Decagon Devices WP4C Operator's Manual

WP

4C

Dewpoint PotentiaMeter

Operator’s Manual

Version 1

Decagon Devices, Inc.

2365 NE Hopkins Court

Pullman WA 99163

tel: [phone]

fax: [phone]

www.decagon.com

[e-mail]

Trademarks

“WP4C PotentiaMeter” is a

registered trademark of Decagon Devices, Inc.

©2010 Decagon Devices, Inc., All rights reserved.

WP4C PotentiaMeter

Table of Contents

i

Contents

ContentsContents

Contents

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

About this Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Customer Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Warranty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Note to our WP4C Users . . . . . . . . . . . . . . . . . . . . . . . 2

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

2. About the WP4C . . . . . . . . . . . . . .4

Instrument Specifications . . . . . . . . . . . . . . . . . . . . . . . 4

WP4C and Water Potential . . . . . . . . . . . . . . . . . . . . 5

How the WP4C works . . . . . . . . . . . . . . . . . . . . . . . . . . 5

WP4C and Temperature . . . . . . . . . . . . . . . . . . . . . . . . 6

3. Getting Started . . . . . . . . . . . . . . .7

Components of your WP4C . . . . . . . . . . . . . . . . . . . . 7

Choosing a Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Preparing the WP4C for Operation . . . . . . . . . . . . 9

Portability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4. The Menus . . . . . . . . . . . . . . . . . . .12

The Main Menu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Changing Languages . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Reading Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

System Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Sample Equilibration Screen . . . . . . . . . . . . . . . . . . . . 20

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5. Calibration and Verification . . 21

Verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Verification Standards . . . . . . . . . . . . . . . . . . . . . . . . . . 21

When to Verify Calibration . . . . . . . . . . . . . . . . . . . . 22

How to Verify and Calibrate the WP4C . . . . . . . 22

6. Sample Preparation . . . . . . . . . . 25

Choosing a Sample Cup . . . . . . . . . . . . . . . . . . . . . . . . . 25

Preparing the Sample . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Dry Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Samples and Temperature . . . . . . . . . . . . . . . . . . . . . . 27

Measuring Plant Samples . . . . . . . . . . . . . . . . . . . . . . . 29

7. Taking a Reading . . . . . . . . . . . . 30

How WP4C takes Readings . . . . . . . . . . . . . . . . . . . . 31

Cautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

8. Computer Interface . . . . . . . . . . . 33

9. Theory: Water Potential . . . . . . 35

Water Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Measuring Water Potential . . . . . . . . . . . . . . . . . . . . . 35

Effect of Temperature on Water Potential . . . . . 37

Estimating Osmotic Potential . . . . . . . . . . . . . . . . . . . 38

10. Cleaning and Maintenance . . . 39

Tools Needed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Accessing the Block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Cleaning Procedure: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Checking Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

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11. Repair Instructions . . . . . . . . . . . .45

Shipping Directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Repair Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Loaner Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

12. Troubleshooting . . . . . . . . . . . . .48

Problems and Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Sensor Performance Screen . . . . . . . . . . . . . . . . . . . . . 54

13. Further Reading . . . . . . . . . . . . . .56

References: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Application Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Appendix A . . . . . . . . . . . . . . . . . . . .58

Preparing Salt Solutions . . . . . . . . . . . . . . . . . . . . . . . . . 58

Index . . . . . . . . . . . . . . . . . . . . . . . . . .61

WP4C PotentiaMeter

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iv

WP4C PotentiaMeter

1. Introduction

1

1. Introduction

Welcome to Decagon’s WP4C Dewpoint PotentiaMeter,

the research standard for measuring water potential.
WP4C is the fastest, most accurate, and most reliable
instrument available for measuring water potential using
the chilled-mirror dewpoint technique. Whether you are a
scientist or a student, WP4C will suit your needs. It is easy
to use and provides fast, accurate results. We hope you
find this manual informative and helpful in understanding
how to maximize the capabilities of your WP4C.

About this Manual

Included in this manual are instructions for setting up
your WP4C, calibration, and maintaining and caring for
your instrument. Please read these instructions before
operating WP4C to ensure that the instrument performs
to its full potential.

