Dewpoint PotentiaMeter
Contents
1. Empty the whole vial of KCl solution into a sam ple cup and place itin the WP4 s sample drawer Carefully slide the drawer closed being espe cially careful so the solution does not splash or spill and contaminate the chamber Turn the drawer knob to the READ position to make a reading Note If you decide at this point that you do not want to continue with the cali bration program just return the knob to the OPEN LOAD position and you will be returned to the main screen After it has finished sampling the verification standard it will bring you to the following screen use these buttons to adjust the value 29 WP4 PotentiaMeter Calibration and Verification Standards 6 At this screen adjust the water potential value to its proper value for the KCI solution at that tem perature 2 19 MPa at 20 C 2 22 MPa at 25 C Press the upper right button to move the value up the lower right button to move it down When it is at the value you want 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 sam pling 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 3322. aaeeea 18 Sample Equilibration Screen 23 Calibration and Verification 25 Verification o on aoaaa 25 Verification Standards WP4 PotentiaMeter Table of Contents When to Verify Calibration 26 How to Verify and Calibrate the WP4 27 6 Sample Preparation 31 Preparing the Sample ys cxsarrashasatmmnarane 3 Dry Samples Autc ccenne cate erie iu tach Aras 32 Samples and Temperature 0 0665 33 7 Taking a Reading 36 How WP4 takes Readings 0 000000000000 37 Cautions eienenn a an e shila 37 Increasing your Accuracy o on 39 8 Computer Interface Al Using Hyperterminal in Windows 98 and Windows 2000 0 00000 00 Al 9 Theory Water potential 44 W ater potential eincce puck fatness anounced fauss 44 Measuring Water Potential 45 Effect of Temperature on Water Potential 46 10 Cleaning and Maintenance 48 Tools Needed esere uT enasi 48 Cleaning the Block and Sensors 49 Checking Calibration 02055 52 ll Repair Instructions 53 Shipping DInSCHONStstia vss Gath eee 53 Repair Costs cert tatd dt Yel aat taint dn wea atedae 55 LOGHERSSIVICE ni vinsnanederoenacearseese chau 55 12 Troubleshooting 56 Problems and Solutions 0000an 56 WP4 PotentiaMeter Table of Contents C
3. 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 The physical temperature of the instrument should be between 5 C 43 C Between these temperatures WP4 will measure samples of sim ilar temperature quickly and accurately Ifyou are sampling and a triangular warning symbol appears in the top right comer this indi cates that the mirror has become too dirty to give accurate measurements and you need to clean the mirror and chamber before continuing to sample For more details about this symbol please refer to Chapter 12 For cleaning instruc tions refer to Chapter 10 e Ifa sample has a water potential drier than about 60 MPa WP4 will display a message accompanied by the flashing light and beeper notifying you that your sample is too dry to be 38 WP4 PotentiaMeter Taking a Reading accurately measured by the WP4 Following is an example This message will stay on the screen until you open the sample drawer If you know that your sample s water potential is below what the screen is telling you your instrument s sensors may have been contaminated and will need to be cleaned see Chapter 10 or serviced see Chapter 11 Increasing your Accuracy Under normal operating conditions
4. the measurement of the head space 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 inter nal equilibrium of the sample itself is important If a system 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 system 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 tem perature 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 1 46 WP4 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 tempere ture errors but achieving 0 005 C accuracy is diffi cult when temperature differences are large Best accuracy is therefore obtained when the sample is near chamber temperature 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
5. 2756 1 800 755 2751 in US and Canada for further instructions 30 WP4 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 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 WP4 is able to accurately mea sure 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 efficiency by providing more stable infrared sample temperatures 2 Do not fill the sample cup more than half full Overfilled cups may contaminate the sensors in the chamber more is not neces sarily better 31 WP4 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 nm 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 positi
6. The top left value is the value the thermocouple is reading It is basi cally the difference in temperature between the block and the mirror If this is zero there is some thing wrong with the thermocouple The top right 63 WP4 PotentiaMeter Troubleshooting 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 for further instruction Press the button next to Exi t to get back to the main menu 64 WP4 PotentiaMeter Further Reading 13 Further Reading References Brye K R 2003 Long term effects of cultivation on particle size and water retention characteris tics 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
