Decagon Devices MPS-2, MPS-6 Operator's Manual

MPS-2 & MPS-6

Dielectric Water Potential

Sensors

Operator’s Manual

Decagon Devices, Inc.

Version: December 1, 2014 — 14:32:49

MPS-2 & MPS-6

Decagon Devices, Inc.

2365 NE Hopkins Court

Pullman WA 99163

Phone: [phone]

Fax: [phone]

Website: www.decagon.com

Email: [e-mail] or [e-mail]

Trademarks

c

2011-2014 Decagon Devices, Inc.

All Rights Reserved

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MPS-2 & MPS-6 CONTENTS

Contents

1 Introduction 1

1.1 Customer Support . . . . . . . . . . . . . . . . . . . . 1

1.2 Specifications . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Warranty Information . . . . . . . . . . . . . . . . . . 3

1.4 Seller’s Liability . . . . . . . . . . . . . . . . . . . . . . 3

2 About the MPS-2 & MPS-6 5

3 Theory 6

3.1 Water Potential Measurement . . . . . . . . . . . . . . 6

3.2 Temperature Measurement . . . . . . . . . . . . . . . 7

4 Range and Accuracy 9

4.1 Measurement Range . . . . . . . . . . . . . . . . . . . 9

4.2 Measurement Accuracy . . . . . . . . . . . . . . . . . 10

4.3 Calibration . . . . . . . . . . . . . . . . . . . . . . . . 12

4.4 Soil Type Dependence . . . . . . . . . . . . . . . . . . 13

4.5 Hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . 13

5 Connecting to a Logger 15

5.1 Connecting to Em50/Em50R/Em50G logger . . . . . . 15

5.2 Connecting to a Non-Decagon Logger . . . . . . . . . 16

5.3 Using Non-Decagon Loggers . . . . . . . . . . . . . . . 18

5.4 Communication . . . . . . . . . . . . . . . . . . . . . . 18

5.5 Installing the Sensors . . . . . . . . . . . . . . . . . . . 20

5.6 Campbell Scientific Programs . . . . . . . . . . . . . . 21

6 Maintenance and Troubleshooting 22

6.1 MPS-2 & MPS-6 in Frozen Soils . . . . . . . . . . . . 22

6.2 Troubleshooting . . . . . . . . . . . . . . . . . . . . . . 23

7 References 25

8 Declaration of Conformity 26

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MPS-2 & MPS-6 1 INTRODUCTION

1 Introduction

Thank you for choosing the Dielectric Water Potential Sensor, model
MPS-2 or MPS-6, for measuring soil water potential and tempera­ture. This manual helps you to understand the sensor features and
how to use it successfully.

Note: The engineering community commonly uses the term soil suc­tion instead of soil water potential. Soil water potential is simply the
negative of soil suction and appears as such throughout this manual.

1.1 Customer Support

There are several ways to contact Decagon if you ever need assistance
with your sensors or if you have questions or feedback. We have cus­tomer service representatives available to speak with you Monday
thru Friday, between 8am and 5pm Pacific time.

Note: If you purchased your sensor through a distributor, please con­tact them for assistance.

E-mail:

[e-mail] or [e-mail]

Phone:
509-332-5600

Fax:
509-332-5158

If contacting us by email or fax, please include as part of your mes­sage your instrument serial number, your name, address, phone, fax
number, and a description of your problem or question.

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1 INTRODUCTION MPS-2 & MPS-6

1.2 Specifications

Water Potential

Range: −9 to −100,000 kPa (pF 1.96 to pF 6.01)

1

Resolution: 0.1 kPa

Accuracy:

MPS-6: ±(10% of reading + 2 kPa) from −9 to −100 kPa

1

MPS-2: ±(25% of reading + 2 kPa) from −9 to −100 kPa

1

Temperature

Range: −40◦C to 60◦C

Resolution: 0.1◦C

Accuracy: ±1◦C

General

Operating Temperature: −40 to 60◦C,20 to 100% RH

Operating Temperature: 0 to 60◦C (no water potential measure-

ment below 0◦C)

Power Requirements: 3.6 to 15 VDC, 0.03 mA quiescent,

10 mA max during 150 ms measurement

Dimensions: 9.6 cm (L) x 3.5 cm (W) x 1.5 cm (D)

Sensor Diameter: 3.2 cm

Dielectric Measurement Frequency: 70 MHz

Measurement Time: 150 ms (milliseconds)

Output: RS232 (TTL) with 3.6 volt levels or SDI-12 communication

protocol

1

Please see Section 4.1 of this manual for more detailed information on range

and accuracy at lower water potentials.

