ML2 manual
Contents
1. This response in aqueous solutions is accurately characterised by the following equation V ES V oco 6 0 bo 1 by 1 e where b 0 175 and b 0 0038 and o is in mS m 7 The response to changes in salinity for soils at fractional volumetric water contents less than 0 6 m m has been shown to be correspondingly less but is difficult to quantify ML2x UM 1 21 May 1999 Page 16 accurately We believe from the data available that the response curve follows the same general shape and that the values for bo and b are similar Effect on the apparent soil moisture There are two important simplifications to note as a result of the above response curve 1 You can ignore salinity effects for soils that are known to be moderately saline or strongly saline i e 400 lt lt 1600 mS m provided you have performed soil specific calibration 2 Changes in salinity due solely to drying wetting cycles do not significantly affect readings The worst case situation occurs when a non saline soil is irrigated with saline irrigation water i e when a calibration is carried out on a non saline soil sample giving values for the coefficients and a as in the previous section but the actual measurement is made on a sample with significant salinity As an indication of the size of the possible errors we will assume that equation 7 above is generally applicable to soils and combine this with equatio2. Do not attempt to straighten the measurement rods while they are still attached to the probe body This may break the rods or damage the case seal Even a small degree of bending in the rods 21mm out of parallel although not enough to affect the inherent ThetaProbe accuracy will increase the likelihood of air pockets around the rods during insertion and so should be avoided Replacements rods are available if required type ML RODS Ensure that they are fully tightened on re assembly but do not apply excessive force as this will cause damage to the probe body seal Pre preparation of holes to accept ThetaProbe measuring rods is recommended in stony soils or other hard materials Extension tubes The probe is designed to be permanently buried if required When used like this optional extension tubes can be fitted to enable easier withdrawal and to protect the cable from damage by animals etc Extension tubes are lengths of the same tubing as is used to make the ThetaProbe cylindrical body They have a female thread in one end and the same sized male thread in the other Two lengths are available ML EX50 is 50cms long and ML EX100 is 100cms long These can be screwed into each other to make longer lengths as required Extension tubes allow the ThetaProbe to be lowered into pre augured holes down to the desired measurement depth and to ease removal from the soil when the application requires burial The outside diameter of t
3. W W L BENE us w 4 It will usually have a value between 7 6 and 8 6 Step 5 By inverting equation 3 and substituting from equation 2 the water content determined from a calibrated ThetaProbe will then be I 1 444V a 5 The corresponding equation using the polynomial relationship is 1 07 64V 64V 4 7V a 0 6 a Page 11 ML2x UM 1 21 May 1999 Using this relationship rather than the linear form will enable the ThetaProbe to achieve full accuracy over the full specified range particularly for wetter soils with 0 5 0 0 6 Example 1 Ina sample of moist soil the ThetaProbe gives an output of 0 43 V This sample weighs 1 18 and has a volume of 0 75 litres From equation 1 3 01 2 After drying the sample of soil the ThetaProbe gives an output of 0 11 V From equation 1 again we can calculate ao 1 59 3 The dry sample now weighs 1 05 kg so the volume of water in the moist sample was 0 13 litres Volumetric water content of the sample 0 173 4 By substituting in equation 4 a 8 19 5 Finally by inserting into equation 5 0 0 54V 0 060 Note It is important to use either the polynomial relationships equations 1 and 6 or the linear relationships equations 2 and 5 consistently when deriving and using the coefficients ay and a In the above example the coefficients have be
4. a DL2e in differential mode and powered through the loggers internal power supply Further details can be found in the SENSORS TXT file which is supplied with the DL2e sensor library codes for the ThetaProbe and in the DL2e manual section on Relay Channels Two sensor configuration codes are supplied with the DL2e M2M which provides generalised conversion from mVolts to volumetric soil water content in m m suitable for generalised mineral soils and M20 which is suitable for generalised organic soils DL3000 connection and configuration Full details including example connection diagrams are available in the on line Help provided with Acquire ML2x UM 1 21 May 1999 Mechanical and electrical specifications 112 60 26 5 dimensions in mm Technical Specifications ML2x Volumetric soil moisture content m m or vol Accuracy figures apply from 0 05 to 0 6 m m Full range is from 0 0 to 1 0 m m Accuracy 0 01 m m 0 to 40 C after calibration to a 0 02 40 to 70 C specific SOIL subject to soil salinity 0 05 0 to 70 C using the supplied soil errors see below calibration in all normal soils Soil salinity errors 0 0 to 250 mS m lt 0 0001 change per mS m 250 to 2000 mS m no significant change Soil sampling volume gt 95 influence within cylinder of 4 0cm diam 6cm long approx 75 surrounding central rod Environment Will withstand bur
