Campbell SR50 User manual

INSTRUCTION MANUAL
SR50
Sonic Ranging Sensor
April 2013
Copyright © 2009 - 2013
Campbell Scientific (Canada) Corp.

1
SR50 Sonic Ranging Sensor
Table of Contents
1. SR50 Specifications.....................................................3
2. Introduction ..................................................................4
3. Operation (General) .....................................................4
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4. Sensor Mounting and Installation ............................21
5. Data Interpretation .....................................................24
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6. Maintenance ...............................................................25
7. Assembly/Disassembly Procedures ........................26
8. Changing the Address/Option Jumpers ..................29
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Formulas
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SR50 Sonic Ranging Sensor
2
Tables
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Appendices
A. Making Concurrent Measurements with the SR50 ....... 32
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SR50 Sonic Ranging Sensor
3
SR50 Sonic Ranging Sensor
1. SR50 Specifications
Power Requirements: 9-16 VOLTS D.C.
Power Consumption: 2 mA (Quiescent)
250 mA (Measurement Peak)
Measurement Time: 0.6 seconds typical
3.0 seconds max.
Output: SDI-12 (version 1.2)
Serial ASCII
- 300 and 1200 Baud TTL or RS232
- 9600 Baud RS232 only
Pulse Train (0 to 5 Volts)

SR50 Sonic Ranging Sensor
4
Measurement Range: 0.5 to 10 meters
Accuracy: ±1 cm or 0.4% of distance to target
(whichever is greater)
requires external temperature compensation
Resolution: 0.1mm
Beam Acceptance Angle: Approximately 22°
Operating Temperature: -30°C to +50°C Standard (extended
temperature available)
Maximum Cable Length: 60 meters (SDI-12 & ASCII)
300 meters (Pulse train)
Cable Type: Part number L9720, 4 conductor,
2-twisted pair, 22 AWG, Santoprene
Dimensions: Length 31 cm
Diameter 7.5 cm
Weight: 1.3 kg
2. Introduction
The SR50 Sonic Ranging Sensor measures the distance from the
sensor to a target. The most common applications are measuring snow
depths and water levels. The sensor is based on a 50 kHz (Ultrasonic)
electrostatic transducer. The SR50 determines the distance to a target
by sending out ultrasonic pulses and listening for the returning echoes
that are reflected from the target. The time from transmissions to return
of an echo is the basis for obtaining the distance measurement. Since
the speed of sound in air varies with temperature, an independent
temperature measurement is required to compensate the distance
reading. A simple calculation is applied to initial readings for this
purpose.
The SR50 is capable of picking up small targets or targets that are
highly absorptive to sound--such as low density snow. The SR50 can
measure multiple targets and it makes use of a unique echo processing
algorithm to help ensure measurement reliability. If desired, the SR50
can also output a data value indicative of measurement quality (not
available for pulse train output).
The SR50 was designed to meet the stringent requirements of snow
depth measurement which makes it well suited for a variety of other
applications. The rugged aluminum housing is built to withstand harsh
environments and offers several mounting options.
3. Operation
The SR50 has several output formats: SDI-12, Pulse Train, and serial
ASCII output. A group of four jumpers inside the SR50 allow setting an

