Campbell TDR, SDM1502, PS1502B, SDMX50 System Manual

CAMPBELL SCIENTIFIC

TDR SOIL MOISTURE MEASUREMENT

SYSTEM MANUAL

REVISION: 2/92

COPYRIGHT (c) 1991, 1992 CAMPBELL SCIENTIFIC, INC.

WARRANTY AND ASSISTANCE

The following warranty applies to the

CONTROL MODULE

, and the

SDM1502 COMMUNICATION INTERFACE, PS1502B POWER

SDMX50 50 OHM COAX MULTIPLEXER

.

These products are warranted by CAMPBELL SCIENTIFIC, INC. to be free from defects in materials and
workmanship under normal use and service for twelve (12) months from date of shipment unless specified
otherwise. Batteries have no warranty. CAMPBELL SCIENTIFIC, INC.'s obligation under this warranty is
limited to repairing or replacing (at CAMPBELL SCIENTIFIC, INC.'s option) defective products. The
customer shall assume all costs of removing, reinstalling, and shipping defective products to CAMPBELL
SCIENTIFIC, INC. CAMPBELL SCIENTIFIC, INC. will return such products by surface carrier prepaid.
This warranty shall not apply to any CAMPBELL SCIENTIFIC, INC. products which have been subjected
to modification, misuse, neglect, accidents of nature, or shipping damage. This warranty is in lieu of all
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purpose. CAMPBELL SCIENTIFIC, INC. is not liable for special, indirect, incidental, or consequential
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CAMPBELL SCIENTIFIC, INC.

RMA#_____
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TDR SOIL MOISTURE MEASUREMENTS

Warranty and Assistance

TABLE OF CONTENTS

PAGE

1. INTRODUCTION

1.1 Soil Moisture Measurement by Time Domain Reflectometry....................................................1

1.2 Probes and Cables.................................................................................................................... 1

1.3 Multiplexing................................................................................................................................ 1

2. SYSTEM COMPONENTS

3. INSTALLATION

3.1 Assigning Addresses................................................................................................................. 3

3.2 Installing 1502B in ENC TDR....................................................................................................4

3.3 Datalogger Wiring...................................................................................................................... 4

3.3.1 SDM Wiring......................................................................................................................... 4

3.3.2 PS1502B............................................................................................................................. 5

3.4 Soil Probes................................................................................................................................5

3.5 Grounding.................................................................................................................................. 6

3.6 Battery Connections ..................................................................................................................6

4. PROGRAMMING INSTRUCTION 100

5. CALIBRATION FOR WATER CONTENT

6. PROGRAMMING EXAMPLES

6.1 Example 1, Measure and Record Every Hour........................................................................... 8

6.2 Example 2, Hourly Average Using Topp's Calibration............................................................... 9

6.3 Example 3, TDR Measurements in Table 2 with Analog Measurements in Table 1............... 10

........................................................................................................................ 1

....................................................................................................... 2

.......................................................................................................................... 2

................................................................................ 6

.......................................................................... 7

.............................................................................................. 8

TABLES

3-1 SDM Addresses......................................................................................................................... 4

4-1 Waveform Resolution Determined by Probe Length................................................................. 7

FIGURES

2-1 TDR System Components......................................................................................................... 2

3-1 Address Switch on SDM1502....................................................................................................3

3-2 Location of Address Jumpers on SDMX50............................................................................... 3

3-3 Mounting 1502B in ENC TDR....................................................................................................4

3-4 SDM Wiring............................................................................................................................... 5

3-5 PB30 Assembly......................................................................................................................... 5

3-6 Terminal Strip Adapters for Connections to Battery.................................................................. 6

5-1 Comparison of Calibrations from Topp and Ledieu................................................................... 8

TDR SOIL MOISTURE MEASUREMENTS

Since the application and advantages of using Time Domain Reflectometry to measure soil water
content were described by Topp, Davis, and Annan in 1980, the method has proven itself through the
work of numerous researchers. Campbell Scientific has developed a system in which the Tektronix
1502B cable tester is coupled with a CR10 or 21X datalogger and multiplexers to provide automated
multiple probe TDR soil moisture measurements.

1. INTRODUCTION

1.1 SOIL MOISTURE MEASUREMENT BY TIME DOMAIN REFLECTOMETRY

A waveform traveling down a coax or
waveguide is influenced by the type of material
surrounding the conductors. If the dielectric
constant of the material is high, the signal
propagates slower. Because the dielectric
constant of water is much higher than most
other materials, a signal within a wet or moist
medium propagates slower than in the same
medium when dry. Ionic conductivity affects the
amplitude of the signal but not the propagation
time. Thus, moisture content can be
determined by measuring the propagation time
over a fixed length probe embedded in the
medium being measured.

