TEROS-12 measurements and how to control irrigation with them
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Precision · Root zone

TEROS-12 measurements and how to control irrigation with them

A TEROS-12 probe in your substrate gives three numbers: the quantity of water in the substrate, the salt concentration of the water, and the temperature. This guide gives information about each number. It shows that you must change the raw reading before it is the correct value. It also shows how to use the reading to make irrigation decisions that you can be sure of for a crop.

Precision12 diagrams8 sources~18 min to read
In this paper
01 Purpose and scope02 Definitions03 Evidence and limitations04 How the probe measures water and does not touch it05 Calibrate the probe to the substrate that you use06 The EC reading and its limits07 Steering of irrigation with TEROS-12 readings08 Find the cause of a bad reading before you think that the sensor is defective09 Expected results and limitations10 References
Start here

Purpose and scope

A TEROS-12 is a small probe that you push into substrate: coco, rockwool or soil. It sends three numbers through one digital wire: the quantity of water in the substrate, the salt concentration of that water, and the temperature. This guide starts from zero. It gives information about each number and about how the probe gets the number. It also gives the cause of an important instruction: do not use only the raw reading to open a valve.

The probe measures a volume of approximately 1010 mL (34.2 fl oz) of substrate around its prongs. It does not measure all the root zone.[8] The next sections show how to change that one local reading, which can have an error, into a number for irrigation control.

  • The TEROS-12 gives volumetric water content (VWC), bulk electrical conductivity (EC) and substrate temperature through the digital protocol SDI-12.
  • The volume of influence of the probe is only approximately 1010 mL (34.2 fl oz) of substrate around the prongs. This volume is one local position. It is not the average of a tray or a zone.
  • This guide changes that local reading, which has noise, into a good estimate of the quantity of water. The guide always gives the uncertainty of the estimate.
  • It is not necessary to know about soil sensors before you read this guide. Each term has a definition where it first occurs.
equipment
The probe
Three steel prongs make a high-frequency electric field and measure the effect of the field on the substrate.
approximately 1010 mL
Volume of influence
The probe measures only approximately 1010 mL (34.2 fl oz) of substrate around the prongs.
not measured
Other parts of the pot
The probe cannot measure the substrate that is not in that volume.
Figure 1. The probe measures a small volume of substrate around the prongs, in the shape of an ellipsoid. It does not measure all the root zone.[8]
Persons for this guide

This guide is for a person who puts a moisture probe in a pot. The person wants to control irrigation with the probe and to include the uncertainty. Use this guide with the guide to smart irrigation (VWC/EC) and the paper on crop steering in coco.

Terms

Definitions

Six terms are necessary for all the information in this guide. Each term has an easy definition here. After this section, the guide uses each term with the same definition. The terms occur again in the next sections.

