Campbell CS120A User manual

CS 120A
and CS125
Visibility and Present
Weather Sensors
Revision
: 10/2020
Copyright ©
2013 – 2020
Campbell Scientific, Inc.

i
Table of Contents
PDF viewers: These page numbers refer to the printed version of this document. Use the
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1. Introduction................................................................ 1
1.1 General safety ......................................................................................2
1.2 Sensor unit safety .................................................................................2
1.3 Principle of operation...........................................................................3
1.4 Recommended Tools............................................................................5
1.5 Quickstart .............................................................................................5
2. Measurement specification....................................... 6
3. Technical specification ............................................. 6
3.1 Electrical specification .........................................................................6
3.2 Optical specification.............................................................................7
4. Communications specification................................. 8
4.1 Communications electrical specifications ............................................8
4.2 Supported data rates and formats .........................................................8
5. Environmental specifications ................................... 9
6. Mechanical specifications......................................... 9
6.1 Dimensions...........................................................................................9
6.2 Weights ..............................................................................................10
6.3 Mounting............................................................................................10
7. Installation procedure ............................................. 10
7.1 Equipment grounding.........................................................................11
7.2 Mounting the sensor...........................................................................12
7.3 Optional Campbell Scientific Mount .................................................15
8. Sensor internal connectors’ description ............... 17
8.1 Sensor recommended wiring using Campbell Scientific cables (this
cable is supplied already connected as standard)............................19
9. HygroVUE 5, HygroVUE 10 or CS215 T/RH
Sensor (CS125 only).............................................. 21
10. Functions of the internal switches ......................... 23

Table of Contents
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11. Message Formats: A breakdown of the different
default outputs of the sensor – Basic/Partial/
Full.......................................................................... 24
11.1 Visibility only messages.....................................................................25
11.2 Messages with SYNOP Present Weather Codes (CS125 only)..........26
11.3 Messages with METAR Present Weather Codes (CS125 only).........27
11.4 Messages with Generic SYNOP Present Weather Codes (CS125
only)................................................................................................28
11.5 Example sensor message outputs .......................................................33
11.6 Custom message format .....................................................................34
12. Interface methods – Device Configuration
Utility/Command line/Menu................................... 37
12.1 Configuring a PC for talking to the sensor.........................................37
13. Definition of the variables that can be set by the
user on the sensor ................................................ 38
14. Command line mode ............................................... 40
14.1 The SET Command............................................................................40
14.1.1 Example of a SET Command......................................................41
14.2 The SETNC Command ......................................................................42
14.2.1 Example of a SETNC Command ................................................42
14.3 The MSGSET Command ...................................................................42
14.4 The GET Command ...........................................................................44
14.5 The MSGGET Command...................................................................46
14.6 The POLL command – Polling the sensor .........................................47
14.7 The ACCRES command – Resetting the accumulation value............48
15. Entering the sensor menu system ......................... 49
16. Calibrating the sensor............................................. 54
16.1 Visibility calibration...........................................................................54
16.2 Dirty window zero calibration............................................................57
16.3 Internal temperature check (CS125 only)...........................................58
17. Performing an operating system update ............... 59
18. Cleaning ................................................................... 60
19. Lubricating the enclosure screws .......................... 61
20. Desiccant ................................................................. 61
Appendices
Addendum – Hail monitoring (CS125 only) ................. 63

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A. CS120A/CS125 block diagram .............................. A-1
B. Example C code of the CRC-16 checksum .......... B-1
C. Present Weather Codes ........................................ C-1
C.1 SYNOP Codes produced by the CS125 ...........................................C-1
C.2 METAR Codes produced by the CS125 ..........................................C-2
C.3 NWS Codes produced by the CS125 ...............................................C-2
Figures
1-1. Particles in the sample volume scatter light in all directions,
including into the detector ................................................................3
1-2. Signals from large, slow falling snowflakes and smaller, faster,
raindrops ...........................................................................................4
1-3. Defining possible precipitation types based on wet bulb and dry
bulb temperatures..............................................................................4
1-4. A typical size/speed map used by the CS125 present weather
algorithm...........................................................................................5
7-1. Airflow ...............................................................................................11
7-2. Grounding boss ..................................................................................12
7-3. Mounting arrangement .......................................................................13
7-4. Mounting to a flat surface ..................................................................14
7-5. Use of band clamps ............................................................................14
7-6. Optical sensor mast ............................................................................15
7-7. Mounting footprint.............................................................................16
8-1. Connections........................................................................................18
8-2. Communications and power connections...........................................20
8-3. Communications and power connector ..............................................20
8-4. Configuration cable............................................................................21
8-5. USB configuration cable ....................................................................21
9-1. Connection for the optional HygroVUE 5, HygroVUE 10 and
CS215 T/RH sensors.......................................................................22
10-1. Internal switches.................................................................................23
16-1. Calibration disk ..................................................................................56
16-2. Mounting calibration disk ..................................................................57
17-1. Sensor DevConfig download instructions..........................................59
17-2. Sensor DevConfig screen when OS update is complete.....................60
Tables
10-1. Internal switch functions ....................................................................24
11-1. Summary of message IDs and descriptions........................................30
11-2. Summary of system alarms and descriptions .....................................31
13-1. User definable settings and descriptions ............................................38