7. 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 instru ment on or off EXIT You may press the EXIT button the lower left but ton 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 20 WP4 PotentiaMeter Menus menu For more details on calibration and how to verify it please refer to the next chapter 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 Adjusting the Setpoint Temperature Use the buttons next to the and buttons to adjust the target setpoint temperature displayed in the lower right corner If you press either button it adjusts in increments of 5 C Note Holding down the button will not increment the value so you must press the button each time you wish to change this value The target setpoint temperature roughly corre sponds to the temperature at which you wish the chamber to read Therefore adjust the setpoint to the temperature that you want then begin measure ments to see how close your WP4 T comes to your desired temperature this works best by putting the WP4 in continuous mode After sever
8. at that temperature At 20 C this should be 2 19 MPa At 25 C this should be 2 22 MPa If you re operating in pF mode it should read 4 35 pF at both 20 and 25 C 27 WP4 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 After cleaning repeat these instructions If after cleaning you consistently get readings that are outside of the proper water potential of the KCl by more than 0 1 a change in calibra tion has probably occurred In this case adjust the reading on the KCl measurement to its cor rect value This is done by the following Adjusting Calibration 1 Once you have determined that your calibration needs adjustment enter the system configuration menu by pressing the lower left button in the main menu Press the upper right button in the system configuration menu to enter the calibra tion menu You will be guided through the cali bration routine The following screen will appear 28 WP4 PotentiaMeter Calibration and Verification Standards 2 5 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
9. button while in the main menu it will bring you to the system configu WP4 PotentiaMeter Menus ration menu This menu allows you to make minor system configuration menu button system changes You can change how the beeper signals after each sample and enter the calibration menu as well Beeper Beeper Calibration Icon Mode indicator menu continuous shown 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 comer of the WP4 s case In normal sampling mode the LED will flash once when a sample is started When it is 19 WP4 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 Beeps four times then stops Beeps continuously until drawer is opened definition of beeper icons The beeper can be tumed off completely it can beep momentarily 4 times when the sample is fin ished and then stop or it can beep continuously until the knob is turned to the OPEN LOAD posi tion After you have
10. changes in 25 menu 20 steps 27 30 when to check 26 calibration drift 25 cautions 37 checking calibration 53 chilled mirror technique 6 cleaning 49 inside case 52 optical sensor 52 71 WP4 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 D Danish 16 data download format 42 dirty mirror indicator 38 distilled water 26 downloading 43 dry samples 61 dry water potential 32 E e mail address 2 error messages 57 sample too dry 61 exit 20 F fan inside sample chamber 7 fax number 2 72 WP4 PotentiaMeter Index features 11 field measurements 13 flashing See LED French 16 further reading 66 fuse changing 59 G German 16 gravitational potential 45 H Hyperterminal using for downloading 41 l Italian 16 K KCI standards 25 26 L languages changing 16 leaf measuring water potential in continuous mode 17 LED 19 linear offset how to adjust for 26 loaners 56 location for sampling 10 73 WP4 PotentiaMeter Index M main menu 13 15 maintenance 49 manual 1 matric potential 45 menus main menu 15 system configuration 18 molality of verification standards 26 N Norwegian 16 O osmotic potential 45 P peltier cooler 6 portability 13
11. follow these steps 1 Press the Start button and select Programs gt Accessories gt Hyperterminal and click on the Hyperterminal icon 2 At the prompt choose a name for this program WP4 is a good one and choose an arbitrary icon above to represent it In future downloads you will be able to click on this icon in have it already set up for you to download Click the OK button 3 A pop up menu labeled Connect To will appear Click on the scroll bar on the bottom of A WP4 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 8 databits no parity 1 stop bit and flow control set to hard ware Click OK 5 Plug your RS 232 cable to the COM port you selected and connect it to your WP4 Begin sam pling Your WP4 s data will be displayed on screen as it samples It will be output in the fol lowing format time since chamber was closed sample tempera ture sample water potential and sample water activity Here is an example Water Water time since temp ae sample was C potential activity started Sr 24 37 2L 0 09839 6 When you are finished sampling you can print the data in the terminal session or cut and paste
12. is only slightly warmer than the sensor block will condense water within the block This will cause errors in the measurement and in subse quent measurements until the condensation disap pears The Ts Tb function helps the user ensure that the sample won t condense on the sensor block 47 WP4 PotentiaMeter Cleaning and Maintenance 10 Cleaning and Maintenance a CT 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 e thin plastic rod or other non metal implement e Distilled Water Isopropyl Alcohol Lintfree or sizing free tissues Kimwipe or e WP4 Cleaning Kit Note Kimwipes are ideal because they don t leave a lint residue like most tissues They also don t have any other compounds in the tissue that may contam inate the sensors in the WP4 s block Also never use 48 WP4 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 2 Unplug your WP4 Remove the case lid screw located on the back panel Carefully remove the lid by pulling the bac