SR50 Sonic Ranging Sensor
5
address for SDI-12 operation or as an option setting for Pulse Train and
ASCII outputs.
The SR50 performs multiple echo processing regardless of output
formats. The SR50 bases every measurement on several readings and
applies an algorithm to improve measurement reliability. When using
SDI-12 or ASCII output, up to 3 targets can be detected with the SR50
provided the difference between their height is > 0.4 meters.
The distance to target readings that are obtained from the sensor are
referenced from the metal mesh on the face of the transducer. The
SR50 has a field of view of approximately 22°. The closest object to the
sensor will be detected if it is within this field of view. If a target is in
motion, the SR50 may reject a reading if the target distance changes at
a rate of 4 centimeters per second or more.
Under most circumstances the SR50 will complete a measurement
within 1 second and begin outputting the data. If the SR50 rejects a
reading or does not detect a target, it will retry. The maximum time limit
to remain in measurement state for retries is 3 seconds. If the reading
is not obtained during this time limit, the SDI-12 and ASCII options will
output zeroes for distance to target and measurement quality numbers
(see below for description of quality numbers). The Pulse Train option
will output a single pulse.
Measurement quality numbers are also available with the SDI-12 and
the ASCII output formats; these give an indication of the measurement
certainty. The quality numbers have no units of measure but can vary
from 162 to 600. Numbers lower than 210 are considered to be
measurements of good quality. A value of zero however, indicates a
reading was not obtained. Numbers greater than 300 indicate that
there is a degree of uncertainty in the measurement. Causes of high
numbers include:
•the sensor is not perpendicular to the target surface
•the target is small and reflects little sound
•the target surface is rough or uneven
•the target surface is a poor reflector of sound (low density snow)
It is not necessary to make use of the quality numbers but they can
provide additional information such as an indication of surface density in
snow monitoring applications.
The SR50 does not include a temperature sensor to compensate for
variations in air temperature. Campbell Scientific recommends the
Model 107 air temperature probe for this purpose. A radiation shield
may be required. Temperature compensation must be applied to the
sensor output using the following formula:
FORMULA 1. Temperature Compensation

SR50 Sonic Ranging Sensor
6
The SR50 calculates a distance reading using the speed of sound at
0°C (331.4 m/s). If the temperature compensation formula is not
applied, the distance values will not be accurate for temperatures other
than 0°C. A temperature compensated distance is obtained by
multiplying the SR50 reading by the square root of the air temperature
in degrees Kelvin divided by 273.15. An example program that includes
this correction is shown in Section 3.2.
On rare occasions, measurements made by the SR50 can
be adversely influenced by strong electromagnetic fields.
Electromagnetic fields can be generated by the operation
of heavy electrical machinery and the presence of radio
frequencies with field strengths greater than 3V/m.
Symptoms of this problem are erroneous measurements
randomly dispersed among accurate measurements.
This problem can be fixed by tying the probe body to earth
ground. This is done by scraping away a small patch of
the clear anodize coating and attaching a bare section of
stranded wire over the scraped area with a band clamp.
Copper wire and a stainless steel band clamp work best. It
is recommended to confirm with an ohmmeter that the
copper wire is making contact to the probe body.
3.1 SDI-12 Operation
SDI-12 is a Serial Digital Interface standard that is used for communication
between data recorders and sensors. Campbell Scientific’s CR10(X),
CR500, CR510, CR23X, CR1000 and BDR320 have the capability to read
SDI-12 sensors (CR10’s with the correct PROM can do SDI-12).
The SR50 can be set to one of ten addresses (0 to 9) which allows up
to ten sensors to be connected to a single digital I/O channel (control
port) of an SDI-12 datalogger.
The SR50 is shipped from the factory with the address set to 0. The
SR50 needs to be opened in order to change the SDI-12 address. Refer
to Section 7.0 for details on opening the SR50 and Figure 8 for the
Address/Option jumper settings.
When it is necessary to measure more than one SR50, it is easiest to
use a different control port for each SR50 instead of changing the
address in each SR50.
SPECIAL CAUTION
But only for SR50’s
with a detachable cable
and connector directly
on the sensor housing.

