Campbell Scientific CS616, CS625 User Manual

INSTRUCTION MANUAL

CS616 and CS625

Copyright © 2002- 2 014

Campbell Scientific, Inc.

Water Content Reflectometers

Revision: 2/14

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Table of Contents

PDF viewers: These page numbers refer to the printed version of this document. Use the
PDF reader bookmarks tab for links to specific sections.

1. Introduction ................................................................. 1

2. Cautionary Statements ............................................... 1

3. Initial Inspection ......................................................... 1

4. Overview ...................................................................... 1

5. Specifications ............................................................. 2

5.1 Dimensions ........................................................................................... 3

5.2 Weight .................................................................................................. 3

5.3 Electrical Specifications ....................................................................... 3

5.4 Operational Details ............................................................................... 4

6. Installation ................................................................... 4

6.1 Orientation ........................................................................................... 4

6.2 Potential Problems with Improper Insertion ......................................... 4

6.3 Wiring .................................................................................................. 5

6.4 Datalogger Instructions and Programming ........................................... 6

6.4.1 Measuring the CS616 Using CRBasic .......................................... 6

6.4.1.1 CS616 Sample CRBasic Program 1 ................................... 7

6.4.1.2 CS616 Sample CRBasic Program 2 ................................... 9

6.4.2 Measuring CS625 Output Using CR200(X) PeriodAvg

Instruction ................................................................................ 10

6.4.3 Sample Programs for CS625 ....................................................... 11

6.4.3.1 CS625 Sample Program 1 ................................................ 11

6.4.3.2 CS625 Sample Program 2 ................................................ 12

6.4.4 Measuring CS616 Using Edlog ................................................... 13

6.4.4.1 Using Edlog Instruction 138 ............................................. 13

6.4.4.2 Using Edlog Instruction 27 (Period Averaging) ............... 14

6.4.5 Sample Edlog Programs for CS616 ............................................ 16

6.4.5.1 CS616 Sample Edlog Program 1 ...................................... 16

6.4.5.2 CS616 Sample Edlog Program 2 ...................................... 17

6.4.5.3 CS616 Sample Edlog Program 3 ...................................... 18

6.4.5.4 CS616 Sample Edlog Program 4 ...................................... 19

6.4.5.5 CS616 Sample Edlog Program 5 ...................................... 21

7. The Water Content Reflectometer Method for

Measuring Volumetric Water Content .................. 24

7.1 Description of Measurement Method ................................................. 24

7.2 Response Curves ................................................................................ 24

7.3 Calibration Equations ......................................................................... 27

i

Table of Contents

7.4 Operating Range ................................................................................ 29

7.4.1 Soil Electrical Conductivity ....................................................... 29

7.4.2 Soil Organic Matter, Clay Content and Soil Bulk Density ......... 29

7.5 Error Sources in Water Content Reflectometer Measurement .......... 30

7.5.1 Probe-to-Probe Variability Error ................................................ 30

7.5.2 Insertion Error ............................................................................ 30

7.5.3 Signal Attenuation Error ............................................................ 30

7.6 Temperature Dependence and Correction ......................................... 31

8. Water Content Reflectometer User-Calibration ...... 32

8.1 Signal Attenuation in Conductive Soils and Need for Site-Specific

Calibration ..................................................................................... 32

8.2 The User-Derived Calibration Equation ............................................ 33

8.3 Collecting Laboratory Data for Calibration ....................................... 34

8.4 Collecting Field Data for Calibration ................................................ 36

8.5 Calculations ....................................................................................... 38

9. Maintenance .............................................................. 39

10. References ................................................................. 39

Appendix

Discussion of Soil Water Content ......................... A-1

A.

Figures

6-1. CS616 Water Content Reflectometer .................................................. 5

6-2. CS625 Water Content Reflectometer .................................................. 5

7-1. CS616 and CS625 linear and quadratic calibrations derived from

loam soil ......................................................................................... 25

7-2. CS616 and CS625 response for low EC loam with bulk density

1.4 g cm
cm
cm

7-3. Difference in % volumetric water content between linear and

quadratic forms of calibrations ...................................................... 28

7-4. Percent volumetric water content error corrected for by

temperature correction equation ..................................................... 32

-3

, a low EC sandy clay loam with bulk density 1.6 g

-3

, and a high EC sandy clay loam with bulk density 1.6 g

-3

. ............................................................................................... 26

Tables

6-1. CS616/625 wiring code. ......................................................................... 5

6-2. CS625 Sample Programs ...................................................................... 11

6-3. CS616 Sample Edlog Programs ............................................................ 16

7-1. Standard calibration coefficients for linear and quadratic forms. ......... 27

7-2. Calibration coefficients for sandy clay loam with bulk density 1.6 g

7-3. Calibration coefficients for sandy clay loam with bulk density 1.6 g

-3

and electrical conductivity at saturation 0.4 dS m-1 for both

cm

linear and quadratic forms. ............................................................ 28

-3

and electrical conductivity at saturation 0.75 dS m-1 for

cm

both linear and quadratic forms. .................................................... 28

ii

CS616 and CS625 Water Content
Reflectometers

1. Introduction

The CS616 Water Content Reflectometer is an improved version of the CS615
Water Content Reflectometer. The CS625 is a modified CS616 for use with
the Campbell Scientific CR200(X) series dataloggers. The difference between
the CS616 and the CS625 is the output voltage level.

Both water content reflectometers are designed to measure volumetric water
content of soils or other porous media. The water content information is
derived from the probe sensitivity to the dielectric constant of the medium
surrounding the probe rods.

Before installing the CS616 or CS625, please study

• Section 2, Cautionary Statements

• Section 3, Initial Inspection

2. Cautionary Statements

• Although the CS616/CS625 is rugged, it should be handled as precision

scientific instrument.

• External RF sources can affect CS616/CS625 measurements.

Consequently, the CS616/CS625 circuitry should be located away from
significant sources of RF such as ac power lines and motors.

• CS616/CS625 probes enabled simultaneously and within approximately 9

inches of each other can cause erratic measurements. If probes must be
close to each other, configure the enable lines to the datalogger control
ports so that the probes are not enabled simultaneously.

3. Initial Inspection

• Upon receipt of the CS616/CS625, inspect the packaging and contents for

damage. File damage claims with the shipping company.

• The 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 are received.

4. Overview

The CS616 output is a square wave output and can be connected to our CR800,
CR850, CR1000, CR3000, CR5000, CR510, CR10X, and CR23X dataloggers.
A special CS616 datalogger instruction is used to measure the probe output
period which is converted to volumetric water content using calibration
equations. Datalogger instructions for period averaging can also be used.

1

CS616 and CS625 Water Content Reflectometers

The CS625 output is a square wave output and can be connected to Campbell
Scientific CR200(X) series dataloggers. A CRBasic program using Period
Averaging is used to measure the probe output period and convert to
volumetric water content using calibration equations.

The water content reflectometer consists of two stainless steel rods connected
to a printed circuit board. A shielded four-conductor cable is connected to the
circuit board to supply power, enable the probe, and monitor the pulse output.
The circuit board is encapsulated in epoxy.

High-speed electronic components on the circuit board are configured as a
bistable multivibrator. The output of the multivibrator is connected to the
probe rods which act as a wave guide. The travel time of the signal on the
probe rods depends on the dielectric permittivity of the material surrounding
the rods and the dielectric permittivity depends on the water content.
Therefore, the oscillation frequency of the multivibrator is dependent on the
water content of the media being measured. Digital circuitry scales the
multivibrator output to an appropriate frequency for measurement with a
datalogger. The water content reflectometer output is essentially a square
wave. The probe output period ranges from about 14 microseconds with rods
in air to about 42 microseconds with the rods completely immersed in typical
tap water. A calibration equation converts period to volumetric water content.

The CS616/CS625's cable can terminate in:

5. Specifications

Features:

CS616 Compatibility
Dataloggers: CR800 series

CR1000
CR3000
CR5000
CR510
CR10X
CR23X

• Pigtails that connect directly to a Campbell Scientific datalogger

(option –PT).

• Connector that attaches to a prewired enclosure (option –PW). Refer

to www.campbellsci.com/prewired-enclosures for more information.