5. a minimum of two relay controlled outputs to provide and control sensor power Each relay called a warm up relay is capable of switching 1A This means that each relay can typically power 50 ThetaProbes Configuring the Warm up Channel Although the probe can be continuously powered and read significant power can be saved by using the Delta T logger warm up relay facility to energise the sensor only just before and during a log For complete stability a warm up time of 5s is recommended although good repeatability can be achieved using times down to 1s the additional error associated with a 1 second warm up is only 40 0025 Shorter times will significantly reduce the battery power consumption of the system Logger input channel configuration ThetaProbe has been designed to make its use with dataloggers straightforward using only a single logger analogue input channel If you simply want to log the probe voltage directly it should be treated as a differential voltage source of range 0 1 5VDC and the logger should be configured accordingly You can convert the data to soil moisture units after logging using the information supplied in the Calibration section The Calibration section also describes how to program your datalogger to automatically convert probe output into soil moisture units before logging DL2e connection and configuration This diagram shows the connections for a ThetaProbe connected to channel 1 of
6. 9 Page 10 Our experience of measurements on soils suggests that below 0 5 there is no significant improvement to the overall accuracy to be achieved by using the 3rd order polynomial equation rather than the linear relationship For very high moisture contents 0 gt 0 5 m m the polynomial equation should be used This is usually only necessary for organic soils Soil specific Calibration Whalley and White Knight Zeggelin and Topp have shown that there is a simple linear relationship beween the complex refractive index which is equivalent to Ve and volumetric water content of the form a a 0 3 Since the relationship beween ThetaProbe output and Ve is already known it is only necessary to determine the two coefficients and We suggest you use the following protocol Step 1 Collect a sample of damp soil disturbing it as little as possible so that it is at the same density as in situ Insert the ThetaProbe into the sample and measure the probe output V Use equation 1 2 to calculate w Weigh the damp sample W and measure its volume ZL Step2 Oven dry the sample insert the ThetaProbe into the dry soil 8 0 and measure the probe output Vo Weigh the dry sample Wo Use equation 1 or 2 to calculate 0 This equals It will usually have a value between 1 0 and 2 0 Step 3 Calculate the volumetric water content of the original sample g
7. Spare parts and accessories for sensors not manufactured by Delta T but supplied by us individually or as part of the weather station or other system may be obtained from the original manufacturer We will endeavour to obtain parts if requested but a certain amount of additional delay is inevitable Should it prove necessary instruments may be returned to our works for servicing We normally expect to complete repairs of our own instruments within 2 days of receiving the equipment Other manufacturers sensors supplied by us and returned for servicing will take longer They will have to be returned to the original manufacture for servicing and may be subject to additional delays of two to four weeks Users in countries that have a Delta T Agent or Technical Representative should contact them in the first instance ML2x UM 1 21 May 1999 Page 22
8. ThetaProbe SOIL MOISTURE SENSOR TYPE ML2x USER MANUAL ML2x UM 1 21 Contents Introduction amp description uice sin prm agna dean aur Ca de V Or ada RRg USE 3 T T aidaa aisats 4 Wiring CONMECHONS isnt ever midi tuas assu GE Cc Du uA TIU Od 6 Connection to Delta T data loggers 012 amp DL3000 7 Mechanical and electrical specifications 8 Use and calibration 9 16 Compatibility with ThetaProbe type 1 17 DG TIONS 20 References Mm 21 Guarantee repairs and 22 Patents The ThetaProbe has been jointly developed by The Macaulay Land Use Research Institute and Delta T Devices and uses novel measurement techniques They are subject to the following patents UK 2300485 Europe 963703190 1 USA 08706675 Copyright Copyright 1999 Delta T Devices Ltd 128 Low Road Burwell Cambridge CB5 OEJ England All rights reserved Under the copyright laws this book may not be copied in whole or in part without the written consent of Delta T Devices Ltd Under the law copying includes translation into another language Delt
