Application Note - Decagon Devices, Inc.
Contents
1. linear calibrations are used for mineral soils that reach maxima at 65 If you would like your sensor to read 100 VWC when submerged in water you can apply a quadratic equation to the output For the EC 10 and EC 20 and 10HS the optimal measurement range in soil is from oven dry to saturated water content which is typically 40 to 50 Above saturation the change in probe output with increasing water content is near 0 The ECH O probe uses an electromagnetic EM field to sense the dielectric of the surrounding medium As water content increases the width of that EM field diminishes somewhat As it approaches 100 VWC water with no soil the EM field diminishes to a fraction of its original size and primarily senses the probe surface itself Therefore changes in water content above that level result in relatively little change in the millivolt output from the EC 10 and EC 20 and 10 HS sensors 11 Can I excite my EC 5 sensor at 5V No in a departure from earlier specifications the EC 5 can only be excited from 2 to 3 6 VDC 13390 01 2009 Decagon Devices Inc All rights reserved support decagon com2. 56 The EC 10 and 20 tend to lose some resolution near saturation However they should still be able to differentiate between different moisture contents even at these high water contents 5 Can the ECH 0 probe be used in soils at a depth between 0 5 cm ECH O probes can be used effectively at shallow depths but temperature fluctuations are a consideration Inherent in ECH2O measurements as with TDR and other capacitance probes is a temperature dependence of 0 003 m 3 m 3 per degree C maximum with typical of 0 001 m m C 1 Obviously temperature fluctuations near the surface would be an issue in these measurements We have worked on a method of removing some of the temperature dependence Still these measurements should be approached with considerable caution 6 What is the effect of temperature variation on ECHO probe output Recently several studies have shown the temperature sensitivity of probes that measure the dielectric constant of soil It is well known that the dielectric of water changes with temperature but theories to explain temperature sensitivity of soil dielectric are relatively new In our tests the ECHO probe has a maximum temperature sensitivity of 0 003 m m per C similar to other sensors we tested and to data found in the literature Details of this study can be found in the application note Response of ECHO Soil Moisture Sensor on Temperature Variation This can be downloaded at ht
3. linear calibration However as it approaches oven dry or saturated soil conditions probe sensitivity changes somewhat If you are concerned about the ends of the spectrum we would recommend using a quadratic calibration equation 7 Can I calibrate my ECH20 probe by submerging it to different depths in water No A calibration that relates the percentage of water covering the probe and the voltage output of the ECHO probe will not give an adequate soil calibration Installation and Operation A demonstration of a typical installation can be found at http www decagon com videos 1 How do I install the ECH O probe Inserting the ECH20 sensors into undisturbed soil will always result in the best accuracy For shallow installations we recommend that you dig a trench and insert the sensors into the undisturbed soil in the side or bottom of the trench With the EC 5 5TE and EC TM sensors the sensor can generally be inserted directly into the soil with no pilot hole The EC 20 EC 10 and 10HS sensors often need a pilot hole which can be effectively support decagon com DECAGON DEVICES made with a blade slightly thinner than the thickness of the sensor or with a pilot tool supplied by Decagon For deep installation you can use Decagon s probe installation tool kit for the EC 10 EC 20 or 10HS This kit is comprised of a probe shaped knife blade slide hammer and extension rods to allow deeper installations with minima
4. require excitation using the SW12 port and one COM port for each sensor You can also multiplex them up to 48 on one AM16 32 support decagon com DECAGON DEVICES 4 Will adding extra ECH O probes to my data logger s excitation port change the output of the probe The ECH20 probes only draw 10 mA of current Thus when the data logger is exciting more than one probe it is very unlikely that the excitation voltage will drop due to current drain Decagon recommends only exciting 3 sensors per excitation port but an unlimited amount of sensors can be excited using the SW12 port Do NOT use the SW12 port to excite the EC 5 EC 10 or EC 20 5 What will happen if I wire my ECH20 probes to a data logger backwards Miswiring the ECH O probes will not damage the probes however the output you read will not be correct According to our tests with Campbell Scientific dataloggers the output should not change at all But some customers have reported changes in ECH 0 readings when it is backwards Thus if you are not finding probe output from 250 mV to 1000 mV with 2 5 V excitation check to see if the probes are hooked up properly 6 How many probes can I use with a CSI AM16 32 multiplexer You can multiplex 48 EC 5 or 10HS sensors or 32 STE or EC TM sensors 7 Do you have recommendations for the wiring with the CSI AM16 32 multiplexer Using the ECH O sensors with a multiplexer is generally only for advanced programmers C