• High accuracy and high precision

• Fast response time

• Designed for long-term unattended water content monitoring

• Probe rods can be inserted from the surface or buried at any

orientation to the surface

2

CS625 Compatibility
Dataloggers: CR200X series

CR200 series

Probe-to-Probe Variability: ±0.5% VWC in dry soil, ±1.5% VWC in

Resolution: better than 0.1% volumetric water content

Water Content Accuracy: ±2.5% VWC using standard calibration with

Precision: better than 0.1% volumetric water content

5.1 Dimensions

Rods: 300 mm (11.8 in) long, 3.2 mm (0.13 in)

Probe Head: 85 x 63 x 18 mm (3.3 x 2.5 x 0.7 in)

CS616 and CS625 Water Content Reflectometers

typical saturated soil

bulk electrical conductivity ≤0.5 deciSiemen

-1

(dS m-1) and bulk density ≤1.55 g cm

meter

-3

in measurement range 0% VWC to 50% VWC

diameter, 32 mm (1.3 in) spacing

5.2 Weight

Probe (without cable): 280 g (9.9 oz)

Cable: 35 g m

5.3 Electrical Specifications

Output
CS616: ±0.7 volt square wave with frequency

CS625: 0 to 3.3 volt square wave with frequency

Power: 65 mA @ 12 Vdc when enabled, 45 µA

Power Supply Requirements: 5 Vdc minimum, 18 Vdc maximum

Enable Voltage: 4 Vdc minimum, 18 Vdc maximum

Maximum Cable Length: 305 m (1000 ft)

Electromagnetic
Compatibility: The CS616/CS625 is Πcompliant with

–1

(0.38 oz per ft)

dependent on water content

dependent on water content

quiescent

performance criteria available upon request.
RF emissions are below EN55022 limits if the
CS616/CS625 is enabled less than 0.6 ms and
measurements are made at a 1 Hz (1 per
second) or slower frequency. The
CS616/CS625 meets EN61326 requirements
for protection against electrostatic discharge
and surge.

3

CS616 and CS625 Water Content Reflectometers

5.4 Operational Details

The accuracy specification for the volumetric water content measurement
using the CS616/CS625 probes is based on laboratory measurements in a
variety of soils and over the water content range air dry to saturated. The soils
were typically sandy loam and coarser. Silt and clay were present in some of
the soils used to characterize accuracy.

Resolution is the minimum change in the dielectric permittivity that can reliably
be detected by the water content reflectometer. The CS616 or CS625 is typically
used to measure soil volumetric water content.

Precision describes the repeatability of a measurement. It is determined for the
CS616 and CS625 by taking repeated measurements in the same material. The
precision of the CS616/CS625 is better than 0.1 % volumetric water content.

Soil Properties

The water content reflectometer operation can be affected when the signal
applied to the probe rods is attenuated. The probe will provide a well-behaved
response to changing water content, even in attenuating soils or other media,
but the response may be different than described by the standard calibration.
Consequently, a unique calibration is required. Change in probe response can
occur when soil bulk electrical conductivity is greater than 0.5 dS m
major contributor to soil electrical conductivity is the presence of free ions in
solution from dissolution of soil salts. Soil organic matter and some clays can
also attenuate the signal.

-1

. The

6. Installation

6.1 Orientation

6.2 Potential Problems with Improper Insertion

The probe rods can be inserted vertically into the soil surface or buried at any
orientation to the surface. A probe inserted vertically into a soil surface will
give an indication of the water content in the upper 30 cm of soil. The probe
can be installed horizontal to the surface to detect the passing of wetting fronts
or other vertical water fluxes. A probe installed at an angle of 30 degrees with
the surface will give an indication of the water content of the upper 15 cm of
soil.

The method used for probe installation can affect the accuracy of the
measurement. The probe rods should be kept as close to parallel as possible
when installed to maintain the design wave guide geometry. The sensitivity of
this measurement is greater in the regions closest to the rod surface than at
distances away from the surface. Probes inserted in a manner which generates
air voids around the rods will reduce the measurement accuracy. In most soils,
the soil structure will recover from the disturbance during probe insertion.

In some applications, installation can be improved by using the CS650G
insertion guide tool. The CS650G is inserted into the soil and then removed.
This makes proper installation of the water content reflectometer easier in
dense or rocky soils.

4

6.3 Wiring

TABLE 6-1. CS616/625 wiring code.

NOTE

power Red

power Red

CS616 and CS625 Water Content Reflectometers

color function datalogger connection

red +12 V +12 V

green output SE analog channel

orange enable control port

black signal ground G

clear shield (power ground) G

Both the black ground wire and the clear shield wire must be
connected to datalogger ground.

output gnd Black

output Green

enable Orange

drain/power gnd Clear

FIGURE 6-1. CS616 Water Content Reflectometer

gnd Black

output Green

enable Orange

drain/gnd Clear

FIGURE 6-2. CS625 Water Content Reflectometer

5

CS616 and CS625 Water Content Reflectometers

NOTE

6.4 Datalogger Instructions and Programming

This section is for users who write their own programs. A
datalogger program can be generated using Short Cut software.
You do not need to read this section if using Short Cut.

The output of the CS616 is a square wave with amplitude of ±0.7 Vdc and a
frequency that is dependent on the dielectric constant of the material
surrounding the probe rods. The CRBasic instruction CS616() is used by the
CR800, CR850, CR1000, CR3000, and CR5000 dataloggers to measure the
CS616 output period. Edlog Instruction 138 is specifically designed for the
CR510, CR10X, and CR23X to measure the output period of the CS616. The
period value is used in the calibration for water content. The period in air is
approximately 14.7 microseconds, and the period in saturated soil with porosity

0.4 is approximately 31 microseconds. Edlog Instruction 27, Period Average,
can also be used to measure CS616 output period.

The output of the CS625 is a square wave with amplitude of 0 to 3.3 Vdc and a
frequency that is dependent on the dielectric constant of the material
surrounding the probe rods. The CRBasic instruction PeriodAvg() is used by
the CR200(X) series dataloggers to measure the CS625 output period. The
period value is used in the calibration for water content. The period in air is
approximately 14.7 microseconds, and the period in saturated soil with porosity

0.4 is approximately 31 microseconds.

6.4.1 Measuring the CS616 Using CRBasic

The CRBasic instruction CS616() is used by the CR800, CR850, CR1000
CR3000, and CR5000 dataloggers to measure the CS616 output period (in
microseconds). The CS616() instruction is used to enable and measure a
CS616 Water Content Reflectometer.

CS616(Dest, Reps, SEChan, Port, MeasPerPort, Mult, Offset)

Dest: The Dest parameter is the variable or variable array in which to store the
results of the measurement. Dest must be dimensioned to at least the number of
Reps.

Reps: The Reps parameter is the number of measurements that should be made
using this instruction. If Reps is greater than 1, Dest must be an array
dimensioned to the size of Reps.

SEChan: The SEChan parameter is the number of the single-ended channel on
which to make the first measurement. If the Reps parameter is greater than 1,
the additional measurements will be made on sequential channels.

Port: The Port parameter is the control port that will be used to enable the
CS616 sensor.

6

Code Description

Program Example 1

#1,2,3,4_Orange

C7

#1,2,3,4_Blk & Clear

GND

1 Control Port 1
2 Control Port 2
3 Control Port 3
4 Control Port 4
5 Control Port 5
6 Control Port 6
7 Control Port 7
8 Control Port 8

MeasPerPort: The MeasPerPort parameter is the number of control ports to be
used to control the CS616 sensor(s). If Reps is set to 4, MeasPerPort = 4 will
result in the same port being used for all measurements. MeasPerPort = 1 will
result in four sequential ports being used for the measurements. MeasPerPort =
2 will result in one port being used for the first two measurements, and the next
port being used for the next two measurements.

Mult, Offset: The Mult and Offset parameters are each a constant, variable,
array, or expression by which to scale the results of the measurement.

6.4.1.1 CS616 Sample CRBasic Program 1

The following CR1000 program uses the CS616() instruction to measure eight
CS616 probes connected to the CR1000 datalogger. Although this example is
for the CR1000, other CRBasic dataloggers are programmed similarly.

CS616 and CS625 Water Content Reflectometers

Wiring for CRBasic

CS616 CR1000

CS616#1_Green 5H

CS616#2_Green 5L

CS616#3_Green 6H

CS616#4_Green 6L

#1,2,3,4_Red 12 V

CS616#5_Green 7H

CS616#6_Green 7L

CS616#7_Green 8H

CS616#8_Green 8L

#5,6,7,8_Orange C8

#5,6,7,8_Blk & Clear GND

#5,6,7,8_Red 12 V

Note: All CS616 “12V_Red”
wires connected to CR1000 12 V
terminal (user supplied common
tie post may be required).

7

CS616 and CS625 Water Content Reflectometers

CR1000 Program Example 1

'Declare Public and Dim Variables
Public batt_volt
Public Panel_temp
Public Period (8)
Public VWC (8)
Public Flag (1)
Dim I

'Declare Constants
'CS616 Default Calibration Constants
const a0= -0.0663
const a1= -0.0063
const a2= 0.0007
'Flag logic constants
const high = true
const low = false

'Define Data Tables
DataTable (Dat30min,1,-1)
DataInterval (0,30,Min,10)
Minimum (1,batt_volt,IEEE4,0,False)
Average (1,Panel_temp,IEEE4,0)
Sample (8,Period(),FP2)
Sample (8,VWC(),FP2)
EndTable

'Main Program
BeginProg
Scan (5,Sec,0,0) 'scan instructions every 5 sec
Battery (Batt_volt)
PanelTemp (Panel_temp,250)
'
'Set flag 1 High every 30 min (Note: User can manually set flag 1 high/low)
If IfTime (0,30,min) Then flag (1) = high '+++++++++++++++++++++++++++
If Flag (1) = high Then
'measure 8ea CS616 probes on CR1000
CS616 (Period(1),4,9,7,4,1.0,0) 'measure 4ea CS616 probes, enable w/ C7
CS616 (Period(5),4,13,8,4,1.0,0) 'measure 4ea CS616 probes, enable w/ C8
'
For I=1 to 8 'convert CS616 period to Volumetric Water Content
VWC(I)=a0 + al*Period(I) + a2*Period(I)^2
Next
'
flag(1)= low 'set Flag 1 = Low
'
EndIf '+++++++++++++++++++++++++++++++
'
CallTable Dat30min 'Call Output Tables
NextScan
EndProg

8

6.4.1.2 CS616 Sample CRBasic Program 2

The following CR1000 program uses the AM16/32-series multiplexer to
measure 48 CS616 probes connected in the 4x16 configuration. The program
also measures datalogger battery voltage and temperature.

