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SM150T User Manual 1.0

Technical Reference

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36

Sampling Volume

The SM150T is most sensitive to signals very close to the two rods, but a
small proportion of the signal reaches up to 50mm from the rods.

Minimum soil sample size: Full accuracy requires a soil volume of one litre but
the additional error from taking a reading in a 0.5 litre sample is negligible

SM150Ts may interact if they are placed too close together

– they should be

separated by at least 100mm.

-1

5

-5

5

15

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

-25 -20

-15 -10

-5

0

5

10

15

20

25

Distance parallel to plane of
rods from a centre line between the rods (mm)

SM150T field of sensitivity surrounding the rods

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Summary of Contents for SM150T

Page 1: ...User Manual for the SM150T Soil Moisture Sensor Delta T Devices Ltd ...

Page 2: ...SA Patent US7944220 Europe Patent EP1836483 Australia Patent AU2005315407 China Patent CN101080631 EMC Compliance See page 38 Design changes Delta T Devices Ltd reserves the right to change the designs and specifications of its products at any time without prior notice User Manual Version SM150T UM 0 f Nov 2016 Delta T Devices Ltd 130 Low Road Burwell Cambridge CB25 0EJ UK Tel 44 1638 742922 Fax 4...

Page 3: ...nnections 10 Installation 11 HH150 Meter 12 HH2 Meter 13 Logger connections and configuration 14 GP1 Logger 14 GP2 Logger Controller 15 DL6 Logger 16 DL2e Logger 17 Other Data Loggers 18 Logger Grounding 21 Calibration 24 Soil calibration 25 Sensor calibration 27 Soil moisture reading 28 Troubleshooting 30 Technical Reference 33 Specifications 33 Volumetric water content 33 ...

Page 4: ... soils 43 Laboratory calibration for clay soils 46 Appendix 2 49 The SM150 Temperature Sensor 49 Global warming and climate studies 49 Civil engineering 49 Soil contamination and hydrogeology 49 Agriculture 49 SM150T Temperature Measurement 50 Effect of Temperature on Water Permittivity 52 Resistance to Temperature Lookup Table 53 Technical Support 54 Index 56 ...

Page 5: ...r meter using the supplied general soil calibrations It can also be calibrated for specific soils Features Soil moisture accurate to 3 Soil temperature to 0 5 C over 0 40 C Low salinity sensitivity Excellent stability Minimal soil disturbance Easy installation at depth in augured holes Waterproof connector to IP68 Rugged weather proof and can be buried Good electrical immunity Choice of cabling sy...

Page 6: ...SM150T User Manual 1 0 Introduction 6 Dimensions Cable connector sealed to IP68 M12 5 pin male Thread inch BSP for connecting to Extension Tube s 67 mm 92 mm 51mm 22mm 12 40 mm ...

Page 7: ...0 100cm SM AUG 100 Spiral Auger 1 2m Extension tubes HH150 SM150T Kit SM150T soil moisture sensor SMCS d HH2 0 9m cable Connects SM150 to HH2 HH150 includes 1m cable Connects to SM150T SMSC lw 05 5m cable 200mm flying leads Connect to data logger EXT 5W 05 5m EXT 5W 10 10m EXT 5W 25 25m Logger extension cables ...

Page 8: ...before pushing the parts together Do not pull the SM150T out of the soil by its cable If you feel strong resistance when inserting the SM150T into soil it is likely you have encountered a stone Stop pushing and re insert at a new location Do not touch the pins particularly when the sensor is attached to a cable An electrostatic discharge from your body can typically cause a temporary 10mV offset i...

Page 9: ...he soil The water content of the soil surrounding the rods dominates its permittivity A measure of a material s response to polarisation in an electromagnetic field Water has a permittivity 81 compared to soil 4 and air 1 The permittivity of the soil has a strong influence on the applied field Vout which is detected by the SM150T resulting in a stable voltage output that Soil Moisture 22 acts as a...

