WP
4C
Dewpoint PotentiaMeter
Operator’s Manual
Version 1
Decagon Devices, Inc.
2365 NE Hopkins Court
Pullman WA 99163
tel: (509) 332-2756
fax: (509) 332-5158
www.decagon.com
support@decagon.com
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
WP4C PotentiaMeter
Table of Contents
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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
WP4C PotentiaMeter
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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 (509) 332-2756.
Fax:
Our fax number is (509) 332-5158. 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 support@decagon.com. 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
WP4C PotentiaMeter
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
WP4C PotentiaMeter
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
WP4C PotentiaMeter
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-
WP4C PotentiaMeter
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
WP4C PotentiaMeter
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
WP4C PotentiaMeter
4. The Menus
17
continuously until the knob is moved to the OPEN/
LOAD position (if not operating in continuous mode).
You cannot turn off or change the LED flashing functions.
There are three beeper options, represented by three
icons:
definition of beeper icons
The beeper can be turned off completely, it can beep
momentarily (4 times) when the sample is finished and
then stop, or it can beep continuously until the knob is
turned to the OPEN/LOAD position. After you have
adjusted the beeper setting, it will remain as you have set it
until you change it again, and will not be affected by turning the instrument on or off.
EXIT
You may press the -Exit- button (the lower left button)
to exit back to the main menu at any time.
Adjusting Calibration
When you need to adjust calibration, press the upper right
button in the system configuration menu, and you will be
brought to the calibration menu. For more details on calibration and how to verify it, please refer to the next chapter.
0x
4x
No beeping.
Beeps four times, then stops.
Beeps continuously until drawer is opened.
x
WP4C PotentiaMeter
4. The Menus
18
Setting the Temperature
The WP4C gives you the ability to manually set your
instrument’s sample chamber temperature. This is done by
pressing the lower right button next to the “
set T
” in
the system configuration menu. The following screen will
appear:
Adjusting the Setpoint Temperature
Use the buttons next to + and - to adjust the target setpoint temperature (displayed in the lower right corner). If
you press either button it adjusts in increments of .1°C.
Note: Holding down the button will rapidly increment the value.
The target setpoint temperature roughly corresponds to
the temperature at which you wish the sample to read.
Therefore, adjust the setpoint to the temperature that you
want, then begin measurements to see how close your
WP4C comes to your desired temperature (this works best
by putting the WP4C in continuous mode). After several
samples, it should show consistent temperature readings.
At this point, make any needed adjustment to the setpoint
index number to reach your desired temperature. You will
be able to adjust the index number between 15° and 50°C.
If you press the - button after you reach 15°, it will disable
Adjust setpoint
-Exit-
25.0
+
-
WP4C PotentiaMeter
4. The Menus
19
the temperature control function until you raise the index
number again. When the temperature control is disabled
the diplay will show ‘off ’ in place of the temperature setting
Important tips with the WP4C
• Before reading, wait for approximately 30 minutes
to let the chamber’s temperature stabilize after
turning it on.
• Cool samples to a temperature slightly below
chamber temperature before starting a reading.
• For slow equilibration samples such as plant and
moist soil samples (> -0.5 MPa), precise reading
mode may yield small errors. For these types of
samples, it is recommended to use continuous
mode and log data over time (See Chapter 8) to
determine when equilibrium conditions are
reached.
• For samples with very little water holding capacity (i.e. dry sand samples), small leaks in the sample chamber can cause water potential to drift
down over time. Fast mode is recommended for
these samples.
• For best results, run in precise mode for most soil
samples.
WP4C PotentiaMeter
4. The Menus
20
• Never place a hot or warm sample in a cooled
chamber, because condensation will form inside
the chamber, causing errors in reading.
Sample Equilibration Screen
To see the temperature difference between your sample
and the WP4C, press the lower right button at the main
menu. This screen can only be accessed when the drawer
knob is in the OPEN/LOAD position. The following
screen will appear:
This screen shows the temperature difference between the
sample (Ts) and the chamber block (Tb), allowing you to
quickly check if the sample is too hot, which may cause
condensation inside the chamber. Press the lower right
button to exit.
Ts - Tb = -0.07
Tb = 24.9
WP4C PotentiaMeter
5. Calibration and Verification
21
5. Calibration and Verification
Verification
WP4C uses the chilled mirror dewpoint technique for measuring water potential. This is a primary method but cleanliness of the instrument can affect the calibration. The
calibration slope is fixed during factory calibration. The user
just adjusts the zero offset. Calibration can be done with
any solution of known water potential. We recommend
using the 0.5 molal KCl verification standard from Decagon.
Verification Standards
Verification standards are specially prepared salt solutions
that have a specific molality and water potential. The
potassium chloride (KCl) verification standards are accurate, easy to use, and readily available from Decagon
Devices. Most importantly, they greatly reduce preparation errors. We therefore recommend using Decagon’s
KCl Performance Verification Standard for the most accurate verification of your WP4C’s performance.