Wiring for CRBasic Program Example 2

CR1000 AM16/32-series (4x16) CS616*

CS616 and CS625 Water Content Reflectometers

'Declare Public and Dim Variables
Public batt_volt
Public Panel_temp
Public Period (48)
Public VWC (48)
Public Flag (1)
Dim I

'Declare Constants
'CS616 Default Calibration Constants
const a0= -0.0663
const a1= -0.0063
const a2= 0.0007
'Flag logic constants
const high = true
const low = false

'Define Data Tables
DataTable (Dat30min,1,-1)
DataInterval (0,30,Min,10)
Minimum (1,batt_volt,FP2,0,False)
Average (1,Panel_temp,FP2,0)
Sample (48,Period(),FP2)
Sample (48,VWC(),FP2)
EndTable

Control/Common

Sensor

Terminals

C4 RES Odd H CS616#1_Green

C5 CLK Odd L CS616#2_Green

12 V 12 V Gnd #1,2,3_Blk & Clear

Gnd Gnd Even H CS616#3_Green

1H COM Odd H Even L #1,2,3_Orange

1L COM Odd L

Gnd Gnd

2H COM Even H

C6 COM Even L

*Three sensors to each set of AM16/32 terminals.

CR1000 Program Example 2

9

CS616 and CS625 Water Content Reflectometers

'Main Program
BeginProg
Scan (5,Sec,0,0) 'scan instructions every 5 sec
Battery (Batt_volt)
PanelTemp (Panel_temp,250)
'
'Set flag 1 High every 30 min (Note: User can manually set flag 1 high/low)
If IfTime (0,30,min) Then flag (1) = high '+++++++++++++++++++++++++++
If Flag (1) = high Then
'measure 48ea CS616 probes on AM16/32 in (4x16) mode
PortSet (4,1) 'Set Mux Reset line High
'
I=1 'Set sub scan loop counter
SubScan (0,mSec,16)
PulsePort (5,10000) 'Clock Mux
CS616 (Period(I),3,1,6,3,1.0,0) 'Measure 3ea CS616 probes
I=I+3
NextSubScan
'
For I=1 to 48 'convert CS616 period to Volumetric Water Content
VWC(I)=a0 + al*Period(I) + a2*Period(I)^2
Next
'
PortSet (4,0) 'Set Mux Reset line Low
flag (1) = low
'
EndIf '+++++++++++++++++++++++++++++++
'
CallTable Dat30min 'Call Output Tables
NextScan
EndProg

6.4.2 Measuring CS625 Output Using CR200(X) PeriodAvg Instruction

The PeriodAvg() instruction is used to measure the period (in microseconds)
or the frequency (in kHz) of a signal on a single-ended channel. This
instruction can be used to measure the CS625 Water Content Reflectometer.

PeriodAvg(Dest, SEChan, Option, Cycles, Timeout, Port, Mult, Offset)

Dest: The Dest parameter is a variable in which to store the results of the
measurement.

SEChan: The SEChan argument is the number of the single-ended channel on
which to make the measurement. Valid options are analog channels 1 through

4. The green wire is connected to this channel number.

Option: The Option parameter specifies whether to output the frequency or the
period of the signal.

Code Description

0 Period of the signal is returned (msec)
1 Frequency of the signal is returned (Hz)

Code 0 is typically used with the CS625 with a multiplier (see below) of 1.

Cycles: The Cycles parameter specifies the number of cycles to average each
scan.

10

CS616 and CS625 Water Content Reflectometers

TABLE 6-2. CS625 Sample Programs

Timeout: The Timeout parameter is the maximum time duration, in
milliseconds, that the datalogger will wait for the number of Cycles to be
measured for the average calculation. An overrange value will be stored if the
Timeout period is exceeded. A value of 1 is recommended if 10 is used for
cycles parameter.

Port: The Port parameter is the control port or analog channel that will be used
to switch power to the CS625 Water Content Reflectometer. Valid options are:

Code Description

0 None
C1 Control Port 1
C2 Control Port 2
3 Analog Channel 3
4 Analog Channel 4
5 Analog Channel 5
P_SW Analog Channel 6/P_SW

Mult, Offset: The Mult and Offset parameters are each a constant, variable,
array, or expression by which to scale the results of the raw measurement. A
multiplier value of 1 is recommended.

6.4.3 Sample Programs for CS625

Sample
Program
Number

1 Measure volumetric water content with 4 CS625 hourly. Store

2 Measure temperature with 109 probe and use the 109

6.4.3.1 CS625 Sample Program 1

Hourly measure temperature with 109 probe and volumetric water content with
4 CS625. Store average hourly readings to final storage every 4 hours.

CS625 leads CR200(X)

greens Single-Ended Channel 1 thru 4 (SE1-4)

Program Description

average hourly readings to final storage every 4 hours.

temperature to correct the period for 1 CS625. Use standard
calibration equation to convert temperature-corrected period to
volumetric water content. Sensors are read hourly and average
water content and temperature are written to storage every 4
hours.

blacks associated grounds for SE1-4

oranges Control Port 1 (C1)

reds SW Battery

clears G

11

CS616 and CS625 Water Content Reflectometers

'CR200(X) program to read 4 CS625s
'Standard calibration is used to convert CS625 output
'period to volumetric water content.
'Sensors are read hourly and average water content are written to storage every 4 hours.

'Declare Variables
Public period(4),vwc(4)
Dim i

'Declare Constants
Const a0=-0.0663
Const a1=-0.0063
Const a2=0.0007

'Define Data Tables
DataTable (ofile,1,10)
DataInterval (0,4,hr)
Average (4,vwc,0)
EndTable

'Main Program

BeginProg
Scan (1,hr)
SWBatt (1)
PeriodAvg (period(1),1,0,10,10,C1,1,0)
PeriodAvg (period(2),2,0,10,10,C1,1,0)
PeriodAvg (period(3),3,0,10,10,C1,1,0)
PeriodAvg (period(4),4,0,10,10,C1,1,0)
For i=1 To 4
vwc(i) = a0 + a1*period(i) + a2*period(i)^2
Next i

CallTable ofile
NextScan
EndProg

6.4.3.2 CS625 Sample Program 2

Measure temperature with 109 probe and use the 109 temperature to correct the
period for 1 CS625. Use standard calibration equation to convert temperature­corrected period to volumetric water content. Sensors are read hourly and
average water content and temperature are written to storage every 4 hours.

CS625 leads CR200(X)

green Single-Ended Channel 1 (SE1)

black ground for SE1

orange Control Port 1 (C1)

red SW Battery

clear G

109 leads CR200(X)

black Switched excitation channel 1 (EX1)

red Single-Ended Channel 5 (SE5)

purple G

clear G

12

CS616 and CS625 Water Content Reflectometers

'CR200(X) program to read 1 109 temperature probe and 1 CS625.
'Use temperature to correct CS625 period.
'Standard calibration is used to convert CS625 output
'period to volumetric water content.
'Sensors are read hourly and average water content and
'temperature are written to storage every 4 hours.

'Declare Variables
Public Tsoil
Public uncorrected,corrected
Public vwc

'Declare Constants
'Water content calibration constants
Const a0=-0.0663
Const a1=-0.0063
Const a2=0.0007
'Temperature correction constants
Const t0=0.526
Const t1=-0.052
Const t2=0.00136
'Reference temperature
Const Tref=20

'Define Data Tables
DataTable (ofile,1,10)
DataInterval (0,4,hr)
Average(1,Tsoil,0)
Average (1,vwc,0)
EndTable

'Main Program
BeginProg
Scan (1,hr)
Therm109 (Tsoil,1,5,Ex1,1.0,0)
SWBatt (1)
PeriodAvg (uncorrected,1,0,10,10,C1,1,0)
SWBatt (0)
corrected=uncorrected+(Tref-Tsoil)*(t0+t1*uncorrected+t2*uncorrected^2)
vwc = a0 + a1*corrected + a2*corrected^2
CallTable ofile
NextScan
EndProg

6.4.4 Measuring CS616 Using Edlog

6.4.4.1 Using Edlog Instruction 138

See Section 6.4.5.1, CS616 Sample Edlog Program 1, for example on using
Instruction 138.