SR50 Sonic Ranging Sensor
7
3.1.1 SDI-12 Wiring
FIGURE 1. SDI-12 Wiring
The order in which the connections are made is critical.
Always connect Ground first, followed by +12V and then
the remaining SDI-12/Data Bus, Sensor Enable Line and
Shield. When disconnecting the sensor, the reverse order
should be followed.
The SDI-12 protocol has the ability to support various measurement
commands. The SR50 supports the commands that are listed in Table 1.
Depending on the application, different measurement commands may
be selected. The different commands are entered as options in
Parameter 2 of instruction 105. The major difference between the
various measurement commands is the data that are returned. The
user has the options to output the distance to target in meters or feet,
include the measurement quality numbers, and to include multiple
target detection.
If outputting a measurement and quality number with SDI
measurements, the multiplier and offset in the P105
instruction will be applied to both. This could cause the
quality numbers to be negative or to over-range.
Therefore, when using SDI output with quality numbers,
ensure that the multiplier and offset in instruction P105 are
set to 1 and 0 respectively. Use P34 for multiplier and P36
for offset.
CAUTION
SPECIAL NOTE
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SR50 Sonic Ranging Sensor
8
The "8" command outputs the first detected echo in meters as in the "0"
command. However, the "8" command disables retries to ensure a fast
measurement. Under poor measurement conditions it may take as long
as 3 seconds to obtain a measurement. For users who find this length
of time unacceptable selecting the "8" command ensures that the
sensor will complete the measurement within a one second period by
disabling retries.
If the SR50 is unable to detect a proper echo for a measurement, the
sensor will return a zero value for the distance to target value. When
the multiple target output option is selected, the SR50 will return three
distance to target values that are arranged in order from closest to the
furthest. If no targets were detected, the SR50 outputs three zeroes.
The "10" command requests the sensor to perform a self check. This
will verify the CPU operation and generate a ROM signature value to
verify the software version that is in the sensor.
SR50’s with software version 1.1 or higher have Concurrent
Measurement capability. With this ability, the datalogger can initiate
measurements with several SDI-12 devices without having to wait for
each individual sensor to complete its sequence before proceeding to
the next program instruction. This can speed up the datalogger
program if that is a concern. For Campbell datalogger users, see the
datalogger manual Section 9 105 SDI-12 RECORDER instruction
description. At present, the CR10X, CR510, CR23X, and CR1000 with
recent OS have this ability.
More detailed instructions and a programming example is located in
APPENDIX A.
FIGURE 2. SR50 Wiring for CR10(X) SDI-12 Example

SR50 Sonic Ranging Sensor
9
3.2 SDI-12 CR10(X) Program Example
In this example, the SR50 is mounted 2 meters above the ground.
Snow depth and a quality of measurement number is the desired
output. This program uses Instruction 105 (refer to the CR10(X)
manual for details on the SDI-12 Recorder Instruction) to obtain a
distance measurement from the SR50. The SR50’s address is 0. It is
assumed that a Campbell Scientific 107 Temperature Probe is used
and that the SR50 SDI-12/DATA Bus is wired to control port 1 (see
Figure 2).
;{CR10X}
;
*Table 1 Program
01: 60 Execution Interval (seconds)
; Measure the 107 temperature probe.
1: Temp (107) (P11)
1: 1 Reps
2: 1 SE Channel ; S.E. Chan #1 used for this example
3: 1 Excite all reps w/E1 ; Excite Chan #1 used for this example
4: 1 Loc [ T_KELVIN ]
5: 1 Mult
6: 273.15 Offset ; This converts the value to degrees Kelvin.
; Measure the SR50 using code ‘1’ for parameter 2. This will cause the SR50 to measure
the distance to target (in meters) and provide a measurement quality number.
2: SDI-12 Recorder (P105)
1: 0 SDI-12 Address
2: 1 Start Measurement (aM1!) ; << code ‘1’ is used (SEE TABLE 1).
3: 1 Port ; CR10X can use any port. CR23X only use ports 5-8 only.
4: 2 Loc [ DEPTH ] ; Two input locations are used. One for distance and
5: -1 Mult ; one for measurement quality.
6: 0 Offset
; A multiplier of -1 is used because the distance to target value will be subtracted from the
distance to ground value to obtain snow depth (see 7th instruction).
; The next four instructions apply temperature compensation to the distance value.
3: Z=F x 10^n (P30)
1: 273.15 F
2: 0 n, Exponent of 10
3: 4 Z Loc [ REF_TEMP ]
4: Z=X/Y (P38)
1: 1 X Loc [ T_KELVIN ]
2: 4 Y Loc [ REF_TEMP ]
3: 5 Z Loc [ MULT ]
5: Z=SQRT(X) (P39)
1: 5 X Loc [ MULT ]
2: 5 Z Loc [ MULT ]
6: Z=X*Y (P36)
1: 2 X Loc [[ DEPTH ]
2: 5 Y Loc [ MULT ]
3: 2 Z Loc [[ DEPTH ]