Volumetric water content (VWC)The pot is a container with a volume that does not change. Some of the volume is solid substrate, some is air in the gaps, and some is liquid water. VWC is the fraction of the total volume that is liquid water. Its unit is cubic meters of water for each cubic meter of substrate (m³/m³). Thus a VWC of 0.34 shows that 34% of the total volume of the pot is liquid water. VWC is the primary number that you use to control irrigation.
Permittivity (dielectric constant)An electric field has an effect on water that is approximately twenty times stronger than the effect on dry substrate. Permittivity is the property that shows this effect. The permittivity of water is approximately 80, of dry substrate approximately 3–5, and of air approximately 1. The probe uses this large difference to find water.[1]
Capacitance sensorA sensor that measures permittivity. It applies a high-frequency electric field and measures how strongly the material around the sensor holds the field. The TEROS-12 is a capacitance sensor. It calculates VWC from the permittivity reading. It does not touch the water directly.
Bulk EC and pore-water ECThe probe measures the conductivity of all the wet substrate together: solids, water and air. This value is bulk EC (0–20000 µS/cm on this probe). The roots touch the water in the gaps between the particles of the substrate. The salt concentration of this water is the pore-water EC. The probe cannot measure pore-water EC directly. You calculate an estimate of it from bulk EC with the Hilhorst (2000) model.
DUL / container capacityDrained upper limit: the quantity of water that this pot holds after gravity removes all the water that it can. It is the maximum value for your steering. It is a property of your pot and substrate, not a general constant. Thus you measure it from the runoff of your pot.
Resolution and accuracyResolution is the smallest change that the probe can show: 0.001 m³/m³ VWC. Accuracy shows how near the number is to the correct value: only ±0.03 m³/m³ with the generic calibration. The probe gives a number with high precision, but precision is not the same as accuracy. A number with high precision can be incorrect each time.[8]
Relative permittivity: water controls the readingThe permittivity of water is much larger than the other values. Thus water controls the reading.0224568901Air4Dry substrate80Water
Figure 2. The permittivity of liquid water (approximately 80) is approximately twenty times the permittivity of dry substrate (approximately 3–5) and approximately eighty times the permittivity of air (approximately 1). A small quantity of water causes a large change in the reading of the probe.[1]
fraction
Solids
The coir, the rockwool fiber or the particles of soil.
fraction
Air
Pore space with air between the solids.
VWC
Water
Liquid in the pores. VWC = this volume / total volume.
Figure 3. One unit of volume of substrate has three parts: solids, air and water. VWC is the fraction of the volume that is water.
Saturation, field capacity and drybackthe same block at three times of the daySaturatedafter irrigation, almostno air→Field capacityfree water drained, peakeach day→Dryback lowplant used water, air +oxygen in
Diagram. The sensor reads these values: saturation, field capacity (container capacity / DUL) and the dryback low.
Water-content cycle for one dayFill to field capacity, hold it, make a controlled dryback, and do it again. The dryback size is the steering control.correct bandtoo dry0255075100offP3 minonP1FCP2offwater content %
Diagram. One day of water content, as the probe measures it.
Quality of the information

Evidence and limitations

Three types of information

We want each paper to be correct. Thus we examine each paper for information that is not solid. This information comes from the view of one grower, from a small number of sources, or from grower methods that have no controlled test. We identify this information, and we do not show it as solid.

Frequently there is no paper for the decision that you must make. Then we use the reports of other growers and the methods that work in the rooms that we know. This information can help you, but it is not the result of a laboratory test. Use the methods that work for your plants, for your room and for your meters. If a table does not agree with your crop, use the data from your crop and record the difference.

Solid
Tests of plants, specifications or many sources that agree show that the information is correct
  • Capacitance probes calculate the VWC from the permittivity. The calibration for the medium is important.
  • The estimate of pore-water EC has limits, as for the Hilhorst model and models of the same type.
Grower method
Many growers use this method in their rooms. Start with it, then change it for your room.
  • The depth of the installation, the volume of influence, and the usual positions of probes in many pots.
Weak
The view of one grower, a small number of sources, one test, or a method that works in one room
  • The accuracy values of the manufacturer as values that you always get with no calibration for your medium.

If you find an error, make a report. We will correct it. Make the report on GitHub, with the name of the paper and the information that is incorrect. If you have a source, include it: Make a report of an accuracy problem. The regulations, labels and licenses of your area always override each procedure in these papers. The label weak in a paper shows the points where the risk of an error is highest.

Primary content

How the probe measures water and does not touch it

The TEROS-12 is a capacitance probe. Its prongs send a high-frequency electric field into the substrate around them. The probe measures how strongly the substrate holds the field. This property is permittivity.

The permittivity of water (approximately 80) is approximately twenty times the permittivity of dry substrate (approximately 3–5) and approximately eighty times the permittivity of air (approximately 1). Thus a change in water content causes a large change in the bulk permittivity of the substrate. You can calculate the size of this change. As a result, you can use permittivity to find VWC.[1]

Then the probe uses a calibration equation to change the measured permittivity to a VWC number. The equation in the probe is a generic curve for mineral soil. The probe gives the VWC number with a resolution of 0.001 m³/m³. There is a problem from the start: the equation is different for different substrates, and the generic curve has an accuracy of only ±0.03 m³/m³.[8]