Page 10: ...re the connection is water tight Extension cables can be joined up to a recommended maximum of 100m for GP1 or GP2 data loggers see Specifications on page 33 Note for full accuracy do not use extension cables with the HH150 Note The HH150 meter does not record temperature SMCS d HH2 0 9m cable Connects SM150 to HH2 HH150 includes 1m cable Connects to SM150T SMSC lw 05 5m cable 200mm flying leads C...

Page 11: ...re insert at a new location Note The SM150T is not suitable for soil surface temperature measurements For soil temperature near the surface dig a trench and install horizontally as shown below Cover both SM150T and the first 30cm of cable with at least 5cm of soil Installing at depth Make a 45mm diameter hole preferably at about 10 to the vertical using the SM AUG 100 auger Connect an extension tu...

Page 12: ...tent of either Mineral Peat Mix Coir Mineral Wool or Perlite soils or to show the sensor output in Volts Press the left on read button to take a reading Repeat as required You may wish to write down the readings Note The HH150 meter does not record temperature Turn on take and display reading Turn on show current setting OFF Mineral 32 2 vol Read Mineral Mineral 34 3 vol OFF Read Peat mix Coir Min...

Page 13: ...he Soil Type option Press Set again and scroll down to the appropriate soil type use Mineral for sand silt or clay soils or Organic for peaty soils Press Set to confirm this choice Choose the units in which you want to display the readings At the start up screen press Set and scroll down to the Display option Press Set again and scroll down to select units Press Set to confirm this choice Press Re...

Page 14: ...hield green CH1 2 GND Temperature grey Temp3 IN Using the DeltaLINK2 logger software configure channel 1 or 2 as soil moisture sensor type SM150T channel 3 or 4 as SM150T Temperature Two more SM150T sensors can be added to Temp3 and or Temp4 channels to measure soil moisture if temperature readings are not required Power 0V brown CH1 2 GND Power V white CH1 2 PWR Signal HI blue CH1 2 Signal LO bla...

Page 15: ... 1 and 2 SM150T wire Colour GP2 terminal Power 0V brown CH1 PGND Power V white CH1 PWR Soil Moisture Signal HI blue CH1 Soil Moisture Signal LO black CH1 Cable shield green CH1 PGND Thermopile HI grey CH2 and CH2 Fit wire link For configuration details see the DeltaLINK software sensor Info Panel and Help or the GP2 User Manual Configure each soil moisture channel as sensor type SM150T and configu...

Page 16: ...s 6 7 SM150T wiring Colour DL6 terminal Power 0V brown 0V Power V white V Signal HI blue IN Signal LO black IN Temperature grey RES IN Cable shield green In DeltaLINK3 configure channel 1 6 as Moisture Probe SM150T and channel 7 as SM150T Temperature See also the DL6 Quick Start Guide and the DeltaLINK online Help 3 You need the PC logger software DeltaLINK version 3 6 or later obtainable online a...

Page 17: ...minal Power 0V brown CH62 or 61 Power V white CH63 NO Signal HI blue CH58 Signal LO black CH58 Temperature grey CH57 and CH57 Cable shield green CH61 or 62 Note If using channel 58 ensure the LAC1 card ribbon is attached to the connector block opposite terminal groups 46 60 See page 3 of DL2e Quick Start Guide Configure the chosen DL2e logger channels by selecting the appropriate mineral or organi...

Page 18: ...l in IN Or Signal Input on single ended loggers 0 1V 0 to 60 vol nominal 3 0 3 vol over 0 to 70 vol and 0 60 C 5 vol over 100 to 1000mS m 1 and 0 60 vol 0 1 5V 0 to 100 vol Reduced accuracy especially at 100 See look up tables and polynomials Signal LO Volumetric water content return black Differential in IN Or 0V On single ended loggers Temperature Thermistor connection grey Resitance measurement...

Page 19: ... 29 Note The SM150T has been optimised for warm up of 0 5 to 1 second duration It is recommended that the sensor is not powered continuously The temperature sensor output should be read as a resistance and the logger configured with a look up table to covert the measured resistance to temperature See SM300 Temperature Measurement on page 50 and Resistance to Temperature Lookup Table on page 53 Log...