The standards are produced under a strict quality assurance regime and are shelf stable for one year. If for some
reason you cannot obtain Decagon’s verification standards
and need to make a salt solution for verification, refer to
Appendix A.
WP4C PotentiaMeter
5. Calibration and Verification
22
When to Verify Calibration
The calibration of your WP4C should be checked with
the KCl standard before each use. It can also be checked
by measuring distilled water, but this is often not a good
choice for checking calibration. When using distilled
water, the humidity of the chamber approaches 100%
which can cause condensation to occur if the sample is
warmer than the chamber. For batch processing, the
instrument should be checked regularly against the KCl
standard. This will alert you to the possibility of contamination of the unit or shifts in the calibration from other
causes.
How to Verify and Calibrate the WP4C
Since errors in the calibration value result in errors in all
values subsequently measured, care should be taken to do
it right.
Checking and Adjusting Calibration
1. Press the upper right button in the system configuration menu to enter the calibration menu. You will be
guided through the calibration routine. The following
screen will appear:
Place standard
in drawer and read
WP4C PotentiaMeter
5. Calibration and Verification
23
2. Empty the whole vial of KCl solution into a sample
cup and place it in the WP4C’s sample drawer. Make
sure you calibrate using the same type of sample cup
(plastic of stainless steel) that you will make subsequent measurements with.
3. Carefully slide the drawer closed, being especially care-
ful so the solution does not splash or spill and contaminate the chamber. Check to be sure the sample
temperature is below chamber temperature (lower
right button).
4. Turn the drawer knob to the READ position to make
a reading. When the reading is complete, the following
screen will appear:
NOTE:
The WP4C will automatically shift to precise sam-
pling mode for the verification/calibration.
The reading should be within ± 0.05 MPa of the correct
reading of the KCl standard at that temperature. At 20 °C
this should be -2.19 MPa. At 25 °C this should be -2.22
MPa.
adjust +
-Exit- -184.95 -
use these buttons to adjust the value
WP4C PotentiaMeter
5. Calibration and Verification
24
5. If your WP4C is reading within 0.05 MPa of the KCl
solution, press
-Exit-
and proceed with reading. If it
is not, a change in calibration may have occurred, or
the sensor chamber may be contaminated. For cleaning
instructions, see Chapter 10. After cleaning, repeat
these instructions.
6. If, after cleaning, you consistently get readings that
differ from the correct water potential of the KCl by
more than ±0.05 MPa, a change in calibration has
probably occurred. Press the upper right button to
move the value up, the lower right button to move it
down. When it is at the correct value for the verification standard, press the -Exit- button. The value
will be stored.
Note: This is the only menu where these buttons can change the calibration, so you won’t hurt anything by pressing these buttons in other
menus.
7. Read the KCl standard again in the normal reading
mode. It should read the proper value.
8. If after adjusting the calibration and cleaning the chamber you still are getting incorrect readings when reading the KCl, contact Decagon at 509 332-2756 (1-800755-2751 in US and Canada) for further instructions.
WP4C PotentiaMeter
6. Sample Preparation
25
6. Sample Preparation
Your WP4C will continually provide accurate water potential measurements as long as its internal sensors are not
contaminated. Careful preparation and loading of samples
will lengthen time between cleanings and will help you
avoid downtime and repairs.
Choosing a Sample Cup
The WP4C comes with two types of samples cups: disposable plastic cups and stainless steel cups. The disposable
plastic cups are adequate for most samples, but are not
good for samples in the wet end. If you are measuring
samples with water potential wetter than -1 MPa, you
should use the stainless steel sample cups. You can also
oven dry soil samples directly in the stainless steel cups to
determine water content gravimetrically, which is convenient if you are generating soil moisture characteristic
curves. It is important to note that you must thoroughly
clean the stainless steel cups using de-ionized water
between uses to prevent solutes from contaminating subsequent samples and causing artificially negative osmotic
potential. Finally, if you calibrate the WP4C (see chapter
5), be sure to calibrate using the same type of sample cup
that you intend to use for subsequent measurements.
WP4C PotentiaMeter
6. Sample Preparation
26
Preparing the Sample
To prepare a sample, follow these steps:
1.
Place the sample in a disposable sample cup, completely covering the bottom of the cup, if possible.
WP4C may not be able to accurately measure a sample
that does not (or cannot) cover the bottom of the cup.
A larger sample surface area speeds up the reading by
shortening the time needed to reach vapor equilibrium.
It also increases instrument accuracy by providing more
stable infrared sample temperature measurements.
2. Do not fill the sample cup more than half full.
Overfilled cups may contaminate the sensors in
the chamber (more is not necessarily better!)
3. Make sure that the rim and outside of the sample
cup are clean. Wipe any excess sample material from
the rim of the cup with a clean tissue. Material left on
the rim or the outside of the cup will contaminate the
sensor chamber and will be transferred to subsequent
samples. The rim of the cup forms a vapor seal with
the sensor block when the drawer knob is turned to
the READ position. Therefore, any sample material
left on the cup rim will be transferred to the block,
preventing this seal and contaminating future samples.