1
CS120A and CS125 Visibility and
Present Weather Sensors
1. Introduction
The CS120A is a visibility sensor. The CS125 additionally detects and reports
present weather in the form of SYNOP, METAR or NWS codes. The CS125
has the same specification for visibility measurement as the CS120A. It is
possible to upgrade a CS120A to a CS125, please contact Campbell Scientific
for more details.
The sensors are infra-red forward scatter visibility and present weather sensors
for automatic weather stations including road, marine and airport based
stations. They both use the well-established forward scatter system for
visibility measurement, utilising a 42º scatter angle. The CS125 uses high
speed sampling to reduce missed events and improves response to other
suddenly changing conditions.
The CS125 has a temperature sensor mounted in the cross arm used as part of
the process for identifying precipitation.
When an optional CS215 temperature and RH sensor is connected, the CS125
can distinguish wet and dry obscuration (for example mist and haze) and make
more precise discrimination between liquid and frozen precipitation.
Dew heaters are provided to keep the sensor optics clear of condensation and
more powerful hood heaters to prevent the build up of snow or ice.
This sensor is certified for Aviation use by the German Meteorological Service,
Deutscher Wetterdienst (DWD).

CS120A and CS125 Visibility and Present Weather Sensors
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1.1 General safety
This manual provides important safety considerations for the installation,
operation and maintenance of the sensor. These safety considerations are
classified into three levels:
Warnings alert the installer or user to serious hazards.
Ignoring these warnings could result in injury or death
and/or irrevocable damage to the sensor unit.
Cautions warn of potential hazards. Ignoring these cautions
could result in the sensor being damaged and data being
lost.
Notes highlight useful information in the installation, use and
maintenance of this product. These should be followed carefully
in
order to gain the maximum benefit from the use of this product.
1.2 Sensor unit safety
The sensor has been checked for safety before leaving the factory and
contains no internally replaceable or modifiable parts.
Do not modify the sensor unit. Such modifications will
lead to damage of the unit and could expose users to
dangerous light levels and voltages.
In unusual failure modes and environmental conditions
the sensor hood could become hot. In normal operation
they will be at ambient temperature or slightly above.
Ensure that the correct voltage supply is provided to the
sensor.
WARNING
CAUTION
NOTE
WARNING
WARNING
CAUTION

CS120A and CS125 Visibility and Present Weather Sensors
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1.3 Principle of operation
Figure 1-1. Particles in the sample volume scatter light in all directions,
including into the detector
The CS120A and CS125 comprise an emitter and detector aligned as in
Figure 1-1. The emitter produces a beam of near infra-red light pulsed at 1
kHz. A detector has a field of view which overlaps the beam and is inclined at
42 degrees to it. Light scattered by a particle (for example a fog droplet or
particle of precipitation) from the overlap or sample volume towards the
detector is detected by a photodiode and recorded as a signal. The size of the
signal is therefore proportional to the extinction of the emitted beam caused by
scattering. The scattering signal averaged over one second is used to calculate
an extinction coefficient or EXCO. Sixty one second averages are then
themselves averaged to give a one minute average EXCO. This is then
converted to a value of Meteorological Optical Range (MOR) using
Koschmieder’s law:
MOR = 3/EXCO where MOR is in km and EXCO in units of km-1.
The CS125 is capable of identifying weather type in addition to measuring
visibility. It does this by analysing the amplitude and width of spikes in the
APD signal corresponding to particles of precipitation passing through the
sample volume. The amplitude of the signal is a guide to the size of the particle
and the width, because it represents the time taken for the particle to fall
through the sample volume, is proportional to the fall speed, see Figure 1-2.

