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Decagon WP4-T Operator's Manual

Dewpoint potentiameter
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WP
4
Dewpoint PotentiaMeter
for models WP4 and WP4-T
Operator' s Manual
Version 2.1
Decagon Devices, Inc.

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Summary of Contents for Decagon WP4-T

  • Page 1 Dewpoint PotentiaMeter for models WP4 and WP4-T Operator’ s Manual Version 2.1 Decagon Devices, Inc.
  • Page 2 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. ©...

  • Page 3: Table Of Contents

    PotentiaMeter Table of Contents Contents 1. Introduction ......1 About this Manual .......1 Customer Service .
  • Page 4 PotentiaMeter Table of Contents When to Verify Calibration .....26 How to Verify and Calibrate the WP4 ..27 6.
  • Page 5 PotentiaMeter Table of Contents Component Performance Screen ....63 13. Further Reading ....65 References: .
  • Page 6 PotentiaMeter Table of Contents...

  • Page 7: Introduction

    PotentiaMeter Introduction 1. Introduction Welcome to Decagon’s WP4 Dewpoint PotentiaMe- ter, 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.

  • Page 8: Warranty

    E-mail: If you need technical support, you can send us mes- sages 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 num- ber, and return e-mail address.

  • Page 9: Note To Our Wp4 Users

    Please include all the information requested on the card. It is necessary for Decagon to have your current mail- ing address and telephone number in case we need to send updated product information to you.
  • Page 10 PotentiaMeter Introduction ered hereby which is not manufactured by Seller shall be covered only by the warranty of its manu- facturer. 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 prof- its), occasioned by or arising out of the installation, operation, use, misuse, nonuse, repair, or replace-...

  • Page 11: About The Wp4

    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 sta- tus of the water in a system.

  • Page 12: How The Wp4 Works

    PotentiaMeter About the WP4 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 mea- surement chamber. For a more detailed description of water potential, please refer to Chapter 9, titled “Theory: Water Potential”...

  • Page 13: Wp4 And Temperature

    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 accord- ing to how it is programmed.
  • Page 14 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...
  • Page 15 PotentiaMeter About the WP4 configuration menu (see page 21) that allows the user to set the temperature.

  • Page 16: Getting Started

    PotentiaMeter Getting Started 3. Getting Started Components of your WP4 Your WP4 should have been shipped with the fol- lowing items: • WP4 main unit • Quickstart guide • Certificate of Calibration • Power cord • RS-232 interface cable • 100 disposable sample cups •...

  • Page 17: Features

    PotentiaMeter Getting Started consistently, place it on a level surface. This reduces the chance that sample material will spill and con- taminate the inside of the instrument. To protect the internal electrical components, and to avoid inaccu- rate readings, place your WP4 in a location where the temperature remains fairly stable.

  • Page 18: Preparing The Wp4 For Operation

    PotentiaMeter Getting Started Lid thumb-screw RS-232 interface Power cord Plug fuse well ON/OFF Switch 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”...

  • Page 19: Portability

    PotentiaMeter Getting Started Then: 0.00 MPa pF 0.00 0.0°C This is the main menu, displaying the water poten- tial in both MegaPascals (MPa) and pF, and the sam- ple 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.
  • Page 20 PotentiaMeter Getting Started 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.

  • Page 21: The Menus

    PotentiaMeter Menus 4. The Menus The Main Menu 0.00 MPa pF 0.00 0.0°C 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 men- tioned earlier, the water potential and sample tem- perature are displayed on the screen.

  • Page 22: Changing Languages

    PotentiaMeter Menus 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.

  • Page 23: Continuous Mode

    PotentiaMeter Menus a brief summary of the normal sampling mode fea- tures: • 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.

  • Page 24: System Configuration

    PotentiaMeter Menus 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 con- nect their WP4 to a computer while in continuous mode in order to log and store data over time.
  • Page 25 Icon Mode indicator (continuous shown) set T -Exit- temp. set (WP4-T model only) 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 flash- ing green LED, located on the left front corner of the WP4’s case.
  • Page 26 PotentiaMeter Menus finished the LED will flash 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: No beeping.
  • Page 27 Setting the Temperature (WP4-T only) If you have a WP4-T, you have the ability to manu- ally set your instrument’s sample chamber tempera- ture. This is done by pressing the lower right button TE set next to the “...
  • Page 28 Note that if it is set to 0, the WP4 will begin taking readings when the temperature is within 4 degrees, but you will not get a final reading until the temperature has stabilized enough for the instrument to make an accurate final value. Important tips with the WP4-T...

