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Development of low cost TDR system for soil moisture measurement


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Soil moisture measurement is one of the difficult at same time much important task. Numbers of methods are available for soil moisture measurement. But after reviewing all these methods at last we come to the conclusion that each of these techniques has some of the limitations. So, it is always said that the ideal method for soil moisture measurement is yet to be perfected. After studying all of these techniques comparatively it is seen that Time Domain Reflectometer method has very much scope to develop most superior system for the task. TDR is nothing but a technology which measures frequency dependent properties of electric and dielectric materials with the use of electromagnetic waves. First off all EM wave is generated and gets transmitted in the sensor transmission waveguide put into the soil. Due to impedance mismatching fraction of the transmitted signal reflected back whose travelling time is a function of dielectric constant of the soil which depends on the amount of moisture present in the soil. High cost of TDR system is most serious limitation to use this technique. A care is taken while designing to provide highly accurate, low cost solution for soil moisture measurement. To reduce the cost instead of using complex FPGA, simpler low cost microcontroller is used. GSM module is used which gives the low cost, wireless communication so that the reading of the TDR can be send wirelessly in short time. A simple 16*2 LCD display is used, to provide the TDR reading at the site itself. Gravimetric method is used as reference for the calibration.

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Development of low cost TDR system for soil moisture measurement

  1. 1. S.S.B.T’s COET, Bambhori, Jalgaon
  2. 2.  Aim  Objectives and Necessity  System Concept  Literature Survey  System Design  Calibration  Results And Discussion  System Measurement And Analysis  Comparative Analysis  Advantages, Limitations and Remedy  Applications  Conclusion  References
  3. 3. Basic aim of this project is to develop an fully Integrated Electronic System, to provide low cost, highly accurate solution for soil moisture measurement Fig. TDR measurement system
  4. 4. OBJECTIVES :  To reduce the cost of the previously developed system  To design universal system to measure soil moisture with high accuracy  To provide most reliable and universal solution for soil moisture measurement  To design a system which can work even at higher frequency (in MHz range) NEED OF SOIL MOISTURE MEASUREMENT :  In agriculture & Plant science field to determine best time to Sow & plow the field.  Various physical & chemical properties of soil changes with amount of moisture present in soil.  To measure changes in infiltration, irrigation.  To study ground water recharge & Evapo-transpiration.  It is also important in the fields like Hydrology, Forestry, Agrology.  To study & determine the parameters like soil profile, surface tension related with civil & soil engineering.
  5. 5.  WHAT IS TIME DOMAIN REFLECTOMETER ? TDR is a very much popular concept related to the measurement of the frequency dependent characteristics such as electric & dielectric properties of various materials & substances.  PRINCIPLE OF OPERATION : Based on the principle of measurement of propagation time required by EM wave for transmission and reflect back. Ka = (c/v)2 = [(c × t)/(2 × L)]2 …………………….(i) Here, c is the velocity of electromagnetic waves in a vacuum and L is the length of the transmission line present in the soil (in m or ft). Fig Working principle of TDR system Graph-Typical waveforms
  6. 6. Fig. a) Equivalent electrical circuit model for the laboratory setup presented Fig. b) Equivalent electrical circuit model for an optimized TDR system, the generator output impedance matches the line impedance EQUIVALENT ELECTRICAL CIRCUIT MODEL: Fig. Soil Sensor Used for Moisture Measurement