4. If a sample will be read at some future time, put
the sample cup’s disposable lid on the cup to
WP4C PotentiaMeter
6. Sample Preparation
27
restrict water transfer. For short-term storage (< 3
hours) the cup lid is acceptable. If it will be a long time
before the measurement is made, seal the lid with tape
or Parafilm completely around the cup/lid junction.
Dry Samples
Samples that have a water potential drier than about -300
MPa cannot be accurately measured with the WP4C.
However, samples with such dry water potential values are
rare. When a sample’s water potential value is drier than
about -300 MPa, WP4C will display an error message indicating the lowest reading it could make on that particular
sample. For example, if you were measuring a dry sample
and the following screen appeared:
this screen indicates that the last water potential reading
the WP4C measured on this sample was -301.8 Megapascals. Therefore, the actual water potential of the sample is
lower than the instrument can measure.
Samples and Temperature
If samples are warmer than the chamber when they are
placed in it (Ts-Tb is a positive number), condensation
may occur and moisture may condense inside the block. In
order to prevent this, do the following:
< -820 MPa
WP4C PotentiaMeter
6. Sample Preparation
28
1. Place your sample in the chamber, slide the drawer
closed and press the lower right button to access the
sample temperature screen and look at the temperature difference. If the sample temperature is shown to
be a positive number, take the sample out immediately
and let it cool on a cold surface with the cup lid on it
to preserve the moisture. Do not cool the sample too
much, or the equilibrium time will be lengthened (ideally the Ts-Tb will be between -0.5 and 0) .
2. After cooling it for a minute or so, place the sample
back in and note the temperature difference. If it is
close enough to the block temperature, turn the knob
to the READ position to begin reading.
There is a linear relationship between the sample’s
dewpoint temperature and its water potential. The
dewpoint decreases -0.12 °C per MPa. For example, a very
dry sample at -40MPa can be 4.8 °C (-.12 x -40) above the
chamber temperature without condensing. A sample at -1
MPa (fairly dry for most soils) can be 0.12°C above the
chamber temperature without condensing. Therefore, if
you know the general range of your sample’s water
potential, you can gage at which temperature it will
condense moisture.
For samples that are more than 1°C below chamber
temperature, the WP4C waits until their temperature
increases to 1 °C below chamber temperature to start a
WP4C PotentiaMeter
6. Sample Preparation
29
reading. Readings are therefore speeded up if sample
temperature is just a little below chamber temperature.
Measuring Plant Samples
The WP4C can be used to measure the water potential of
leaves and plant material. Please refer to the application
note: Measurement of Leaf Water Potential Using the
WP4, which can be found at www.decagon.com.
WP4C PotentiaMeter
7. Taking a Reading
30
7. Taking a Reading
Once you have prepared your sample, you are ready to
take readings.
1. Turn the sample drawer knob to the OPEN/LOAD
position and pull the drawer open.
2. Place your prepared sample in the drawer. Check the
top lip of the cup to make sure it is free from sample
residue (remember, an over-filled sample cup will contaminate the chamber’s sensors).
3. Carefully slide the drawer closed, being especially careful if you have a liquid sample that may splash or spill
and contaminate the chamber.
4. Access the sample temperature menu (press lower
right button) to watch the temperature difference
between the sample and the instrument.
5. Turn the sample drawer knob to the READ position
to seal the sample cup with the chamber. The instrument will beep once, and the green light will flash
once to indicate that the reading cycle has started. In
about 40 seconds, the first measurement will be displayed.
WP4C PotentiaMeter
7. Taking a Reading
31
How WP4C takes Readings
WP4C’s cooled mirror is controlled at the chamber dew
point and its temperature is measured.. When the instrument has finished its read cycle, the water potential is displayed, accompanied by the LED flash and beeper (if you
have the beeper enabled).
Cautions
• Never leave a sample in your WP4C after a reading has
finished. The sample may spill and contaminate the
instrument’s chamber if the instrument is accidentally
moved or jolted.
• Never try to move your instrument after a sample has
been loaded. Movement may cause the sample material to spill and contaminate the sample chamber.
• Take special care not to move the sample drawer too
quickly when loading or unloading liquid samples, in
order to avoid spilling.
• If a sample has a temperature that is higher than the
WP4C’s chamber (Ts-Tb is a positive number), take
the sample out immediately, put a cap on it, and cool
it. Warm samples can cause condensation in the
chamber and adversely affect subsequent readings.
WP4C PotentiaMeter
7. Taking a Reading
32
• The physical temperature of the instrument should be
between 5°C—43°C. Between these temperatures,
WP4C will measure samples of similar temperature
quickly and accurately.
• If you are reading and a triangular warning symbol
appears in the top right corner of the display, this indicates that the mirror has become too dirty to give
accurate measurements. You will need to clean the
mirror and chamber before continuing to sample. For
more details about this symbol, please refer to Chapter
12. For cleaning instructions, refer to Chapter 10.
C
pF 7.38 24.9°C
-1.84 MPa
p
WP4C PotentiaMeter
8. Computer Interface
33
8. Computer Interface
Your WP4C was shipped to you with a standard RS-232A
interface cable. Using this, you can use your computer’s
terminal program1 to send water potential data to your
computer for further analysis and storage. Set up your terminal program as follows:
1. Select the COM Port your RS-232 cable is connected to.
2. Settings are the following: Bits per second, 9600; 8
databits, no parity, 1 stop bit, and flow control set to
hardware. Click OK.