  • Page 29: Sample Equilibration Screen

    PotentiaMeter Menus • Before sampling, wait for approximately 30 minutes to let the chamber’s temperature to stabilize after turning it on. • The instrument may have a difficult time measuring the temperature of a sample that is being cooled if the ambient temperature is high.
  • Page 30 PotentiaMeter Menus tion. The following screen will appear: Ts = 24.9 Ts - Tb = -0.07 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.

  • Page 31: Calibration And Verification

    When this occurs, it changes the accuracy of your readings. In final test- ing at Decagon, the slope of the instrument is fixed. However, due to the reasons noted above, some drift is expected over time. This is compensated for by verification of the WP4’s calibration before use.

  • Page 32: When To Verify Calibration

    Decagon Devices. Most importantly, they greatly reduce preparation errors. Because of these reasons, we recommend using Decagon’s KCl Performance Verification Standard for the most accurate verification of your WP4’s performance.

  • Page 33: How To Verify And Calibrate The Wp4

    PotentiaMeter Calibration and Verification Standards How to Verify and Calibrate the Since errors in the calibration value result in errors in all values subsequently measured, care should be taken to do it right. Checking Calibration To check the calibration of your WP4, do the fol- lowing: 1.
  • Page 34 PotentiaMeter Calibration and Verification Standards 4. If your WP4 is reading within 0.1 MPa of the KCl solution, proceed with sampling. If it is not, a change in calibration may have occurred, or the sensor chamber may be contaminated. For cleaning instructions, see Chapter 10.
  • Page 35 PotentiaMeter Calibration and Verification Standards 2. If you wish to continue, press the button next to “yes.” To return to the main menu, press the but- ton next to “no.” After selecting “yes,” the fol- lowing screen will appear: Place standard in drawer and read 3.
  • Page 36 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 (1-800-755-2751 in US and Canada) for further instructions.

  • Page 37: Sample Preparation

    PotentiaMeter Sample Preparation 6. Sample Preparation Your WP4 will continually provide accurate water potential measurements as long as its internal sen- sors are not contaminated. Careful preparation and loading of samples will lengthen time between cleanings and will help you avoid downtime and repairs.

  • Page 38: Dry Samples

    PotentiaMeter Sample Preparation 3. Make sure that the rim and outside of the sample cup are clean. Wipe any excess sam- ple 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.

  • Page 39: Samples And Temperature

    If you have a WP4-T, you can set the chamber temperature to eliminate this problem. If you have a standard WP4, when the...
  • Page 40 PotentiaMeter Sample Preparation Ts-Tb = -0.49 As mentioned earlier, this screen shows you the dif- ference in temperature between the sample temper- ature (Ts) and the block temperature (Tb). When Ts-Tb is in the 0 to -0.5 range, the temperatures are close enough that your read time should not be very long.
  • Page 41 PotentiaMeter Sample Preparation cup lid on it to preserve the moisture. Try not to cool the sample too much, since this will lengthen the equilibrium time. 2. After cooling it for a minute or so, place the sample back in and note the temperature differ- ence.

  • Page 42: Taking A Reading

    PotentiaMeter Taking a Reading 7. Taking a Reading Once you have prepared your sample, you are ready to take readings. The process is simple: 1. Turn the sample drawer knob to the OPEN/ LOAD position and pull the drawer open. 2.

  • Page 43: How Wp4 Takes Readings

    PotentiaMeter Taking a Reading light will flash once to indicate that the reading cycle has started. In about 40 seconds, the first measurement will be displayed. How WP4 takes Readings WP4’s cooled mirror crosses the dew threshold numerous times to ensure the accuracy of readings. 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...
  • Page 44 PotentiaMeter Taking a Reading higher (Ts-Tb is a positive number) than the WP4’s chamber, take it out immediately, put the cap on it, and let it cool for a few minutes before sampling. Warm samples can cause con- densation in the chamber and adversely affect subsequent readings.

  • Page 45: Increasing Your Accuracy

    ±.05 MPa: • For successful operation, the WP4 requires very precise temperature measurement and control. Therefore, the WP4-T model will work best for eliminating these variables. For a standard WP4 (not temperature-controlled), it is best to make measurements in a room with fairly constant, or...
  • Page 46 Adjust your sample readings up or down according to the amount of variation from the 0.5 molal KCl. • If using a WP4-T, set the block temperature to best match the temperature of your samples. This will help reduce error resulting from...