Page 20: ...ng equation 1 14 4396 V 31 2587V2 49 0575V3 36 5575V4 10 7117V5 Where V is the SM150T soil moisture output converted from milliVolts to Volts 2 Convert the ε value to soil moisture θ using θ ε a0 a1 Where a0 and a1 are constants For temperature measurement see page 50 a0 a1 Mineral 1 6 8 4 Organic 1 3 7 7 Peat mix 1 16 7 09 Coir 1 16 7 41 Min wool 1 04 7 58 Perlite 1 06 6 53 ...

Page 21: ...e SM150T In addition if used in an electrically noisy environment such as a farm or greenhouse then the presence of a ground loop can permit AC signals to inject noise and so disrupt readings To minimise sensor cable screen currents that can result from the DC power supply unit and its cabling to the logger earth the logger using a 1m long copper clad earth stake Figure 1 Showing the use of a 1m l...

Page 22: ...ns power source allowing noise to be injected into readings and increasing the risk of galvanic corrosion to the M12 connector on the SM150T Figure 2 This diagrams shows that if the logger is not well grounded then a potential ground loop can exist between the sensor and the power supply and or mains power There is also the risk of galvanic corrosion to buried M12 connectors on the sensor and cabl...

Page 23: ...rby AC powered machinery injecting noise into the readings The impact of the potential ground loop between logger and sensor is mitigated by the differential wiring of the SM150T Logger grounded Figure 4 This diagram shows a battery or high quality power supply with no leakage current to ground The logger is grounded The potential ground loop between logger and sensor is mitigated by the different...

Page 24: ... The SM150T measures volumetric soil moisture by detecting the dielectric properties of the damp soil the permittivity or more conveniently the refractive index which is closely equivalent to The SM150T response is best understood in these stages 0 0 2 0 4 0 6 0 8 0 0 0 0 2 0 4 0 6 0 8 1 0 1 2 Soil refractive index ε SM150Toutput Volts SM150Tdielectric performance 0 20 40 60 80 0 0 0 2 0 4 0 6 0 8...

Page 25: ...ve index measured by the SM150T is only affected by changes to the water content This relationship simplifies to 1 0 a a where the coefficients a0 and a1 conveniently parameterise the dielectric properties of soils Note that soil dry a _ 0 is usually between 1 3 to 2 3 a1 corresponds approximately to 1 water and usually takes a value about 8 0 Real soil values for a0 and a1 can vary significantly ...

Page 26: ... Instead of adopting these general calibrations you may wish to determine specific calibration values of a0 and a1 for your soil This procedure is fairly straightforward if you can get access to standard laboratory equipment and is described in detail in Appendix 1 on page 42 Soil specific calibration can significantly improve the accuracy of individual readings but make less of an improvement to ...

Page 27: ...be approximated either by a polynomial below or by a linearisation table see next page Polynomial for use over the full range of SM150T readings 𝜖 1 0 14 4396𝑉 31 2587𝑉2 49 0575𝑉3 36 5575𝑉4 10 7117𝑉5 where V is the SM150T output in Volts 0 0 2 0 4 0 6 0 8 0 0 0 0 2 0 4 0 6 0 8 1 0 1 2 Soil refractive index ε SM150Toutput Volts SM150Tdielectric performance ...

Page 28: ... 3 144 0 525 4 713 0 825 6 342 1 125 8 270 1 425 12 242 Soil moisture reading Polynomial conversion Combining the Soil calibrations and Sensor calibration steps the conversion equation becomes 𝜃 1 0 14 4396𝑉 31 2587𝑉2 49 0575𝑉3 36 5575𝑉4 10 7117𝑉5 𝑎0 𝑎1 where a0 and a1 are the calibration coefficients For a generalised mineral soil this becomes 𝜽𝒎𝒊𝒏𝒆𝒓𝒂𝒍 0 0714 1 7190𝑉 3 7213𝑉2 5 8402𝑉3 4 3521𝑉4 1 ...