CS120A and CS125 Visibility and Present Weather Sensors
4
Figure 1-2. Signals from large, slow falling snowflakes and smaller,
faster, raindrops
The CS125 also has a temperature sensor. These three parameters, fall speed,
size and temperature are used to identify the type of particle. If an additional
external temperature and relative humidity probe is connected then a wet-bulb
temperature can be calculated. This provides useful additional information
identifying particles more accurately especially, between liquid and frozen
around 0°C.
Figure 1-3 shows how these temperatures are used to define possible
precipitation types around 0°C.
Figure 1-3. Defining possible precipitation types based on wet bulb and
dry bulb temperatures
The processing algorithm then works with several ‘maps’ such as Figure 1-4 to
identify each particle.

CS120A and CS125 Visibility and Present Weather Sensors
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Figure 1-4. A typical size/speed map used by the CS125 present
weather algorithm
1.4 Recommended Tools
The following tools are recommended:
10 mm open spanner (for grounding boss, must be open)
13 mm spanner
19 mm open spanner (for cable glands, must be open)
2 mm flat screwdriver
Number 2 cross head screwdriver
1.5 Quickstart
The sensor is shipped set to the following default communication RS-232, 8N1,
38400 baud, a sensor ID = 0 and set to transmit default messages, full format,
visibility only for the CS120A, SYNOP present weather full format for the
CS125, at 1 minute intervals (see Section 11, Message Formats: A breakdown
of the different default outputs of the sensor – Basic/Partial/Full (p. 24)).
To start using the CS120A or CS125, first connect a DC supply matching the
specification in Section 3.1, Electrical specification (p. 6), to the red and black
wires on the ‘D-connector’ (see Figure 8-3) and connect to a PC
communications port with a terminal emulator set to RS-232, 38400 baud,
8N1. After a couple of minutes, data messages will be received. Typing ‘open

CS120A and CS125 Visibility and Present Weather Sensors
6
0’ will access the menu structure, see Section 15, Entering the sensor menu
system (p. 49).
2. Measurement specification
Minimum
Value
Nominal
Value
Maximum
Value
Visibility characteristics
Reported visibility (metric)
5 metres
-
75,000
metres
Reported visibility (imperial)
16 feet
-
46 miles
Visibility accuracy calibration
against factory calibration disk
-
+/- 2%
-
Visibility accuracy up to 600 m
-
+/-8%
-
Visibility accuracy up to 10,000 m
-
+/-10%
-
Visibility accuracy up to 15,000 m
-
+/-15%
-
Visibility accuracy above
15,000 m
-
+/-20%
-
Precipitation characteristics, water equivalent (CS125 only)
Reported accumulation range
0 – 999.9 mm
Accumulation accuracy**
+/-15%
Accumulation resolution
0.1 mm
Reported intensity range (up to *)
0 - 999.99 mm/hr
Intensity accuracy**
+/-15%
Intensity resolution
0.01 mm/hr
*The maximum intensity reported is dependent on the mixture of precipitation falling.
**Against factory calibration standards in the laboraory, for liquid precipitation.
3. Technical specification
3.1 Electrical specification
Minimum
Value
Nominal
Value
Maximum
Value
Main power supply for DSP and dew heaters
Power supply, (DC only)
7V
12V
30V(1)
Current consumption sampling
continuously with dew heaters ON and
RS-232 communications active(2, 3) (at
12V DC)
-
200 mA
248 mA
Current consumption sampling
continuously with dew heaters
disabled (at 12V DC)
-
110 mA
151 mA
Current consumption without any
sampling occurring and dew heaters
disabled (at 12V DC)
-
21 mA
30 mA

CS120A and CS125 Visibility and Present Weather Sensors
7
Hood heater power supply
Hood heater voltage (AC or DC)
-
24V(3)
30V(4)
Hood heater wattage (at 24V AC or
DC)
-
60W(5)
-
User alarm outputs
User output high level (at 85ºC)
3.8V
-
-
User output high level (at 25ºC )
4.13V
-
-
User output low (All temperatures)
0.25V
-
0.55V
User output current
-
-
32 mA
(1) If a CS215 is being used with a CS125 the supply voltage should not exceed 28V.
(2) The RS-232 communications interface will automatically turn itself off when not transmitting.
(3) If hood heaters are not being used ensure ‘Hood heater override’ (details in Section 13,
Definition of the variables that can be set by the user on the sensor (p. 38)) is set to off.
(4) It is recommended that the hood heaters are run at 24V AC/DC. It is possible to run the
heaters at any voltage below 24V but the heaters will generate proportionally less heat
reducing their ability to prevent ice build-up.
(5) Each hood takes 30W, 60W is the total for both hoods on the sensor together.
If a CS215 is being used the supply voltage should not
exceed 28V.
3.2 Optical specification
Minimum
Value
Nominal
Value
Maximum
Value
Optical characteristics
LED centre wavelength
-
850 nm
LED spectral bandwidth
-
+/-
35 nm
Pulse characteristics
Light pulse rate
-
1KHz
CAUTION