13. open windows outside doors refriger ator exhausts or other items that may cause rapid temperature fluctuation Features LED indicator Function Keys Front view of WP4 WP4 PotentiaMeter Getting Started Lid thumb screw RS 232 interface PI ewel 9 ON OFF Switch ee 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 tum the instrument on The ON OFF switch is located on the lower right comer of the WP4 s back panel The following screens will appear on the LCD WP4 PotentiaMeter Getting Started Then This is the main menu displaying the water poten tial in both MegaPascals MPa and pE 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 When you insert a sam ple into the chamber drawer and tum 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
14. to adjust e Dirty thermopile e If using own salt solu tion it may not be in equilibrium 4 Sample too dry e Sample too dry to accu rately measure e Dirty chamber or mirror 5 Triangle appears in upper right corner e Mirror is dirty 6 Block ROM failed appears on screen after turning on WP4 e Block is disconnected from motherboard e Block memory compo nent has failed 7 TE set option no longer appears on screen Temperature control mod ule in WP4 is broken or not functioning 1 PROBLEM WP4 won t turn on SOLUTION 57 WP4 PotentiaMeter Troubleshooting e 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 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 as well as 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 instru ment on If the fuse blows a
15. water potential reading to the vapor pressure of air in equilibrium with the sample The relationship between the sam ple s water potential and the vapor pressure of the air is RT p se M ae where p is the vapor pressure of the air pp 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 p is computed from sample temperature The WP4 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 head space In the WP4 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 tem perature sensor and an infrared thermometer The dew point sensor measures the dew point tempera ture of the air and the infrared thermometer mea sures the sample temperature The purpose of the 45 WP4 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
16. Portuguese 16 preparing salt solutions 68 R read time affected by sample temp 33 long read time 58 60 readings cautions 37 how Aqualab takes 36 how WP4 takes 37 taking readings 36 references 66 74 WP4 PotentiaMeter Index repair costs 56 instructions for 54 shipping 54 S 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 not at room temperature 33 sampling modes continuous 17 normal 16 saturated salts 68 seller s liability 3 shipping for repair 54 Spanish 16 spilling the sample 37 Swedish 16 T TE set 21 technical support 1 75 WP4 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 long read times 60 tollfree number 2 triangle 38 62 troubleshooting 57 V vapor pressure 6 46 verification standards 25 compared to saturated salts 69 long read times for 60 water potential readings too high low for 60 W warmup 13 warranty 2 warranty card 3 water content definition 45 vs water potential 45 76 WP4 Po
17. 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 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 Microbiol ogy Soil Science Society of America Madi son Wisconsin pp 1 22 65 WP4 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 e Generating a Soil Moisture Characteristic with the WP4 Measuring Leaf Water Potential using the WP4 Field Portability Instructions for the WP4 e Water Potential The Key to Successful Seed Priming e Seed Longevity in Storage is Enhanced by Con trolling Water Activity e Classification of Expansive Soils using the WP4 Dewpoint Potentiameter 66 WP4 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 2 3 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 potential and to an interm
18. WP4 Dewpoint PotentiaMeter for models WPA and VWP4 T Operator s Manual Version 2 2 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 WP4 PotentiaMeter Table of Contents Contents Introduction 1 About this Manual n 1 Customer Service 0 cece eee eee 1 Warranty ek 2b Dea bl cealeri alehesesttantraein ha ie 2 Note to our WP4 Users 0 000 200 3 Seller s Liability sh es cabaatee tanta eats 3 About the WP4 5 WP4 Models and Options 055 5 WPA and Water Potential 0 00 5 How the WP4 works 0000ceeeee eee 6 WP4 and Temperature 0 0000008 7 Getting Started 10 Components of your WP4 00000 0 10 Choosing a Location eur k ete ga dh ee 10 FOGIORGS ni onae a hoe AN ei Shit cabinets 1 Preparing the WP4 for Operation 12 Portability secerneren aA aiaa 13 The Menus 0 a 15 The Main Meru ace duet caa deuce nomedeumine 15 Changing Languages 0cece renee 16 Normal sampling mode 0 000e 00 16 Continuous mode naianei 17 System configuration
19. al samples it 21 WP4 PotentiaMeter Menus should show consistent temperature readings At this point make any needed adjustment to the set point index number to reach your desired tempera ture You will be able to adjust the index number between 40 5 and 15 C If you press the button after you reach 15 it will disable the temperature control function until you raise the index number again Setting Temperature Equilibration Time In the upper left comer before the words Adjust Setpoint there is a number default is 0 This value can be one of three integers 0 1 or 2 and is adjusted by pressing the upper left button This number sets the level of temperature equilibration desired after the knob is turned to the READ posi tion and before the WP4 begins to make measure ments A setting of 0 begins measurement immediately as long as sample is not above 4 C of block temperature 1 requires it to be within 1 degree and 2 requires it to be within 1 2 of a degree 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 22 WP4 PotentiaMeter Menus e Before sampling wait for approximately 30 minutes to let the chamber s temperature to stabilize after turning it on The ins