9. a T Devices Ltd 128 Low Road Burwell Cambridge CB5 OEJ England Telephone 01638 742922 Fax 01638 743155 E mail delta t co uk ML2x UM 1 21 May 1999 Page 2 Introduction amp description The ThetaProbe measures volumetric soil moisture content by the well established method of responding to changes in the apparent dielectric constant These changes are converted into a DC voltage virtually proportional to soil moisture content over a wide working range Volumetric soil moisture content is the ratio between the volume of water present and the total volume of the sample This is a dimensionless parameter expressed either as a percentage vol or a ratio m m Thus 0 0 m m corresponds to a completely dry soil and pure water gives a reading of 1 0 There are important differences between volumetric and gravimetric soil moisture contents The section on Definitions gives details of the relationship between these two parameters and soil matric potential as measured by Tensiometers Operating principles ThetaProbe consists of a waterproof housing which contains the electronics and attached to it at one end four sharpened stainless steel rods that are inserted into the soil The probe generates a 100 MHz sinusoidal signal which is applied to a specially designed internal transmission line that extends into the soil by means of the array of four rods The impedance of this array varies with the impedance
10. able and the overall reading errors will typically increase by 0 01 as in this rework of the example in the section on Acieveable Accuracy Error soil generalised source of error category specific calibration calibration ThetaProbe 0 01 0 01 repeatability beween ML2x probes errors compatibility 0 025 0 025 max differences beween ML2x and errors probes non random error Calibration 0 02 0 04 errors in values of ap and errors Sampling 0 04 0 04 soil variability and insertion errors errors RSS value Using the ML2x with the ThetaMeter type HH1 The ThetaMeter is optimised for use with the ML1 rather than the ML2x It is compatible with the ML2x and the mVolt readings will be accurate but the mineral and organic conversions use the old ML1 values for slope and offset The effect of this is to add approximately 0 01 m m error to the readings as in the example above This can probably be ignored for the general purpose use for which the ThetaMeter was intended Page 19 ML2x UM 1 21 May 1999 Definitions Volumetric Soil Moisture Content is defined as e V where V is the volume of water contained in the sample and V is the total volume of the soil sample The preferred units for this ratio are m m though 96vol is also frequently used The usefulness of this definition depends in part on the fact that the volume of the dry soil does not change as water is
11. added This is not true of shrink swell soils but for the most part is a reasonable approximation Soil Moisture Content varies from approx 0 02 m m for sandy soils at the permanent wilting point through approx 0 5 m m for clay soils at their field capacity up to values as high as 0 85 m m in saturated peat soils Soil water content is usually expressed volumetrically because it is then possible to ignore the bulk density of the soil sample Volumetric versus Gravimetric soil water content Gravimetric Soil Moisture Content is defined as gg where is the mass of water in the sample and is the total mass of dry sample To convert from volumetric to gravimetric water content use the equation 6 9 Pw where py is the density of water 1 5 M and p is the bulk density of the soil sample a S Soil Water Content versus Soil Matric Potential Studies of plant growth need to characterise the availability of water to the plant and this is usually done using the soil matric potential which measures the suction necessary to extract a unit voume of water from the soil pores and is measured in units of pressure hPa The soil matric potential is highly dependent on both soil type and soil water content and varies from 0 hPa at field capacity down to approximately 1500 hPa at the permanent wilting point The relationship beween soil water content 0 and soil matric potenti
12. al is called the water release curve or characteristic or retention curve and varies greatly for different soil types There is no generalised method of converting from soil water content to matric potential though a number of expressions have been found which have been successfully applied to a restricted list of soil types ML2x UM 1 21 May 1999 Page 20 References 1 Gaskin G J and Miller J D 1996 Measurement of soil water content using a simplified impedance measuring technique J Agr Engng Res 63 153 160 2 Topp G C J L Davis A P Annan 1980 Electromagnetic determination of soil water content Water Resour Res 16 3 574 582 3 Whalley W R 1993 Considerations on the use of time domain reflectometry TDR for measuring soil moisture content Journal of Soil Sci 44 1 9 4 White I Knight J H Zegelin S J and Topp GC 1994 Comments on Considerations on the use of time domain reflectometry TDR for measuring soil water content by W R Whalley Journal of Soil Sci 45 503 508 5 Roth C H Malicki M A and Plagge R 1992 Empirical evaluation of the relationship between soil dielectric constant and volumetric water content as the basis for calibrating soil moisture measurements Journal of Soil Sci 43 1 13 6 Knight J H 1992 Sensitivity of Time Domain Reflectometry measurements to lateral variations in soil water content Page 21 ML2x UM 1 21 May 1999 Guarantee repairs