  • Page 47: Computer Interface

    PotentiaMeter Computer Interface 8. Computer Interface Your WP4 was shipped to you with a standard RS- 232A interface cable. Using this, you can use your computer’s terminal program to send water poten- tial data to your computer for further analysis and storage.
  • Page 48 PotentiaMeter Computer Interface the screen labeled “Connect Using” and select the COM Port your RS-232 cable is connected to. 4. A pop-up menu labeled “COM Properties” will appear, showing the port settings for the COM port you selected. Make sure the settings are the following: Bits per second, 9600;...
  • Page 49 PotentiaMeter Computer Interface save the data, go into the Transfers menu and select “Capture text,” and designate where you would like it saved.

  • Page 50: Theory: Water Potential

    PotentiaMeter Theory 9. Theory: Water potential Water potential Water Potential is defined as the potential energy of water per unit mass of water in the system. The total water potential of a sample is the sum of four com- ponent potentials: gravitational, matric, osmotic, and pressure.

  • Page 51: Measuring Water Potential

    PotentiaMeter Theory Measuring Water Potential The water potential of a solid or liquid sample can be found by relating the sample water potential reading to the vapor pressure of air in equilibrium with the sample. The relationship between the sam- ple’s water potential (Ψ) and the vapor pressure of the air is: Ψ...

  • Page 52: Effect Of Temperature On Water Potential

    PotentiaMeter Theory fan is to speed equilibrium and to control the boundary layer conductance of the dew point sen- sor. From these measurements, the vapor pressure of the air in the headspace is computed as the satura- tion vapor pressure at dewpoint temperature. When the water potential of the sample and the headspace air are in equilibrium, the measurement of the head- space vapor pressure and sample temperature (from...
  • Page 53 PotentiaMeter Theory MPa, temperature difference measurements need to be accurate to 0.005°C. WP4’s infrared thermometer measures the differ- ence in temperature between the sample and the block. It is carefully calibrated to minimize tempera- ture errors, but achieving 0.005°C accuracy is diffi- cult when temperature differences are large.

  • Page 54: Cleaning And Maintenance

    PotentiaMeter Cleaning and Maintenance 10. Cleaning and Maintenance The accuracy of your WP4 is vitally dependent on keeping your instrument clean. Dust and sampling debris can contaminate the sampling chamber and must therefore be regularly cleaned out. To clean your instrument, carefully follow these instructions. Tools Needed •...

  • Page 55: Cleaning The Block And Sensors

    PotentiaMeter Cleaning and Maintenance 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. Cleaning the Block and Sensors Accessing the Block 1.
  • Page 56 2. Wrap 1.5 — 2cm wide strips of Kimwipe around the plastic spatula 3. Place a few drops of Decagon’s cleaning solu- tion on the Kimwipe (this works best). Other- wise, use a distilled water-moistened tissue to gently wipe the mirror.
  • Page 57 PotentiaMeter Cleaning and Maintenance 4. Wrap a new dry Kimwipe around the spatula, and use it to thoroughly wipe the cleaning solu- tion from the mirror surface and chamber. 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.

  • Page 58: Checking Calibration

    KCl standard. If a change has occurred, refer to chapter 5 for direc- tions on how to recalibrate. If, after adjusting cali- bration your instrument is still not reading samples correctly, contact Decagon for technical support.

  • Page 59: Repair Instructions

    11. Repair Instructions If your WP4 ever needs to be sent in for service or repair*, call Decagon at (509) 332-2756 or (US and Canada) 1-800-755-2751, or fax us at (509) 332-5158. We will ask you for your address, phone number, and serial number.
  • Page 60 PotentiaMeter Repair Instructions instrument and ship it back to you in good time. Following are steps that will help in safely shipping your instrument back to us: 1. If possible, ship your WP4 back to us in its orig- inal 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.

  • Page 61: Repair Costs

    We have a $50 minimum repair charge. 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.

  • Page 62: Troubleshooting

    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 or (509) 332-2756 (for those not in the US or Canada).
  • Page 63 PotentiaMeter Troubleshooting Table 1: Troubleshooting Problem Possible Solutions 2. Long read time • Dirty sample chamber • Sample desorbs slowly • Broken chamber fan blade 3. Readings on KCl • Dirty thermopile standards are too high/ • If using own salt solu- low to adjust tion, it may not be in equilibrium...
  • Page 64 PotentiaMeter Troubleshooting • Check to make sure your power cord is securely attached the back of the instru- ment, and into the power outlet. • A power surge may have caused a fuse to blow. To change the fuses, follow these instruc- tions: 1.
  • Page 65 PotentiaMeter Troubleshooting Decagon to make arrangements for repairs. 2. PROBLEM: Readings are slow or inconsistent. SOLUTION: • The sample chamber may be dirty. Refer to Chapter 10 of the manual for directions on cleaning the sample chamber. • Some samples absorb or desorb moisture...
  • Page 66 Refer to Chapter 10 for directions on cleaning. • If you weren’t using Decagon’s KCl verifica- tion standards, high readings may indicate that the salt solution you are using is not in equilib- rium.
  • Page 67 PotentiaMeter Troubleshooting moisture to condense on the mirror and provide a reading. • The mirror may be dirty. Try cleaning the mir- ror and chamber and measuring the sample again. 5. PROBLEM: A small triangle appears in the upper right corner after sampling: -1.84 MPa 7.38...
  • Page 68 If this message appears and you continue to sample, Decagon cannot be liable for errors in reading that may occur. Contact Decagon for a solution to this problem.