1: CS616 Water Content Reflectometer (P138)
1: 1 Reps
2: 00 SE Channel
3: 00 Control Port Code
4: 0000 Loc [ _________ ]
5: 1.0 Mult
6: 0.0 Offset

Reps: Enter the number of CS616s that will be measured with the instruction.
The sensors must be wired in consecutive channels. Each measurement uses
the same multiplier and offset. The option chosen in parameter 3 will
determine whether subsequent repetitions are enabled with the next higher
control port or with the same control port.

13

CS616 and CS625 Water Content Reflectometers

SE Chan: Enter the single-ended analog channel where the sensor’s green wire
is connected. When Reps is greater than 1, this entry is the channel for the first
CS616.

Port: Enter an integer to specify the control port that will be used to enable the
CS616 sensor. For the CR510, only C1 can be used to enable a CS616.

Code Control Port Option

X X specifies the first control port that will be used. Subsequent

repetitions will be enabled with the next sequential control port.
Control port 1 follows control port 8 in a sequence.

1X All repetitions will be enabled with a single specified control port.

Simultaneously enabling several CS616s can result in exceeding
power supply capacity.

Loc: Enter the input location that will store the period measurement. The
period output is in microseconds.

An Input Location is a place in the datalogger's memory where a measurement
is temporarily stored until it is used in intermediate storage, output, or
overwritten. An input location is reserved for each measurement in the
datalogger program. Additional locations can be created by the user.

Mult: A factor that the input location value is multiplied by. Enter a 1 for
probe output period in microseconds.

A multiplier is often used for calibration or to convert the input location value
to different units.

Offset: A constant that is added to the input location value. An offset of 0 has
no effect on the input location value.

An offset is often used for calibration or to convert the data to different units.

With a multiplier of 1 and an offset of 0, P138 returns the period in
microseconds.

6.4.4.2 Using Edlog Instruction 27 (Period Averaging)

See Section 6.4.5.2, CS616 Sample Edlog Program 2, for example on using
Instruction 27.

1: Period Average (SE) (P27)
1: 1 Reps
2: 00 Range Option
3: 00 SE Channel
4: 0000 No. of Cycles
5: 0000 Timeout (units = 0.01 seconds)
6: 0000 Loc [ _________ ]
7: 1.0 Mult
8: 0.0 Offset

14

CS616 and CS625 Water Content Reflectometers

Reps: Enter the number of CS616s that will be measured with the instruction.
The sensors must be wired in consecutive channels. Each measurement uses
the same multiplier and offset.

Range Option: Enter 2-digit integer to choose output as period in
microseconds and range. Suggested value for CS616 is 04.

Code Max. Freq.
x1 8 kHz @ 2 mV peak-to-peak
x2 20 kHz @ 3 mV peak-to-peak
x3 50 kHz @ 12 mV peak-to-peak
x4 200 kHz @ 2 V peak-to-peak

Where: x = 0 Output period in microseconds
x = 1 Output frequency in kHz

SE Chan: Enter the single-ended analog channel where the sensor green wire
is connected. When Reps are greater than 1, this entry is the channel for the
first CS616.

No. of Cycles: Enter the number of cycles of the input signal the instruction
uses to determine period. A value of 100 is recommended.

Timeout: Enter the maximum amount of time for the no. of cycles to occur. A
value of 1 (10 millisecond) is recommended.

Loc: Enter the input location that will store the period measurement. The
period output is in microseconds.

An Input Location is a place in the datalogger's memory where a measurement
is temporarily stored until it is used in intermediate storage, output, or
overwritten. An input location is reserved for each measurement in the
datalogger program. Additional locations can be created by the user.

Mult: A factor that the input location value is multiplied by.

A multiplier is often used for calibration or to convert the input location value
to different units.

Offset: A constant that is added to the input location value. An offset of 0 has
no effect on the input location value.

An offset is often used for calibration or to convert the data to different units.

With a multiplier of 1 and an offset of 0, P27 returns the period in
microseconds or frequency in kHz as selected by parameter 2.

15

CS616 and CS625 Water Content Reflectometers

TABLE 6-3. CS616 Sample Edlog Programs

6.4.5 Sample Edlog Programs for CS616

Sample
Program
Number

Program Description

1 Set Flag 1 to read output of one CS616 using CR10X instruction

138 and convert output period to volumetric water content

2 Every 5 minutes, measure CS616 output period using instruction

27 and convert to volumetric water content. Write hourly
average to datalogger final storage.

3 Read datalogger battery voltage and 3 CS616s hourly. Convert

CS616 period to volumetric water content. Write average of
hourly readings to final storage every 4 hours.

4 Measure soil temperature and CS616 output period every 4

hours. Correct CS616 output period for temperature and write
result to datalogger final storage.

5 Hourly, measure datalogger battery voltage, internal temperature

and 48 CS616 probes using AM16/32 multiplexer.

6.4.5.1 CS616 Sample Edlog Program 1

Set Flag 1 to read output of one CS616 using CR10X instruction 138 and
convert output period to volumetric water content

;{CR10X}
;
;Set Flag 1 high for single water content reading.
;Result stored in input storage only
;
*Table 1 Program
01: 1 Execution Interval (seconds)

1: If Flag/Port (P91)
1: 11 Do if Flag 1 is High
2: 30 Then Do

2: CS616 Water Content Reflectometer (P138)
1: 1 Reps
2: 1 SE Channel
3: 1 C1 is first of sequential Control Ports used
4: 1 Loc [ period ]
5: 1.0 Mult
6: 0.0 Offset

CS616 CR10X

green Single-Ended Channel 1 (SE1)

orange Control Port 1 (C1)

The red lead is connected to 12 Vdc and the black and shield are connected to
ground.

16

CS616 and CS625 Water Content Reflectometers

3: Polynomial (P55)
1: 1 Reps
2: 1 X Loc [ period ]
3: 2 F(X) Loc [ VWC ]
4: -0.0663 C0
5: -0.0063 C1
6: 0.0007 C2
7: 0.0 C3
8: 0.0 C4
9: 0.0 C5

4: Do (P86)
1: 21 Set Flag 1 Low

5: End (P95)

*Table 2 Program
02: 0.0000 Execution Interval (seconds)

*Table 3 Subroutines

End Program

-Input Locations­1 period 1 0 1
2 VWC 1 0 1

6.4.5.2 CS616 Sample Edlog Program 2

;{CR10X}
;
*Table 1 Program
01: 300 Execution Interval (seconds)

1: Do (P86) ;Enable/Turn On the CS616 probe.
1: 41 Set Port 1 High

2: Period Average (SE) (P27)
1: 1 Reps
2: 4 200 kHz Max Freq @ 2 V Peak to Peak, Period Output
3: 1 SE Channel
4: 100 No. of Cycles
5: 1 Timeout (0.01 sec units)
6: 1 Loc [ period ]
7: 1.0 Mult
8: 0.0 Offset

Every 5 minutes, measure CS616 output period using instruction 27 and
convert to volumetric water content. Write hourly average to datalogger final
storage.

CS616 CR10X

green Single-Ended Channel 1 (SE1)

orange Control Port 1 (C1)

The red lead is connected to 12 Vdc and the black and shield are connected to
ground.

17

CS616 and CS625 Water Content Reflectometers

3: Polynomial (P55)
1: 1 Reps
2: 1 X Loc [ period ]
3: 2 F(X) Loc [ vwc ]
4: -0.0663 C0
5: -0.0063 C1
6: 0.0007 C2
7: 0.0 C3
8: 0.0 C4
9: 0.0 C5

4: Do (P86) ;Turn Off CS616 probe
1: 51 Set Port 1 Low

5: If time is (P92) ;Turn On Data Storage every hour
1: 0 Minutes (Seconds --) into a
2: 60 Interval (same units as above)
3: 10 Set Output Flag High (Flag 0)

6: Real Time (P77) ;Store a time stamp.
1: 1220 Year,Day,Hour/Minute (midnight = 2400)

7: Average (P71) ;Store the average VWC.
1: 1 Reps
2: 2 Loc [ vwc ]
*Table 2 Program
02: 0.0000 Execution Interval (seconds)

*Table 3 Subroutines

End Program

-Input Locations­1 period 1 1 1
2 vwc 1 1 1

6.4.5.3 CS616 Sample Edlog Program 3

Read datalogger battery voltage and 3 CS616s hourly. CS616 enable wires are
connected to sequential datalogger control ports for automatic incrementing.
Convert CS616 period to volumetric water content. Write average of hourly
readings to final storage every 4 hours.

CS616x CR10X

green Single-Ended Channel x (SEx)

orange Control Port y (Cy)

x = 1,2,3 for 3 CS616 outputs

y = 6,7,8 for 3 CS616s enables

The red leads are connected to 12Vdc and the blacks and shields are connected
to ground.