20. 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 continuous normal language selection Fa mode T MPs SS sl A 8 c system setup sample equilibration screen 15 WP4 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 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 lan guage 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 nor mal sampling mode In this mode a single measure ment is made each time you read a sample Here is WP4 PotentiaMeter Menus a brief summary of the normal sampling mode fea tures e single sampling not continuous e instrument may or may not beep after mea sur
21. 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 Put your instrument in a plastic bag to avoid dis figuring marks from the packaging Don t ship your WP4 to us with the power cord we have plenty here to use with your instru ment and it will damage the instrument in ship ment If you aren t using the foam inserts pack the box moderately tight with packing material like styrofoam peanuts Tape the box in both directions so it cannot be broken open in shipment Include necessary paperwork so your repair can be processed quickly This should include your name address serial number phone and fax numbers purchase order and a description of 54 WP4 PotentiaMeter Repair Instructions the problem Ship to Decagon Devices Inc ATTN Repair Department 950 NE Nelson Court Pullman WA 99163 Repair Costs Manufacturer s defects and instruments within the one year warranty will be repaired at no cost For non warranty repairs including cleanings for instru ments in their warranty period costs for parts labor and shipping will be billed to you 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 instrum
22. de the drawer closed being espe cially careful if you have a liquid sample that may splash or spill and contaminate the cham ber Access the sample temperature menu press lower right button to watch the temperature dif ference between the sample and the instrument Turn the sample drawer knob to the READ posi tion to seal the sample cup with the chamber The instrument will beep once and the green 36 WP4 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 enabled Cautions e Never leave a sample in your WP4 after a read ing has been taken 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 sam ple has been loaded Movement may cause the sample material to spill and contaminate the sample chamber e Take special care not to move the sample drawer too quickly when loading or unloading liquid samples in order to avoid spilling Ifa sample has a temperature that is much 37 WP4 PotentiaMeter Taking a Reading
23. e 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 Essen tially it means that there is not enough sample 60 WP4 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 SOLUTION The mirror needs to be cleaned along with the rest of the sample chamber until it disappears This tri angle is a mirror performance indicator When the WP4 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 sam pling 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 6 WP4 PotentiaMeter Troubleshooting machine SOLUTIONS 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 ci
24. ed 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 con densation 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 reflec tance 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 WP4 PotentiaMeter About the WP4 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 tempera tures are simultaneously measured the need for complete thermal equilibrium is eliminated which reduces measurement times to less than five min utes The WP4 T gives you the option of controlling the sample temperature by means of an inte
25. ediate depth for intermediate salts Add distilled water in increments of about 2mL stirring constantly Add water until the salt can absorb no more water as evidenced by the presence of free liq uid Keep the amount of free liquid to the mini mum needed to keep the solution saturated with water If you intend on using this solution over a long term period make sure to seal the solution well to prevent losses from evaporation 67 WP4 PotentiaMeter Appendix A Following is a table showing the water potential at given concentrations of NaCl and KCI 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 KCI in Megapascals MPa ann NaCl KCI 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 68 WP4 PotentiaMeter Appendix A 69 WP4 PotentiaMeter Appendix A 70 WP4 PotentiaMeter Index Index A accessories 10 accuracy steps for increasing 39 application notes 67 B beeper 19 changing block sensors 50 block ROM failure 58 62 buttons for linear offset settings 28 for menu selection 15 C c for continuous mode 18 calibration
26. ement is made depending on the system setting e green LED blinks until you turn the drawer knob to the OPEN LOAD position continuous normal mode eS 5i 5 0 system setup sample temperature menu 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 use ful in doing long term monitoring of samples that take an especially long time to come to vapor equi librium such as plants and leaves In this mode it WP4 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 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 for continuous mode 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