Customer Service

If you ever need assistance with your WP4C, or if you just
have questions, there are several ways to contact us:

NOTE: If you purchased your WP4C through a distributor, please
contact them for assistance.

WP4C PotentiaMeter

1. Introduction

2

Phone:

Our toll-free customer service number is available to our
customers in the US, Monday through Friday, between 8
a.m. and 5 p.m. Pacific time at 1-800-755-2751. For our
customers outside of the US, our regular telephone num-

ber is [phone].

Fax:

Our fax number is [phone]. When you fax us,

please include your WP4C’s serial number, your name,
address, phone and fax number along with a description
of your problem.

E-mail:

If you need technical support or have questions about
your application with WP4C, you can send us messages via

e-mail at [e-mail]. Again, please include

as part of your message your WP4C’s serial number, your
name, address, phone, fax number, and return e-mail
address.

Warranty

WP4C has a 30-day satisfaction guarantee and a one-year
warranty on parts and labor.

Note to our WP4C Users

This manual is written to aid the end user in understand­ing the basic concepts of water potential, enabling them to
use our instrument with confidence. Every effort has been

WP4C PotentiaMeter

1. Introduction

3

made to ensure that the content of this manual is correct
and scientifically sound.

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), 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.

WP4C PotentiaMeter

2. About the WP4C

4

2. About the WP4C

Instrument Specifications

Range: 0 to -300MPa*
Accuracy: ±0.05MPa* from 0 to -5 MPa

1% from -5 to -300 MPa

Measurement time:

~10-15 min. for most soil samples in precise mode
~20 min. for plant tissue samples
<5 minutes in fast mode (reduced accuracy)

Temperature Control: 15 to 40° C (±0.2° C)
Operating Environment: 5 to 40° C (41 to 104° F)

Sensor Type: 1) Chilled-mirror dewpoint sensor

2) Infrared temperature sensor

Sample Cup Capacity: 7 ml recommended (15 ml full)
Dimensions: 24.1 x 22.9 x 8.9 cm (9.5 x 9.0 x 3.5 in)
Weight: 3.2 Kg (7.1 lbs)
Case Material: Powder Painted Aluminum
Display: 20 x 2 alphanumeric LCD with backlighting
Data Communication: RS232A compatible, 8-data bit

ASCII code, 9600 baud, no parity, 1 stop bit

Power: 110VAC to 220 VAC, 50/60 Hz
Interface Cable: Standard RS232 serial cable (included)
Compatible Standards: ASTM D6836-07
Warranty: 1 year parts and labor

* Note that the WP4C (and all vapor pressure instruments) are

WP4C PotentiaMeter

2. About the WP4C

5

limited by accuracy in the wet end of the water potential range. The
range of 0 to -5 MPa has an accuracy of 0.05 MPa. For example,
a measurement at -0.1MPa has an accuracy of ±50% of the read­ing, and -1 MPa has an accuracy of ±5% of the reading. Function­ally this means that the WP4C will not measure water potential
accurately near field capacity.

WP4C and Water Potential

Water potential is a measurement of the energy status of
the water in a system. It indicates how tightly water is
bound, structurally or chemically, within a substance.
Water potential can be computed from the vapor pressure
of air in equilibrium with a sample in a sealed measure­ment chamber. For a more detailed description of water
potential, please refer to Chapter 9, titled “Theory: Water
Potential” of this manual.

How the WP4C works

WP4C uses the chilled-mirror dewpoint technique to mea­sure the water potential of a sample. In this type of instru­ment, the sample is equilibrated with the headspace of a
sealed chamber that contains a mirror and a means of
detecting condensation on the mirror. At equilibrium, the
water potential of the air in the chamber is the same as the
water potential of the sample. In the WP4C, the mirror
temperature is precisely controlled by a thermoelectric
(Peltier) cooler. Detection of the exact point at which con­densation first appears on the mirror is observed with a
photoelectric cell. A beam of light is directed onto the

WP4C PotentiaMeter

2. About the WP4C

6

mirror and reflected into a photodetector. The photode­tector senses the change in reflectance when condensation
occurs on the mirror. A thermocouple attached to the
mirror then records the temperature at which condensa­tion occurs. Values begin to be displayed indicating that
initial measurements are being taken. WP4C then signals
you by flashing a green LED and/or beeping when final
values are reached. The final water potential and tempera­ture of the sample is displayed.