3. Plug your RS-232 cable to the COM port you selected
and connect it to your WP4C. Begin reading. Your
WP4C’s data will be displayed on screen as it samples.
It will be output in the following format:
4. When you are finished reading, you can print the data
3, 24.3, -2.19, 4.16
time (minutes)
since sample
was started
temp
(°C)
Water
potential
pF
(MPa)
WP4C PotentiaMeter
8. Computer Interface
34
in the terminal session, or cut and paste it to a spreadsheet or text editor.
1
A good, free terminal program can be found at
http://ttssh2.sourceforge.jp.
WP4C PotentiaMeter
9. Theory: Water Potential
35
9. Theory: Water Potential
Water Potential
Water potential is defined as the potential energy per unit
volume of water in a sample. The total water potential of a
sample is the sum of four component potentials: gravitational, matric, osmotic, and pressure. Gravitational potential depends on the position of the water in a gravitational
field. Matric potential depends on the adsorptive forces
binding water to a matrix. Osmotic potential depends on
the concentration of dissolved substance in the water.
Pressure potential depends on the hydrostatic or pneumatic pressure on the water.
The WP4C measures the sum of the osmotic and matric
potentials in a sample. Often one or the other of these
potentials will be the dominant factor in determining the
total potential. For example, solutions like the KCl calibration standard have only an osmotic component. Soils bind
water mainly through matric forces, and therefore have
mainly a matric component (though salt-affected soils can
have a significant osmotic component).
Measuring Water Potential
The water potential of a solid or liquid sample can be
found by relating the sample water potential reading to the
WP4C PotentiaMeter
9. Theory: Water Potential
36
vapor pressure of air in equilibrium with the sample. The
relationship between the sample’s water potential (Ψ) and
the vapor pressure of the air is:
where p is the vapor pressure of the air, po is the saturation
vapor pressure at sample temperature, R is the gas constant (8.31 J/mol K), T is the Kelvin temperature of the
sample, and M is the molecular mass of water. The vapor
pressure of the air can be measured using a chilled mirror,
and po is computed from sample temperature.
The WP4C measures water potential by equilibrating the
liquid phase water of the sample with the vapor phase
water in the headspace of a closed chamber, then measuring the vapor pressure of that headspace. In the WP4C, a
sample is placed in a sample cup, which is sealed against a
sensor block. Inside the sensor block is a fan, a dew point
sensor, a temperature sensor, and an infrared thermometer. The dew point sensor measures the dew point temperature of the air, and the infrared thermometer measures
the sample temperature. The purpose of the fan is to
speed equilibrium and to control the boundary layer conductance of the dew point sensor.
From these measurements, the vapor pressure of the air in
the headspace is computed as the saturation vapor pres-
Ψ
RT
M
-------
p
p
o
-----ln⋅=
WP4C PotentiaMeter
9. Theory: Water Potential
37
sure at dewpoint temperature. When the water potential
of the sample and the headspace air are in equilibrium, the
measurement of the headspace vapor pressure and sample
temperature (from which saturation vapor pressure is calculated) gives the water potential of the sample.
In addition to equilibrium between the liquid phase water
in the sample and the vapor phase, the internal equilibrium of the sample itself is important. If the sample is not
at internal equilibrium, one might measure a steady vapor
pressure (over the period of measurement) which is not
the true water potential of the sample.
Effect of Temperature on Water
Potential
Temperature plays a critical role in water potential determinations. Most critical is the measurement of the difference between sample and dew point temperature. If this
temperature difference were in error by 1°C, an error of 8
MPa would result. In order for water potential measurements to be accurate to 0.05 MPa, temperature difference
measurements need to be accurate to 0.006°C.
WP4C’s infrared thermometer measures the difference in
temperature between the sample and the block. It is carefully calibrated to minimize temperature errors, but
achieving 0.006°C accuracy is difficult when temperature
differences are large. Best accuracy is therefore obtained
when the sample is near chamber temperature.
WP4C PotentiaMeter
9. Theory: Water Potential
38
Another effect of temperature on water potential occurs
with samples that are near saturation (like many soil samples). A sample that is close to 0.00 MPa and is only
slightly warmer than the sensor block will condense water
within the block. This will cause errors in the measurement, and in subsequent measurements until the condensation disappears. The Ts-Tb function helps the user
ensure that the sample won’t condense on the sensor
block.
Estimating Osmotic Potential
The WP4C measures the sum of osmotic and matric
potential. An approximate value for the osmotic potential
can be found by measuring the electrical conductivity
(EC) of the saturation extract of the soil. The osmotic
potential of the saturation extract is computed from:
Ψ
os
(MPa) = -0.036EC (dS/m)
The osmotic component of the water potential is then
computed from:
where θ is the volumetric water content of the sample and
θs is the volumetric water content at saturation. The
matric potential is the total potential minus the osmotic.