  • Permittivity is the physical quantity that the probe measures. VWC is a calculated and calibrated estimate. It is one more step after the measurement.
  • The curve from permittivity to VWC is not a straight line. It is most different from a straight line near saturation. There the curve becomes flat. A substrate that is almost full can give a ‘full’ reading when it is not full.
  • The temperature of the substrate changes the dielectric response. The change is a known physical effect. It can cause a change in the reading that is the same as a change in water content. Correct the reading for this effect.[4]
  • The probe sends data through SDI-12. Three values show a fault in the equipment or the cable. The first is a constant value at 0 or at full scale. The second is a ‘NaN’ value (not a number). The third is an expired value (the probe sends the last value again). These values are not readings of data.
Permittivity to VWC: a curve that is not straightThe bottom axis is permittivity. The curve becomes flat at the top. Thus a reading near saturation can show 'full'.00001381524354862VWC (m³/m³)
Figure 4. VWC increases when the permittivity increases, but the curve bends and becomes flat near saturation. The same step of VWC is a different step of permittivity in different parts of the curve.[1]
The measurement chain1Electric fieldprongs send thefield→2Permittivitysubstrate holdsthe field→3Calibrationcurve gives theVWC→4VWC + EC + °Csent throughSDI-12Bulk EC and temperature come from the same measurement. All three numbers go through the SDI-12 wire.
Figure 5. From electric field to a number: each output comes from the same measurement. The error for each substrate occurs in the calibration.
Primary content

Calibrate the probe to the substrate that you use

When you receive the TEROS-12, it has a generic calibration for mineral soil. This calibration has an accuracy of only ±0.03 m³/m³. A substrate-specific calibration for your coco or rockwool decreases the error to ±0.01–0.02 m³/m³.[3] This difference is important when you use the probe. Frequently, the dryback windows in crop steering are smaller than the error band of the generic calibration (±0.03). If you use a VWC without calibration for steering, the changes that you want are in the noise.

An example of safe headroom shows the result of this difference. An estimate that ignores the error gives 256 mL (8.7 fl oz) of ‘room to apply water’. When you include an accuracy of ±0.02, the safe quantity is approximately 109 mL (3.7 fl oz). With the generic calibration (accuracy ±0.03), the safe quantity is only approximately 54 mL (1.8 fl oz). The pot and the probe are the same. The only difference is how you include the error band when you calculate.[7]

How accuracy decreases safe headroom (same pot)A wider error band gives less water that you can add with no risk of overshoot.075150225300256 mLRaw estimate109 mLCalibrated ±0.0254 mLGeneric ±0.03
Figure 6. The safe headroom decreases by a large quantity when the calibration error increases. It decreases from 256 mL (8.7 fl oz) in the raw estimate to approximately 54 mL (1.8 fl oz) with the generic curve.[7]
The resolution is the same for the two types. The accuracy is different. Calibrate the probe to your substrate before you use it for steering.
Calibration typeVWC accuracyResolutionAccurate steering?
Generic calibration for mineral soil (in the probe)±0.03 m³/m³0.001 m³/m³No. The error band is wider than a usual dryback window.
Substrate-specific calibration±0.01–0.02 m³/m³0.001 m³/m³Yes. It is necessary for accurate steering.
Calibration does not correct all errors

Make sure that you use a substrate-specific calibration: it is necessary for accurate steering. Calibration corrects only an additive offset in the reading. The gain error and the nonlinearity near saturation stay, and they do not cancel when you calculate with the reading. Include the residual error that stays after the calibration.

Primary content

The EC reading and its limits

The probe measures bulk EC, the conductivity of all the wet substrate (0–20000 µS/cm). Growers must know pore-water EC: the salt concentration of the solution that touches the roots. To calculate pore-water EC, use the Hilhorst (2000) model with bulk EC, VWC and temperature.[2] The bulk EC is the conductivity of the water and the substrate together. The Hilhorst model then gives an estimate of the conductivity of the water only.