13. and spares Our Conditions of Sale ref COND 91 11 set out Delta T s legal obligations on these matters For your information the following paragraphs summarise Delta T s position but reference should always be made to our Conditions of Sale which prevail over the following explanation Instruments supplied by Delta T are guaranteed for one year against defects in manufacture or materials used The guarantee does not cover damage through misuse or inexpert servicing or other circumstances beyond our control For the UK this means that no charges are made for labour materials or return carriage for guarantee repairs For other countries the guarantee covers free exchange of faulty parts during the guarantee period Alternatively if the equipment is returned to us for guarantee repair we make no charge for labour or materials but we do charge for carriage and UK customs clearance We strongly prefer to have such repairs discussed with us first and if we agree that the equipment does need to be returned we may at our discretion waive these charges Service and spares We recognise that some users of our instruments may not have easy access to technically specialised backup Please refer to the Care and Maintenance section of this Manual for specific information on this product Spare parts for our own repairable instruments can be supplied from our works These can normally be despatched within 1 working day of receiving an order
14. e uses the same characteristic to convert from its mV output to the square root of the apparent dielectric constant VE of the soil However the conversion from Ve to percent moisture content depends on the soil type encountered If you want to log data in soil moisture content units you will have to provide the logger with information about how to convert the data Three main methods of configuring dataloggers to ThetaProbe are recommended e Polynomial equation conversion e Linearisation table conversion e Slope and offset conversion Each method is described in the following section The need for calibration The relationship beween ThetaProbe output and soil moisture content is a non linear curve of this form for generalised mineral and organic soils 1 000 p f 0 900 d 0 800 organic BH mineral 0 700 p E c 0 600 E g 5 0 500 2 0 400 0 300 0 200 0 100 0 000 0 02 04 0 6 0 8 1 12 ThetaProbe output V Page 9 ML2x UM 1 21 May 1999 These two curves are generalised examples for mineral and organic soils The calibration curve for any specific soil would be slightly different from either of these because the ThetaProbe is actually sensing the dielectric constant of the soil and the relationship between the measured dielectric constant of a soil and its water content 0 depends on the particular compositi
15. en derived using the linear relationship in equation 2 so equation 5 and not equation 6 must be used for generating the conversion formula ML2x UM 1 21 May 1999 Page 12 Generalised calibration If it is not necessary to perform a soil specific calibration we suggest using the following parameters which have been derived from the measurements taken on a large number of mineral and organic soils la Mineral 1 6 soils Organic 1 3 7 7 soils These parameters have been used to generate the following linearisation tables and slope and offset conversions Linearisation table conversion Delta T dataloggers are able to store a linear or non linear conversion characteristic permanently in their software using a linearisation table This enables almost instantaneous logger channel configuration that is required is to select a Sensor Type code number from a list displayed on your computer screen For users of existing Delta T dataloggers you can add the necessary linearisation tables using the following data soil mV mV soil mV mV moisture organic mineral moisture organic mineral Soil 3 soil For details of how to do this refer to your Datalogger User Manual Use of this feature will give improved accuracy and give indicative moisture readings at high moisture levels Note that non linearity has been introduced at both ends of the table to avoid out of range logged readings outside o
16. f the probes valid working range Slope and offset conversion Using linear fit equations from experimental data offset and slope parameters have been calculated that can be programmed into any data logger capable of accepting this conversion method For details of how to do this refer to your data logger Page 13 ML2x UM 1 21 May 1999 documentation Delta T loggers can be programmed in this way if preferred although linearisation tables are capable of achieving higher accuracy For DL2 AND DL2e loggers the Engineering Factor used for slope conversion is the reciprocal of the Slope figures below This conversion method applies to a probe output range of 0 900mV beyond which the output becomes excessively non linear The parameters to convert from probe output in mV to organic and mineral moisture content for soils are From probe to moisture Mineral 0 050 5 0 soil moisture Organic 0 055 2 0 soil Organic and Mineral definitions The generalised calibrations have been optimised to cover a wide range of soil types based on the following definitions Soil type optimised use for organic bulk density use for bulk around organic contents range g cm densities content ML2x UM 1 21 May 1999 Page 14 Page 15 Achievable accuracy The errors associated with the ThetaProbe are shown in the Specifications table They are the errors associated with the instrument itself and do