This process of sending pulses and observing
the reflected waveform is called Time Domain
Reflectometry (TDR). TDR is also used to
determine the location of failures in
telecommunications cables and, on cables
grouted in boreholes, to monitor rock mass
deformation.

The reflectometer used in our system is the
Tektronix 1502B TDR Cable Tester equipped
with Campbell Scientific's SDM1502
Communications Interface and PS1502B Power
Control Module. The Tektronix 1502C will also
work but cannot use the PS1502B for
controlling power.

1.2 PROBES AND CABLES

The simplest soil probe consists of two parallel
rods inserted into the soil. These are attached
directly to twin lead cable. The two rod probe
and the twin lead cable that attaches to it carry
a "balanced" signal (the signal travels on both
leads).

The 1502B has a 50 ohm coax connector; coax
carries an unbalanced signal. To convert from
an unbalanced to a balanced signal, and to
match impedances, a balancing transformer
(balun) is needed. The PB30B (-RG8 or ­RG58) is a two rod probe with 30 cm stainless
steel rods; a balun molded into the cable joins
the twin lead cable from the probe to the 1502B
(or multiplexer).

Another type of probe that has come into use
recently is the unbalanced probe. The probe
has three or more rods. A central rod is
connected to the signal lead of the coax. The
other rods are arranged radially around the
center and are connected to the shield of the
coax. The volume of soil sampled with this
configuration is smaller than with a balanced
design and is concentrated around the center
electrode. The probes connect directly to the
50 ohm coax cables used throughout the
system. In dry soils, the unbalanced probe may
produce a wave form that cannot be interpreted
by the algorithm used to determine propogation
velocity for the probe.

1.3 MULTIPLEXING

The SDMX50 is an eight to one 50 ohm coax
multiplexer with BNC connectors. The coax
cable coming from the 1502B connects to the
common.

The eight multiplexed connections are used to
connect additional multiplexers or probes. The
PB30B connects directly to the SDMX50. Up to
three levels of multiplexers may be used. Up to
512 soil probes may be measured on a fully
expanded system.

1

TDR SOIL MOISTURE MEASUREMENTS

FIGURE 2-1. TDR System Components

2. SYSTEM COMPONENTS

1502B - Tektronix 1502B TDR Cable Tester.
SDM1502 - Communication Interface; this module plugs into the 1502B or 1502C and provides

a Synchronous Device for Measurement (SDM) interface to a CR10 or 21X datalogger.

PS1502B Power Control Module plugs into the battery receptacle of the 1502B; provides for

connection to an external 12 VDC source and allows the datalogger to control power to
the 1502B.

SDMX50 - 50 Ohm Coax Multiplexer - 8 to 1, BNC connectors, enclosure included. This

multiplexer is used to connect additional multiplexers or probes. Balanced probes
attached to this multiplexer require a balun for each probe.

PB30B (-RG8 or -RG58) - 30 cm soil probes; two rods, balanced design, with balun molded in

cable. A BNC connector on the coax cable attaches directly to SDMX50. The -RG8

version uses a low loss coax cable. The -RG58 version uses RG58 coax cable.
COAX TDR - 50 ohm coax cable with BNC connectors for connecting multiplexer.
6549 - 5 conductor cable used for SDM connection between datalogger and multiplexers.
21X or CR10 - The datalogger communicates with the 1502B and multiplexers with the SDM

interface using control ports 1-3 (and single ended channel 1 on the 21X). An additional

control port is used to switch power to the 1502B.
ENC TDR - Enclosure to hold 1502B and datalogger. The ENC TDR includes a transient

suppressor and cabling for connecting the power supply and datalogger. The 1502B

mount allows it to swing out to view screen.
Datalogger Software - Special PROMS for the CR10 or 21X datalogger include Instruction 100

for controlling the 1502B and multiplexers.
6590 - Transient suppressor attaches to cable from 1502B and is required if ENC TDR is

purchased.

3. INSTALLATION

The 1502B and datalogger are housed in the
ENC TDR. In most instances power is supplied
from an external 12 volt deep cycle battery
charged by a MSX18R solar panel. Each
SDMX50 multiplexer has its own enclosure.

not

Coax cable connects the 1502B and the
SDMX50 and SDMX50 to other SDMX50s. In
addition to the coax cable that carries the TDR
signal, the multiplexers must be connected to
the datalogger by a 5 conductor cable which
provides power to the multiplexers and allows
the datalogger to control the multiplexer
switching.

2