18

CS616 and CS625 Water Content Reflectometers

;{CR10X}
;
*Table 1 Program
01: 3600 Execution Interval (seconds)

1: Batt Voltage (P10)
1: 7 Loc [ batt ]

2: CS616 Water Content Reflectometer (P138)
1: 3 Reps
2: 1 SE Channel
3: 6 C6 is first of sequential Control Ports used
4: 1 Loc [ period_1 ]
5: 1.0 Mult
6: 0.0 Offset

3: Polynomial (P55)
1: 3 Reps
2: 1 X Loc [ period_1 ]
3: 4 F(X) Loc [ vwc_1 ]
4: -0.0663 C0
5: -0.0063 C1
6: 0.0007 C2
7: 0.0 C3
8: 0.0 C4
9: 0.0 C5

;
4: If time is (P92)
1: 0000 Minutes (Seconds --) into a
2: 240 Interval (same units as above)
3: 10 Set Output Flag High (Flag 0)

5: Average (P71)
1: 4 Reps
2: 4 Loc [ vwc_1 ]

*Table 2 Program
02: 0.0000 Execution Interval (seconds)

*Table 3 Subroutines

End Program

-Input Locations­1 period_1 5 1 1
2 period_2 9 1 1
3 period_3 17 1 1
4 vwc_1 5 1 1
5 vwc_2 9 1 1
6 vwc_3 17 1 1
7 batt 1 0 1

6.4.5.4 CS616 Sample Edlog Program 4

Measure soil temperature with 107 probe and CS616 output period every four
hours. Correct CS616 output period for temperature and write result to
datalogger final storage.

19

CS616 and CS625 Water Content Reflectometers

Sensor lead CR10X

CS616 green Single-Ended Channel 2 (SE2)

CS616 orange Control Port 4 (C4)

107 red Single-Ended Channel 1 (SE1)

107 black Excitation 1 (E1)

107 purple Analog Ground (AG)

107 clear Ground (G)

The CS616 red leads are connected to 12 Vdc and the blacks and shields are
connected to ground.

;{CR10X}
;
*Table 1 Program
01: 60 Execution Interval (seconds)

;set reference temperature for temperature correction
1: Z=F x 10^n (P30)
1: 20 F
2: 0 n, Exponent of 10
3: 2 Z Loc [ Tref ]

2: If time is (P92)
1: 0000 Minutes (Seconds --) into a
2: 240 Interval (same units as above)
3: 30 Then Do

3: Temp (107) (P11) ;Measure soil temperature.
1: 1 Reps
2: 1 SE Channel
3: 1 Excite all reps w/E1
4: 1 Loc [ TSoil ]
5: 1.0 Mult
6: 0.0 Offset

4: CS616 Water Content Reflectometer (P138)
1: 1 Reps
2: 2 SE Channel
3: 4 C4 is first of sequential Control Ports used
4: 3 Loc [ CS616 ]
5: 1.0 Mult
6: 0.0 Offset

;The following four instructions correct CS616 period for soil temperature
;The corrected period is stored as variable NewCS616.

5: Polynomial (P55)
1: 1 Reps
2: 3 X Loc [ CS616 ]
3: 4 F(X) Loc [ TempCS616 ]
4: 0.526 C0
5: -0.052 C1
6: 0.00136 C2
7: 0.0 C3
8: 0.0 C4
9: 0.0 C5

6: Z=X-Y (P35)
1: 2 X Loc [ Tref ]
2: 1 Y Loc [ TSoil ]
3: 5 Z Loc [ TFactor ]

20

CS616 and CS625 Water Content Reflectometers

7: Z=X*Y (P36)
1: 5 X Loc [ TFactor ]
2: 4 Y Loc [ TempCS616 ]
3: 4 Z Loc [ TempCS616 ]

8: Z=X+Y (P33)
1: 3 X Loc [ CS616 ]
2: 4 Y Loc [ TempCS616 ]
3: 6 Z Loc [ NewCS616 ]

9: Polynomial (P55) ;Convert corrected probe period to water
content.
1: 1 Reps
2: 6 X Loc [ NewCS616 ]
3: 7 F(X) Loc [ WaterCont ]
4: -0.0663 C0
5: -0.0063 C1
6: 0.0007 C2
7: 0.0 C3
8: 0.0 C4
9: 0.0 C5

10: Do (P86)
1: 10 Set Output Flag High (Flag 0)

11: Real Time (P77) ;Record time of measurement.
1: 0220 Day,Hour/Minute (midnight = 2400)

12: Sample (P70) ;Write water content to datalogger final
storage.
1: 1 Reps
2: 7 Loc [ WaterCont ]

13: End (P95)

*Table 2 Program
02: 0.0000 Execution Interval (seconds)

*Table 3 Subroutines

End Program

-Input Locations­1 TSoil 1 1 1
2 Tref 1 1 1
3 CS616 1 2 1
4 TempCS616 1 2 2
5 TFactor 1 1 1
6 NewCS616 1 1 1
7 WaterCont 1 1 1

6.4.5.5 CS616 Sample Edlog Program 5

Hourly, measure datalogger battery voltage, internal temperature and 48 CS616
probes. The AM16/32-series multiplexer slide switch on the top panel is set to
4X16. Can execute measurement sequence manually by setting Flag 1 high.

21

CS616 and CS625 Water Content Reflectometers

Sensor lead AM16/32 CR10X

CS616-1 green 1H

CS616-2 green 1L

CS616-3 green 2H

CS616-1,2,3 orange 2L

CS616-4 green 3H

CS616-5 green 3L

CS616-6 green 4H

CS616-4,5,6 orange 4L

... …

CS616-46 green 31H

CS616-47 green 31L

CS616-48 green 32H

CS616-46,47,48 32L

RES C1

CLK C2

COM ODD H SE1

COM ODD L SE2

COM EVEN H SE3

COM EVEN L C3

;{CR10X}
;
*Table 1 Program
01: 60 Execution Interval (seconds)

1: Batt Voltage (P10) ;Measure battery voltage
1: 1 Loc [ Bat_Volt ]

2: Internal Temperature (P17) ;Measure datalogger internal temperature
1: 2 Loc [ DL_Temp ]

3: If time is (P92) ;Every 60 Minutes Start Multiplexer Measurement Loop
1: 0 Minutes (Seconds --) into a
2: 60 Interval (same units as above)
3: 11 Set Flag 1 High

4: If Flag/Port (P91) ;User can start Measurement Loop by setting Flag 1 High
1: 11 Do if Flag 1 is High
2: 30 Then Do

5: Do (P86) ;Set control port 1 high (i.e. Reset Multiplexer)
1: 41 Set Port 1 High

6: Beginning of Loop (P87) ;16 loops X 3ea CS616 measured/loop
1: 0 Delay
2: 16 Loop Count

22

CS616 and CS625 Water Content Reflectometers

7: Step Loop Index (P90) ;Increment input locations by 3/loop pass
1: 3 Step

8: Do (P86)
1: 72 Pulse Port 2 ;Pulse control port 2 (i.e. Multiplier Clock)

9: CS616 Water Content Reflectometer (P138) ;Measure Period on 3ea CS616
1: 3 Reps
2: 1 SE Channel
3: 13 All reps use C3
4: 3 -- Loc [ Period_1 ]
5: 1.0 Mult
6: 0.0 Offset
;Note: Input "Loc" must be indexed "--" in a loop! How? Use the "F4" Key.

10: End (P95) ;end of loop

11: Do (P86) ;Set user flag 1 low
1: 21 Set Flag 1 Low

12: End (P95) ;Do Loop End instruction

13: Do (P86) ;Set control port 1 "low" (i.e. Multiplexer Reset)
1: 51 Set Port 1 Low

14: Polynomial (P55) ;Convert 48ea Period measurements into water content
1: 48 Reps
2: 3 X Loc [ Period_1 ]
3: 52 F(X) Loc [ VWC_1 ]
4: -0.0663 C0
5: -0.0063 C1
6: 0.0007 C2
7: 0.0 C3
8: 0.0 C4
9: 0.0 C5

15: If time is (P92) ;Store hourly averages every 4 hours
1: 0 Minutes (Seconds --) into a
2: 240 Interval (same units as above)
3: 10 Set Output Flag High (Flag 0)

16: Real Time (P77) ;Data Storage time stamp
1: 1220 Year,Day,Hour/Minute (midnight = 2400)

17: Average (P71) ;Data Storage Average, Location 1 & 2
1: 2 Reps
2: 1 Loc [ Bat_Volt ]
18: Average (P71) ;Data Storage Average, Location 52,53,..99
1: 48 Reps
2: 52 Loc [ VWC_1 ]

*Table 2 Program
02: 0.0000 Execution Interval (seconds)

*Table 3 Subroutines

End Program

-Input Locations­1 Bat_Volt 1 1 1
2 DL_Temp 1 1 1
3 Period_1 7 1 1
4 Period_2 11 1 1
.
.
50 Period_48 19 1 0
51 _________ 0 0 0
52 VWC_1 5 1 1

23

CS616 and CS625 Water Content Reflectometers

53 VWC_2 9 1 1
.
.
99 VWC_48 17 1 1

7. The Water Content Reflectometer Method for Measuring Volumetric Water Content

7.1 Description of Measurement Method

The water content reflectometer method for measuring soil water content is an
indirect measurement that is sensitive to the dielectric permittivity of the
material surrounding the probe rods. Since water is the only soil constituent
that (1) has a high value for dielectric permittivity and (2) is the only
component other than air that changes in concentration, a device sensitive to
dielectric permittivity can be used to estimate volumetric water content

The fundamental principle for CS616/CS625 operation is that an
electromagnetic pulse will propagate along the probe rods at a velocity that is
dependent on the dielectric permittivity of the material surrounding the line.
As water content increases, the propagation velocity decreases because
polarization of water molecules takes time. The travel time of the applied
signal along 2 times the rod length is essentially measured.