In addition to the technique described above, WP4C uses an
internal fan that circulates the air within the sample chamber
to reduce time to equilibrium. Since both dewpoint and
sample surface temperatures are simultaneously measured,
the need for complete thermal equilibrium is eliminated.

The WP4C controls the sample temperature by means of
an internal thermo-electrical module that monitors and
stabilizes the sample block temperature according to how
it is set.

WP4C and Temperature

Large temperature differences, between sample and block,
will cause longer reading times, since a complete and accu­rate reading will not be made until the difference between
the sample tempeature and the block temperature is less than

1.0 degree. To help you monitor the temperature difference
between your sample and the block, you can access a sample
equilibration screen at the main menu.

WP4C PotentiaMeter

3. Getting Started

7

3. Getting Started

Components of your WP4C

Your WP4C should have been shipped with the following
items:

• WP4C main unit

• Quickstart guide

• Certificate of Calibration

• Power cord

• RS-232 interface cable

• 25 plastic sample cups and lids

• 10 Stainless Steel sample cups

• Operator’s Manual

• 12 vials of 0.5 Molal KCl

• Cleaning Kit

WP4C PotentiaMeter

3. Getting Started

8

Choosing a Location

To ensure that your WP4C operates correctly and consis­tently, place it on a level surface. This reduces the chance
that sample material will spill and contaminate the inside
of the instrument. To protect the internal electrical com­ponents, and to avoid inaccurate readings, place your
WP4C in a location where the temperature remains fairly
stable. This location should be well away from air condi­tioner and heater vents, open windows, outside doors,
refrigerator exhausts, or other items that may cause rapid
temperature fluctuation.

Features

Front view of WP4C

LED indicator

light

LCD

Function Keys

Sample drawer

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3. Getting Started

9

Back view of WP4C

Preparing the WP4C for Operation

After finding a good location for your WP4C, plug the
power cord to the back of the unit. Before turning it on,
pull open the sample drawer (turn the knob to the
“OPEN/LOAD” position). An empty disposable sample
cup is usually placed upside-down in the drawer to protect
it during shipment. Remove this sample cup and turn the
instrument on. The ON/OFF switch is located on the
lower right corner of the WP4C’s back panel. The follow­ing screens will appear on the LCD:

RS-232 interface

fuse well

ON/OFF Switch

Power cord

Plug

Fan

Lid thumb-screw

Model WP4C

PotentiaMeter

v4.03

WP4C PotentiaMeter

3. Getting Started

10

Then:

This is the main menu, displaying the water potential in
both MegaPascals (MPa) and pF, and the sample tempera­ture in °C.

In order to provide the most accurate readings, WP4C
should ideally be allowed a warm-up period of 15-30 min­utes after turning it on. When you insert a sample into the
chamber drawer and turn the drawer knob to the
“READ” position, the instrument will begin the read cycle
to measure the water potential of your sample.

Portability

On occasion you may want to take water potential mea­surements in the field where it is not feasible to take sam­ples and return to the lab. The following is a procedure for
powering your WP4C using your vehicle as a power source
at sites where AC power is not readily available.

1. Purchase a portable power inverter that plugs into the
12V output (cigarette lighter) of your vehicle. The
inverter should have a continuous output of at least
140 Watts.

2. Place the WP4C on a level surface. Care should be

0.0°C

0.00 MPa

pF 0.00

p

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3. Getting Started

11

taken to minimize temperature gradients that may
affect the instrument while in the field. A Styrofoam
box, for example, will help minimize temperature
effects.

3. Plug the 12-volt inverter to the 12-volt output of the

vehicle, or directly to the battery itself.