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8
4. Communications specification
4.1 Communications electrical specifications
Minimum
Value
Nominal
Value
Maximum
Value
RS-232 Communications(1)
RS-232 input threshold Low
0.8V
1.5V
-
RS-232 input threshold High
-
2.0V
2.4V
RS-232 input absolute maximum
-15V
-
+15V
RS-232 input resistance
12KΩ
-
-
RS-232 output voltage low
-
-
0.4V
RS-232 output voltage high (into
3KΩ)
4.4V
-
-
RS-485 Communications
RS-485 input threshold voltage
-0.2V
-
+0.2V
RS-485 output (Unloaded)
-
-
5V
RS-485 output (Load 50Ω)
2V
-
-
Maximum voltage at any terminal(2)
-7V
-
+7V
(1) The RS-232 communications interface will automatically turn itself off when not
transmitting.
(2) The ground of the sensor and the ground of any RS-485 equipment cannot be further apart
than this voltage. The sensor ground (pin 1) on connector B, see page 18, can be connected
to the ground of the host equipment. This will reduce any parasitic currents.
4.2 Supported data rates and formats
Serial setting 8N1
Supported data rates
•1200 bps
•2400 bps
•9600 bps
•19200 bps
•38400 bps - default
•57600 bps
•115200 bps
Supported formats
•RS-232 (Full duplex only), default
•RS-485 (Half duplex)
•8 bit data bytes
•1 stop bit
•Parity checking is not supported as most communication protocols used
by the CS125 have built in checksums as well as checks that
communications have been understood.

CS120A and CS125 Visibility and Present Weather Sensors
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5. Environmental specifications
Minimum
Value
Nominal
Value
Maximum
Value
Sensor temperature ranges
Operating temperature
-25°C
-
+60°C
Extended operating temperature
-40°C
-
+70°C (1)
Storage temperature
-40°C
-
+85°C
Sensor humidity ranges
Operating humidity range
0%
-
100%
Sensor heater thresholds
Dew heater Turn On
-
<35°C
-
Dew heater Turn Off
-
>40°C
-
Hood heater Turn On
-
<15°C
-
Hood heater Turn Off
-
>25°C
-
(1) Extended temperature ranges are only guaranteed if the sensor has been tested by Campbell
Scientific and verified within this temperature range. Some degradation of absolute accuracy
can be expected at the extremes of the extended ranges.
6. Mechanical specifications
6.1 Dimensions

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10
6.2 Weights
Sensor weight: 3 Kg
Shipping weight: 6 Kg (including packing box)
6.3 Mounting
Sensor mounting: Bracket mounts on a vertical pole 32-52.5 mm diameter.
The mounting bracket has cut-outs for band clamps for
larger diameter masts.
7. Installation procedure
The sensor measures environmental variables and is designed to be located in
harsh weather conditions. However, there are a few considerations to take into
account if accurate and representative data from a site are to be obtained.
The descriptions in this section are not exhaustive. Please refer to
meteorological publications for further information on locating
weather instruments
The sensor should be sited in a position representative of local weather
conditions and not of a specific microclimate (unless the analysis of
microclimate weather is being sought).
The sensor has good resistance to background light but it is a good idea to
avoid locations where the transmitter is pointing at a light scattering or
reflecting surface. Ideally, the receiver should point north in the northern
hemisphere or south in the southern hemisphere but this is not critical if the
field of view does not include a bright and scattering surface.
To give non-microclimatic measurements the sensor should be sited away from
possible physical obstructions that could affect the fall of precipitation. The
sensor should also be positioned away from sources of heat, electrical
interference and in such a position as to not have direct light on the sensor
lenses. Whenever possible, the sensor should be located away from
windbreaks.
Several zones have been identified upwind and downwind of a windbreak in
which the airflow is unrepresentative of the general speed and direction. Eddies
are generated in the lee of the windbreak and air is displaced upwind of it. The
height and depth of these affected zones varies with the height and to some
extent the density of the obstacle.
Generally, a structure disturbs the airflow in an upwind direction for a distance
of about twice the height of the structure, and in a downwind direction for a
distance of about six times the height. The airflow is also affected to a vertical
distance of about twice the height of the structure. Ideally, therefore, the sensor
should be located outside this zone of influence in order to obtain
representative values for the region.
NOTE