The model gives a good estimate, but its sensitivity to its parameters is approximately ±20%. It does not give correct results when the VWC is less than 0.10 m³/m³. Do not use it in this condition.

A more important limit is representativeness: the probe measures one position of approximately 1010 mL (34.2 fl oz). Channeling, dry areas or bad contact between the probe and the substrate can cause a number that is different from the value of the zone. The probe operates correctly at this time.[3]

  • The probe measures bulk EC (0–20000 µS/cm) directly. You calculate an estimate of pore-water EC from bulk EC. The two values are not the same.
  • The conversion of Hilhorst (2000) has a sensitivity of approximately ±20%. It does not give correct results when the VWC is less than 0.10 m³/m³.[2]
  • A representativeness fault: the 1010 mL (34.2 fl oz) of substrate that the probe measures can have a value that is different from the value of the zone. Channeling, an air gap or a probe that is not fully in the substrate can cause the difference. The probe operates correctly at the same time.
  • The sensor cannot measure the volume of runoff from each pot. It cannot measure the effective substrate volume. This volume decreases when the roots fill the pot. It cannot find if the system supplied an irrigation shot that the controller started.
measured
Bulk EC
The conductivity of solids, water and air together. The probe measures this directly.
approximately ±20%
Hilhorst model
The model uses bulk EC, VWC and temperature. The sensitivity is approximately ±20%. It is not correct when the VWC is less than 0.10.
calculated
Pore-water EC
The salt in the liquid that touches the roots. You must find this value.
Figure 7. The probe measures bulk EC directly. You calculate pore-water EC with the model of Hilhorst (2000). This estimate becomes less accurate quickly when the substrate becomes dry.[2]
The information that one TEROS-12 can and cannot give, and the second witness (a second measurement with a different method) that fills each gap.
The probe CAN measureThe probe CANNOT measureOther method
VWC (local position)The correct average of the zone for all the potsMany probes, a cohort model
Bulk ECRunoff volume for each potRunoff trays / drain sensors
Substrate temperatureEffective substrate volume (decreases when the roots fill the pot)Measure DUL again at intervals
Calculated pore-water ECIf an emitter operatedFlow meter, or a sudden change of the weight from a load cell
How-to

Steering of irrigation with TEROS-12 readings

The method has two parts. Make the raw VWC reading less important. Do not use it as a correct value. Use it as one witness that has noise, with a confidence score (a number that shows how accurate the reading is).

Make a small water-balance model more important. The model holds the best estimate of the water in the substrate. A reading with a high confidence score can change the estimate only by a small quantity.

Monitor dryback (the quantity by which the VWC decreases between shots) and specific yield (the quantity by which the VWC increases for each mL supplied). Specific yield shows when the pot comes near to its capacity. Set the capacity anchor (the DUL value that the system uses) from the DUL that you measured, and not from a number without a measurement. Control irrigation with trends (the shape and the slope of the dryback) more than with the absolute value, because an additive calibration offset does not change trends.[6] Do not use only a trend. Always use a second witness, for example the time of runoff or the weight of the pot.

  1. 1
    Calibrate to your substrate first
    A substrate-specific calibration is the first necessary step. If you do not have it, your steering is in the error band. You cannot be sure that the reading is sufficiently accurate.
  2. 2
    Make sure that the probe contact and position are correct
    Put the probe tightly in the substrate, in a representative position that does not change. A loose probe or an air gap gives the reading of the substrate around it. It does not give the reading of your root zone.
  3. 3
    Measure the DUL of this pot from irrigation events with a second witness
    Set the capacity anchor from approximately five irrigation events with runoff or weight data. Do not use only one irrigation event. Give the capacity anchor as a volume of water, and not as a raw VWC number.
  4. 4
    Steering with the slope of the dryback, with a limit from the safe headroom
    Use the slope of the dryback and the specific yield. Use a limit from a headroom estimate that includes the calibration error, and not from the raw estimate.
  5. 5
    Use a second witness before you apply water
    A second witness (the start of runoff or the mass from a load cell) must give the same result before a signal goes to a valve. The probe reading must not operate a valve if there is no second witness.
A day of dryback: shots, drying and the DUL limitA sharp increase is a shot. The decrease between shots is the dryback. The top is the DUL that you measured.00000P0 endshot+2h+4hshot+2hday endVWC (m³/m³)
Figure 8. Irrigation shots increase the VWC to the DUL. The steering signal is the slope of the VWC when it decreases between shots, and not the absolute value at one time.
The steering loop: the reading is not the only signal for the valve1Probe readingraw VWC / EC /°C→2Confidence scorehow accurate itis→3Water modelwater-balanceestimate→4Second witnessrunoff or weightagreesA valve operates only if the confidence score is high AND a second witness agrees.
Figure 9. Each reading has three steps before the system can apply water: a confidence score, a water-balance model and a check by a second witness.[6]
When a problem occurs