  7. 7. Technique Principle used and Methodology Photograph 1. Gravimetric Method (GM) Depends on the weight of original sample and oven dried sample. 1. Take Weight of the original sample (Wt) 2. Apply oven drying at 105oC for 24 Hr & weight (Ws). 3. 2. Neutron Moderation (NM) Depends on the amount collision between fast neutrons and Hydrogen atoms present in moisture 1. Insert probe into access tube installed in soil. 2. Linear calibration between the count rate of slowed neutrons gives the reading of % moisture content. 3. TDR Depends on the propagation time required by EM wave to transmit and reflect back from sensor transmission waveguide 1. Insert probe into access tube & transmit EM wave. 2. The propagation time required for transmit & reflect back gives the % moisture content depending on the dielectric constant  REVIEW OF DIFFERENT SOIL MOISTURE MEASUREMENT TECHNIQUES:
  8. 8. Technique Principle used and Methodology Photograph 4. FDR It is a dielectric method obtaining moisture content by observing response at different frequencies 1. Probe is introduced into soil 2. After applying electric field gives reading due to capacitance effect 5. ADR Depends on the change in amplitude of transmitted wave after reflecting from the section of different impedance depending on content of moisture 1. EM wave is generated & transmitted using sensor 2. Amplitude change gives the reading 6. PT Based on the property of the travelling sinusoidal wave showing relative phase after travelling a fixed distance. 1. EM is generated and transmitted 2. Phase difference at the start & end fives the reading 7. TDT Similar to TDR measurement of the propagation time only change is that measured over known distance. 1. Methodology is exactly similar to TDR system 8. Tensiometer Depends on the suction produced by water into sealed tube coming into equilibrium with the soil solution through porous medium 1. Tip of ceramic cup is placed into the soil 2. Water is drawn out side to form equilibrium a suction is created inside tube 3. Depending on the amount of suction produced moisture content is indicated
  9. 9. Parameter GM NM TDR FDR ADR PT TDT Tensiometer 1) Requirement of Specific Calibration N N Y Y Y Y Y N 2) Affected by Soil Salinity & air gaps N Y N/Y Y Y Y Y N 3) Measurement at different depths Y Y N Y N N N N 4) Connection with Data Logger N N Y Y Y Y Y N 5) Time efficient N N Y Y Y Y Y N 6) Permanent Installation N Y Y Y Y Y Y N 7) Safety Y N Y Y Y Y Y Y 8) Automation N N Y Y Y Y Y N COMPARISION OF DIFFERENT SOIL MOISTURE MEASUREMENT TECHNIQUES
  10. 10. NAME ADVANTAGES DISADVANTAGES Campbell Scientific TDR Integration in automated measurement system Required additional ext. control setup Lee et al Random equivalent sampling TDR Required two extremely stable oscillators Xudong et al Developed TDR based cable fault diagnosis system Expensive off the shelf laboratory equipment Purisima et al Developed field programmable gate array based TDR system Direct sampling scheme is limited Negrea and Rangu Small microcontroller-based sequential sampling with three programmable delay lines Resolution is only 250 ps Schimmer et al Portable high-frequency TDR meter. Ability to be Used even at very high frequency components up to a few GHz Very low overall recording time, limited temporal resolution, high power consumption, need for calibration, & sometimes, limited availability on the market Sokoll and Schimmer Replaced the programmable delay lines by two programmable but free-running oscillators. excellent performance and high accuracy The system cannot easily be adapted for long transmission lines required in many geological and agricultural applications  REVIEW OF DIFFERENT TDR SYSTEM:
  11. 11.  BASIC BLOCK DIAGRAM Fig. Basic block diagram of Proposed System
  12. 12. Sr. No. COMPONENT FUNCTION COST 01 Microcontroller Board To implement Microcontroller and assembly 125 02 MICROCONTROLLER (AT89S52) CPU of system 90 03 MAX-232ECPE Serves as TTL Converter 25 04 RS 232 Cable Provide communication between controller and GSM module 45 05 Regulator IC-L7805CV Provide regulated 5V supply 25 06 Relay (VK8FF-S-DC5V-C) Provide switching between GSM and sensor 85 07 LCD (JHD162A16*2) Display VWC and propagation Time to travel EM wave 180 08 MICRO-CHIP (PIC16F1516-I/SO) Generate, transmit and receive reflected EM wave to and from transmission waveguide 2000 09 GSM (SIM-300 V702) Provide wireless communication 1600 10 Supply Unit To provide Power supply 80 11 Other Expenses 120 Total Project Cost 4375/-Rs  COMPONENTS REQUIRED AND FUNCTION Table : List of Components required, functions provided and cost