17. he extension tubes is 4cms so an auger of approximately 5cms is recommended Extension tubes can be easily added to the ThetaProbe by following these instructions Carefully remove the black plastic thread protection cover from the ThetaProbe This cover protects the extension tube mating thread when extension tubes are not needed It can be removed by sliding it up the cable Pass the ThetaProbe cable through the hole in the extension tube ensuring that the female thread in the extension tube is towards ThetaProbe Screw the extension tube onto ThetaProbe and hand tighten only Repeat this process for additional extension tubes to make up the length required Finally thread the cable through the black plastic thread protection cover and slide the cover into place over the male thread on the end of the extension tube This will minimise water ingress into the tube Page 5 ML2x UM 1 21 May 1999 Wiring connections ThetaProbe is supplied with a four core screened cable which provides these connections Red Power supply positive Blue Power supply zero volts Yellow Output signal HI load resistance 1OKQ minimum Green Output signal LO Braid Cable screen Not connected within probe The Blue and Green leads are connected internally The braid screen should be connected to an analogue earth on the logger or other measuring unit If not using Delta T equipment please refer to the manufacturer s instructions Some ThetaProbe var
18. ial in wide ranging soil types or water for long periods without malfunction or corrosion IP68 to 5m Stabilization time 1 to 5 sec from power up depending on accuracy required Less than 0 5 sec to 99 of change Duty cycle 100 96 Continuous operation possible Interface Input requirements 5 15V DC unregulated Current consumption 19mA typical 23mA max Output signal approx 0 1V DC for 0 0 5m3m 3 PVC Stainless steel Standard 5m Maximum length 100m 350 gm approx with 5m cable ML2x UM 1 21 May 1999 Page 8 Use and calibration ThetaProbe can be used to provide an instantaneous reading of soil conditions or in conjunction with a datalogger comprehensive moisture data over time The millivolt output from the probe although requiring a non linear conversion to soil moisture units for optimum accuracy can provide reasonable results using a straightforward linear conversion To use the probe without a datalogger you will need to provide it with 5 15VDC at about 20mA Voltage readings can be taken with any general purpose voltmeter and the conversion to soil moisture units made using the information given in a later section This measurement method produces good instantaneous results easily but for optimum accuracy automatic conversion to soil moisture units for known types of soil and of course permanent data records ThetaProbe should be used with a datalogger Data conversion methods Every ThetaProb
19. iants have or require a connector to be fitted to mate to appropriate Delta T instrumentation Connection details are given in the relevant instrumentation User Manual Electromagnetic Compatibility EMC ThetaProbe has been assessed for compatibility under the European Union EMC Directive 89 336 EEC and conforms to the appropriate standards provided the moisture measuring rods and probe body are completely immersed in the soil or other material being measured The cable connecting the ThetaProbe to its associated instrumentation should also be routed along the surface of the soil If the probe is not installed in this way some interference may be experienced on nearby radio equipment Under most conditions moving the equipment further from ThetaProbe typically 1 2 metres will stop the interference ThetaProbes installed near to each other will not malfunction due to interference ML2x UM 1 21 May 1999 Page 6 Connection to Delta T data loggers DL2 amp DL3000 Page 7 Power Connections ThetaProbe can be directly powered by Delta T data loggers using their internal batteries However if several probes are to be used or if the logger has to supply significant power to other sensors or accessories we recommend powering the logger and sensors from an external power supply Battery power consumed by a probe for a single measurement taken with a 1 second warm up time is typically 19mA 15 0 005 mA h Delta T loggers include
20. ific calibration see below When the differences in ao and are allowed for the measurements of soil moisture can differ by up to 0 025 33 m m Adapting ML1 calibrations for ML2x If you ve done a soil specific calibration with the ML1 and generated values of and those values should be unchanged when used with the ML2x because they describe the dielectric properties of the soil see equation 3 and are theoretically independent of the response of the ThetaProbe However the changes to the nature of the curve or linearisation used to model the response of the ThetaProbe does have a slight effect on the calculated values of a and a and so you may choose to use slightly different values and for ML2x formaximum mineral soils no change compatibility with readin s uses ede soils Go Am lmm 94 e for greatest accuracy generate new values of ao and a using the ML2x and following the procedure on page 11 ML2x UM 1 21 May 1999 Page 18 Mixed installations of ML1 and ML2x ThetaProbes If you have a mixture of ML1 and ML2x ThetaProbes in an installation obviously the best choice is to use ML1 calibrations for the ML1 and ML2x calibrations for the ML2x Sometimes that will be impractical and you need to know whether it s possible to use both ML2x and ML1 with the same conversion If you do there will be an extra source of errors in the errors t
