Campbell CS325DM User manual

Revision: 07/2020
Copyright © 2020
Campbell Scientific, Inc.

Table of contents
1. Introduction 1
2. Precautions 1
3. Initial inspection 1
4. Overview 2
5. Specifications 2
6. Siting 4
7. Mounting 4
8. Wiring 4
9. Programming 6
9.1 RS-485 programming 6
9.1.1 Register map 6
9.2 Analog programming 7
10. Maintenance and troubleshooting 8
Appendix A. Example programs 9
Table of Contents - i

1. Introduction
The CS325DM, manufactured by Atonometrics as the RC18, is a silicon solar irradiation sensor
commonly used as a reference cell in solar PV monitoring applications. The sensor element was
designed to correspond to that of a photovoltaic (PV) module, including spectral selectivity and
incident angle modifier. The data signals are Modbus RTU RS-485 or analog.
2. Precautions
lREAD AND UNDERSTAND the Safety section at the back of this manual.
lDo not remove the screws on the top cover of the CS325DM, as this may compromise the
weather-proof seal.
lTo minimize the potential for water entry to the sealed housing, mount the CS325DM with
the cable facing down or to the side.
lWhen opening the shipping package, do not damage or cut the cable jacket. If damage to
the cable is suspected, consult with a Campbell Scientific support engineer.
lAlthough rugged, the CS325DM should be handled as a precision scientific instrument.
3. Initial inspection
lUpon receipt of the CS325DM, inspect the packaging and contents for damage. File
damage claims with the shipping company.
lThe model number and cable length are printed on a label at the connection end of the
cable. Check this information against the shipping documents to ensure the expected
product and cable length were received.
CS325DM Silicon Irradiance Reference Sensor 1

4. Overview
The CS325DM is specifically designed for low-cost, high-accuracy solar irradiance measurements
for outdoor monitoring of PV systems. It includes both user-configurable analog outputs and
digital communication via Modbus (RS-485). The CS325DM includes a microprocessor that stores
the calibration data, eliminating the need to reprogram data loggers or SCADA systems when
installing a new unit.
The CS325DM measures irradiance using a crystalline silicon PV cell. The PV cell short-circuit
current is measured with a precision shunt resistor and PV cell temperature is measured with a
back-of-cell resistance temperature device (RTD).
The IP67-rated cast aluminum enclosure provides for solid mounting and protection from the
elements. The M12 circular connector allows easy installation and cable replacement.
Features
lRugged design with wide temperature operating range
lAnalog and/or digital outputs available
lSpectral selectivity and incident angle modifier correspond to crystalline PV module
lBuilt-in cell temperature measurement and signal compensation
5. Specifications
Measurement range: 0 to 1500 W/m2
Operating temperature: –35 to 80 °C
Input power: 8 to 28 VDC
Photovoltaic cell: Crystalline Si, 20 mm x 20 mm, ~135 mA @ 1 sun
Window: Low-iron solar glass, or CdTe-matching filter
Cell temperature
measurement: –40 to +100 °C, Pt100 RTD
Calibration data: Internally calibrated; no calibration data to manage
Enclosure material: Powder-coated cast aluminum housing
Enclosure outdoor rating: IP67
Cable type: Shielded, weather-resistant, UV-rated, 24 AWG/0.2 mm2
CS325DM Silicon Irradiance Reference Sensor 2

Cable connector: M12 circular connector, IP67
Response time: 0.15 s
Electronics non-linearity: ±0.03% of range
Repeatability: ±0.02% of range
Temperature drift: ±0.4% at 1000 W/m2(–35 to 80 °C)
Resolution: 0.1 W/m2
Factory calibration
of electronics: ±0.1% of reading ±0.2% of range
Irradiance calibration: ±1.2%, calibrated to NREL-traceable reference standard
Overall measurement
uncertainty: ±2.0% @ 1500 W/m2, ±2.9% @ 100 W/m2
Stability: 0.5% per year
Mounting: Four mounting holes with diameter 5.50 mm (0.217 in)
Dimensions: 11.5 x 6.5 x 3.0 cm (4.5 x 2.6 x 1.2 in)
Weight: 0.3 kg (0.6 lb)
Digital output
Communications
protocols: Modbus over RS-485, user-settable Modbus address
Baud rate: Up to 57.6 kbps
Current consumption: Typically 8 to 15 mA
Analog output
Analog output options: 0 to 1.5 V; 0 to 10 V; 4 to 20 mA
Output signals: Irradiance, cell temperature, short-circuit current
Output impedance: 2 kohm (0 to 1.5 V or 0 to 10 V mode)
Internal voltage drop: Allow 3.5 V minimum (4 to 20 mA mode)
CS325DM Silicon Irradiance Reference Sensor 3