The applied signal travels the length of the probe rods and is reflected from the
rod ends traveling back to the probe head. A part of the circuit detects the
reflection and triggers the next pulse.

The frequency of pulsing with the probe rods in free air is about 70 MHz. This
frequency is scaled down in the water content reflectometer circuit output
stages to a frequency easily measured by a datalogger. The probe output
frequency or period is empirically related to water content using a calibration
equation.

7.2 Response Curves

FIGURE 7-1 shows calibration data collected during laboratory measurements
in a loam soil with bulk density 1.4 g cm
saturation of 0.4 dS m
conductivity of 0.4 dS m
using extraction methods of about 2 dS m

The response is accurately described over the entire water content range by a
quadratic equation. However, in the typical water content range of about 10%
to about 35% volumetric water content, the response can be described with
slightly less accuracy by a linear calibration equation. The manufacturer
supplied quadratic provides accuracy of ± 2.5% volumetric water content for
soil electrical conductivity ≤ 0.5 dS m
measurement range of 0% VWC to 50% VWC.

-3

-1

. For this soil, the saturation bulk electrical

-1

corresponds to laboratory electrical conductivity

and bulk electrical conductivity at

-1

.

-1

and bulk density ≤ 1.55 g cm-3 in a

24

CS616 and CS625 Water Content Reflectometers

16 18 20 22 24 26 28 30 32

0

0.1

0.2

0.3

0.4

calibration data
linear fit
quadratic fit

Output period (microseconds)

Volumetric Water Content (fractional)

FIGURE 7-1. CS616 and CS625 linear and quadratic calibrations

derived from loam soil

25

CS616 and CS625 Water Content Reflectometers

16 18 20 22 24 26 28 30 32 34

0

0.1

0.2

0.3

low EC soil
low EC, compacted soil
high EC, compacted soil

CS616 output peri od (usec)

Volumetric Water Content (fractional)

FIGURE 7-2 compares the CS616/CS625 response in the FIGURE 7-1 loam
soil to a higher density sandy clay loam for two different electrical
conductivities. The bulk density for both sandy clay loam soils is 1.6 g cm
The electrical conductivity at saturation for the sandy clay loam labeled
compacted soil is 0.4 dS m
conductivity at saturation of 0.75 dS m

-1

. The compacted soil, high EC had an electrical

-1

.

-3

.

FIGURE 7-2. CS616 and CS625 response for low EC loam with bulk

-3

, a low EC sandy clay loam with bulk density 1.6 g

density 1.4 g cm

-3

, and a high EC sandy clay loam with bulk density 1.6 g cm-3.

cm

The compacted soil response shows the effect of compaction and high clay
content. The signal attenuation caused by compaction or high clay content
causes an offset in the response as shown by the near-parallel curves at water
contents above 10%. This is the effect of attenuation by the solid phase.

The effect of increased electrical conductivity for the same soil is shown by the
response curve high EC, compacted soil. Higher electrical conductivity causes
a decrease in the slope of the response curve. This is the effect of attenuation

26

by the solution phase.

7.3 Calibration Equations

TABLE 7-1. Standard calibration coefficients for linear and quadratic forms.

TABLE 7-1 lists the calibration coefficients derived in the Campbell Scientific
soils laboratory. Both linear and quadratic forms are presented. The choice of
linear or quadratic forms depends on the expected range of water content and
accuracy requirements. These coefficients should provide accurate volumetric
water content in mineral soils with bulk electrical conductivity less than

0.5 dS m

Linear Quadratic

C0 C1 C0 C1 C2

–0.4677 0.0283 –0.0663 –0.0063 0.0007

The linear equation is

The quadratic equation is

-1

, bulk density less than 1.55 g cm-3, and clay content less than 30%.

CS616 and CS625 Water Content Reflectometers

VWC = -0.4677+0.0283*period .

VWC = -0.0663 - 0.0063*period + 0.0007*period

2

.

Period is in microseconds. The result of both calibration equations is
volumetric water content on a fractional basis. Multiply by 100 to express in
percent volumetric water content.

FIGURE 7-3 shows the difference between the linear and quadratic calibration
forms over the typical range. A CS616/CS625 output period of 16
microseconds is about 1.2% VWC and 32 microseconds is 44.9%. The linear
calibration is within ± 2.7% VWC of the quadratic. The linear calibration
underestimates water content at the wet and dry ends of the range and
overestimates it by up to about 2.6 % VWC at about 20% VWC.

27

CS616 and CS625 Water Content Reflectometers

TABLE 7-2. Calibration coefficients for sandy clay loam with bulk density 1.6 g cm-3 and

TABLE 7-3. Calibration coefficients for sandy clay loam with bulk density 1.6 g cm-3 and

FIGURE 7-3. Difference in % volumetric water content between linear

and quadratic forms of calibrations

The linear and quadratic coefficients for the sandy clay loam data in FIGURE
7-3 follow and can be used in similar soils.

electrical conductivity at saturation 0.4 dS m-1 for both linear and quadratic forms.

Linear Quadratic

C0 C1 C0 C1 C2

–0.6200 0.0329 0.0950 –0.0211 0.0010

electrical conductivity at saturation 0.75 dS m-1 for both linear and quadratic forms.

Linear Quadratic

C0 C1 C0 C1 C2

–0.4470 0.0254 –0.0180 –0.0070 0.0006

28

7.4 Operating Range

σ σ

θ σ

bulk solution

=

+

v solid

Τ

7.4.1 Soil Electrical Conductivity

The quality of soil water measurements which apply electromagnetic fields to
wave guides is affected by soil electrical conductivity. The propagation of
electromagnetic fields in the configuration of the CS616/CS625 is
predominantly affected by changing dielectric constant due to changing water
content, but it is also affected by electrical conductivity. Free ions in soil
solution provide electrical conduction paths which result in attenuation of the
signal applied to the waveguides. This attenuation both reduces the amplitude
of the high-frequency signal on the probe rods and reduces the bandwidth. The
attenuation reduces oscillation frequency at a given water content because it
takes a longer time to reach the oscillator trip threshold.

It is important to distinguish between soil bulk electrical conductivity and soil
solution electrical conductivity. Soil solution electrical conductivity refers to
the conductivity of the solution phase of soil. Soil solution electrical
conductivity, σ

methods to separate the solution from the solid and then measuring the
electrical conductivity of the extracted solution.

The relationship between solution and bulk electrical conductivity can be
described by (Rhoades et al., 1976)

solution

CS616 and CS625 Water Content Reflectometers

can be determined in the laboratory using extraction

with σ

bulk

solution; σ

being the electrical conductivity of the bulk soil; σ

, the solid constituents; θv , the volumetric water content; and Τ,

solid

solution

, the soil

a soil-specific transmission coefficient intended to account for the tortuosity of
the flow path as water content changes. See Rhoades et al., 1989 for a form of
this equation which accounts for mobile and immobile water. This publication
also discusses soil properties related to CS616/CS625 operation such as clay
content and compaction. The above equation is presented here to show the
relationship between soil solution electrical conductivity and soil bulk
electrical conductivity.

Most expressions of soil electrical conductivity are given in terms of solution
conductivity or electrical conductivity from extract since it is constant for a
soil. Bulk electrical conductivity increases with water content so comparison
of the electrical conductivity of different soils must be at same water content.
Discussion of the effects of soil electrical conductivity on CS616/CS625
performance will be on a soil solution or extract basis unless stated otherwise.

-1

When soil solution electrical conductivity values exceed 2 dS m

, the response
of the CS616/CS625 output begins to change. The slope decreases with
increasing electrical conductivity. The probe will still respond to water content
changes with good stability, but the calibration will have to be modified. (See
Section 8, Water Content Reflectometer User-Calibration.) At electrical

-1

conductivity values greater than 5 dS m

the probe output can become unstable.

7.4.2 Soil Organic Matter, Clay Content and Soil Bulk Density

The amount of organic matter and clay in a soil can alter the response of
dielectric-dependent methods to changes in water content. This is apparent
when mechanistic models are used to describe this measurement methodology.

29

CS616 and CS625 Water Content Reflectometers

The electromagnetic energy introduced by the probe acts to re-orientate or
polarize the water molecules. If other forces are acting on the polar water
molecules, the force exerted by the applied signal will be less likely to polarize
the molecules. This has the net effect of ‘hiding’ some of the water from the
probe. Additionally, some clays sorb water interstitially and thus inhibit
polarization by the applied field.

Organic matter and some clays are highly polar. These solid constituents can
affect CS616/CS625 response to water content change and require specific
calibration. This affect is opposite to that of the ‘hiding’ effect. It would be
convenient if the calibration of water content to CS616/CS625 output period
could be adjusted according to some parameter of the soil which reflects the
character of the signal attenuation. However, such a parameter has not been
identified.