21. ised that these figures are given purely for illustration purposes the actual accuracy achieved in a particular measurement will depend mainly on e 501 heterogeneity e the number of samples taken e the extent to which it is possible to perform an accurate calibration on an undisturbed sample e inserting the ThetaProbe without causing air pockets or localised soil compression ML2x UM 1 21 May 1999 Salinity The output of the ThetaProbe is affected by the ionic conductivity of salts dissolved in the soil moisture This effect is not major and is limited to salinity levels below about 250 mS m The potential error is discussed below Units The preferred units for ionic conductivity units mS m where S is Siemens a measure of electric conductance The following conversions apply 1mS m 0 01 mS cm 0 001 mmho cm 10 uS cm Soil salinity is also partitioned into the following descriptive categories non saline 0 200 mS m slightly saline 200 400 mS m moderately saline 400 800 mS m strongly saline 800 1600 mS m extremely saline 1600 mS m ThetaProbe response The ThetaProbe has been tested extensively in saline conditions both in fluids and soils even up to 6000 mS m The response in water of varying salinity is as follows MLURI conductivity data 9 Vmeasured ThetaProbe output V 0 500 1000 1500 2000 2500 lonic conductivity mS m
22. n t take into account errors you may introduce when carrying out a calibration and they assume you insert the probe perfectly into a perfectly uniform material They assume an error budget like this Error category soil generalised source of error specific calibration calibration ThetaProbe errors repeatability beween ML2x probes Calibration errors typical error in values of ao and a RSS value In practice when determining the overall reading errors sampling and insertion errors need to be considered As described in the Installation section these can be as large as 0 1 m m With care and by taking appropriate numbers of samples 10 to 20 samples might be required for each reading you may reduce this but unless your soil is unusually homogeneous these errors are unlikely to be less than 0 04 m m Based on that you might expect the errors associated with your readings to look like this example values are 5 Error category soil generalised source of error specific calibration calibration ThetaProbe errors repeatability beween ML2x probes Sampling errors soil variability and insertion errors RSS value If a linear rather than Linearisation Table or polynomial conversion is used the following additional error will apply resulting in the overall error shown Linearity errors 0 015 0 015 if linear rather than polynomial conversion used 0 to 900mV RSS value It should be emphas
23. ns 2 and 3 to give a t a 0 11 7 4441 b 1 e 8 For example suppose a calibration has determined that the coefficients for ao and a should be 1 5 and 7 8 and a reading of 0 71Volts is obtained with the ThetaProbe If it was assumed that the salinity was 0 0 mS m the soil moisture content calculated from equation 8 would be 0 0 35 m m However if the sample s salinity was in fact 100 0 mS m the real moisture content would have been 0 0 37 m m resulting in an error of 0 02 Minimising errors due to salinity Calibrate the ThetaProbe in a sample of soil which is towards the lower end of but not below the range of salinities that are likely to be encountered If you are using the ThetaProbe in situations where the salinity varies widely and includes non saline conditions you may need to measure the salinity and then apply a correction using equation 8 Compatibility with ThetaProbe type ML1 Page 17 ML2x UM 1 21 May 1999 ML2 output compared to ML1 6 00 5 00 2 00 1 00 0 00 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 1 output V The ML2x performs very similarly to the previous version of the ThetaProbe This graph shows a comparison of their response to dielectric constant These differences beween the ML1 and ML2x result in very slightly different values of a and when doing a soil spec