2

Customers may use sensors at higher temperatures under certain conditions,

please contact Decagon for assistance.

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MPS-2 & MPS-6 1 INTRODUCTION

Connector Types: 3.5 mm (stereo) plug or stripped & tinned lead

wires (3)

Cable Length: 5 m standard; custom lengths available upon request

Data Logger Compatibility (not exclusive):

Decagon: Em50, Em50R, Em50G (rev 2.13+)
Campbell Scientific: Any logger with serial I/O including

CR10X, CR23X, any CRBasic type logger (CR850, 1000,
3000, etc.)

Other: Any data acquisition system capable of 3.6 to 15 V

excitation and serial or SDI-12 communication

Handheld Reader Compatability: ProCheck (rev 1.53+)

Software Compatibility: ECH2O Utility (rev 1.71+) and DataTrac

3 (rev 3.9+)

1.3 Warranty Information

The Dielectric Water Potential Sensor has a 30-day satisfaction guar­antee and a one-year warranty.

1.4 Seller’s Liability

Seller warrants new equipment of its own manufacture against defec­tive workmanship and materials for a period of one year from date
of receipt of equipment (the results of ordinary wear and tear, ne­glect, misuse, accident and excessive deterioration due to corrosion
from any cause are not to be considered a defect); but Seller’s lia­bility for defective parts shall in no event exceed the furnishing of
replacement parts F.O.B. the factory where originally manufactured.

Material and equipment covered hereby which is not manufactured
by Seller shall be covered only by the warranty of its manufacturer.
Seller shall not be liable to Buyer for loss, damage or injuries to per­sons (including death), or to property or things of whatsoever kind
(including, but not without limitation, loss of anticipated profits),

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1 INTRODUCTION MPS-2 & MPS-6

occasioned by or arising out of the installation, operation, use, mis­use, nonuse, repair, or replacement of said material and equipment,
or out of the use of any method or process for which the same may be
employed. The use of this equipment constitutes Buyer’s acceptance
of the terms set forth in this warranty.

There are no understandings, representations, or warranties of any
kind, express, implied, statutory or otherwise (including, but with­out limitation, the implied warranties of merchantability and fitness
for a particular purpose), not expressly set forth herein.

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MPS-2 & MPS-6 2 ABOUT THE MPS-2 & MPS-6

2 About the MPS-2 & MPS-6

The MPS-2 and MPS-6 measure the water potential and temperature
of soil and other porous materials. These sensors have a low power
requirement which makes them ideal for permanent burial in the soil
and continuous reading with a data logger or periodic reading with
a handheld reader.

The only functional difference between the MPS-2 and MPS-6 is
the amount of effort spent calibrating the sensor. Each MPS-2 has a
two point calibration that results in a fair degree of accuracy, while
the MPS-6 has a six point calibration that results in research-grade
accuracy.

Figure 1: MPS-2 & MPS-6 Components

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3 THEORY MPS-2 & MPS-6

3 Theory

3.1 Water Potential Measurement

There are two basic parameters that describe the state of water in
soil: one is soil water content, or the amount of water per unit of soil,
and the other is soil water potential, or the energy state of water in
the soil. Although water content is useful when trying to describe
the water balance of a soil, i.e. how much water is moving in, out,
or being stored, water potential is often preferred over water content
because it determines how water moves in a soil or from the soil to
the plant. In addition, you can use water potential to determine
plant availability of water, schedule irrigation, or determine the me­chanical stress state of soil.

All soil water potential measurement techniques measure the poten­tial energy of water in equilibrium with water in the soil. The Second
Law of Thermodynamics states that connected systems with differing
energy levels move toward an equilibrium energy level. When an ob­ject comes into hydraulic contact with the soil, the water potential of
the object comes into equilibrium with the soil water potential. For
example, tensiometers make use of this principal to measure the po­tential energy of a liquid water reservoir in equilibrium with the soil
water (liquid equilibration), and psychrometers/dew point hygrome­ters measure the potential energy of water vapor in equilibrium using
soil water (vapor equilibration).

Another category of water potential sensors use a solid matrix equi­libration technique to measure the water potential of the soil. This
technique introduces a known material with a static matrix of pores
into the soil and allows it to come into hydraulic equilibrium ac­cording to the Second Law of Thermodynamics. Because the two
are in equilibrium, measuring the water potential of the solid matrix
gives the water potential of the soil. Historically, instruments have
measured the thermal conductivity or electrical conductivity of the
solid matrix to determine its water potential with varying degrees of
success.

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