SR50 Sonic Ranging Sensor
10
7: Z=X+F (P34)
1: 2 X Loc [ DEPTH ]
2: 2.0 F ; <<<<<< Insert the distance from sensor to bare ground
3: 2 Z Loc [ DEPTH ] ; This is actual snow depth.
;Hourly data output…
8: If time is (P92) ; Output data at the interval you require for your data analysis.
1: 0 Minutes (Seconds --) into a
2: 60 Interval (same units as above)
3: 10 Set Output Flag High (Flag 0)
9: Set Active Storage Area (P80)
1: 1 Final Storage Area 1
2: 60 Array ID; This sets the array ID# to be ‘60’.
10: Real Time (P77); Output a time stamp with the data.
1: 1220 Year,Day,Hour/Minute (midnight = 2400)
11: Sample (P70) ; Sample the snow depth and measurement
1: 2 Reps ; quality number.
2: 2 Loc [ DEPTH ]
-Input Locations-
1 T_KELVIN
2 DEPTH
3 SIG_QUAL1
4 REF_TEMP
5 MULT

SR50 Sonic Ranging Sensor
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
TABLE 1. SR50 SDI-12 Command List
CR10X and SDI-12 FUNCTION DATA OUTPUT
CR23X P105 COMMANDS
Parameter 2 When using
Options non-Campbell
Loggers
0aM First Target (Meters) D1
1aM1 First Target (Meters) D1,Q1
with Quality Number
2aM2 Multiple Targets (Meters) D1,D2,D3
3aM3 Multiple Targets (Meters) D1,D2,D3,Q1,Q2,Q3
with Quality Numbers
4aM4 First Target (Feet) D1
5aM5 First Target (Feet) D1,Q1
with Quality Number
6aM6 Multiple Targets (Feet) D1,D2,D3
7aM7 Multiple Targets (Feet) D1,D2,D3,Q1,Q2,Q3
with Quality Numbers
8aM8 First Target (Meters) D1
No Retries
N/A aD0 Send Data DEPENDENT UPON
COMMAND SENT
10 aV Start Verification calculates NUMERIC SIGNATURE
ROM Signature
aI Send Identification 012CAMPBELLSR50 1.1
NOTE: There are 2 spaces between SR50 and 1.1
0-- aC Concurrent Measurement* D1
n-- aCnConcurrent Measurements* Same as M – M8
Where a = address of SDI-12 device.
Where n= numbers 1 to 8.
* = see APPENDIX A for details about Concurrent Measurements.

SR50 Sonic Ranging Sensor
12
3.3 Pulse Train Output
This option is most commonly used with 21X or CR10s without SDI-12.
The SR50 sensor can also output a pulse train to represent the distance
to target value. The pulse train option outputs a series of 5 Volt pulses
on the SDI-12/Data Bus with each pulse representing a distance to
target unit. Several options are available in the type of pulse train that
is output. The pulse frequency can be selected as either 1000 Hz or
100 Hz. Selectable units are 2.5 mm/pulse, 1cm/pulse, or 0.1
inch/pulse.
Because the 21X datalogger does not have SDI-12 capabilities, the
SR50’s pulse train output is measured using the datalogger’s P3 Pulse
Count instruction. Configure the pulse channel as “high frequency” with
option 00.
All CR10X, CR510, and CR23X dataloggers have SDI-12 capabilities.
Typically the SR50 is measured with the SDI-12 output format with
CR10 dataloggers. If your CR10 does not have SDI-12 capabilities, the
SR50’s pulse train output is measured using the datalogger’s P3 Pulse
Count instruction. Configure the CR10 pulse channel as “low level ac”
with option 01.
To place the sensor in Pulse Train mode, select the desired option by
setting the internal Address/Option jumpers (see Figure 8) according to
the list in Table 2. Only addresses 0 to 5 are applicable for the Pulse
Train output. The sensor initiates the measurement when the Sensor
Enable Line is pulsed high to +5 Volts. The pulse width should be a
minimum of 5 ms and a maximum of 250 ms in width. When the SR50
detects that the Sensor Enable Line is high, it makes a measurement.
When complete, the SR50 outputs the appropriate pulse train on the
SDI-12 Bus. The SR50 will output the number of pulses based on the
closest target that was detected. If the SR50 is unable to detect any
targets, the sensor will output a single pulse to indicate that no reading
was obtained.
Please note that it is more critical to consider the time it takes to make a
measure and transmit the result with this option. After the SR50 detects
the pulse on the Sensor Enable Line, the measurement begins and can
take up to 3 seconds before the pulse train begins. The time required
to transmit the pulse train varies significantly depending on the pulse
option selected and the distance to target. For example, if a target was
5 meters from the sensor and the 1000 Hz, 1 cm/pulse option was
selected, the pulse train would require 0.5 seconds to transmit. If the
100 Hz, 2.5mm/pulse option were selected for the same distance, the
pulse train would require 20 seconds.
It is also important not to leave the Sensor Enable Line high for too
long. If the sensor completes the measurement, and detects that the
Sensor Enable Line is still high it will immediately initiate another
measurement and output a second pulse train. This may cause a
second pulse train to be counted and added with the first pulse train
creating an error in the measurement recorded.