5. DECAGON DEVICES Application Note Frequently Asked Questions about the Decagon s Soil Moisture Probes and Accessories From this point forward all of Decagon s soil moisture sensors will be referred to as ECH20 probes General 1 How does the ECH O probe measure volumetric water content VWC The ECHO probes use capacitance to measure the dielectric permittivity of the surrounding medium The volume of water in the total volume of soil most heavily influences the dielectric permittivity of the soil because the dielectric of water 80 is much greater than the other constituents of the soil mineral soil 4 organic matter 4 air 1 Thus when the amount of water changes in the soil the ECH O probe will measure a change in capacitance from the change in dielectric permittivity that can be directly correlated with a change in water content Circuitry inside the ECHO probe changes the capacitance measurement into a proportional millivolt output 2 What is the difference between volumetric water content VWC and gravimetric water content GWC Gravimetric water content is the weight of soil water per unit weight of dry soil Volumetric water content is the volume of soil water per unit of total volume Volumetric and gravimetric water content are related by the bulk density of the soil Gravimetric water content is the most easily measured requiring only a glass jar with a lid an accurate scale and an oven Indeed w
6. TL also provides a program writing service for multiplexing Decagon sensors To familiarize yourself with the multiplexing program you can read Decagon s short application note including wiring diagram and sample program on our website at http www decagon com appnotes soil_moisture An alternative to writing a program to multiplex is by having the Short Cut generator from CSI create the program for you Details on how to add in ECH20 probes Short Cut files are located at http www decagon com appnotes soil_moisture and the program itself can be downloaded at http www decagon com appnotes soil_moisture At this time only the LOHS EC 5 EC 10 and EC 20 are available in the shortcut menu 800 755 2751 www decagon com Application Note 8 Do I need special inductor adapter cables for each probe when using the multiplexer No You have two options in this case First you can purchase standard 3 5 mm adapters that break out into three wires available from Decagon for all probes made before January 2006 check the circular date stamp on the black probe overmolding Probes made after January 2006 can be plugged into jack adapters or the cables can be cut and wired directly into the datalogger see the ECH2O system user s manual Second you can order the sensor with pig tailed cables bare wires with no connector These have the advantage of being available in custom lengths 9 When are inductors required in the
7. connection between the ECH 20 probe and other dataloggers As of January 2006 inductors are located on the probe board so no inductors are needed When using probes manufactured before January 2006 you should use a 3 5 mm pigtail adapter a stereo cable that splits into three wires which can be purchased from Decagon These allow you to use the inductor that is contained in the probe cable connector Calibration 1 Do ECHO probes need to be calibrated It depends on how much accuracy you would like from your results and what type of medium you are working in The accuracy of the EC 10 and 20 probes is 3 to 4 in medium to fine textured soils but in organic soils soils with high electrical conductivity EC or coarse textured soils the accuracy of these probes can decrease significantly On the other hand the EC 5 5TE EC TM and 10HS appear after extensive testing not to require calibration in the majority of mineral soils even those with high EC values with an accuracy of 3 As with anything there will be exceptions that will require individual calibration so we still encourage users to calibrate their probes to become familiar with them before putting them in the field If you are concerned about accuracy we offer a calibration service to calibrate your probes for you please refer to our calibration page or we recommend you calibrate the probes yourself you support decagon com DECAGON DEVICES can find de
8. e accuracy of the ECHO because of the air gaps between the probe and the grain It may be useful to use an evaluation ECH O unit to determine whether the ECH 0 probe could be used in your application 2 How would the ECH20 probe function in a low density medium like Perlite We recommend using an EC 5 the EC TM or 5TE in low density media like Perlite From our testing so far these probes perform very well in low density media A factory calibration for perlite can be found in the user s manual 3 Can the ECHO probe measure the water content of organic matter and compost We would also recommend the EC 5 EC TM or STE in these applications Because they have a forked design instead of a blade they can measure over a much larger range of water contents all the way up to 100 volumetric water content VWC with the right calibration The maximum temperature that the soils can function is 60 degrees C so compost measurements are only possible below this temperature We are currently conducting tests to make compost measurement more reliable 4 How does the probe respond to high soil moisture content The EC 5 EC TM or the 5TE are a good option to measure soils or soilless substrates with high VWC 800 755 2751 www decagon com Application Note because its output does not fall off when soil reaches saturation The 10HS is accurate at high soil moisture contents but not at high soilless substrate moisture contents gt