  13. 13. Circuit Connections: Component Pins Connections LCD Display DB0-DB7 P2.0-P2.7 of Microcontroller RS,R/W,E P1.0,P1.1,P1.2 of Microcontroller Vss LED+ and GND pin of Soil Sensor Vcc LED- and Supply pin of Soil Sensor Relay N/C Tx/out Pin of Soil Sensor N/O GSM module through MAX232 and P3.0 Com Collector terminal of Relay Driver circuitry Soil Sensor Supply Vcc pin of LCD Display GND With Vss pin of the LCD Display Tx/out N/C pin of Relay Microcontroller Port 2 Data pins of LCD Display. P1.0,P1.1,P1.2 Control pins of LCD Display. P0.0 Emitter terminal of Relay Driver circuitry P0.2 Base terminal of Relay Driver circuitry P3.0 N/O pin of Relay, GSM module through MAX232
  14. 14. Driver Circuitry of Relay: Driver Circuitry of Relay used Photograph of the Relay Used
  15. 15. COMPLETE CIRCUIT DIAGRAM OF SYSTEM Fig. Complete circuit diagram of the Developed System
  16. 16. PHOTOGRAPH Fig. Photograph of the Developed System
  17. 17. Available methods for calibrating TDR instruments : 1. Using empirical function- This is the traditional method which relates dielectric constant of soil and soil moisture content 2. Using Neutron probe – Much popular but require authentication and also it is risky one. 3. Using dielectric mixing – Here relationship between modelled bulk dielectric constant with the individual dielectric values of specific components of the soil system like water, mineral grains, bound water and air. 4. Using bulk density values, volumetric water content and depth- Serious limitation of this method that it is not practical for heterogeneous soil samples. 5. Using Gravimetric method - It is the most accurate and proven method for calibrating moisture measuring instrument. So, this method is selected for TDR calibration
  18. 18. CALIBRATION USING GRAVIMETRIC METHOD : Requirements : • Oven with 1000-1050C temperature • A balance of precision of ±0.001 g. • Aluminium weigh tins • Tool to collect soil samples Procedure : i. A weight of original soil sample is taken and noted as Wt . ii. Place the sample in the oven 1050C, and dry for 24 hours. iii. Weight of oven dried sample is taken and noted as Ws iv. % Volumetric Water Content is calculated using following formula: v. Measurement of original soil sample using TDR meter is taken in parallel so travelling TIME (Ts) and it’s proportional COUNT is noted. vi. Now, % VWC by Gravimetric Method per TDR COUNT is determined as: % VWC (yn) = 100 x The soil moisture content may be expressed by Weight as the ratio of the mass of water present to the dry to the dry weight of the soil sample, or by volume as ratio of volume of water to the total volume of the soil sample.
  19. 19. vi. Same procedure is repeated for number of samples and then Average of all the % VWC per Count is taken. In this way TDR instrument is calibrated. vii. % VWC by TDR now can be calculated simply as:
  20. 20. Table indicates the results of soil moisture measurement by Gravimetric Method and TDR Method. Percent moisture i.e. volumetric water content measurement by gravimetric method is calculated from oven drying technique by determining the weights of the original soil sample (Wt) and oven dried sample(Ws). % VWC (yn) = 100 x Here: Wt = Weight of soil and water, Ws = Dry soil weight Percent soil moisture measurement is done with the reference of Gravimetric method, as GM is used as calibration technique for TDR method. TDR indicates the propagation time (Ts) required by EM wave to transmit and reflect back through the soil sample and a count proportional this Time. Percent per count is calculated as: % VWC by TDR now can be calculated as:
  21. 21. Results for different soil samples: Gravimetric Method TDR Method Sample Wt (gm) Ws (gm) % VWC (yn) TIME Ts (ms) COUNT % VWC Per COUNT Avg. % VWC Per COUNT % VWC (xn) I. 150 128.08 17.11 11.25 96 0.1782 0.1789 17.17 150 124.30 20.68 13.24 113 0.1830 20.21 150 116.77 28.46 19.69 168 0.1694 30.05 150 111 35.14 22.27 190 0.1849 33.99 II. 150 125.66 19.37 18.05 154 0.1258 0.1288 19.84 150 123.95 21.02 18.75 160 0.1314 20.61 150 122.60 22.35 20.27 173 0.1292 22.28 III. 150 127.66 17.50 12.42 106 0.1651 0.1618 17.15 150 116.67 28.51 20.74 177 0.1611 28.64 150 115.50 29.87 21.45 183 0.1632 29.61 150 111.34 34.72 25.78 220 0.1578 35.60 Table :Final results of Soil Moisture Measurement for Different Soil Sample