SR50 Sonic Ranging Sensor
13
3.3.1 Pulse Train Wiring
FIGURE 3. Pulse Train Wiring
The order in which the connections are made is critical.
Always connect ground first, followed by +12V and then
the remaining SDI-12/Data Bus, Sensor Enable Line and
Shield. In disconnecting the sensor the reverse order
should be followed.
CAUTION
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SR50 Sonic Ranging Sensor
14
3.4 Two 21X Program Examples for Pulse Train Output
FIGURE 4. SR50 Wiring for 21X Pulse Example
The programming examples assume that the sensor is mounted 2 meters
above the ground and the desired output is snow depth in meters. In these
examples a 21X micrologger is used to read the SR50 with the pulse train
option. The number of pulses are counted on Pulse channel 1.
3.4.1 Pulse Train Programming Example 1
Example 1 will measure every execution interval.
* Table 1 Program
01: 10.0 Execution Interval (seconds)
The Execution Interval must be long enough to count all
pulses generated by the SR50.
The number of pulses is dependent on the SR50 jumper setting and
distance to target.
NOTE

SR50 Sonic Ranging Sensor
15
The following instructions set the sensor enable line high long enough
for the SR50 to detect it, then set it low.
1: Set Port (P20)
1: 1 Set High
2: 1 Port Number
2: Excitation with Delay (P22)
1: 1 Ex Chan
2: 1 Delay w/Ex (units = 0.01 sec)
3: 0000 Delay After Ex (units = 0.01 sec)
4: 0.0 mV Excitation
3: Set Port (P20)
1: 0 Set Low
2: 1 Port Number
The P3 instruction counts the pulse train being transmitted from the
SR50. Distance-to-surface will automatically be reset to 0.00 after the
distance is calculated and stored for later processing and output.
4: Pulse (P3)
1: 1 Reps
2: 1 Pulse Input Chan
3: 00 Config Option
4: 21 Loc [ Dis2Surf ]
5: -.0025 Mult
6: 0.0 Offset
Measure air temperature and convert to degrees Kelvin. This example
assumes air temperature is being measured with Campbell Scientific’s
Model 107 probe.
5: Temp 107 Probe (P11)
1: 1 Reps
2: 1 In Chan
3: 1 Excite all reps w/Exchan 1
4: 22 Loc [ T_Kelvin ]
5: 1 Mult
6: 273.15 Offset
Compute the air temperature compensation factor and correct the
distance-to-surface for air temperature (see formula 1).
6: Z=F (P30)
1: 273.15 F
2: 23 Z Loc [ 273_15 ]
7: Z=X/Y (P38)
1: 22 X Loc [ T_Kelvin ]
2: 23 Y Loc [ 273_15 ]
3: 24 Z Loc [ ATempComp ]