9. e opportunities to increase the field life of the sensors 6 What is the difference between the different sensor models All of the ECH2O sensors measure water content using a dielectric measurement the EC 5 EC TM STE and 10HS use a measurement frequency at 70 MHz compared to the EC 10 and EC 20 sensors at about 10 MHz Because of this increased measurement frequency the 70 MHz sensors has low sensitivity to differences in electrical conductivity EC tested to 8 dS m and soil texture In addition to VWC the 5TE also measures bulk electrical conductivity EC and temperature The EC TM measures temperature in addition to VWC The EC 5 STE and EC TM all have prongs with a length of 5cm The IOHS has prongs of 10cm length For more detailed comparisons and specifications please visit our specs page support decagon com DECAGON DEVICES 7 In what case would I choose the EC 5 over the 1OHS Any time where large sampling volume is not essential 8 Is the ECH20 sensor itself exposed to the soil No The copper traces used to measure water content are sealed between two pieces of epoxy impregnated fiberglass circuit board material The EM field generated by the traces travels through the fiberglass and into the soil surrounding the probe Measurement and Applications 1 Can I measure the moisture content of grain using the ECH20 probe We have not conducted any studies on this application There may be limits on th
10. hat you take advantage of our custom cable option if your project requires long cable leads Please read the cable length limits below Analog Sensors EC 5 10 20 1OHS In our tests we have successfully connected up to 5 of the 50 ft 15 5m extension cables sold by Decagon for a total length of 250 ft 78m with acceptable results The issue with the length of the cable is resistance The longer the cable the more resistance in the wire which leads to a small signal loss we saw a 14 mV loss under maximum load over 250 ft If you decide to add your own cable it must have a braid shield or foil connected to the probe ground to protect it from external noise We do not recommend that you use single cable with multiple strands to wire several sensors to the datalogger Each set of excitation and output should be surrounded by the ground shield Digital Sensors STE EC TM Our tests have shown that custom cable lengths of over 6000 feet 2000m communicated with the datalogger support decagon com DECAGON DEVICES Although we do not recommend this length it shows that more conventinal lengths are easily obtainable 8 If I use 50 ft 15 5m extension cables from Decagon how can I seal the 3 5mm connectors from moisture Sealing connections in the field is inherently difficult Some researchers have successfully used sections of 34 inch PVC pipe cut in half filled with plumber s silicone and clamped over the top
11. hile GWC may be more easily obtained and more intuitive the output of most soil water content sensors including ECHO probes is correlated with VWC One common calibration error is to relate probe output with GWC 3 Does the probe read water content at a certain point on the sensor or does it average over the entire length The probe averages the volumetric water content over the entire length of the probe The volume of influence depends on which Decagon sensor that 800 755 2751 www decagon com you are using the EC 5 EC TM and 5TE have a volume of influence of 0 3 L while the 10HS sensor has a volume of influence of 1 L 4 Where is the probe the most sensitive The electromagnetic field produced by the probe decreases with distance from the probe surface 5 How long will the ECH20O probes last in the field ECH20 probes should last at least 3 years in the field In fact we expect them to last much longer than that Since we began selling the probes in the spring of 2001 we have been constantly improving the probe to increase their longevity in the field The sensors utilize a chemical on the circuit board that creates a physical bond between two plastics to decrease the chance of water entering the sensor circuitry The sensor also utilizes a rodent repellant cable additive to prevent rodent damage Ongoing testing has shown these new probes to be exceptionally robust even in extreme conditions We continue to explor
12. l soil disturbance The EC 5 STE and EC TM can be pushed into an augered hole using a notched PVC pipe 2 Should I place the sensors flat side up or flat side vertically when installing the sensors Pooling may occur if the flat side of the sensor is facing up parallel planes with the soil surface so we recommend installing the sensors flat side vertically 3 How good does the contact with the soil need to be For example would the ECH O probe function if it is in partial contact with the soil like a coarse gravel material where there might be small air voids between the instrument and the soil Because the ECH O probe is most sensitive at the surface even small air gaps that are not reflective of the overall soil water content would cause problems with the measurement In this type of application we would recommend that you obtain a loaner ECH20 probe from Decagon when available to determine if it could be calibrated for your particular setup 4 Can I measure water content at several locations by inserting an ECH20 probe into the ground taking a reading and then removing it The ECH O probes are designed to be installed in the ground and left in place for long term monitoring Inserting the probes in the ground and taking a reading will give a value of water content at that location but because water content varies considerably over small areas changes in water content may be difficult to distinguish using this me