Page 29: ...oil moisture Mineral soil Organic soil vol Volts Volts vol Volts Volts 4 2 090 2 090 52 0 758 0 638 0 0 046 0 022 56 0 818 0 695 4 0 076 0 046 60 0 876 0 750 8 0 110 0 074 64 0 933 0 805 12 0 149 0 105 68 0 987 0 859 16 0 195 0 140 72 1 039 0 910 20 0 248 0 180 76 1 087 0 962 24 0 308 0 226 80 1 130 1 010 28 0 373 0 279 84 1 170 1 056 32 0 440 0 336 88 1 206 1 099 36 0 507 0 397 92 1 238 1 138 40 ...

Page 30: ...mware version 2 8 or later and PC software HH2Read version 2 8 or later GP1 GP2 DL6 Loggers DeltaLINK version 3 6 or later is required DL2e Logger Ls2Win 1 0 SR10 or later is required Consult the user manuals or the on line help for these devices and their related software Try alternative types of equipment if you have them available Check that you are using an appropriate soil calibration and the...

Page 31: ...ge from your body can create a temporary 10mV offset in sensor readings lasting an hour In air an SM150T gives an output of 0 4mV Note the HH150 reports under range if the reading is less than zero Mid range reading dip rod tips in water If you wish to take a quick reading to check the sensor is working you can dip the sensor into water With the pins half immersed in tap water an HH2 set to read a...

Page 32: ...owing the effect of salinity on SM150T sensor output when fully immersed in water with no soil present 0 00 10 00 20 00 30 00 40 00 50 00 60 00 70 00 80 00 90 00 100 00 0 0 0 2 0 4 0 6 0 8 1 0 1 2 1 4 1 6 1 8 0 100 200 300 400 500 600 Output vol Output Volts ConductivityECp mS m 1 SM150T fully immersed in water as read by HH150 and HH2 meters Volts vol mineral vol organic TOO WET Soil moisture rea...

Page 33: ...2 DL6 any cable length Negligible for DL2e with 5m cable 9 Maximum cable length 100 m GP1 GP2 DL6 data loggers 100 m DL2e water content measurement 25 m DL2e temperature measurement Power requirement 5 14 VDC 1 8mA for 0 5 to 1 s Operating range 20 to 60 C Environment IP68 Sample volume 55 x 70 mm diameter Dimensions 143 x 40 mm diameter Weight 77 gm without cable 6 In water no soil present the re...

Page 34: ...vels 0 0 0 2 0 4 0 6 0 8 1 0 1 2 1 4 1 6 1 8 0 100 200 300 400 500 600 SM150T output V ConductivityECp mS m 1 SM150T conductivity response at different water contents 100 water 60 45 38 30 20 0 200 400 800 1600 600 1000 800 1200 1400 non saline slightly saline moderately saline strongly saline extremely saline mS m 1 0 2 4 6 8 10 12 14 16 dS m 1 Classification of salinity ...

Page 35: ...s a much lower refractive index than free water The percentage of bound water decreases with temperature and this produces a positive temperature response particularly in clay soils at lower water contents The last two effects partially offset each other but in soil conditions where one or the other effect dominates the SM150T will appear to have a significant temperature response This illustratio...

Page 36: ...accuracy requires a soil volume of one litre but the additional error from taking a reading in a 0 5 litre sample is negligible SM150Ts may interact if they are placed too close together they should be separated by at least 100mm 15 5 5 15 0 0 01 0 02 0 03 0 04 0 05 0 06 0 07 0 08 25 20 15 10 5 0 5 10 15 20 25 Distance parallel to plane of rods from a centre line between the rods mm SM150Tfieldof ...

Page 37: ...utside the pot This behaviour is shown in the graph below For best results keep a gap of at least 25mm 1 inch between the body of the sensor and the wall of the plant pot 6 5 4 3 2 1 0 1 0 5 10 15 20 25 30 35 40 Error of reading Gap between SM150 body and wall of pot mm SM150T Errorclose to wall of plant pot live pin nearest to pot wall ...