  • Page 69: Component Performance Screen

    Troubleshooting SOLUTION: This indicates that the temperature control module inside the WP4-T is broken or not functioning cor- rectly. When the instrument senses that there is a problem with the temperature control module, it removes that function as an option to the user as a precaution.
  • Page 70 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 for further instruction. Press the button next to -Exit-...

  • Page 71: Further Reading

    PotentiaMeter Further Reading 13. Further Reading References: 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. Campbell. (1992) Rapid measurement of low soil water potentials using a water activity meter.

  • Page 72: Application Notes

    PotentiaMeter Further Reading Application Notes: The following WP4 application notes are available from Decagon by request and via our website at: www.decagon.com/instruments/agdownload.html. • Generating a Soil Moisture Characteristic with the WP4. • Measuring Leaf Water Potential using the WP4. • Field Portability Instructions for the WP4.

  • Page 73: Preparing Salt Solution

    PotentiaMeter Appendix A Appendix A Preparing Salt Solution If you choose to mix a saturated salt solution for use as a verification standard, we recommend that you use the approved AOAC method. This method is as follows: 1. Select a reagent-grade salt and place it in a test container to a depth of about 4cm for more solu- ble salts (higher water potential), to a depth of about 1.5 cm for less soluble salts (low water...
  • Page 74 NaCl and KCl at 20°C. Satu- rated salt solutions are very temperature-sensitive and their values are not as accurate as the verifica- tion standards offered by Decagon. Table 2: Water Potential of NaCl and KCl in Megapascals (MPa)
  • Page 75 PotentiaMeter Appendix A...
  • Page 76 PotentiaMeter Appendix A...
  • Page 77 PotentiaMeter Index Index accessories 10 accuracy steps for increasing 39 application notes 67 beeper 19 changing block sensors 50 block ROM failure 58, 62 buttons for linear offset settings 28 for menu selection 15 c for continuous mode 18 calibration changes in 25 menu 20 steps 27–30...
  • Page 78 PotentiaMeter Index sensor block 50 thermopile 52 tools needed for 49 communication settings 42 component performance indicator 64 components 10 computer interface 41 continuous sampling mode 17 cotton swabs not used for cleaning 50 customer service 1 Czech 16 Danish 16 data download format 42 dirty mirror indicator 38...
  • Page 79 PotentiaMeter Index features 11 field measurements 13 flashing. See LED French 16 further reading 66 fuse changing 59 German 16 gravitational potential 45 Hyperterminal using for downloading 41 Italian 16 KCl standards 25, 26 languages changing 16 leaf measuring water potential in continuous mode 17 LED 19 linear offset how to adjust for 26...
  • Page 80 PotentiaMeter Index main menu 13, 15 maintenance 49 manual 1 matric potential 45 menus main menu 15 system configuration 18 molality of verification standards 26 Norwegian 16 osmotic potential 45 peltier cooler 6 portability 13 Portuguese 16 preparing salt solutions 68 read time affected by sample temp.
  • Page 81 PotentiaMeter Index repair costs 56 instructions for 54 shipping 54 salt standards. See verification standards sample slow water-emitting 60 sample cups cleaning 32 filling level 31 sealing 32 sample equilibration screen 8, 23 sample preparation 31 sample temperature screen 33 sample too dry 61 samples dry water potential 32...
  • Page 82 PotentiaMeter Index telephone number 2 temperature effect on read time. 39 effects on reading 7 effects on water potential 47 hot samples 37 of instrument 37 samples not at room temp. 33 temperature control reasons for 8 Terminal for Windows using for downloading 43 time format in data download 42...
  • Page 83 30 definition 5, 45 displayed 13 equation 46 measuring 46 theory 45 WP4 and 5 and chilled mirror dewpoint technique 6 and temperature 7 measuring water potential 46 preparing for operation 12 theory 45 WP4-T description 8 important tips 22...

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