CS120A and CS125 Visibility and Present Weather Sensors
11
Figure 7-1. Airflow
In order to reduce the service frequency with the unit, the sensor should be
placed away from sources of contamination, in the case of roadside monitoring;
larger mounting poles can be used. More regular maintenance will be required
when the instrument is placed in areas where contamination is unavoidable or
where measurements may be safety critical.
The WMO recommend a sample volume height of 1.5 m. However, for
applications such as aviation or road visibility other heights may be
appropriate.
If operating a sensor indoors it is likely that there will be sources
of light and/or reflections that will create false readings and erratic
results.
If carrying out simple checks, blocking a lens or the sample
volume will simulate an INCREASE in visibility not a decrease.
7.1 Equipment grounding
The sensor must be properly grounded. It is sufficient to ground the mounting
bracket and if the sensor is connected to a grounded metal mast, and in
electrical contact with it, then this will be sufficient. Otherwise, the mounting
bracket should be earthed and a grounding boss is supplied to allow this.
A ground wire with a minimum cross section of 6 mm2and maximum length of
5 m should be used.
The pole and foundations of a pole mounted installation will provide some
basic lightning protection and protection against radio frequency interference
and should also be correctly grounded.
NOTE
NOTE

CS120A and CS125 Visibility and Present Weather Sensors
12
Figure 7-2. Grounding boss
7.2 Mounting the sensor
A quick release pole mounting kit is supplied with the sensor.
If a power supply enclosure has been supplied with the sensor it can be
mounted on the pole, near its base using the brackets supplied with the
enclosure. Alternatively the power supply can be mounted elsewhere, e.g. on a
wall at some distance from the sensor. The power supply enclosure should be
mounted away from the sensor head to avoid wind flow disturbance or rain
drops bouncing back up into the sensor’s sensing volume.
Take care not to overtighten the nuts on the bolts, as it may
be possible to distort and/or damage the brackets or DSP
plate by doing so, and/or the nuts may seize up. Only tighten
the nuts to a degree necessary to hold the sensor firmly in
place.
Where the sensor is to be mounted onto another type of mast, please refer to
the manual for that mast for mounting details.
Ensure that the sensor is mounted according to Figure 7-3.
Do not reposition, once fixings are tightened, by forcing the
arms of the unit as this can cause damage.
CAUTION
CAUTION

CS120A and CS125 Visibility and Present Weather Sensors
13
Do not remove the mounting plate as this will compromise
resistance to water ingress.
Figure 7-3. Mounting arrangement
If you need to mount the sensor to a flat surface, remove the plastic formers
from the mounting brackets and use the holes as shown in Figure 7-4.
If mounting to a flat surface ensure that there is no obstruction to
airflow through the sample volume.
CAUTION
NOTE

CS120A and CS125 Visibility and Present Weather Sensors
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Figure 7-4. Mounting to a flat surface
Slots are provided to allow band clamps to be used with larger diameter masts,
see Figure 7-5.
Figure 7-5. Use of band clamps

CS120A and CS125 Visibility and Present Weather Sensors
15
7.3 Optional Campbell Scientific Mount
A Campbell Scientific ‘optical sensor mount’ is available. This will put the
sample volume at about 1.5 m in compliance with the WMO ‘Guide to
Meteorological Instruments and Methods of Observation’, 7th Edition, Section
9.3.4.
Figure 7-6. Optical sensor mast

CS120A and CS125 Visibility and Present Weather Sensors
16
If applicable, use the following installation instructions.
Install the mount on a concrete foundation. If one does not already exist then a
concrete foundation should be constructed at least 600 mm square and 600 mm
deep. Ensure the ground consistency is not too loose and will be able to support
the mount and concrete foundation.
Drill four 12 mm diameter holes using the mount base as a template or
following Figure 7-7 to a depth of 77 mm.
Figure 7-7. Mounting footprint
Clean the holes of all debris.
Place washers and nuts on the ends of the wedge anchors supplied (to protect
the threads during installation).
Hammer the wedge anchors into the holes until the start of the threads are
below the surface.
Tighten the nuts until about 25 mm of thread protrudes above the surface.
Remove the washers and nuts from the protruding length screw. Then lower
the mount into place.