27. ents 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 WP4 or WP4 T needs service during critical operations 55 WP4 PotentiaMeter Troubleshooting 12 Troubleshooting WP4 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 or 509 332 2756 for those not in the US or Canada 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 e Power cord discon nected e Blown fuse 56 WP4 PotentiaMeter Troubleshooting Table 1 Troubleshooting Problem Possible Solutions 2 Long read time e Dirty sample chamber e Sample desorbs slowly e Broken chamber fan blade 3 Readings on KCI standards are too high low
28. epts of water potential enabling them to use our instrument with confi dence Every effort has been made to ensure that the content of this manual is correct and scientifi cally sound Seller s Liability Seller warrants new equipment of its own manufac ture 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 deteriora tion due to corrosion from any cause are not to be considered a defect but Seller s liability for defec tive parts shall in no event exceed the furnishing of replacement parts FO B the factory where origi nally manufactured Material and equipment cov WP4 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 ment 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 con stitutes Buyer s acceptance of the terms set forth in this warranty There are no understandings repre sentations or warranties of a
29. er just resets the zero offset Usually any changes in performance requiring recalibration are the result of chamber contamination 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 Calibration can be done with any solution of known water potential We recommend checking the cali bration by measuring a fresh sample of 0 5 molal KCl and adjusting for any drift Verification Standards Verification standards are specially prepared salt solutions that have a specific molality and water 25 WP4 PotentiaMeter Calibration and Verification Standards 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 Because of these reasons we recommend using Decagon s KCl Performance Verification Standard for the most accurate verification of your WP4 s performance The standards are produced under a strict quality assurance regime The accuracy of the standards is verified by an independent third party and are shelf stable for one year If for some reason you cannot obtain Decagon s verification standards and need to make a saturated salt solution for verification refer to A
30. gain a failed com ponent may be causing the problem Contact 58 WP4 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 very slowly causing measurements to take longer than usual and nothing can be done to speed up the process Refer to Chapter 6 for fur ther explanation A fan blade may be broken inside the block If even the KCl standard takes a long time to read and the sample chamber is clean you may have a broken chamber fan blade This is espe cially likely if you have just cleaned the cham ber If you suspect this may have happened contact Decagon for details on replacement 3 PROBLEM Water potential readings on KCI standards are too high low and a calibration adjustment cannot be made any higher lower 59 WP4 PotentiaMeter Troubleshooting 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 KC verifica tion standards high readings may indicate that the salt solution you are using is not in equilib rium 4 PROBLEM Message on screen displays the following exam ple SOLUTION The sampl
31. h positive number take the sample out immedi ately and let it cool on a cold surface with the 34 WP4 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 If it is close enough to the block tempere ture 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 35 WP4 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 Tum the sample drawer knob to the OPEN LOAD position and pull the drawer open 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 may contaminate the cham bers sensors Carefully sli
32. it to a spreadsheet or text editor If you want to 42 WP4 PotentiaMeter Computer Interface save the data go into the Transfers menu and select Capture text and designate where you would like it saved 43 WP4 PotentiaMeter Theory 9 Theory Water potential ee ad 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 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 sub stance in the water Pressure potential depends on the hydrostatic or pneumatic pressure on the water The WP4 measures the sum of the osmotic and mat ric potentials in a sample Often one or the other of these potentials will be the dominant factor in deter mining 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 44 WP4 PotentiaMeter Theory Measuring Water Potential The water potential of a solid or liquid sample can be found by relating the sample
33. k of the lid upward and then sliding the lid back away from the front of the case and off Unscrew the two thumbscrews that secure the sensor block 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 Carefully lift the block straight up from its mount Turn the block over to expose the cham ber cavity as shown in the illustration 49 WP4 PotentiaMeter Cleaning and Maintenance mirror optical sensor chamber fon m View of inside block chamber Mirror 1 Wash hands with soap and water to prevent oils from contaminating the Kimwipe tissue and being transferred to the mirror 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 miror If the mirror is very dirty clean it with reagent grade isopropanol isopro pyl alcohol followed by multiple distilled water moistened tissues 50 WP4 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 Clea