13. nd using the soil weight loss over time to correlate probe output with VWC Although fundamentally the evaporative weight loss of a soil should determine the change in VWC the amount of water lost from the system will not be uniform across the vertical profile of the soil Thus even if the overall soil VWC has changed 5 the actual VWC of the soil nearest the probe may have changed very little Essentially it is a problem of different sampling volumes See Methods of Soil Analysis Part 4 Physical Methods J Dane and G C Topp Because of the likelihood of non uniform wetting and drying we encourage calibration using 800 755 2751 www decagon com Application Note the procedure we define in our application note referenced above 5 Does the factory calibration that was supplied with my ECH 0 probe work for all soils The EC 5 5TE EC TM and the 10HS sensors showed consistent calibration across all soil types and salinites that we tested According to our tests most fine textured soils with low ECs have a similar calibration function for the EC 10 and EC 20 However if you are concerned about the accuracy of your water content measurement we would encourage you to use Decagon s calibration service or calibrate your probe s using our calibration procedure 6 Although you show a linear calibration my results show some nonlinearity at very low and very high water contents Why is this Generally the ECHO probe has a
14. of the connector Our experience with heat shrink even epoxy lined has shown to be unreliable in some circumstances Another good option is to purchase a length of raw cable from Decagon and splice it into the middle of the cable to increase the length Decagon offers splice kits that create reliable and waterproof connection Decagon also offers custom cable lengths that can be specified when you order 9 What should I do if my probe reads outside the 250 to 1000 mV range that Decagon states is normal for probe output If an ECH20 EC 5 10 or 20 or 1OHS probe reads below 200 mV or above 1200 mV or between 275 to 1700 ADC there is a good chance that it is defective assuming the probe is wired and operated correctly If you have another ECHO probe try plugging that probe into the readout device to ensure that the connector and readout device are functioning properly If there is a problem with the probe contact Decagon for a replacement The EC TM should read from 400 to 1350 Raw and the STE should read from 50 to 4095 Decagon Devices Inc 2365 NE Hopkins Ct Pullman WA 99163 USA 1 800 755 2751 support decagon com www decagon com soil_moisture 800 755 2751 www decagon com Application Note 10 Why does my ECHO sensor not measure 100 when submerged in water Our tests on the EC 5 EC TM and 5TE probe indicate that their response allows for measurements up to 100 However for good accuracy and ease of use
15. provided by CTL a daughter company of Decagon Please see the Decagon website for additional resources for using the STE and EC TM with Campbell Scientific data loggers 2 How much resolution does my data logger need to read the analog ECHO probes This depends on your preference The analog ECH 0 probe changes approximately 10 mV per change in VWC For a datalogger with 8 bits of resolution and a range of 0 to 2500 mV you will have a resolution of 10 mV or 1 VWC for your probe output For loggers with 12 bits of accuracy and the same range you will resolve greater than ImV or 0 1 VWC Not surprisingly the graphed data from the 12 bit dataloggers appears much smoother than the 8 bit dataloggers There are methods to double the resolution of the 8 bit loggers to SmV resolution over the ECHO range though use of operation amplifiers Contact your data logger supplier for details 3 Can the ECH O probe be incorporated into a CSI weather station running on a CRI10 X datalogger All of the ECH20 probes will function with most of the Campbell Scientific data loggers However see the statements in the above questions regarding differences between digital and analog sensors The EC 5 EC 10 and EC 20 will require one excitation voltage location for up to 3 probes and one single ended input location for each probe The 10HS requires excitation using the SW12 port and one single ended port for each sensor The STE and the EC TM