4. Plug the WP4C to the inverter, and turn it on. When

the instrument is on, it draws up to 1 amp. Check the
rating of your battery if you want to know how long it
will power the instrument (for example, if your battery
is rated for 60 amp hours, it will work for 60 hours
when the vehicle isn’t running.

5. Allow the instrument to warm up for 15-30 minutes as

you would in the lab. Check the calibration of the
instrument before proceeding with reading.

WP4C PotentiaMeter

4. The Menus

12

4. The Menus

The Main Menu

Every time you turn on your WP4C, it will come to this
screen. If this screen doesn’t appear, refer to Chapter 12
for troubleshooting instructions. As mentioned earlier, the
water potential and sample temperature are displayed on
the screen. On each side of the LCD there are buttons.
Each button performs a different function depending on
which mode you want. Following is a description of the
modes and options you may use, and the buttons used to
set them.

0.0°C

0.00 MPa

pF 0.00

p

sample temperature menu system setup

language

selection

0.0°C

0.00 MPa

pF 0.00

p

precise/continuous/fast mode

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4. The Menus

13

Changing Languages

The WP4C comes to you with English as the default on­screen user language. If you prefer not to use English, you
can change it to one of a variety of other languages: Ger­man, French, Spanish, Italian, Swedish, Danish, Norwe­gian, Czech, Portuguese, Japanese, Polish or Finnish. This
is done simply by pressing the upper right button of the
instrument while it is not reading a sample. You will see
the following screen:

Press the upper right key again, and the next language
option (German) will appear:

Each time you press the right button, the display will scroll
to the next language option. Select the desired language,
then press the lower left button to exit.

Reading Modes

Precise Mode

The first time you turn on the WP4C, it will be in precise
mode. In this mode, measurements on a sample are

English

-Exit-

Deutsch

-zurück-

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4. The Menus

14

repeated until successive readings agree within a preset

tolerance (0.03MPa for Ψ>-40MPa; otherwise 0.3MPa) .

The WP4C always starts in precise mode. To toggle
between the precise, continuous and fast modes, press the
top left button. The display will show a small p, c or f to
the left of the water potential readings:

main menu with continuous mode enabled

Here is a brief summary of the precise mode features:

• Precise water potential value is insured by repeat­ing measurements on a sample.

• Read time is typically 10-15 minutes.

• Green LED blinks until you turn the drawer knob
to the OPEN/LOAD position.

Continuous Mode

Continuous mode measures the water potential of your
sample continuously until you turn the drawer knob to the
OPEN/LOAD position. This can be useful in doing long
term monitoring of samples that take an especially long

c

“c” for continuous mode

0.0°C

0.00 MPa

pF 0.00

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4. The Menus

15

time to come to vapor equilibrium, such as plant samples
and moist soil samples with water potential > -0.5MPa. In
this mode the WP4C will measure the sample, stop to dis­play the water potential and sample temperature, then
begin another read cycle. Between samples, it will signal
you with the green LED flash, accompanied by the
beeper, if it is enabled. Some find it helpful to connect
their WP4C to a computer while in continuous mode in
order to log and store data over time. For instructions on
how to do this, see Chapter 8.

Fast Mode

In fast mode, the sample is measured only once. Read time
is typically 3-5 minutes. Readings are less precise, particu­larly in the wet range. However, fast mode is recom­mended for dry soil samples with water potential <-40
MPa.

System Configuration

If you press the bottom left button while in the main
menu, it will bring you to the system configuration menu.
This menu allows you to make minor system changes.

WP4C PotentiaMeter

4. The Menus

16

You can change how the beeper signals after each reading
and enter the calibration menu as well.

System configuration menu

Changing the beeper

When you are reading, the WP4C has two ways of notify­ing you that the water potential reading is completed for
your sample: the beeper and a flashing green LED, located
on the left front corner of the WP4C’s case. In fast and
precise reading modes, the LED will flash once when a
sample is started. When it is finished the LED will flash

system configuration menu button

0.0°C

pF 0.00

p

0.00 MPa

Beeper

Beeper
Mode indicator

Calibration

menu

Icon

(4 beeps shown)

temperature set

+

-

-Exit-

4x

set T