  22. 22. 1 2 3 4 S I 0.1782 0.183 0.1694 0.1849 S II 0.1258 0.1314 0.1292 S III 0.1651 0.1611 0.1632 0.1578 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 %VWCPerCOUNT % VWC Per COUNT For Different Soil Samples 0 50 100 150 200 250 1 2 3 4 12.42 ms 20.74 ms 21.45 ms 25.78 ms 106 177 183 220 17.15% 28.64% 29.61% 35.60% TDRCOUNT TIME, COUNT and % VWC for soil sample III TIME COUNT % VVC Graph : Plot of TIME, COUNT and % VWC for Soil Sample-III Graph : Plot of Percent VWC by GM per TDR COUNT for Different Soil Samples
  23. 23. Statistical analysis is much important in the study of any analytical instrument. Analysis of the measurement results stated in the table done using the standard formulae used for statistical analysis. All of these formulae are considered with the reference of book Electronic Instrumentation and Measurement by H. S. Kalsi [18]. 1) Percentage error: “It is the deviation of the true value (measured value) from the expected value”. It is determined by Here, 2) Relative accuracy is determined by: “It is the degree of exactness (closeness) of a measurement compared to the expected (desired) value”. It is determined as: Percent accuracy is determined by:
  24. 24. Sample (yn) (xn) e = yn - xn %E A % a I. 17.11 17.17 -0.06 -0.35 0.9965 99.65 20.68 20.21 0.47 2.27 0.9773 97.73 28.46 30.06 -1.6 -5.62 0.9438 94.38 35.14 33.99 1.15 3.27 0.9673 96.73 Average % E = 2.88 % Average % a = 97.12 % II. 19.37 19.84 -0.47 -2.43 0.9757 97.57 21.02 20.61 0.41 1.95 0.9805 98.05 22.35 22.28 0.07 0.31 0.9969 99.69 Average % E = 1.56 % Average % a = 98.44 % III. 17.50 17.15 0.35 2.00 0.9800 98 28.51 28.64 -0.13 -0.45 0.9955 99.55 29.87 29.61 0.28 0.93 0.9907 99.07 34.72 35.60 -0.88 -2.53 0.9747 97.47 Average % E = 1.48 % Average % a = 98.52 % Table : Percent Error and Percent Accuracy of the system
  25. 25. 3) Arithmetic mean: Here, xn = Value of nth measurement n = Total numbers of measurements 4) Precision (P) : 5) Deviation from mean (dn): 6) Average Deviation (Davg): 7) Standard Deviation (δ): “It is the most probable value of a measured variable of the taken number of readings”. “It is a quantitative or numerical indication of the closeness of measured value with which a repeated set of measurement of the same variable agree with the average set of measurements”. It is determined as: “The departure of a given reading from the arithmetic mean of the group of readings“. “It is an indication of the precision of the instrument used in measurement”.
  26. 26. Sample xn P Davg δ I. x1 = 0.1849 0.1789 0.9665 d1 = 0.0060 0.0051 0.0081 x2 = 0.1830 0.9771 d2 = 0.0041 x3 = 0.1782 0.9961 d3 = -0.0007 x4 = 0.1694 0.9469 d4 = -0.0095 II. x1 = 0.1258 0.1288 0.9767 d1 = -0.0030 0.0020 0.0040x2 = 0.1314 0.9798 d2 = 0.0026 x3 = 0.1292 0.9969 d3 = 0.0004 III. x1 = 0.1651 0.1622 0.9821 d1 = 0.0029 0.0020 0.0036 x2 = 0.1611 0.9932 d2 = -0.0011 x3 = 0.1632 0.9938 d3 = 0.0010 x4 = 0.1592 0.9815 d4 = 0.0020 Table- Precision, Average Deviation and Standard Deviation of the system Precision, Average Deviation and Standard Deviation of the system:
  27. 27. 0.00% 20.00% 40.00% 60.00% 80.00% 100.00% 1 2 3 4 17.15% 28.64% 29.61% 35.60% 17.50% 28.51% 29.87% 34.72% 98.00% 99.55% 99.07% 97.47% % VWC and % Accuracy (% a) for Soil Sample III % VWC by TDR % VWC by GM % a Graph : Plot of Percent VWC and % Accuracy for Soil Sample III Graph- Plot of Avg. Deviation, Standard Deviation (δ) and Avg. % Error for different Soil Samples GRAPHICAL PRESENTATION OF CALCULATED DATA 0 0.005 0.01 0.015 0.02 0.025 0.03 S-I S-II S-III 0.0051 0.002 0.002 0.0069 0.0028 0.0022 0.0288 0.0156 0.0148 Avg. Deviation, Standard Deviation and avg. Error Davg δ Average (E)