Ψ Ψ
os
θ
s
θ
-----
=
WP4C PotentiaMeter
10. Cleaning and Maintenance
39
10. Cleaning and Maintenance
The accuracy of your WP4C is vitally dependent on keeping your instrument clean. Dust and sample debris can
contaminate the sampling chamber and must therefore be
regularly cleaned out. To clean your instrument, carefully
follow these instructions.
Tools Needed
• WP4C Cleaning Kit
Note: Kimwipes® are included in the WP4C Cleaning Kit. They
are ideal for cleaning because they don’t leave much of a lint residue
like most tissues. They also don’t have any other compounds in the
tissue that may contaminate the sensors in the WP4C’s block. Never
use cotton swabs to clean the block sensors. Most cotton swabs contain adhesives and other compounds that are released and transferred
to the mirror and other surfaces, contaminating them.
Accessing the Block
1. Unplug your WP4C.
2. Remove the case lid screw located on the back panel.
Carefully remove the lid by pulling the back of the lid
WP4C PotentiaMeter
10. Cleaning and Maintenance
40
upward and then sliding the lid back (away from the
front of the case) and off.
3. Unscrew the two thumbscrews that secure the sensor
block.
4. Unplug the cable with the 20-pin socket that attaches
the block to the main circuit board by releasing the
two locking levers that are on either side of the socket.
5. Carefully lift the block straight up from its mount.
Turn the block over to expose the chamber cavity as
shown in the illustration:
View of inside block chamber
thermopile
mirror
optical sensor
chamber fan
WP4C PotentiaMeter
10. Cleaning and Maintenance
41
Cleaning Procedure:
Cleaning your WPC4 is a multi-step procedure which
involves washing, rinsing, and drying for each specific area
as outlined below:
1. Cleaning the Inside Chamber
Note: Be extremely careful not to damage the fan blades in the chamber. The fan blades are very fragile; if one of them breaks, your
instrument won’t work properly. Therefore, take extra care when
cleaning this portion.
a. Remove any debris that may have collected within or
around the sample chamber.
b. Wrap a NEW Kimwipe around the end of the thin
plastic rod (spatula) and moisten it with isopropyl
alcohol or Decagon Cleaning Solution. Note: Do
NOT dip a used Kimwipe into your container of IPA
or cleaning solution (the IPA or cleaning solution will
become contaminated).
c. WASH--Clean all surface edges of the samples cham-
ber including the edge where the sample cup seals to
the chamber block. You may need to replace the Kimwipe if it becomes too dirty during this process.
d. RINSE--Repeat steps b and c using new Kimwipes
with distilled water.
e. DRY--Repeat steps b and c using new, dry Kimwipes
to help remove any moisture remaining from the
cleaning.
WP4C PotentiaMeter
10. Cleaning and Maintenance
42
f. Visually inspect the sample chamber for cleanliness.
Re-clean if necessary. Note: Do not reuse Kimwipes.
2. Cleaning the Mirror
NOTE: Wash hands with soap and water (to prevent oils from contaminating the Kimwipe tissue and being transferred to the mirror).
a. Wrap a NEW Kimwipe around the end of the thin
plastic rod (spatula) and moisten it with isopropyl
alcohol or Decagon Cleaning Solution.
b. WASH--Carefully clean the mirror with the moist
Kimwipe.
c. RINSE--Repeat steps b and c using new Kimwipes
with distilled water.
d. DRY--Repeat steps b and c using new, dry Kimwipes
to help remove any moisture remaining from the
cleaning.
e. Visually inspect the mirror for cleanliness. Re-clean if
necessary. Note: Do not reuse Kimwipes.
3. Cleaning the Optical Sensor
You will probably clean the optical sensor while you are
cleaning the mirror, since they face each other in a very
small gap. Clean it in the same manner as described above
for the mirror.
WP4C PotentiaMeter
10. Cleaning and Maintenance
43
4. Cleaning the Thermopile
a. Wrap a new Kimwipe around the end of the thin plas-
tic rod (spatula) and moisten it with isopropyl alcohol
or Decagon Cleaning Solution.
b. WASH--Swipe the moistened Kimwipe across ther-
mopile. (A single swipe is usually sufficient to remove
contaminants.)
c. RINSE--Repeat steps a-b using new Kimwipes moist-
ened with distilled water instead of cleaning solution.
d. DRY--Repeat steps a-b but use a new, dry Kimwipe to
help remove any moisture remaining from the cleaning.
e. Visually inspect the thermopile for cleanliness. This
sensor must be free of all dirt and lint. Re-clean if necessary.
5. Inside Case
1. Clean the sample drawer and drawer base as described
above for the thermopile. Remove any debris that may
have collected inside the case.
2. Check once more to make sure there is no contamina-
tion of the sample chamber cavity.
3. Replace the block, and insert the ribbon cable socket
into to the 20-pin plug on the block. Lock it in place
with the locking levers.