Find the cause of a bad reading before you think that the sensor is defective

Most problems with the TEROS-12 do not occur because the sensor is defective. The problem is that a person thinks that the reading is correct when it is not. First, find the difference between two conditions. An incorrect reading is a representativeness fault. The probe operates correctly, but the 1010 mL (34.2 fl oz) that it measures has a different value from the zone. With an incorrect reading, decrease your confidence in the absolute VWC number.

A constant, NaN or expired value is a fault in the equipment or the cable. With this value, stop all automatic operation.

Monitor for a VWC that changes with the temperature cycle of the substrate each day. This change is not a change in the quantity of water. It is an artifact: the probe does not touch the substrate correctly, or the calibration is incorrect.[5] Monitor the wetting curve and the drying curve of each pot. If they are different from the usual curves, the cause is possibly hydrophobic substrate or channeling.

Monitor each pot that is different from the pots near it that have the same treatment. The cause is possibly a blocked emitter or a defective probe. It is not a problem of the plant.

Temperature and EC change your confidence, not the water number

Do not let temperature and EC change the estimate of stored water directly. Let them change your confidence in the reading. The temperature cycle of each day can cause a dielectric change in dry substrate. This change is a physical effect. It has the same effect on the reading as a change in water content.[4]

Do not let that change write the VWC. If you do, a physical effect causes the irrigation, and not the water uptake of the plant.

Fault diagnosis: equipment first, then artifacts, then local faults1Constant, NaN, expired?equipment fault→ stop→2Temperature effect?artifact → lessconfidence→3One pot different?local fault →examine→4If noneuse it as awitness withnoiseEquipment faults stop the operation. Representativeness faults only decrease confidence.
Figure 10. Do the checks in sequence: first for equipment faults, then for artifacts and then for local faults. Use the number only after these checks.
From symptom to cause to task. The ‘It is NOT’ column is the most important. The primary cost is an incorrect diagnosis.
SymptomPossible causeIt is NOTTask
The VWC is at the limit of the range, is constant, is ‘NaN’ or is an expired valueFault in the equipment or the cableA correct reading of waterStop steering. Operate a safe procedure with limits. Tell a person.
The VWC changes with the temperature each dayBad contact, or an artifact of the calibrationA change in the quantity of waterDecrease the confidence in the absolute VWC. Examine the position of the probe.[5]
The wetting curve and the drying curve are very differentChanneling, or hydrophobic substrateDefective sensorExamine the substrate. Make the substrate wet again. Examine the position of the probe.
One pot is different from the pots near itBlocked emitter or defective probeA problem of the plant, at this timeExamine the emitter and the probe before you think that the plant has a problem.
Use more than one irrigation event to set a capacity anchor

Do not let one manual reading or one observation by a person set a capacity anchor. Do not let it override a safety interlock. A capacity anchor must come from more than one irrigation event, with a second witness for each irrigation event. One good observation is not sufficient.