Page 38: ...VD Regulatory information Europe This device conforms to the essential requirements of the EMC directive 2004 108 EC based on the following test standards EN61326 1 2006 Electrical requirement for measurement control and laboratory use EMC requirements Group 1 Class A equipment emissions section only EN61326 1 2006 Electrical requirement for measurement control and laboratory use EMC requirements ...

Page 39: ...pprox 0 02 m3 m 3 for sandy soils at the permanent wilting point through approx 0 4 m3 m 3 for clay soils at their field capacity up to values as high as 0 85 m3 m 3 in saturated peat soils Gravimetric Soil Moisture Content is defined as where is the mass of water in the sample and is the total mass of the dry sample To convert from volumetric to gravimetric water content use the equation where is...

Page 40: ... C denotes percentage Carbon and is a measure of organic content Salinity The preferred SI units for ionic conductivity are mS m 1 where S is Siemens the unit of electric conductance Dimensionally it is equivalent to the inverse of resistance i e Ohm 1 The following conversions apply 1 mS m 1 0 01 dS m 1 0 01 mS cm 1 10 µS cm 1 Soil salinity can be classified using the following descriptive catego...

Page 41: ... C 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 M A Malicki and R Plagge 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 1...

Page 42: ... soils Laboratory calibration for clay soils We use the term substrate to refer to any artificial growing medium Underlying principle Soil moisture content is proportional to the refractive index of the soil as measured by the SM150T see Calibration section The goal of calibration is to generate two coefficients a0 a1 which can be used in a linear equation to convert probe readings into soil moist...

Page 43: ... the soil or substrate This sample needs to be unchanged from its in situ density to be 0 5 litre to have the correct dimensions to fit the beaker and to be generally uniform in water content For cohesive soils this is most easily done with a soil corer Sandy soils can be poured into the beaker but you should take the subsequent measurements immediately as the water will quickly begin to drain to ...

Page 44: ...mple to avoid burning off any volatile factions Weigh the dry sample in the beaker W0 627 2g Re insert the SM150T into the dry sample and record this reading V0 0 051V Calculate a0 For the SM150T In the dry soil V V0 0 051 Volts Substitute this into the equation 𝜖 1 0 14 4396𝑉 31 2587𝑉2 49 0575𝑉3 36 5575𝑉4 10 7117𝑉5 gives 𝜖 1 66 Since 0 0 this is the value needed for a0 a0 1 66 Calculate w The wat...

Page 45: ...1 8 0 25 0 66 1 79 3 0 0 1 w w a a1 8 51 Result a0 1 66 a1 8 51 In this example this soil is now calibrated You can now use these two numbers in place of the standard mineral or organic calibration factors to convert SM150T readings into volumetric water content θ using 1 0 a a See also page Underlying principle on page 42 ...

Page 46: ...eter Soil corer Heat resistant beaker 500ml Weighing balance accurate to 1g Temperature controlled oven Process Notes and example Collect a wet sample of the clay soil 25 to 30 water content would be ideal This sample needs to be unchanged from its in situ density to be 500ml to have the correct dimensions to fit the beaker and to be generally uniform in water content This is most easily done with...

Page 47: ...for the water content to equilibrate throughout the sample before taking a reading Reweigh Wm 693 2g Re measure with the SM150T Vm 0 180V Dry the sample thoroughly With clay soils this is usually achieved by keeping it in the oven at 105 C for several hours or days the time required depends on the sample size and porosity Weigh the dry sample in the beaker W0 627 2g Calculations Substituting in th...

Page 48: ...0 180 gives 𝜀𝑚 2 84 𝑎0 𝑎1 𝜃 for 𝜃𝑚 693 2 627 2 463 5 0 14 Calculate a1 Then 𝑎1 𝜀𝑤 𝜀𝑚 𝜃𝑤 𝜃𝑚 9 0 a1 9 00 Calculate a0 and 𝑎0 𝜀𝑤 𝑎1 𝜃𝑤 1 58 a0 1 58 Result a1 9 00 a0 1 58 In this example this soil is now calibrated You can now use these two numbers in place of the standard mineral or organic calibration factors to convert SM150T readings into volumetric water content θ using 1 0 a a See also page Und...