  28. 28. Technique Operating Range (ft3 per ft3) Accuracy (ft3 per ft3) Measurement volume Cost Neutron Moderation 0 to 0.6 ± 0.005 Sphere(radius 6-16 inches) $10,000-15,000 TDR 0.05 to saturation ± 0.01 About 1.2 inches radius around waveguide $400-23,000 FDR 0 to saturation ± 0.01 Sphere(radius 1.6 inches) $100-3,500 ADR 0 to saturation ± 0.01 to 0.05 Cylinder (radius 1.2 inches) $500-700 PT 0.05 to 0.5 ± 0.01 Cylinder $200-400 TDT 0 to 0.7 ± 0.05 Cylinder (radius 2 inches) $400-1,300 Tensiometer 0-0.80 bar ±0.01 bar Sphere (Greater than 4 inch radius) $75-250  COMPARISION WITH DIFFERENT SOIL MOISTURE MEASUREMENT TECHNIQUES :
  29. 29.  ADVANTAGES :  Low cost.  Higher accuracy.  Simple software and hardware design.  Wireless transmission of the readings is possible.  Fault detection and correction is easy as compared to existing system.  No need of frequent maintenance, low maintenance cost.  Easily expanded using multiplexing.  Varity of sensor probes availability.  Very less soil disturbance.  Insensitive to normal salinity.  LIMITATIONS : • Encounters problem at high salinity condition • Specific calibration required  REMEDY : • Problem at high salinity condition can be easily solved using coated TDR probes with Polyolefin coating. • Remedy to second limitation is to gather data base of large numbers of soil samples
  30. 30.  APPLICATIONS : i. In agriculture and plant science field to determine best time to sow and plow the field. ii. In the Drainage engineering to measure changes in infiltration and irrigation. iii. To study ground water recharge and Evapo-transpiration. iv. It is also important in the fields like Hydrology, Forestry and Agrology. v. In the study of various physical and chemical properties of soil which changes with amount of moisture present in soil. vi. To study and determine the parameters like soil profile, surface tension related with civil and soil engineering. vii. Automation of Irrigation and Mushroom cultivation viii. For solid waste management for decomposition of waste
  31. 31. CONCLUSION :  We are succeeded to provide low cost solution for soil moisture measurement. Ideal method for soil moisture measurement is yet to be perfected.  A TDR instrument also has limitations like high cost, need of specific calibration, inaccuracy at higher salinity conditions. This system design is a step towards the perfection of this technique as it removes the limitation of the high cost.  Need of the specific calibration can be minimize in the future by collecting large data base from large number of different soil samples.  Solution for the reduction of inaccuracy problems faced at higher salinity condition is also available; some of the researchers are trying to solve this problem with the use of polyolefin coated TDR probes.
  32. 32. FUTURE SCOPE :  Increasing accuracy and system performance using different material for TDR probe design.  Use of polyolefin coated probes to reduce problem faced at higher salinity conditions.  Reducing need of specific calibration by collecting large amount of data base. Fig. Coaxial Cable Construction [28] (A) Insulating Jacket (B) Braided Shield Wire (C) Insulating Dielectric Material (D) Inner Conducting Wire (Source: Belden Cable)
  33. 33. Publications in International Journal: 1. Bhushan N. Patil, P. H. Zope & K. S. Patil, “A Review of Various Soil Moisture Measurement Techniques”, Cyber Times International Journal of Technology & Management Vol. 7 Issue 2, April 2014 – September 2014. P.P. 247-254. Available at: 2. Bhushan N. Patil, P. H. Zope & K. S. Patil, “Development of Low Cost TDR System for Soil Moisture Measurement”, International Journal of Advanced Research in Education & Technology Vol. 2 Issue 3, July to Sept, 2015. 3. Bhushan N. Patil, “A Review of ECG Monitoring System Using Wavelet Transform”, International Journal of Research in Advent Technology (IJRAT) Volume 2, Issue 4, 15 April 2014. Publication in International Conference: Bhushan N. Patil, P. H. Zope, K. S. Patil, “A Review of Various Soil Moisture Measurement Techniques”, International Conference on Global Trends in Engineering, Technology and Management, Jan 9th & 10th 2015.
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  36. 36. Soil Moisture Measurement using Gypsum Block Technique at Irrigation & Drainage Engineering Department of College of Agricultural Engineering and Technology, Akola. Fig- Soil Moisture Measurement using Gypsum Block Technique  Visit to College of Agricultural Engineering and Technology, Akola.