The response of the water content reflectometer to changing water content has
been shown to change for some soils when bulk density exceeds 1.5 g cm
The response to changing water content is still well behaved, but the slope will
decrease with increasing bulk density.

7.5 Error Sources in Water Content Reflectometer Measurement

7.5.1 Probe-to-Probe Variability Error

All manufactured CS616s/CS625s are checked in standard media. The limits
for probe response in the standard media ensure accuracy of ± 2% volumetric
water content.

7.5.2 Insertion Error

The method used for probe insertion can affect the accuracy of the
measurement. The probe rods should be kept as close to parallel as possible
when inserted to maintain the design wave guide geometry. The sensitivity of
this measurement is greater in the regions closest to the rod surface than at
distances away from the surface. Probes inserted in a manner that generates air
voids around the rods will indicate lower water content than actual. In some
applications, installation can be improved by using insertion guides or a pilot
tool. Campbell Scientific offers the CS650G insertion tool.

7.5.3 Signal Attenuation Error

-3

.

30

Section 7.1, Description of Measurement Method, presents a detailed
description of CS616/CS625 operation. In summary, the CS616/CS625 is
primarily sensitive to the dielectric permittivity of the material surrounding the
probe rods. The propagation of electromagnetic energy along the probe rods
depends on the dielectric properties of the medium. When the reflection of the
applied signal from the end of the rods is detected by the CS616/CS625 circuit,
another pulse is applied. The time between pulses depends on the propagation
time, and the associated period is empirically related to volumetric water
content.

The applied signal is subject to attenuation from losses in the medium being
measured. While this does not directly affect propagation time, it causes
delays in detection of the reflected signal. Attenuation of the signal will occur

CS616 and CS625 Water Content Reflectometers

τ

uncorrected

T

soil

( )

( )

( )

τ

τ τ τ

corrected soil

uncorrected soil

uncorrected uncorrecte

d

T T

= + −

− +20

0 526 0

052 0 00136

2

* . . * .

*

if there are free ions in soil solution, polar solid constituents such as organic
matter or some clay, or conductive mineral constituents.

The general calibration equation for the CS616/CS625 will provide good
results with attenuation equivalent to about 0.5 dS m

-1

conductivity. Between 0.5 dS m

and 5 dS m-1, the CS616/CS625 will
continue to give a well-behaved response to changes in water content but a soil
specific calibration is required. See Section 8, Water Content Reflectomer
User-Calibration, for calibration information.

7.6 Temperature Dependence and Correction

The error in measured volumetric water content caused by the temperature
dependence of the CS616/CS625 is shown in FIGURE 7-4. The magnitude of
the temperature sensitivity changes with water content. Laboratory
measurements were performed at various water contents and over the
temperature range from 10°C to 40°C to derive a temperature correction for
probe output period. The following equation can be used to correct the
CS616/CS625 output period,

. See sample datalogger programs. The temperature correction assumes

that both the water content and temperature do not vary over the length of the
probes rods.

, to 20°C knowing the soil temperature,

-1

bulk electrical

31

CS616 and CS625 Water Content Reflectometers

10 15 20 25 30 35 40

4

2

0

2

4

6

8

Wa ter Con tent = 30%
Wa ter Con tent = 12%

Water Content Error with Temperature

Soi l Temperature (C)

Water Content Error (%VWC)

8. Water Content Reflectometer User-Calibration

FIGURE 7-4. Percent volumetric water content error corrected for by

temperature correction equation

8.1 Signal Attenuation in Conductive Soils and Need for Site­Specific Calibration

A shift in water content reflectometer response results if the applied signal is
attenuated significantly. There is a voltage potential between the probe rods
when a pulse is applied to them. If the material between the rods is electrically
conductive, a path for current flow exists and the applied signal is attenuated.
Since the parallel rod design in soil is inherently a lossy medium and
attenuation is frequency dependent, both the amplitude of the reflection and the
rise-time or bandwidth are affected. Instead of a relatively short rise-time
return pulse, the rise-time is greater and the amplitude is less.

The reflected signal must exceed a set amplitude before the next pulse is
triggered. Reflections that are attenuated and have longer rise-times will take
longer to be detected and trigger the next pulse leading to decreased frequency
or increased period in conductive materials.

32

CS616 and CS625 Water Content Reflectometers

( )

θ τ τ τ

v

C

C C= + +

0 1 2

2

* *

( )

θ τ τ

v

C C= +

0 1

*

Some clays are very polar and/or conductive and will also attenuate the applied
signal. Additionally, if the clayey soil is compacted, increased bulk density,
the conductivity is increased and the response is affected.

Given the water content reflectometer response to changing water content in
attenuating media changes as described above, the accuracy of the volumetric
water content measurement can be optimized by characterizing the probe
response in the specific medium to be measured. The result is a specific
calibration equation for a particular medium.

The precision and the resolution of the water content reflectometer
measurement are not affected by attenuating media. Both precision and
resolution are better than 0.1% volumetric water content.

8.2 The User-Derived Calibration Equation

The probe output response to changing water content is well described by a
quadratic equation, and, in many applications, a linear calibration gives
required accuracy.

Quadratic form:

with

θ

, the volumetric water content (m3 m

v

(microseconds); and C
coefficients are derived from factory laboratory measurements using curve

fitting of known volumetric water content to probe output period.

Linear form:

θ

with
reflectometer period (microseconds); C

Two data points from careful measurements can be enough to derive a linear
calibration. A minimum of 3 data points is needed for a quadratic. With 3
evenly spaced water contents covering the expected range, the middle water
content data point will indicate whether a linear or quadratic calibration
equation is needed.

Note from FIGURE 7-1 and FIGURE 7-2 that the calibration function
describing the CS616/CS625 response to changing water content is always
concave up. If calibration data suggests a different shape, there may be a
problem with the data or method.

, the volumetric water content (m3 m

v

, the calibration coefficient. The standard calibration

n

-3

); τ, the CS616/CS625 period

-3

); τ, the water content

, the intercept; and C1 , the slope.

o

33

CS616 and CS625 Water Content Reflectometers

( )

θ τ τ τ

v

C C C= + +

0 1 2

2

* *

( )

θ τ τ

v

C C= +

0 1

*

8.3 Collecting Laboratory Data for Calibration

Water content reflectometer data needed for CS616/CS625 calibration are the
CS616/CS625 output period (microseconds) and an independently determined
volumetric water content. From this data, the probe response to changing
water content can be described by a quadratic calibration equation of the form

with

θ

being the volumetric water content (m3 m-3); τ, the CS616 period

v

(microseconds); and C

The linear form is

, the calibration coefficient (n = 0..2).

n

with

θ

, the volumetric water content (m3 m

v

(microseconds); C

, the intercept; and C1 , the slope.

o

-3

); τ, the CS616 period

Required equipment:

1. CS616/CS625 connected to datalogger programmed to measure output
period

2. Cylindrical sampling devices to determine sample volume for bulk density;
for example, copper tubing of diameter ≥ 1 in. and length about 2 in.

3. Containers and scale to measure soil sample weight

4. Oven to dry samples (microwave oven can also be used)

The calibration coefficients are derived from a curve fit of known water
content and probe output period. The number of data sets needed to derive a
calibration depends on whether the linear or quadratic form is being used and
the accuracy requirement. Consider the expected range of soil water content
while viewing FIGURE 7-1 and FIGURE 7-2. If the expected response is
nearly linear, fewer laboratory measurements are needed to derive the
calibration. A linear response is best described by data taken near the driest
and wettest expected water contents.

The measurement sensitive volume around the probe rods must be completely
occupied by the calibration soil. Only soil should be in the region within 4
inches of the rod surface. The probe rods can be buried in a tray of soil that is
dry or nearly dry. The soil will be homogeneous around the probe rods if it is
poured around the rods while dry. Also, a 10 cm diameter PVC pipe with
length about 35 cm can be closed at one end and used as the container.

34

It is important that the bulk density of the soil used for calibration be similar to
the bulk density of the undisturbed soil. Using dry soil without compaction will

-3

give a typical bulk density, 1.1 - 1.4 g cm
bulk density is greater than 1.55 g cm

. This is especially important when

-3

. Compaction of the calibration soil to

similar bulk density may be necessary.

CS616 and CS625 Water Content Reflectometers

The typically used method for packing a container of soil to uniform bulk
density is to roughly separate the soil into three or more equal portions and add
one portion to the container with compaction. Evenly place the first loose soil
layer in the bottom of the container. Compact by tamping the surface to a level
in the container that is correct for the target bulk density. Repeat for the
remaining layers. Prior to placing successive layers, scarify the top of the
existing compacted layer.

The container to hold the soil during calibration should be large enough that
the rods of the probe are no closer than about 4 in. from any container surface.

Pack the container as uniformly as possible in bulk density with relatively dry
soil (volumetric water content <10%).