16. tails on our recommended calibration procedure on our website at http www decagon com appnotes soil_moisture 2 Can the ECH20 probe be calibrated to measure the water content of organic matter and compost STE EC T and EC 5 probes are well suited for measuring organic matter and soil less media So far in testing rockwool and potting soil consistent calibration curves were obtained regardless of electrical conductivity Field tests of the probes have also show excellent response to soil water content changes The results of EC 20 calibration in organic matter and peat moss can be found on our website at http www decagon com appnotes soil_moisture ph p Both of these media were consolidated when they were tested Accurate calibration would be much more difficult if the medium were loose such as compost For calibration equations and information for the EC 5 and ECH O TE probes see the application note Calibration Equations for the ECH gt 20O EC 5 and ECH2O TE Sensors which can be downloaded at http www decagon com appnotes soil_moisture 3 How can I calibrate my ECH20 probes for a medium that Decagon has not tested We would recommend you take advantage of Decagon s calibration service or follow the calibration procedure described in our application note located on the web at http www decagon com appnotes soil_moisture 4 What is wrong with calibrating by adding a known weight of water to a known volume of soil a
17. the ECH O probe with every data logger that you could plug it into However below are some recommendations for integrating the sensors into different data loggers EC 5 10HS EC 10 EC 20 analog output The EC 10 and 20 probes require an excitation of 2 to 5V at 3mA for 10 milliseconds The EC 5 requires an excitation of 2 3 6V at 10mA for 10 milliseconds Although longer excitation times will not change probe output Decagon does not recommend that the probe be constantly excited The output of the probe is approximately proportional to the input about 10 to 40 for 2 5V excitation IOHS analog output Because the 10HS contains a voltage regulator the sensor can be powered with any voltage between 3 and 15 VDC In addition the output of the probe is independent of excitation voltage so the factory calibration can be used with any excitation voltage used STE EC TM digital output 800 755 2751 www decagon com Application Note The STE and the EC TM sensors have a digital output and therefore require a data logger that can read a digital signal Many Campbell Scientific data loggers can read the output from the EC TM and the STE however programming for the digital sensors is more complicated than that of analog sensors and is recommended only for those with advanced programming abilities Decagon can provide basic programming support for programming the Campbell Scientific data loggers with more advanced support being
18. thod In addition additional variability in probe water content readings could occur because of variable soil to probe contact If you need to make push in and read type measurements then the EC 5 STE or EC TM would be the best probes to use 800 755 2751 www decagon com Application Note 5 After installing the ECH O probes I see some variability between readings even though they are all buried at the same depth Why Soil water content is inherently variable Studies have shown soil water content to vary as much as 3 over a small area Therefore installing water content sensors in close proximity to each other does not guarantee that they will read the same value Installation mistakes can also lead to difference in reading Soil to probe contact is critical for accurate results Therefore during installation you should make sure that the probe surfaces are snug against the soil surface If a probe does not appear to be reading right we recommend that you reinstall it in the soil 6 The entire probe does not fit in my plant pot Can I use just a portion of the probe to measure water content For potted plants we recommend using a smaller probe like the 5TE EC TM or EC 5 they were designed to fit in small pots 7 What is the longest recommended cable length for connecting an ECH20 sensor to a datalogger Decagon provides both extension cables and custom cable lengths for all of our sensors We recommend t
19. tp www decagon com appnotes soil_moisture These results may be complicated by highly saline soil A method for correcting the effects of temperature on VWC can also be found at http www decagon com appnotes soil_moisture 7 Will the ECHO probe be damaged by high soil surface temperatures Soil temperatures up to 50 C should not damage the ECH O probe However it is important to consider the temperature dependence of the probe output support decagon com DECAGON DEVICES when determining whether the ECHO probe is appropriate for a given installation 8 I want to determine the water availability water potential of my soil Are moisture release curves for soil available from Decagon to correlate water content and water potential Moisture release curves are different and specific for each soil in which you are working with and can be constructed by relating water content with water potential Decagon offers a service to construct a moisture release curve for your soil please contact Decagon for details If you would like to do it on your own we may be able to help by suggesting methods for constructing a moisture release curve or you can read our application note Generating a Soil Moisture Characteristic using the WP4 at http www decagon com appnotes soil_moisture Compatibility with Other Data loggers and Devices 1 Is the ECH2O probe compatible with my data logger As much as we d like to we cannot test
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