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       Welcome to the first message of the 

  Soil Water Content Sensor TDR discussion group!



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Run by Bruce Metelerkamp         bruce@icfr.unp.ac.za



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Recently several soil water  content sensors have become 

available which offer alternatives to the 

reflectometer-based TDR systems, neutron probes and capacitance 

probes.  One such product is the CS615 Water Content

Reflectometer marketed by Campbell Scientific Inc. (CSI).

This has prompted the creation of this discussion group.



We will start by reviewing this sensor and the discussion

below will hopefully answer most of the questions I have

been sent. (There has been a wonderful response to this

list and particularly to the hope that this sensor is "the

answer").



I have put together a number of questions in CAPITALS.

Below each question, I have attempted to answer BOTH by quoting

directly from the CS615 manual, and have given my first bash at the

answer/s.

              -------_________________--------

To keep the messages to a manageable size, PLEASE edit the

message and only include the question relevant to the comment you

want to make, and then add your comments in at the appropriate

point. Add new questions at the BOTTOM ONLY.



I will collate all the replies and send these out as soon as

the flow of responses slows to a trickle, so you should get

it back within 36 hours. (This is all run manually by me alone, so

please be patient!) (A related but separate discussion group 

on a new capacitance-based length-measuring sensor, the 

Rubbery Ruler, is also being initated).

             =========______________===========



WHAT IS THIS NEW SENSOR?

CS615MANUAL: The CS615 Water Content Reflectometer provides

a measure of the volumetric water content of porous media using

time-domain measurement methods.The water content information is

derived from the effect of changing dielectric constant on the

propagation velocity of electromagnetic waves along a wave guide.

It consists of a pair of parallel stainless steel

rods 300 mm long held at one end by a resin block which

also contains the electronic circuits.



CS615MANUAL: The CS615 effectively integrates the water content

over the length of the probe rods and a distance from the probe rods

of about 2 cm. This must be considered when choosing the orientation

of the probe during installation.  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.





It can therefore be buried permanently, as part of a

network of sensors linked to a datalogger. Alternatively it can

be pushed into the ground for an instantaneous reading, in order to

sample over many points.



HOW DOES IT WORK?

CS615MANUAL: The Water Content Reflectometer consists of two

stainless steel rods connected to a printed circuit board.  A

five-conductor cable is connected to the circuit board to supply

power, enable the probe, and monitor the pulse output.  The circuit

board is potted in an epoxy block.

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.  When the

multivibrator switches states, the transition travels the length of the

rods and is reflected by the rod ends.  This reflection provides

feedback to switch the state of the multivibrator.  The travel time to

the end of the rods and back is dependent on the dielectric constant

of the material surrounding the rods.  The dielectric constant is

predominantly dependent on the water content.  Digital circuitry

scales the multivibrator output to an appropriate frequency for

measurement with a datalogger. 

The output of the CS615 is essentially a square wave with

an amplitude of (2.5 VDC). The measured frequency is dependent on the dielectric

constant of the material surrounding the probe rods.  The frequency

range is approximately 400 to 1000 Hz.  The period or frequency

value is used in the calibration for water content.

The frequency or period of the square wave is used for the calibration

of water content.





DOES IT MEASURE WATER CONTENT OR SOIL WATER POTENTIAL?

CS615MANUAL:The datalogger period or frequency output can be

converted to volumetric water content using calibration values.



It measures the soil water content - which must be converted

via the soil's water retention curve to soil water

potential if that is required.



CAN IT BE SETUP TO MEASURE SOIL WATER POTENTIAL

DIRECTLY

If the water retention curve is known for the soil, then

yes, OR if the calibration has been done (for the soil) on

a water potential basis.





HOW MUCH DOES ONE COST?

About US$200 - considerably cheaper than conventional TDR

equipment.



WHERE DO I GET THEM?

Campbell Scientific Incorporated or any of their agents:

In the USA  csi@csius.com           

In South Africa jvisagie@iafrica.com



WHO ELSE HAS USED THEM?

Some of the members of this list who will be contributing.

(I AM DOING THEM THE COUTRESY OF ALLOWING THEM

THE CHOICE TO REMAIN ANONYMOUS OR NOT) Please respect this.



DO THEY NEED CALIBRATION?

Apparently, yes, - but what sensor doesn't?

(Perhaps we will deal with calibration separately as it is

such a large topic - but send your ideas now anyway).

Generaly calibrations can be applied for a limited 

range of soil types.  Accuracy will vary depending on the water 

content range and soil type.  Soil-specific calibrations are 

necessary if accuracy must be optimized.





CS615MANUAL:Since the intrinsic dielectric constant of soils in the

dry state may vary due to derivation from different parent materials,

using a single calibration for a range of soils may not provide

optimum accuracy. It has been found that the following calibration

yields an accuracy of (2.5% for a range of typical mineral soils.

Soils with significantly different intrinsic dielectric constants will

show an error which appears as an offset. The calibration can be

optimized by taking measurements at several known water contents of

the soil type to be measured. The last term in the calibration (zero

order term) is then adjusted to compensate.





WHAT IS THE RANGE AND ACCURARACY OF WATER

CONTENT MEASURED?

Seemingly the entire range of water contents, 

CS615MANUAL:

 Accuracy

A calibration of volumetric water content as a function of CS615

output period using third degree polynomial provides an accuracy of

(2.5% when applied to typical mineral soils.

 Resolution

The resolution for volumetric water content depends on which

datalogger instruction is used to measure the output of the probe. 

When the CR10X or CR10 Instruction 27, Period Measurement, is

used, the resolution is on the order of 10-6 m3 m-3 and is not a

factor.