Probe rods can be buried in a tray or inserted into a column. When using a
column, insert the rods carefully through surface until rods are completely
surrounded by soil. Movement of rods from side-to-side during insertion can
form air voids around rod surface and lead to measurement error.

Collect the probe output period. Repeat previous step and this step 3 or 4
times.

Determine volumetric water content by subsampling soil column after
removing probe or using weight of column. If subsampling is used, remove
soil from column and remix with samples used for water content measurement.
Repack column.

Water can then be added to the top of the container. It must be allowed to
equilibrate. Cover the container during equilibration to prevent evaporation.
The time required for equilibration depends on the amount of water added and
the hydraulic properties of the soil. Equilibration can be verified by frequently
observing the CS616/CS625 period output. When period is constant,
equilibration is achieved. Collect a set of calibration data values and repeat
the water addition procedure again if needed.

With soil at equilibrium, record the CS616/CS625 period value.

Take subsamples of the soil using containers of known volume. This is
necessary for measurement of bulk density. Copper tubing of diameter ≥ 1 in.
and length about 2 in. works well. The tubes can be pressed into the soil
surface.

It is good to take replicate samples. Three carefully handled samples will
provide good results.

The sample tubes should be pushed evenly into the soil. Remove the tube and
sample and gently trim the ends of excess soil. Remove excess soil from
outside of tube.

Remove all the soil from tube to a tray or container of known weight that can
be put in oven or microwave. Weigh and record the wet soil weight.

Water is removed from the sample by heating with oven or microwave. Oven
drying requires 24 hours at 105°C. Microwave drying typically takes 20
minutes depending on microwave power and sample water content. ASTM

35

CS616 and CS625 Water Content Reflectometers

θ

g

wet dry

dry

m m

m

=

−

ρ

bulk

dry

cylinder

m

volume

=

θ θ ρ

v g bulk

= *

( )

θ τ τ τ

v

C C C= + +

0 1 2

2

* *

( )

θ τ τ

v

C C= +

0 1

*

Method D4643-93 requires heating in microwave for 3 minutes, cooling in
desiccator then weighing and repeating this process until weigh is constant.

Gravimetric water content is calculated after the container weight is accounted
for.

For the bulk density

the dry weigh of the sample is divided by the sample tube volume.

The volumetric water content is the product of the gravimetric water content
and the bulk density

The average water content for the replicates and the recorded CS616/CS625
period are one datum pair to be used for the calibration curve fit.

8.4 Collecting Field Data for Calibration

Required equipment

1. CS616/CS625 connected to datalogger programmed to measure probe
output period

2. Cylindrical sampling devices to determine sample volume for bulk density,
e.g. copper tubing of diameter ≥ 1” and length about 2”

3. Containers and scale to measure soil sample weight

4. Oven to dry samples (microwave oven can also be used)

Data needed for CS616/CS625 calibration are the CS616/CS625 output period
(microseconds) and an independently determined volumetric water content.
From this data, the probe response to changing water content can be described
by a quadratic calibration equation of the form

with

θ

being the volumetric water content (m3 m

v

period (microseconds); and C

, the calibration coefficient (n = 0..2).

n

-3

); τ , the CS616/CS625

36

The linear form is

CS616 and CS625 Water Content Reflectometers

with θ

, the volumetric water content (m3 m

v

(microseconds); C

, the intercept; and C1 , the slope.

o

-3

); τ, the CS616/CS625 period

The calibration coefficients are derived from a curve fit of known water
content and CS616/CS625 period.

The number of data sets needed to derive a calibration depends on whether the
linear or quadratic form is being used and the accuracy requirement. Consider
the expected range of soil water content while viewing FIGURE 7-1 and
FIGURE 7-2. If the expected response is nearly linear, fewer laboratory
measurements are needed to derive the calibration. A linear response is best
described by data taken near the driest and wettest expected water contents.

Collecting measurements of CS616/CS625 period and core samples from the
location where the probe is to be used will provide the best soil-specific
calibration. However, intentionally changing water content in soil profiles can
be difficult.

A vertical face of soil can be formed with a shovel. If the CS616/CS625 is to
be used within about 0.5 meters of the surface, the probe can be inserted into
the face and water added to the surface with percolation. After adding water,
monitor the CS616/CS625 output period to determine if the soil around the
rods is at equilibrium.

With soil at equilibrium, record the CS616/CS625 period value.

Soil hydraulic properties are spatially variable. Obtaining measurements that
are representative of the soil on a large scale requires multiple readings and
sampling. The average of several core samples should be used to calculate
volumetric water content. Likewise, the CS616/CS625 should be inserted at
least 3 times into the soil recording the period values following each insertion
and using the average.

Remove the CS616/CS625 and take core samples of the soil where the probe
rods were inserted. This is necessary for measurement of bulk density. Copper
tubing of diameter ≥ 1” and length about 2” works well. The tubes can be
pressed into the soil surface.

It is good to take replicate samples at locations around the tray surface. Three
carefully handled samples will provide good results.

The sample tubes should be pushed evenly into the soil surface. Remove the
tube and sample and gently trim the ends of excess soil. Remove excess soil
from outside of tube.

Remove all the soil from tube to a tray or container of known weight that can
be put in oven or microwave. Weigh and record the wet soil weight.

Water is removed from the sample by heating with oven or microwave. Oven
drying requires 24 hours at 105 C. Microwave drying typically takes 20
minutes depending on microwave power and sample water content. ASTM
Method D4643-93 requires heating in microwave for 3 minutes, cooling in
desiccator then weighing and repeating this process until weigh is constant.

37

CS616 and CS625 Water Content Reflectometers

θ

g

wet dry

dry

m m

m

=

−

ρ

bulk

dry

cylinder

m

volume

=

θ θ ρ

v g bulk

= *

volume

d

h=











π* *

2

2

θ

g

wet dry

dry

m m

m

=

−

Gravimetric water content is calculated after the container weight is accounted
for.

For the bulk density,

the dry weight of the sample is divided by the sample tube volume.

The volumetric water content is the product of the gravimetric water content
and the bulk density

The average water content for the replicates and the recorded CS616 period are
one datum pair to be used for the calibration curve fit.

8.5 Calculations

The empty cylinders used for core sampling should be clean and both empty
weight and volume are measured and recorded. For a cylinder, the volume is

where d is the inside diameter of the cylinder and h is the height of the
cylinder.

During soil sampling it is important that the cores be completely filled with soil
but not extend beyond the ends of the cylinder.

Once soil core samples are obtained, place the soil-filled cylinder in a small
tray of known empty weight. This tray will hold the core sample during drying
in an oven.

To obtain m
weight from the weight of the soil filled cylinder in the tray. Remove all the

soil from the cylinder and place this soil in the tray. Dry the samples using
oven or microwave methods as described above.

To obtain m
weight for m

, subtract the cylinder empty weight and the container empty

wet

, weigh the tray containing the soil after drying. Subtract tray

dry

. Calculate gravimetric water content, θg, using

dry

38

.

To obtain soil bulk density, use

ρ

bulk

dry

cylinder

m

volume

=

θ θ ρ

v g bulk

= *

Volumetric water content is calculated using

9. Maintenance

The CS616/CS625 does not require periodic maintenance.

10. References

Rhoades, J.D., P.A.C. Raats, and R.J. Prather. 1976. Effects of liquid-phase
electrical conductivity, water content and surface conductivity on bulk soil
electrical conductivity. Soil Sci. Soc. Am. J., 40: 651-653.

Rhoades, J.D., N.A. Manteghi, P.J. Shouse, W.J. Alves. 1989. Soil electrical
conductivity and soil salinity: New formulations and calibrations. Soil Sci.
Soc. Am. J., 53:433-439.

CS616 and CS625 Water Content Reflectometers

.

39

CS616 and CS625 Water Content Reflectometers

40

θ

g

water

soil

wet dry

dry

m

m

m m

m

= =

−

θ

ρ

ρ

θ ρ

ρ

v

water

soil

water

water

soil

soil

g soil

water

volume

volume

m

m

=

= =

*

ρ

bulk

dry

sample

m

volume

=

ε

ρ
ρ

= −1

bulk

solid

Appendix A. Discussion of Soil Water Content

The water content reflectometer measures volumetric water content. Soil water
content is expressed on a gravimetric and a volumetric basis. To obtain the
independently determined volumetric water content, gravimetric water content
must first be measured. Gravimetric water content (θg) is the mass of water
per mass of dry soil. It is measured by weighing a soil sample (m

the sample to remove the water, then weighing the dried soil (m

wet

dry

), drying

).

Volumetric water content (θ
Volume is the ratio of mass to density (ρ

) is the volume of liquid water per volume of soil.

v

) which gives:

b

The density of water is close to 1 and often ignored.

Soil bulk density (ρ

) is used for ρ

bulk

and is the ratio of soil dry mass to

soil

sample volume.

Another useful property, soil porosity (ε), is related to soil bulk density as
shown by the following expression.

The term ρ

2.65 g cm

is the density of the soil solid fraction and is approximately

solid

-3

.

Notes:

A-1

Appendix A. Discussion of Soil Water Content

A-2

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