6. Siting
A CS325DM used for monitoring PV installations must be installed with the same alignment and
inclination as the PV generator. The mounting location should be free of shading. To facilitate
maintenance and cleaning, mount the CS325DM in an easily accessible location such as near
windows or skylights.
7. Mounting
Use the CM261-LP to mount to a single axis tracker or the CM256-LP to mount the sensor at a
fixed angle.
1. Place the sensor on the top plate.
2. Use the mounting screws shipped with the sensor to secure the sensor to the top plate.
3. Using a diopter in combination with a solar compass, install and orient the crossarm on the
tripod or the mast.
4. Place the V-bracket or U-bolt on the crossarm and tighten the bolts or U-bolt nuts.
5. Use the bubble level and leveling bolts to level the sensor.
6. If using the CM256-LP, tilt to the correct azimuth and tighten bolts.
7. Verify mounting hardware is firmly tightened, and that the mounting bracket is horizontal
or at the desired angle.
8. Route the sensor cable to the enclosure.
9. Secure the cable to the crossarm and tripod or tower mast.
8. Wiring
The CS325DM outputs either a Modbus RTU RS-485 or an analog signal. Table 8-1 (p. 5) provides
the Modbus RS-485 wiring and Table 8-2 (p. 5) provides analog wiring. The RS-485 output can
be directly read by a MeteoPV, CR6, CR1000X, or Modbus RTU RS-485 network. Other Campbell
Scientific data loggers can use an MD485 multidrop interface to read the RS-485 output. Refer to
the MD485 manual for more information.
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Table 8-1: RS-485 pin-out, wire color, function, and data logger connection
Wire color1Pin-out Function Data logger2connection
Blue 8 RS-485 A- A-,C(odd)
White/blue stripe 7 RS-485 B+ B+,C(even)
Brown 1 8 to 24 VDC 12V
White/brown stripe 2 Power and signal ground G
Orange 3 G ⏚(analog ground)
Green 4 G ⏚(analog ground)
Clear N/A Shield ⏚(analog ground)
1The white/orange striped and white/green striped wires are not used.
2Assumes the sensor directly connects to the data logger.
Table 8-2: Analog pin-out, wire color, function, and data logger connection
Wire color1Pin-out Function Data logger connection
White/orange
stripe 5Irradiance input
Uconfigured for single-ended analog input2,
SE (single-ended, analog-voltage input)
White/green
stripe 6Temperature input
Uconfigured for single-ended analog input2,
SE (single-ended, analog-voltage input)
Brown 1 8 to 24 VDC 12V
White/brown
stripe 2Power and signal ground G
Orange 3 G ⏚(analog ground)
Green 4 G ⏚(analog ground)
Clear N/A Shield ⏚(analog ground)
1The white/blue striped and blue wires are not used.
2Uterminals are automatically configured by the measurement instruction.
CS325DM Silicon Irradiance Reference Sensor 5

9. Programming
Programming basics for CRBasic data loggers are provided in the following sections. Complete
program examples for measuring the Modbus over RS-485 and analog outputs can be found in
Example programs (p. 9). Campbell Scientific recommends using Modbus over RS-485.
9.1 RS-485 programming
The RS-485 output can be directly read by a MeteoPV, CR6-series, CR1000X-series, or Modbus
RTU RS-485 network. Other Campbell Scientific data loggers can use an MD485 multidrop
interface to read the RS-485 output. Refer to the MD485 manual for information about using the
MD485. Refer to www.campbellsci.com/videos/meteopv for information about using the
MeteoPV.
A CR6 or CR1000X data logger programmed as a Modbus Master can retrieve the values stored in
the Input Registers (Register map (p. 6)). To do this, the CRBasic program requires SerialOpen()
followed by ModbusMaster(). The SerialOpen instruction has the following syntax:
SerialOpen (ComPort, Baud, Format, TXDelay, BufferSize, Mode)
The Format parameter is typically set to logic 1 low; even parity, one stop bit, 8 data bits. The
Mode parameter should configure the ComPort as RS-485 half-duplex, transparent.
The ModbusMaster() instruction has the following syntax:
ModbusMaster (Result, ComPort, Baud, Addr, Function, Variable, Start, Length,
Tries, TimeOut, [ModbusOption])
The Addr parameter must match the sensor Modbus address. To collect all of the values, the
Start parameter needs to be 1 and the Length parameter needs to correspond with the
register count (see Register map (p. 6)). ModbusOption is an optional parameter described in
the CRBasic Editor Help.
9.1.1 Register map
Table 9-1 (p. 7) provides the register map for the most commonly used values. Calculated
irradiance uses the following equation:
CS325DM Silicon Irradiance Reference Sensor 6

Table 9-1: RS-485 register map
Starting register
number
Register
count Data format Units Description
2 2 Single-precision 32 bit
floating point W/m² Calculated irradiance
4 2 Single-precision 32 bit
floating point A
Measured short-circuit
current (Isc) of the PV cell,
without temperature
correction
6 2 Single-precision 32 bit
floating point °C Measured temperature (T) of the PV cell
105 4 Char x 2 n/a
Serial number of the unit,
up to eight characters,
two characters per register
121 5 Signed 16 bit integer n/a
Part number of the unit,
up to 10 characters, two
characters per register
9.2 Analog programming
Two VoltSE() CRBasic instructions are required to measure the analog output: one instruction
for the irradiance measurement and the other instruction for the temperature measurement.
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 VoltSE()
instruction; the autorange adds a few milliseconds to the measurement time. 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.
CS325DM Silicon Irradiance Reference Sensor 7

10. Maintenance and
troubleshooting
The CS325DM 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 2 years.
The most common reason for communication failure for the CS325DM is because the sensor is
not configured properly. Ensure that the sensor has a unique Modbus address and proper RS-485
settings.
CS325DM Silicon Irradiance Reference Sensor 8