WP4C PotentiaMeter
10. Cleaning and Maintenance
44
4. Screw the thumb-screws on the block back in until
they are hand-tight.
5. Replace the case lid and secure the lid screw.
6. Connect the WP4C’s power cord.
Checking Calibration
After you have cleaned the chamber and other parts of
your WP4C, it is important to check the instrument’s performance in order to correct for any calibration change
that may have occurred during cleaning procedures.
Check the calibration of your instrument by measuring the
water potential of the KCl standard. If a change has
occurred, refer to chapter 5 for directions on how to recalibrate. If, after adjusting calibration your instrument is
still not reading samples correctly, contact Decagon for
technical support.
WP4C PotentiaMeter
11. Repair Instructions
45
11. Repair Instructions
If your WP4C ever needs to be sent in for service or
repair*, call Decagon at (509) 332-2756 or 1-800-755-
2751, or fax us at (509) 332-5158. We will ask you for
your address, phone number, and serial number. For nonwarranty repairs, we will also ask for a payment method
(such as a purchase order or credit card number), a repair
budget, and billing address.
*Note: If you purchased your WP4C from one of our international
distributors, please contact them before contacting Decagon. They
may be able to provide you with local service and help you remedy the
problem.
Shipping Directions
When you ship your instrument back to us, please include
with it a document with the complete shipping address,
name and department of the person responsible for the
instrument, and (most importantly) a description of the
problem. This information will better help our technicians
and our shipping department to expedite repair on your
instrument and ship it back to you in good time.
All WP4Cs returning to Decagon for servicing must be
accompanied with a Return Material Authorization (RMA)
WP4C PotentiaMeter
11. Repair Instructions
46
form. Prior to shipping the instrument, please contact a
Decagon customer support representative to obtain a
Return Merchandise Authorization (RMA).
Following are steps that will help in safely shipping your
instrument back to us:
1. If possible, ship your WP4C back in its original cardboard box with foam inserts. If this is not possible, use
a box that has at least 4 inches of space between your
instrument and each wall of the box. If you aren’t
using the foam inserts, pack the box moderately tight
with packing material, like styrofoam peanuts
2. Put your instrument in a plastic bag to avoid disfiguring marks from the packaging.
3. Don’t ship your WP4C to us with the power cord; we
have plenty here to use with your instrument, and it
may damage the instrument in shipment.
4. Please review the RMA form and verify the ship to
and bill to information, contact name, and problem
description. If anything is incorrect, please contact a
Decagon representative.
5. Tape the box in both directions so it will not break
open in shipment.
6. Include the RMA number in the attention line on the
shipping label.
WP4C PotentiaMeter
11. Repair Instructions
47
Ship to:
Decagon Devices Inc.
ATTN: Repair Department RMA#_______
2365 NE Hopkins Court
Pullman, WA 99163
Repair Costs
Manufacturer’s defects and instruments within the oneyear warranty will be repaired at no cost. For non-warranty repairs (including cleanings for instruments in their
warranty period), costs for parts, labor, and shipping will
be billed to you. We have a minimum repair charge, and an
extra fee will be charged for rush work. Decagon will provide an estimated repair cost, if requested.
Loaner Service
We have loaner instruments that can be provided while
your instrument is being serviced. There is, however, a
limited number of loaner instruments. They are granted
on a “first-come-first-served” basis. This service is in
place to help you if your WP4C needs service during critical operations.
WP4C PotentiaMeter
12. Troubleshooting
48
12. Troubleshooting
WP4C is a high performance instrument, designed to have
low maintenance and few problems if used with care.
Unfortunately, sometimes even the best operators using
the best instruments encounter technical difficulties. Here
is a list of some problems that may occur. If you have
encountered a problem that isn’t addressed here, or if
these remedies still don’t resolve your problem, contact
Decagon at 1-800-755-2751 (US only) or (509) 332-2756
(for those outside the US). If purchased from a distributor, please contact the distributor for assistance first.
Problems and Solutions
The following table is a brief guide to help you quickly
define solutions to your problems. For more detailed
descriptions of these problems and their solutions, see the
explanations below the table.
Table 1: Troubleshooting
Problem Possible Solutions
1. Won’t turn on • Power cord discon-
nected
• Blown fuse
WP4C PotentiaMeter
12. Troubleshooting
49
2. Long read time • Dirty sample chamber
• Sample desorbs slowly
• Broken or bent chamber
fan blade
3. Readings on KCl
standards are too high/
low to adjust
• Dirty thermopile
• If using own salt solu-
tion, it may not be in
equilibrium
4. < -301.8 MPa • Sample too dry to accu-
rately measure
• Dirty chamber or mirror
5. Triangle appears in
upper right corner
• Mirror is dirty
6. “block failure”
appears on screen after
turning on WP4C
• Block is disconnected
from motherboard
• Block memory compo-
nent has failed
7.
7.7.
7. “set T” option no
longer appears on system config screen
Temperature control mod-
ule in WP4C is broken
or not functioning
Table 1: Troubleshooting
Problem Possible Solutions
WP4C PotentiaMeter
12. Troubleshooting
50
1. PROBLEM:
WP4C won’t turn on.