24. of the soil which has two components the apparent dielectric constant and the ionic conductivity The 100 MHz signal frequency has been chosen to minimise the effect of ionic conductivity so that changes in the transmission line impedance are dependent almost solely on the soil s apparent dielectric constant Because the dielectric of water 81 is very much higher than soil typically 3 to 5 and air 1 the dielectric constant of soil is determined primarily by its water content The impedance of the rod array affects the reflection of the 100 MHz signal and these reflections combine with the applied signal to form a voltage standing wave along the transmission line The output of the ThetaProbe is an analogue voltage proportional to the difference in amplitude of this standing wave at two points and this forms a sensitive and precise measure of soil water content Work published over many years by Whalley White Knight Zegelin and Topp and others shows almost linear correlation between the square root of the dielectric constant Ve and volumetric moisture content and this has been documented for many soil types Each ThetaProbe is adjusted during manufacture to provide a consistent output when measuring media of known dielectric constant making them readily interchangeable without system re calibration The output signal is 0 tol V DC for a range of soil dielectric constant between 1 and 32 which corresponds to a
25. on of the soil You will need to calibrate the ThetaProbe for your specific soil if you want to minimise the errors associated with converting the ThetaProbe output V to soil water content The repeatability of the ML2x is 40 01 m m from 0 to 40 C Theoretically if a soil specific calibration is performed with no additional errors this will be the probe error in this situation If using a generalised calibration typical errors of 0 05 m m should be expected In practice whether you need to do a soil specific calibration will depend on what accuracy you need to work to and the size of your sampling errors see section on Achievable Accuracy Response to dielectric constant Performing a soil specific calibration is relatively straightforward because all ML2x ThetaProbes respond to dielectric constant in the same stable uniform way so it is only necessary to do this once for one probe The relationship beween ThetaProbe output V and square root of dieletric constant Ve is like this 6 00 5 00 4 00 9 3 00 polynomial linear 2 00 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 1 ThetaProbe output V In the range 0 to 1 Volt corresponding to a soil moisture range 0 to 0 55 by volume this relationship can be fitted very precisely by a 3rd order polynomial Je 2107 4 64V 64V 4 7V R 0 998 1 or by the linear relationship Ve 11 4 44V R 0 99 2 ML2x UM 1 21 May 199
26. pproximately 0 5 m m volumetric soil moisture content for mineral soils Page 3 ML2x UM 1 21 May 1999 Installation Essentially installation is very simple you just push the probe into the soil until the rods are fully covered connect up the power supply and take readings from the analogue output Optional extension tubes are available for monitoring a soil layer below the surface However it is quite possible to get sampling errors greater than 0 1 m m and in extreme cases damage the probe if you don t pay due care and attention to the details of the installation of ThetaProbes when making measurements of soil water content You will need to consider each of the following factors when setting up a measurement Air pockets The ThetaProbe is sensitive to the water content of the soil sample held within its array of 4 stainless steel rods but this sensitivity is biased towards the central rod and falls off towards the outside of this cylindrical sampling volume The presence of air pockets around the rods particularly around the central rod will reduce the value of soil moisture content measured In particular you need to be very careful when removing and re inserting the probe into a previous location Insertion angle If the probe is going to be left in situ and measurements taken during and after rainfall it is a good idea to insert it at an angle say 20 so that any water running down the side of the probe housing tends
27. to be carried away from the rods This is particularly important if the probe is being installed below the soil surface using a probe extension tube Soil sampling points The soil water content measured by a ThetaProbe within one small locality can be affected by Variations in soil density and composition Stones close to the rods Roots either nearby or pierced by the rods Earth worm holes or even mole holes Subsoil drainage Small scale variability in transpiration and evaporation losses It is important to take the degree of variability of these various parameters into account when deciding on the number of probes to use at any particular location If the soil is known to be very heterogeneous it will be necessary to take measurements from at least three closely spaced locations Care and maintenance ThetaProbes are sealed after calibration require no routine maintenance and are constructed of materials selected for robust field operation However please pay careful attention to the following 3 points 1 Do not remove the cross head sealing screws This may damage the seal and will invalidate your guarantee No internal maintenance or repair can be performed by the user 2 Do not remove the ThetaProbe from soil by pulling on the cable ML2x UM 1 21 May 1999 Page 4 If the measurement rods become bent in use they can be carefully unscrewed from the body and straightened They have a right handed thread 3
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