SR50 Sonic Ranging Sensor
16
8: Z=SQRT(X) (P39)
1: 24 X Loc [ ATempComp ]
2: 24 Z Loc [ ATempComp ]
9: Z=X*Y (P36)
1: 21 X Loc [ Dis2Surf ]
2: 24 Y Loc [ ATempComp ]
3: 21 Z Loc [ Dis2Surf ]
Add distance-to-ground to distance-to-snow (which is negative) to
calculate snow depth.
In the following instruction, Parameter 2 is a constant. You enter the
distance-to-ground at your site.
10: Z=X+F (P34)
1: 21 X Loc [ Dis2Surf ]
2: 2.00 F
3: 10 Z Loc [ SnowDepth ]
An hourly output example, yours may vary.
11: If time is (P92)
1: 0 Minutes into a
2: 60 Minute Interval
3: 10 Set Output Flag High
12: Real Time (P77)
1: 0220 Day,Hour/Minute (prev day at midnight, 2400 at midnight)
13: Sample (P70)
1: 1 Reps
2: 10 Loc [ SnowDepth ]
* Table 2 Program
02: 0.0 Execution Interval (seconds)
* Table 3 Subroutines
End Program
-Input Locations-
10 SnowDepth
21 Dis2Surf
22 T_Kelvin
23 273_15
24 ATempComp

SR50 Sonic Ranging Sensor
17
3.4.2 Pulse Train Programming Example 2
Example 2 will measure every execution interval.
This program will measure the SR50 when Flag 1 is set high, which can
be done manually and/or at a specified interval. All instructions above
those that define the Output Interval must remain together. Note that in
this example the datalogger program executes every 10 seconds, but
the SR50 measures and processes only every 15 minutes. This results
in considerable power savings at remote sites without giving up data.
This also allows the user to trigger a scan at any time.
* Table 1 Program
01: 10.0 Execution Interval (seconds)
The execution interval must be long enough to count all
pulses generated by the SR50. The number of pulses is
dependent on the SR50 address setting and distance to
target.
As configured from the factory (Address 0), it will take 4
seconds to measure 10 meters. Shorter distances will take
less time.
Measure air temperature and convert to Kelvin. This measurement must
represent the temperature of air where the SR50 is operating.
1: Temp 107 Probe (P11)
1: 1 Reps
2: 1 In Chan
3: 1 Excite all reps w/Exchan 1
4: 22 Loc [ T_Kelvin ]
5: 1 Mult
6: 273.15 Offset
Enable the Pulse Count instruction (P3) to count pulses from the SR50
when they are generated. The negative multiplier will generate a
negative distance-to-surface so that when the distance-to-ground is
added distance-to-surface, snow depth is computed.
The Distance-to-surface will be set to 0.00 after measurement.
2: Pulse (P3)
1: 1 Reps
2: 1 Pulse Input Chan
3: 00 Config Option
4: 21 Loc [ Dis2Surf ]
5: -.0025 Mult
6: 0.0 Offset
NOTE

SR50 Sonic Ranging Sensor
18
If Flag 2 is high, compute the air temperature compensation factor and
correct the distance-to-surface measurement for air temperature (see
Formula 1).
3: If Flag/Port (P91)
1: 12 Do if Flag 2 is High
2: 30 Then Do
4: Z=F (P30)
1: 273.15 F
2: 23 Z Loc [ 273_15 ]
5: Z=X/Y (P38)
1: 22 X Loc [ T_Kelvin ]
2: 23 Y Loc [ 273_15 ]
3: 24 Z Loc [ ATempComp ]
6: Z=SQRT(X) (P39)
1: 24 X Loc [ ATempComp ]
2: 24 Z Loc [ ATempComp ]
7: Z=X*Y (P36)
1: 21 X Loc [ Dis2Surf ]
2: 24 Y Loc [ ATempComp ]
3: 21 Z Loc [ Dis2Surf ]
Add distance-to-ground to distance-to-snow (which is negative) to
calculate snow depth.
In the following instruction, Parameter 2 is a constant. You enter the
distance-to-ground at your site.
8: Z=X+F (P34)
1: 21 X Loc [ Dis2Surf ]
2: 2.0 F
3: 25 Z Loc [ Temporary ]
Move snow Depth to another location so that it can be viewed.
9: Z=X (P31)
1: 25 X Loc [ Temporary ]
2: 10 Z Loc [ SnowDepth ]
10: Do (P86)
1: 22 Set Flag 2 Low
11: End (P95)
Measure the sensor every 15 minutes by setting Flag 1 high.