SOLUTION:
• Check to make sure your power cord is securely
attached the back of the instrument, and into the
power outlet.
• A power surge may have caused a fuse to blow. To
change the fuses, follow these instructions:
1. Unplug the power cord from the wall and the instrument.
2. Locate the panel where the power cord plugs in. The
fuse box is on the right side of that panel. Press in on
the release tab and pull the fuse-holder out.
3. Pull the broken fuse(s) out and replace with a 2.0 Amp
250V fuse.
Caution: Do not use any other kind of fuse or you will risk damage
to your instrument and void your warranty.
4. Replace the fuse-holder and push it into the fuse-well
until the release tab snaps in place.
5. Re-connect the power cord and turn your instrument
WP4C PotentiaMeter
12. Troubleshooting
51
on. If the fuse blows again, a failed component may be
causing the problem. Contact Decagon to make
arrangements for repairs.
2. PROBLEM:
Readings are slow or inconsistent.
SOLUTION:
• The sample chamber may be dirty. Refer to Chap-
ter 10 of the manual for directions on cleaning the
sample chamber.
• Some samples absorb or desorb moisture very
slowly, causing measurements to take longer than
usual, and nothing can be done to speed up the process. Refer to Chapter 6 for further explanation.
•
The fan blade inside the block chamber may be broken.
If even the KCl standard takes a long time to read,
and the sample chamber is clean, you may have a broken or
bent chamber fan blade. This is especially likely if you have
just cleaned the chamber. If you suspect this may have happened, contact Decagon for details on replacement.
3. PROBLEM:
Water potential readings on KCl standards are too
high/low and a calibration adjustment cannot be made
any higher/lower.
WP4C PotentiaMeter
12. Troubleshooting
52
SOLUTION:
• The thermopile in your chamber, which measures
sample temperature, may have become contaminated. Refer to Chapter 10 for directions on cleaning.
• If you weren’t using Decagon’s KCl verification
standards, high readings may indicate that the salt
solution you are using is not in equilibrium.
4. PROBLEM:
Message on screen displays the following (example):
SOLUTION:
• The sample is too dry for the instrument to read
accurately. If your sample has a water potential that is
above the detection limits of the instrument, this message will come up. Essentially, it means that there is
not enough sample moisture to condense on the mirror and provide a reading.
• The mirror may be dirty. Try cleaning the mirror
and chamber and measuring the sample again.
< -820 MPa
WP4C PotentiaMeter
12. Troubleshooting
53
5. PROBLEM:
A small triangle appears in the upper right corner after
reading:
SOLUTION:
The mirror needs to be cleaned, along with the rest of the
sample chamber, until it disappears. This triangle is a mirror
performance indicator. When the WP4C senses that the
mirror performance has dropped to unacceptable levels, it
will display the triangular warning sign after the sample has
been measured. When this appears, you should stop sampling and clean the chamber. If the triangle is still on the
screen after cleaning, the mirror is most likely still dirty and
you will need to clean it until the triangle disappears.
6. PROBLEM:
The following screen comes up after turning on the
machine:
C
pF 7.38 24.9°C
-1.84 MPa
p
block failure
WP4C PotentiaMeter
12. Troubleshooting
54
SOLUTION:
• The block is not plugged in to the motherboard. Open
the case and check to make sure that the small ribbon
cable that connects the block to the motherboard is
snapped and locked in place.
• One or more components has failed on the block’s circuit board. If the block is properly plugged in to the
motherboard and this message appears, it is likely that
one or more of the components have failed on the
block’s circuit board.
Sensor Performance Screen
If, after cleaning your instrument and reading the other
troubleshooting hints, you have reason to believe that one
of the components of your WP4C may be causing measurement error, you can access a screen that will display
values for component performance. This is done by holding down the lower right button while turning on the
instrument. After it initializes, it will beep and come to the
following screen:
This screen gives you four values. The top left value is the
value the thermocouple is reading. It is basically the differ-
-Exit-
sensors
3.36 0.030
23.5 0.665
WP4C PotentiaMeter
12. Troubleshooting
55
ence in temperature between the block and the mirror. If
this is zero, there is something wrong with the thermocouple. The top right value is the value read by the thermopile, which is the temperature difference between the
block and what it “sees” below it (the sample, when reading). This value is variable, but should never be zero. The
bottom left value is the block temperature. This value
should be around ambient temperature. The bottom right
value is the mirror reflectance voltage, in units of volts.
This value should normally be around 0.5 or above, but if
it drops below 0.3, there is something wrong.
You can’t change anything in this screen, but it is here to
give you an indication of the component performance. If
you notice that any of these values are not what they
should be, contact Decagon at support@decagon.com
for further instruction. Press the button next to
-Exit-
to get back to the main menu.
WP4C PotentiaMeter
13. Further Reading
56
13. Further Reading
References:
Brye, K.R. (2003) Long-term effects of cultivation on par-
ticle size and water-retention characteristics determined using wetting curves. Soil Science Society of
America 168:7 459-468.
