(T) = Sensitivity (10^-6 V/(W/m^2)) at instrument body temperature, T
'So = Sensitivity at 20 DegC
'a,b,c are temperature coefficients determined from second order polynomial
'fit
'Calibration Constants for Irradiance and Temperature (SN:4971)
Const SR20T2_SENS_So = 16.12 'uV/W*m^-2 @ 20 DegC
Const SR20_A = -7.3905E-6 'Deg C^-2
Const SR20_B = -0.3711E-4 'Deg C^-1
Const SR20_C = 1.0037
'Resistor built into CSI cable (RES 0.1% 10K 1/8W 5PPM)
Const Rf = 10000
Public SR20T2_mV
Public SR20T2_IRR 'using SR20T2_SENS_So (sensitivty at 20 DegC)
Public SR20T2_IRR_TC 'using temperature characterization result S(T)
Public SR20T2_BodyTemp
Public SR20T2_SENS_TC
Public Rs
SR20-T2 ISO Spectrally Flat Class A (Secondary Standard) Pyranometer 17

CRBasic Example 2: CR1000X 4-wire configuration for measuring the SR20-T2
'Thermistor Constants for Steinhart-Hart Linearization
'Constants for YSI 44031 type 10K thermistor
Const A = 1.0295*10^-3
Const B = 2.391*10^-4
Const C = 1.568*10^-7
Units SR20T2_IRR = W/m^2
Units SR20T2_IRR_TC = W/m^2
Units SR20T2_BodyTemp = DegC
Units SR20T2_SENS_TC = uV/(W/m^2)
DataTable (OneMin,1,-1)
DataInterval (0,1,Min,10)
Average (1,SR20T2_IRR,IEEE4,False)
Average (1,SR20T2_IRR_TC,IEEE4,False)
Maximum (1,SR20T2_IRR_TC,IEEE4,False,False)
Minimum (1,SR20T2_IRR_TC,IEEE4,False,False)
StdDev (1,SR20T2_IRR_TC,IEEE4,False)
Average (1,SR20T2_BodyTemp,IEEE4,False)
EndTable
DataTable (SR20_T2_MetaData,1,-1)
DataInterval (0,1,Hr,10)
Average (1,SR20T2_SENS_TC,IEEE4,False)
Maximum (1,SR20T2_SENS_TC,IEEE4,False,False)
Minimum (1,SR20T2_SENS_TC,IEEE4,False,False)
StdDev (1,SR20T2_SENS_TC,IEEE4,False)
EndTable
BeginProg
Scan (1,Sec,0,0)
VoltDiff (SR20T2_mV,1,mV200,1,True ,0,_60Hz,1.0,0)
SR20T2_IRR = SR20T2_mV * (1000/SR20T2_SENS_So)
BrHalf4W (Rs,1,mV5000,mV5000,2,Vx1,1,2500,True ,True ,0,60,Rf,0)
SR20T2_BodyTemp=(A+(B*(LN(Rs))+C*(LN(Rs))^3))^-1 - 273.15
SR20T2_SENS_TC = SR20T2_SENS_So * (SR20_A*SR20T2_BodyTemp^2 + _
SR20_B*SR20T2_BodyTemp + SR20_C)
SR20T2_IRR_TC = SR20T2_mV * (1000/SR20T2_SENS_TC)
'SW12V heater control function
If TimeIsBetween (0,7,24, hr) Then
SW12 (SW12_1,1,0)
Else
SR20-T2 ISO Spectrally Flat Class A (Secondary Standard) Pyranometer 18