Facts and limits

Expected results and limitations

The facts

One TEROS-12 cannot give you the correct condition of the root zone for each zone. If you think that it can, you make the most frequent error and the error with the highest cost. With a substrate-specific calibration, you can find the trends of dryback. You can also calculate an estimate of the water in the substrate with an accuracy of approximately ±0.01–0.02 m³/m³ in the position of the probe. This accuracy is sufficient for steering if you include the uncertainty in your decisions and compare the data with a second witness.[3]

Confidence: one probe, or probe + witness + calibrationA calibration and a second witness change the probe from a source with many limits to a source for control.only one probe, wide error band, many limitsprobe + second witness + calibration: for steering0% trust50% trust100% trust
Figure 11. Only one probe has a wide error band and many limits. When you add a substrate-specific calibration and a second witness, the error band becomes smaller. You can use this range for steering.[7]
  • With a substrate-specific calibration, the best result is a correct shape of the dryback. The accuracy for the stored water in the local position of the probe is approximately ±0.01–0.02 m³/m³.
  • One probe cannot give the runoff for each zone. It cannot show that the system supplied water. It cannot give the correct average of the zone for all the pots.
  • A load cell (pot weight) is the most important sensor to add, because it measures the water in the substrate directly. It does not use the dielectric properties of the substrate.
  • Make the system give a clear ‘I cannot tell’ output. Do not give a precision that is higher than the accuracy. Let the system control only when the data show that it can.

The best method is to make the system show when it cannot tell. The system must not give a number with more confidence than the data give. Calibrate first and add a second witness. Then read the guide to smart irrigation (VWC/EC) for information on how those signals control the shots. Read the paper on signal and noise for information on the difference between a trend and sensor noise.

Related papers
Irrigation control · VRWE
How the controller makes irrigation decisions
Coco and crop steering
Use water to control the plant
Signal and noise
Find the difference between plant changes and sensor noise

References

  1. Topp, G. C., Davis, J. L., & Annan, A. P. (1980). Electromagnetic determination of soil water content: Measurements in coaxial transmission lines. Water Resources Research, 16(3), 574-582. https://doi.org/10.1029/WR016i003p00574
  2. Hilhorst, M. A. (2000). A Pore Water Conductivity Sensor. Soil Science Society of America Journal, 64(6), 1922-1925. https://doi.org/10.2136/sssaj2000.6461922x
  3. Fragkos, A., Loukatos, D., Kargas, G., & Arvanitis, K. G. (2024). Response of the TEROS 12 Soil Moisture Sensor under Different Soils and Variable Electrical Conductivity. Sensors, 24(7), 2206. https://doi.org/10.3390/s24072206
  4. Nasta, P., Coccia, F., Lazzaro, U., Bogena, H. R., Huisman, J. A., Sica, B., Mazzitelli, C., Vereecken, H., & Romano, N. (2024). Temperature-Corrected Calibration of GS3 and TEROS-12 Soil Water Content Sensors. Sensors, 24(3), 952. https://doi.org/10.3390/s24030952
  5. Kapilaratne, R. G. C. J. & Lu, M. (2012). Correcting the Temperature Influence on Soil Capacitance Sensors Using Diurnal Temperature and Water Content Cycles. Sensors, 12(7), 9773-9790. https://doi.org/10.3390/s120709773
  6. Tavan, M., Wee, B., Brodie, G., Fuentes, S., Pang, A., & Gupta, D. (2021). Optimizing Sensor-Based Irrigation Management in a Soilless Vertical Farm for Growing Microgreens. Frontiers in Sustainable Food Systems, 4, 622720. https://doi.org/10.3389/fsufs.2020.622720
  7. Nemali, K. S. & van Iersel, M. W. (2006). An automated system for controlling drought stress and irrigation in potted plants. Scientia Horticulturae, 110(3), 292-297. https://doi.org/10.1016/j.scienta.2006.07.009
  8. METER Group, Inc. (2023). TEROS 11/12 User Manual & Specifications. METER Group, Pullman, WA. (source not from a journal, for example from a manufacturer) https://metergroup.com/products/teros-12/

Each citation in the paper, for example [1], refers to an item in this list. Each item is a primary source or an instruction from an authority, unless the item shows a different type. The results of cannabis tissue culture change much with the genotype. Before you use the dilutions, hormone doses and local regulations, make sure that they agree with the primary sources.