Campbell, E.C., G.S. Campbell, and W.K. Barlow. (1973) A
dewpoint hygrometer for water potential measurement. Agric. Meteor. 12:113-121.
G.W. Gee, M.D. Campbell, G.S. Campbell, and J.H. Camp-
bell. (1992) Rapid measurement of low soil water
potentials using a water activity meter. Soil Science
Society of America 56:4 1068-1070.
Papendick, R.I. and G.S. Campbell. (1980) Theory and
measurement of water potential. in Water Potential
Relations in Soil Microbiology. Soil Science Society of America. Madison, Wisconsin. pp.1-22.
WP4C PotentiaMeter
13. Further Reading
57
Application Notes
The following WP4C application notes are available from
Decagon by request and via our website under the education tab, choose WP4C.
• Generating a Soil Moisture Characteristic with the
WP4C.
• Measuring Leaf Water Potential using the WP4C.
• Field Portability Instructions for the WP4C.
• Water Potential: The Key to Successful Seed Priming.
• Seed Longevity in Storage is Enhanced by Controlling
Water Activity.
• Classification of Expansive Soils using the WP4C
Dewpoint PotentiaMeter
WP4C PotentiaMeter
Appendix A
58
Appendix A
Preparing Salt Solutions
Following is a table showing the water potential at given
concentrations of NaCl and KCl at 20°C.
Table 2: Water Potential of NaCl and KCl in
Megapascals (MPa)
Concentration
(Moles/kg)
NaCl KCl
0.05 -0.232 -0.232
0.10 -0.454 -0.452
0.20 -0.901 -0.888
0.30 -1.349 -1.326
0.40 -1.793 -1.760
0.50 -2.242 -2.190
0.60 -2.699 -2.622
0.70 -3.159 -3.061
0.80 -3.618 -3.501
0.90 -4.087 -3.931
1.00 -4.558 -4.372
WP4C PotentiaMeter
Appendix A
59
WP4C PotentiaMeter
Appendix A
60
Index
A
Accessories 7
Application notes 57
B
Beeper 16
changing
Block
sensors 39
Block failure 49,53
Buttons
for menu selection 12
C
c for continuous mode 14
Calibration
changes in 21
menu 17
steps 22–24
when to check 22
Calibration drift 21
Cautions 31
Checking calibration 44
Chilled-mirror technique 5
Cleaning 39
inside case 43
optical sensor 42
sensor block 39
Components 7
Computer interface 33
Continuous mode 14
Cotton swabs
not used for cleaning 39
Customer service 1
Czech 13
D
Danish 13
Dirty mirror indicator 32
Distilled water 22
Dry samples 52
Dry water potential 27
E
E-mail address 2
Error messages 48,52
Exit 17
F
Fan
inside sample chamber 6
Fast mode 15
Fax number 2
Features 8
Field measurements 10
Flashing. See LED
French 13
Further reading 56
Fuse
changing 50
G
German 13
Gravitational potential 35
I
Instrument 4
Italian 13
K
KCl standards 21
L
Languages
changing 13
Leaf
measuring water potential in continuous mode 15
LED 16
Linear offset
how to adjust for 22
Loaners 47
Location
for reading 8
M
Main menu 10, 12
Maintenance 39
Manual 1
Matric potential 35
Menus
main menu 12
system configuration 15
Molality
of verification standards 21
N
Norwegian 13
O
Osmotic 35
Osmotic potential 38
P
Peltier cooler 5
Plant samples 29
Portability 10
Portuguese 13
Precise mode 13
Preparing salt solutions 58
Pressure potential 35
R
Read time
affected by sample temp. 27
long read time 49, 51
Reading modes 13
Continuous 14
Fast 15
Precise 13
Readings
cautions 31
how WP4C takes 31
taking readings 30
References 56
Repair
costs 47
instructions for 45
shipping 45
S
Salt standards. See verification standards
Sample
dry water potential 27
not at room temperature 27
preparation 25
slow water-emitting 51
too dry 52
too hot 31
Sample cups
cleaning 26
filling level 26
sealing 26
Sample equilibration screen 6, 20
Saturated salts 58
Seller’s liability 3
Sensor performance screen 54
Shipping for repair 45
Spanish 13
Spilling the sample 31
Swedish 13
T
TE set 18
Technical support 1
Telephone number 2
Temperature
effects on reading 6
effects on water potential 37
hot samples 31
of instrument 31
samples not at room temp. 27
Time
long read times 51
Toll-free number 2
Triangle 32
Triangle 53
Troubleshooting 48
V
Vapor pressure 5, 36
Verification standards 21
long read times for 51
water potential readings too high/low for 51
W
Warm-up 10
Warranty 2
Water content
definition 35
vs. water potential 35
Water potential 58
definition 5, 35
displayed 10
equation 36
measuring 35
theory 35
WP4C
and temperature 6
Chilled mirror dewpoint technique 5
important tips 19
measuring water potential 36
preparing for operation 9
theory 35
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