Decagon WP4-T, WP4 Operator's Manual
WP
4
Dewpoint PotentiaMeter
for models WP4 and WP4-T
Operator’s Manual
Ve r s i o n 2 . 1
Decagon Devices, Inc.
Decagon Devices, Inc.
950 NE Nelson Court
Pullman WA 99163
tel: (509) 332-2756
fax: (509) 332-5158
www.decagon.com/wp4
wp4@decagon.com
Trademarks
“WP4 Dewpoint PotentiaMeter” is a
registered trademark of Decagon Devices, Inc.
©
1998-2003 Decagon Devices, Inc., All rights reserved.
WP
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PotentiaMeter
Table of Contents
i
Contents
1. Introduction
. . . . . . . . . . . . . . . . . . . . . . . . 1
About this Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
Customer Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
Warranty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
Note to our WP4 Users . . . . . . . . . . . . . . . . . . . . . . . . . .3
Seller’s Liability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
2. About the WP4
. . . . . . . . . . . . . . . . . . . . 5
WP4 Models and Options . . . . . . . . . . . . . . . . . . . . . .5
WP4 and Water Potential . . . . . . . . . . . . . . . . . . . . . . .5
How the WP4 works . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
WP4 and Temperature . . . . . . . . . . . . . . . . . . . . . . . . . .7
3. Getting Started
. . . . . . . . . . . . . . . . . . . 10
Components of your WP4 . . . . . . . . . . . . . . . . . . . . . . .10
Choosing a Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Preparing the WP4 for Operation . . . . . . . . . . . . . .12
Portability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
4. The Menus
. . . . . . . . . . . . . . . . . . . . . . . . . 15
The Main Menu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Changing Languages . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
Normal sampling mode . . . . . . . . . . . . . . . . . . . . . . . . . .16
Continuous mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
System configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
Sample Equilibration Screen . . . . . . . . . . . . . . . . . . . . .23
5. Calibration and Verification
. . . . 25
Verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25
Verification Standards . . . . . . . . . . . . . . . . . . . . . . . . . .25
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When to Verify Calibration . . . . . . . . . . . . . . . . . . . . .26
How to Verify and Calibrate the WP4 . . . . . . . . .27
6. Sample Preparation
. . . . . . . . . . . . . . 31
Preparing the Sample . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
Dry Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32
Samples and Temperature . . . . . . . . . . . . . . . . . . . . . . .33
7. Taking a Reading
. . . . . . . . . . . . . . . . 36
How WP4 takes Readings . . . . . . . . . . . . . . . . . . . . . . .37
Cautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37
Increasing your Accuracy . . . . . . . . . . . . . . . . . . . . . . .39
8. Computer Interface
. . . . . . . . . . . . . . . 41
Using Hyperterminal in Windows 98
and Windows 2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41
9. Theory: Water potential
. . . . . . . . 44
Water potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44
Measuring Water Potential . . . . . . . . . . . . . . . . . . . . .45
Effect of Temperature on Water Potential . . . . . .46
10. Cleaning and Maintenance
. . . . . 48
Tools Needed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48
Cleaning the Block and Sensors . . . . . . . . . . . . . . . . .49
Checking Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . .52
11. Repair Instructions
. . . . . . . . . . . . . . . 53
Shipping Directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53
Repair Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55
Loaner Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55
12. Troubleshooting
. . . . . . . . . . . . . . . . . . 56
Problems and Solutions . . . . . . . . . . . . . . . . . . . . . . . . . .56
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Component Performance Screen . . . . . . . . . . . . . . . .63
13. Further Reading
. . . . . . . . . . . . . . . . . . 65
References: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65
Application Notes: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .66
Appendix A
. . . . . . . . . . . . . . . . . . . . . . . . . . 67
Preparing Salt Solution . . . . . . . . . . . . . . . . . . . . . . . . . .67
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WP
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PotentiaMeter
Introduction
1
1. Introduction
Welcome to Decagon’s WP4 Dewpoint PotentiaMeter, the research standard for measuring water
potential. WP4 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,
WP4 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 WP4.
About this Manual
This manual is for use with the standard model WP4
and the temperature-controlled model WP4-T.
Included in this manual are instructions for setting
up your WP4, calibration, and maintaining and caring for your instrument. Please read these instructions before operating WP4 to ensure that the
instrument performs to its full potential.
Customer Service
If you ever need assistance with your WP4, or if you
just have questions, there are several ways to con-
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Introduction
2
tact us:
Phone:
Our toll-free customer service number is available
to our customers in the US and Canada, 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 and Canada,
our regular telephone number is (509) 332-2756.
Fax:
Our fax number is (509) 332-5158. When you fax
us, please include your WP4’s serial number, your
name, address, phone and fax number along with a
description of your problem.
E-mail:
If you need technical support, you can send us messages via e-mail at support@decagon.com. Again,
please include as part of your message your WP4’s
serial number, your name, address, phone, fax number, and return e-mail address.
If you have a question about your application with
WP4, please send your message with the above
information to wp4@decagon.com.
Warranty
WP4 has a 30-day satisfaction guarantee and a one-
WP
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Introduction
3
year warranty on parts and labor. To validate your
warranty, simply contact us with the information on
the warranty card included with this manual. You
can return your warranty information by fax, e-mail,
phone or by mailing the postage-paid card. Please
include all the information requested on the card. It
is necessary for Decagon to have your current mailing address and telephone number in case we need
to send updated product information to you.
Note to our WP4 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 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 cov-
WP
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Introduction
4
ered 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.
WP
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About the WP4
5
2. About the WP4
WP4 is the fastest instrument for measuring water
potential, giving readings directly in MegaPascals
within five minutes. It measures water potential
from 0 to -60 MPa with an accuracy of ±0.1 MPa
from 0 to -10MPa and ±1% from -10 to -60 MPa. The
instrument is easy to use and maintain, with simple
checking of calibration.