34. n it in the same man ner as described above for the mirror Thermopile Sensor Wrap a strip of Kimwipe tissue around a plastic cleaning kit spatula and moisten it with water or alcohol to clean the thermopile sensor This sensor must be free of all dirt and lint Inside Chamber Clean all other portions of the chamber block with a moist Kimwipe Note Be extremely careful not to damage the fan blades in the chamber The fan blades are very frag ile if one of them breaks your instrument won t work properly Therefore take extra care when cleaning this portion Inside Case 1 Clean the sample drawer and drawer base Remove any debris that may have collected 5 WP4 PotentiaMeter Cleaning and Maintenance inside the case 2 Check once more to make sure there is no debris in the sample chamber cavity 3 Replace the block and insert the nbbon cable socket into to the 20 pin plug on the block Lock it in place with the locking levers 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 Plug the WP4 s power cord back in Checking Calibration After you have cleaned the chamber and other parts of your WP4 it is important to check the instru ment s performance in order to correct for any cali bration drift that may have occurred during cleaning procedures Check the calibration of your instrument by measur ing the water
35. 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 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 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 WP4 PotentiaMeter Introduction 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 mail ing 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 under standing the basic conc
36. ny kind express implied statutory or otherwise including but with out limitation the implied warranties of merchant ability and fitness for a particular purpose not expressly set forth herein WP4 PotentiaMeter About the WP4 2 About the WP4 aaa eee 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 sta tus of the water in a system It indicates how tightly WP4 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 of this manual How the WP4 works WP4 uses the chill
37. omponent Performance Screen 63 13 Further Reading 65 References uh Ti lit Gree nas ey Ol a 65 Application Notes sc cerka econ cages tot dete 66 Appendix A 0205 67 Preparing Salt Solution 0eee ee 67 WP4 PotentiaMeter Table of Contents WP4 PotentiaMeter Introduction 1 Introduction ee Se 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 It is easy to use and pro vides 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 car ing for your instrument Please read these instruc tions before operating WP4 to ensure that the instrument performs to its full potential Customer Service If you ever need assistance with your WP4 orif you just have questions there are several ways to con WP4 PotentiaMeter Introduction tact us Phone Our toll free customer service
38. on Therefore any sample mate rial 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 restrict water transfer For short term storage lt 3 hours the cup lid is accept able If it will be a long time before the measure ment 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 WP4 However samples with such dry water potential values are rare When a sample s water potential value is higher than about 300 MPa WP4 will display an error message indicating the last reading it could make on that particular sample For 32 WP4 PotentiaMeter Sample Preparation example if you were measuring a dry sample and the following screen appeared this screen indicates that the last water potential reading the WP4 measured on this sample was 301 8 Megapascals Therefore the actual water potential of the sample is higher than the instru ment s components can measure Samples and Temperature Samples that are 4 degrees colder or warmer than the instrument chamber temperature will need to be close to the WP4 s temperature before a fast accurate reading can be made Rapid changes in temperature over sh
39. ort periods of time will cause the water potential readings to rise or fall until the temperature stabilizes If you have a WP4 T you can set the chamber temperature to eliminate this problem If you have a standard WP4 when the temperature stabilizes within one or two degrees of the chamber temperature you can proceed with normal measurements This is why the sample tem perature screen is available for you to access see chapter 4 before you read a sample 33 WP4 PotentiaMeter Sample Preparation 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 If the temperature difference is outside this range like 0 5 to 1 0 you may still begin sampling by turning the knob to the READ position however your readings may take longer until the temperatures equilibrate If samples are significantly warmer than the cham ber when they are placed in it Ts Tb is a high posi tive number condensation may occur and moisture will condense inside the block In order to prevent this do the following 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 very high a hig
40. potential of the KCI 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 52 WP4 PotentiaMeter Repair Instructions Il Repair Instructions ee ee ee 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 For non warranty repairs we will also ask for a payment method such as a pur chase order or credit card number a repair budget and billing address Note If you purchased your WP4 from one of our international distributors please contact them before contacting Decagon They may be able to pro vide 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 ship ping address name and department of the person responsible for the instrument and most impor tantly a description of the problem This informa tion will better help our technicians and our shipping department to expedite repair on your 53 WP4 PotentiaMeter Repair Instructions instrument and ship it back to you in good time Following are steps that will help in safely shipping your instrument