When Instruction 3, Pulse, is used, the resolution with an execution

interval of 1.0 second is 0.01% volumetric water content when pulse

period is 2.5 milliseconds.  The resolution improves--decreases

linearly--both as the water content decreases and as the execution

interval decreases.  An execution interval of 0.1 seconds yields a

resolution of 1% volumetric water content at the same water content.



HOW MANY DO I NEED FOR A PARTICULAR APPLICATION?

That can be answered by asking how many you consider

necessary to get an idea of the soil profile in how ever

many dimensions you are working.



HOW FAR FROM THE DATA LOGGER CAN THE SENSOR BE

PLACED?

The makers claim no problem swith 300m (with their

cabling at US$0.65 per foot).

CS615MANUAL:Probe cable length is not a limitation under typical

applications. Laboratory measurements show no degradation in

measurement quality with cable lengths up to 300 meters. The

performance may be degraded if cable other than that provided with

the probe is used.





CAN I USE NORMAL EXTENSION CABLING, OR SHOULD I

BUY THE UNIT WITH THE REQUIRED LENGTH OF CABLING?

Looking at the cabling, it appears to be normal multi [5?] core cable.

HOWEVER - CSI advises:..

This is a very important point.  The individual conductor 

insulation of multwire cables can significanlty affect sensor 

performance.  In this application, it is easy to see the affect by 

considering how a RC network affects a square wave--just what we 

have in this system.  As the time constant increases (R times C), the 

rate of voltage change is decreased.  If the period is small enough, 

the peak amplitude will be reduced.  This can affect the results 

depending on the configuration of the electronics ie zero-crossing or 

peak detector.  Cabling is always an important consideration.



This means that one needs to specify the cable lengths on

ordering probes.





WHAT TYPE OF LOGGER IS REQUIRED?

CS615MANUAL:The reflectometer output can be connected to

Campbell Scientific dataloggers CR10X, CR10, 21X, or CR7.

The Pulse Count instruction of a CR10,  21X or CR7 dataloggers can

be used with the CS615 output connected to a pulse count channel. 

The Period Measurement instruction of the CR10 can be used with the

CS615 output connected to a single-ended analog channel.

The enable line is set high to put the probe in the measuring mode. 

A 1 second delay is sufficient for probe warm-up.  The output is very

stable and no averaging is necessary.

The Period Measurement instruction is available only on the CR10

and CR10X.  The resolution of this instruction can be much better

than with the Pulse Count instruction though it is not a factor in the

range of frequencies for the CS615.





Any datalogger that is capable of measuring a frequency

signal of approximately  400 TO 1000 Hz. An enabling current of

5V is also required just prior to measuring. The current demand 

of the enable circuitry is very small.  The  enable voltage 

requirement is nominally 5VDC.

The Campbell Scientific Incorporated range of loggers all

are capable, and the CR10 is eminently suited to the task.



HOW MANY SENSORS CAN BE CONNECTED TO ONE DATA LOGGER?



With a CR10 only, 14 probes could conceivably be monitored.

( 12 on the SE inputs and 2 on the pulse inputs. ) Since there are only 8 

control ports for enables, more than one probe would be enabled at a 

time--no problem.

With a multiplexer (CSI's AM416), this can be expanded to 48. 

Using 6 AM416s, 288 probes could be monitored with a single

CR10!

 

WHAT ARE THE POWER REQUIREMENTS?

CS615MAN: 70 milliamps @ 12VDC



SIZE?

CS615MAN: Rods: 30.0 cm long  3.2 mm diameter 3.2 cm spacing

Head:          11.0 cm x 6.3 cm x 2.0 cm



WEIGHT?

CS615MAN: Probe: 280 g    Cable: 35 g m-1



MAINTENANCE?

CS615MAN: The CS615 does not require periodic maintenance.





WHAT ARE THE HIDDEN CATCHES?

(Apart from heaps of data, and the points below) - I wish I knew!  



CS615MANUAL: Soil Electrical Conductivity

High soil electrical conductivity affects the accuracy and stability of

the CS615. When electrical conductivity values exceed 2 dS m-1, the

slope of the period output with water content begins to change. At

electrical conductivity values greater than 20 dS m-1 the probe output

becomes unstable.



CS615MANUAL: Temperature Dependence

The CS615 output is sensitive to temperature, and compensation can

be applied to enhance accuracy. The magnitude of the temperature

coefficient varies with water content. Laboratory measurements show

temperature coefficients of 0.0012, 0.0014, and 0.00165 m3 m-3 (C-1

at volumetric water contents of 0.11, 0.13 and 0.23 m3 m-3,

respectively. The calibration presented in Section 9 is for a

temperature of 20(C. The following equation can be used to

interpolate the temperature coefficient for a range of volumetric water

content ((v) values.

An example for using the temperature correction is a measurement

taken on a soil at a water content of about 0.23 and a temperature of

25(C.  The temperature coefficient value is 0.00164 m3 m-3 (C-1

which means that the measured water content is 5(C *(0.00164 m3 

m-3 (C-1) or 0.8% high.



CS615MANUAL: Potential problems with improper insertion

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 some applications, installation

can be improved by using insertion guides or a pilot tool.









OTHER

The CS615MANUAL provides the folllowing Sample Programs

Simple program using the Period Averaging Instruction (P27) of a

CR10 datalogger to read a single CS615.  

Simple program using the Pulse Instruction (P3) of a CR10 or 21X

datalogger to read a single CS615. 

Program using the Pulse Instruction (P3) of 21X datalogger and

AM416 multiplexer to read 48 CS615 probes. (The AM416

multiplexer is a 16 channel multiplexer with 4 lines per channel.)

Program using the Pulse Averaging Instruction (P27) of CR10

datalogger and AM416 multiplexer to read 48 CS615 probes.