WP4 Models and Options
The WP4 comes in two models to suit the needs of
different users. Here is a brief description of each:
WP4: Our standard model, adequate for most water
potential needs.
WP4-T: User-selectable internal temperature control
model, uses thermoelectric (Peltier) components to
maintain a constant internal temperature.
WP4 and Water Potential
Water potential is a measurement of the energy status of the water in a system. It indicates how tightly
WP
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About the WP4
6
water is bound, structurally or chemically, within a
substance. Water potential is 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 WP4 works
WP4 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 WP4, 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 mirror and reflected into a photodetector
cell. 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. WP4
then signals you by flashing a green LED and/or
beeping. The final water potential and temperature
of the sample is then displayed.
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About the WP4
7
In addition to the technique described above, WP4
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, which
reduces measurement times to less than five minutes.
The WP4-T gives you the option of controlling the
sample temperature, by means of an internal
thermo-electrically controlled module that monitors
and stabilizes the sample block temperature according to how it is programmed. This can also help in
making more rapid measurements.
WP4 and Temperature
Temperature gradients can often be of concern for
making stable water potential measurements. Since
water potential is influenced by temperature, best
results are obtained by controlling the temperature.
Samples that are not at room temperature during the
read cycle will need to equilibrate to the temperature of the WP4 before accurate measurements of
water potential can be made. Large temperature differences will cause longer reading times, since a
complete and accurate reading will not be made
until the sample and the instrument are within 0.1
degree of each other. To help you monitor the tem-
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About the WP4
8
perature difference between your sample and the
instrument, you can access a sample equilibration
screen at the main menu.
Though temperature control is usually unnecessary
for most applications, there are some instances
where it is desired. For this reason, Decagon offers
a temperature-controlled WP4 model: the WP4-T.
There are several advantages in having a temperature-controlled model. Here are a few major reasons:
1. Research purposes. To study the effects of
temperature on the water potential of a sample,
comparison of the water potential of different
samples independent of temperature, or other
water potential studies where temperature control is critical.
2. To minimize extreme ambient temperature
fluctuations. If the laboratory and WP4 temperatures fluctuate by as much as ± 5°C daily, water
potential readings can vary as much as ± 0.5
MPa on a dry soil sample.
WP4-T
The WP4-T has thermoelectric components installed
that allow the instrument to maintain a set chamber
temperature without the need of an external water
bath. The WP4-T has an extra option in the System
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About the WP4
9
configuration menu (see page 21) that allows the
user to set the temperature.
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Getting Started
10
3. Getting Started
Components of your WP4
Your WP4 should have been shipped with the following items:
• WP4 main unit
• Quickstart guide
• Certificate of Calibration
• Power cord
• RS-232 interface cable
• 100 disposable sample cups
• Operator’s Manual
• 12 vials of 0.5 Molal KCl
• Cleaning Kit
Choosing a Location
To ensure that your WP4 operates correctly and
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Getting Started
11
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 WP4 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 WP4
LED indicator
light
LCD
Function Keys
Sample drawer
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Getting Started
12
Back view of WP4
Preparing the WP4 for Operation
After finding a good location for your WP4, 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 WP4’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 WP4
PotentiaMeter
v3.0
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Getting Started
13
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, WP4
should ideally be allowed a warm-up period of 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 WP4 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. We strongly recommend that this
0.0°C
0.00 MPa
pF 0.00
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Getting Started
14
inverter have a continuous output of at least 140
Watts.
2. Place the WP4 on a level surface. Care should be
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 WP4 to the inverter, and turn it on.
When the instrument is on, it draws 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 30 minutes
as you would in the lab. Check the calibration of
the instrument before proceeding with sampling.
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Menus
15
4. The Menus
The Main Menu
Every time you turn on your WP4, 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
sample equilibration screen system setup
continuous/normal
language
selection
mode
0.0°C
0.00 MPa
pF 0.00
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Menus
16
Changing Languages
The WP-4 comes to you with English as the default
on-screen 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, or
Japanese. 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.
Normal sampling mode
The first time you turn on the WP4, it will be in normal sampling mode. In this mode, a single measurement is made each time you read a sample. Here is
English
-exit-
Deutsch
-zurück-
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Menus
17
a brief summary of the normal sampling mode features:
• single sampling (not continuous).
• instrument may or may not beep after mea-
surement is made, depending on the system
setting.
• green LED blinks until you turn the drawer
knob to the OPEN/LOAD position.
diagram of button options from main menu
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 time to come to vapor equilibrium, such as plants and leaves. In this mode, it
sample temperature menu
system setup
continuous/normal mode
0.0°C
0.00 MPa
pF 0.00
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18
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 WP4 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.
To toggle between the normal and continuous
modes, press the top left button. The display will
show a small “c” to the left of the water potential
readings:
main menu with continuous mode enabled
If you press the upper left button again, the “c” will
disappear and you will be back in normal sampling
mode.
System configuration
If you press the bottom left button while in the
main menu, it will bring you to the system configu-
c
“c” for continuous mode
0.0°C
0.00 MPa
pF 0.00
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Menus
19
ration menu. This menu allows you to make minor
system changes. You can change how the beeper
signals after each sample and enter the calibration
menu as well.
System configuration menu
Changing the beeper
When you are sampling, the WP4 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 WP4’s case. In normal sampling mode, the LED
will flash once when a sample is started. When it is
system configuration menu button
0.0°C
0.00 MPa
pF 0.00
-Exit-
+
-
set T
Beeper
Beeper
Mode indicator
Calibration
menu
Icon
(continuous shown)
temp. set (WP4-T model only)
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