Page 49: ...oning which in turn is strongly influenced by the effect of soil moisture on thermal conductivity Civil engineering Most civil engineering projects depend critically on the mechanical properties of soils Those properties are effected by many different parameters but moisture content and temperature are the two variables that are most likely to change over time so may be measured together in order ...

Page 50: ...esponse curve is given in the Resistance to Temperature Lookup Table on page 53 B The Thermistor circuit shares the Power 0V wire If the thermistor is measured when the SM150T is powered the measured resistance measurement may need to be corrected for 18 mA SM150T supply current Allow 20 minutes for the temperature reading to stabilise after installation Do not log at frequencies under 1 minute in...

Page 51: ...5 0 9 x Rc 0 297 kΩ where R5 value supplied in the table for the SM150T Temp 5m sensor type Lex length of extension cable excluding the 5m of SMSC lw 05 cable Rc total cable resistance including resistance of SMSC lw 05 cable if fitted Other loggers If your logger can be programmed so that the soil moisture and temperature readings can be taken sequentially i e the sensor is not powered during the...

Page 52: ...f a1 1 Ic Rc Vref Ic 18 mA SM150T sensor supply current For Delta T EXT 5W xx series cables Rc 0 066 Ω m 1 For the SMSC lw 05 5m logger cable Rc 0 33 Ω Current Excited You need to know the excitation current Iex The corrected resistance is given by the equation using terms defined above R Rmeas Ic Rc Iex Effect of Temperature on Water Permittivity See Temperature response of soil moisture readings...

Page 53: ...all accuracy of readings Temperature Resistance degrees C Kohms 25 90 538 22 77 683 19 66 854 16 57 713 13 49 968 10 43 379 7 37 759 4 32 957 1 28 844 2 25 299 5 22 244 8 19 608 11 17 321 14 15 334 17 13 606 20 12 098 23 10 780 26 9 623 29 8 611 32 7 720 35 6 935 38 6 241 41 5 627 44 5 080 47 4 595 50 4 162 53 3 775 56 3 430 59 3 121 62 2 843 65 2 593 0 10 20 30 40 50 60 70 80 90 100 40 20 0 20 40...

Page 54: ...r Delta T requires that such returns are discussed with them in advance and may at their discretion waive these charges Delta T shall not be liable to supply products free of charge or repair any goods where the products or goods in question have been discontinued or have become obsolete although Delta T will endeavour to remedy the buyer s problem Delta T shall not be liable to the buyer for any ...

Page 55: ...elay We normally expect to complete repairs within one or two weeks of receiving the equipment However if the equipment has to be forwarded to our original supplier for specialist repairs or recalibration additional delays of a few weeks may be expected For contact details see below Technical Support Users in countries that have a Delta T distributor or technical representative should contact them...

Page 56: ...s linearisation table 26 27 polynomial 26 27 Copyright 2 D Data logger 19 29 30 31 DL2e 17 29 DL6 30 GP1 14 16 30 other 18 Definitions 39 Description 5 Dielectric performance 27 refractive index 24 25 42 Dimensions 6 E EMC 38 Extension cables 10 F FCC compliance 38 Features 5 G GP1 14 16 30 H HH150 meter 12 HH2 31 HH2 Meter 13 I Installation buried 11 insertion rod 11 surface 11 L Linearisation ta...

Page 57: ...26 28 40 44 stony 11 soil calibrations 26 Specifications 2 T Technical support 54 55 Temperature Lookup Table 53 Temperature response of soil moisture readings 35 Temperature sensor 50 Troubleshooting 30 V Volumetric Soil Moisture 39 W Warning Do not touch the pins 31 Water reading 32 Technical Support Delta T Devices Ltd 130 Low Road Burwell Cambridge CB25 0EJ England UK Tel 44 1638 742922 Fax 44...

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