41. ppendix A When to Verify Calibration The calibration of your WP4 should be checked with the KCl standard before each use It should never be checked by measuring distilled water The WP4 will read distilled water but it is not a good choice for checking calibration since as the humid ity of the chamber approaches 100 the sensor may be adversely affected For batch processing the instrument should be checked regularly against the KCI standard This will alert you to the possibil ity of contamination of the unit or shifts in the cali bration from other causes 26 WP4 PotentiaMeter Calibration and Verification Standards How to Verify and Calibrate the WP4 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 Empty the whole vial of KCl solution into a sam ple cup and place it in the WP4 s sample drawer For best results make sure you wait for the KCl to come to temperature equilibrium at the sam ple temperature menu Ts Tb before measur ing it 2 Carefully slide the drawer closed being espe cially careful so the solution does not splash or spill and contaminate the chamber 3 Turn the drawer knob to the READ position to make a reading Make two readings The second reading should be within 0 1 of the correct reading of the KCl standard
42. rcuit board If the block is properly plugged in to the motherboard and this message appears it is likely that one or more of the com ponents have failed on the block s circuit board If you press amp xi 4 at this prompt the instru ment will not have values for the component that has failed which will lead to incorrect read ings 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 7 PROBLEM TE Set option does not appear anymore in the WP4 T s system configuration menu 62 WP4 PotentiaMeter 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 Contact Decagon for service informa tion Component 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 WP4 may be causing measurement error you can access a screen that will display values for component per formance 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
43. 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 13 WP4 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 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 tum 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 WP4 PotentiaMeter Menus 4 The Menus es 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 men tioned earlier the water potential and sample tem perature
44. rnal thermo electrically controlled module that monitors and stabilizes the sample block temperature accord ing 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 tempera ture of the WP4 before accurate measurements of water potential can be made Large temperature dif ferences 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 WP4 PotentiaMeter About the WP4 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 tempera ture controlled model Here are a few major rea sons 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 temperat
45. tentiaMeter Index water potential 69 adjusting for offset 30 definition 5 45 displayed 13 equation 46 measuring 46 theory 45 WP4 and 5 WP4 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 77
46. trument may have a difficult time measuring the temperature of a sample that is being cooled if the ambient temperature is high e When changing samples the opening and closing of the drawer may cause the temper ature in the chamber to fluctuate Therefore make sure to take more than one reading per sample to ensure that the temperature and water potential are stable between readings e 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 WP4 press the lower night button at the main menu This screen can only be accessed when the drawer knob is in the OPEN LOAD posi 23 WP4 PotentiaMeter Menus tion 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 24 WP4 PotentiaMeter Calibration and Verification Standards 5 Calibration and Verification eel Verification WP4 uses the chilled mirror dewpoint technique for measuring water potential Due to the nature of this technique periodic calibration is essential for the instrument to perform properly The calibration slope is fixed during factory calibration The us
47. ure or other water potential studies where temperature con trol is critical 2 To minimize extreme ambient temperature fluctuations If the laboratory and WP4 temper atures 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 8 WP4 PotentiaMeter About the WP4 configurationmenu seepage 21 thatallowstheuserto set the temperature WP4 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 Operator s Manual 12 vials of 0 5 Molal KCl Cleaning Kit Choosing a Location To ensure that your WP4 operates correctly and 10 WP4 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 This location should be well away from air conditioner and heater vents
48. your WP4 should be reading with an accuracy of 0 1 MPa from 0 to 10MPa and 1 from 10 to 60 MPa You can increase the accuracy of your WP4 provided you take certain precautions Here are some steps you can take to improve your instrument s accuracy up to 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 39 WP4 PotentiaMeter Taking a Reading slowly varying temperature Large temperature variations from cycling air conditioning or forced air heating will degrade performance Intermittently run the 0 5 KCl in continuous mode between samples 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 changes in ambient temperature 40 WP4 PotentiaMeter Computer Interface 8 Computer Interface ee 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 Using Hyperterminal in Windows 98 and Windows 2000 To use Hyperterminal with your WP4
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