Proposal of a Novel Heat Dissipation Soil Moisture Sensor

Pedro Carvalhaes Dias, Wellington Roque, Elnatan C. Ferreira and José A. Siqueira DiasUniversity of Campinas - UNICAMP

Department of Electronics and Microelectronics - DEMIC/FEECAv. Alber Einstein, 400

13083-852 Campinas, SPBrazil

welroque@demic.fee.unicamp.br, p.c.dias@demic.fee.unicamp.br,elnatan@demic.fee.unicamp.br, siqueira@demic.fee.unicamp.br

Abstract: A novel heat dissipation soil moisture sensor with only one bipolar transistor performing as both heatingand temperature sensing element is presented. Due to the fact that the power dissipated in the transistor is a functionof both the voltage and the current in the transistor, it is possible to obtain the required levels of heating power withlow currents, increasing the battery life of the sensor’s interrogator. Experimental results measured in a fabricatedsensor show that in soils with high moisture levels the proposed sensor presents a sensitivity that is almost 10 timeshigher than those found in conventional sensors with separated heating and sensing elements. Since the transistoris used as both the heating and sensing elements, for the same applied energy (80 mW during 45 s) the temperatureincrease in the proposed sensor is about 7.5◦C, while the conventional porous block probe presents a temperatureincrease of only 0.8◦.

Key–Words: Soil moisture measurement, Heat dissipation sensor, Porous ceramic sensors, Precision agriculture.

1 Introduction

One of the goals of modern precision agriculture isto achieve a high efficiency in what concerns waterirrigation control. Proper soil moisture measurementcan optimize water usage, resulting in less water beingwasted, and better crop development [1, 2].

Although the gravimetric technique [3] can leadto very precise results in soil moisture measurement,it is not possible to use this technique on a plantation,since it is a slow procedure (tens of hours to obtain oneresult) and it is necessary to take samples of the soiland make the measurements in laboratory. To over-come this problem, several types of sensors were de-veloped to measure soil moisture in-situ, like the ca-pacitive sensors [4], resistive sensors [5], Time Do-main Reflectometry [6], and heat dissipation sensors[7, 8].

For field applications, the heat dissipation soilmoisture measurement probes prevailed and are wellknown in the market. There are basically two typesof probes, and both require a heating element and atemperature measuring device. One type does not usea porous ceramic block and the heating/sensing ele-ments are inserted in metal tubes and buried directlyin the ground, at a specific distance [9]. The heater isturned on during a determined time, heating the soilaround it. The temperature sensor detects the amount

of heat that is transferred through the soil from oneprobe to the other. Since the heat transfer ratio is afunction of the thermal conductivity of the soil, whichdepends on the amount of water in the soil, the max-imum temperature detected at the sensing probe is afunction of the soil moisture.

The other type of heat dissipation sensor uses aporous ceramic body with one heater and a temper-ature sensor assembled inside it [10]. Similarly, theheater is turned on during a determined time, heatingthe porous block. The temperature sensor (usually athermistor or a thermocouple) detects the temperaturethat the inner part of the sensor body reaches after thepower is applied for a given time. Since the amountof water absorbed from the soil by the porous ceramicchanges the thermal conductivity of the water+porousceramic complex, the probe temperature is a functionof the soil moisture.

The novel sensor presented in this paper also usesa porous block, but instead of using one heating andone temperature sensing devices, it is proposed the useof a single bipolar transistor inside the porous block,since the bipolar transistor is an electronic device thatcan perform both as a heating source and as the tem-perature sensor.

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2 Materials and MethodsThe soil moisture sensor proposed in this paper iscomposed of a single transistor, encapsulated insidea porous block. By forcing a voltage VCE and a cur-rent IC in the transistor, it dissipates a power that isequal to P = VCEIC . This power is dissipated in theporous block+water complex and heats the body of thesensor, just like the resistive heating elements used inthe conventional heat dissipation sensors. To measurethe temperature that the probe has reached, the wellknown characteristic of VBE x T for the bipolar tran-sistor is used.

Since for field applications the measurementequipment is powered by batteries, it is important tonotice that the collector current IC is inversely pro-portional to VCE (IC = P/VCE) and, for a de-sired power, by increasing the voltage VCE the cur-rent drained from the battery is reduced, increasingthe battery life.

Thus, the proposed sensor presents two importantfeatures: i) it can achieve a high heating power withlow current; ii) the temperature of the probe is mea-sured using one of the best characterized relationshipin the bipolar transistor (VBE x T ), leading to highaccuracy in the temperature measurement.

Also, since the only electronics device in theprobe is a single low-power bipolar transistor (anextremely low-cost device), the manufacturing costwould be lower than for the standard probes, whereone heater and one sensing elements are needed. Fur-thermore, since there is a single active element, theerrors due to the variations in both heater and sensingelement contacts with the porous block and physicaldistance between heating/sensing elements [8] can beminimized.

Using a small SMD component, the transistorcan be considered a punctual heat source, irradiat-ing evenly across the probe the heat generated by thepower applied to the transistor. It is also expectedthat a higher sensitivity should be reached, since thetemperature measurement is performed exactly at thesame point of the heating element, as both elementsare the same.

2.1 Probe DesignTo test the concept, a simple probe prototype was de-signed and constructed. The probe is comprised of alow-cost off-the-shelf BC556 NPN bipolar transistorand a porous block, as seen in Figure 1. The tran-sistor is soldered to three long plastic insulated wiresthat will be connected to the driver/measuring equip-ment. The emitter, base and collector must be insu-lated from each other and from the porous block, to

avoid any leakage currents that could flow through thepores filled with water, leading to false measurementresults.

Figure 1: Cutaway drawing of the proposed soil mois-ture probe.

In the constructed prototype, a silicone protectivecoating was used to electrically insulate the transis-tor leads. It is very important to leave the transis-tor plastic header exposed, and cover only the metalleads with the silicone. The fact that the sensor de-pends on the heat transfer between the transistor andthe porous block, a silicone layer over the transistorheader could interfere with that heat exchange ratio.After the silicone insulation is cured, a gypsum mix ismade, placed into a tube and the insulated transistoris then inserted in the middle of the wet gypsum. Af-ter the porous block is completely dried, the probe isremoved from the tube, and is ready to be used.

2.2 Measurement SetupAll measurement tests were made in laboratory, withbench-top programmable equipments, all from Keith-ley [11]. As shown in Fig. 2, the emitter of the transis-tor was connected to a programmable current source(Model 220), the base was grounded and the collec-tor was connected to a programmable voltage source(Model 230). The value of VBE is read using a Model177 4-1/2 digits multimeter (actually the value read is−VBE).

The program created for the measurement isstarted with 100µA in the current source and VCB =0, in order to keep the power dissipated by the tran-sistor extremely low (approximately 55µW if VBE =550 mV @ IC = 100µA). In the next step, both thevoltage source and the current source are simultane-ously raised to 10 V and 8 mA respectively, and thevalue of the initial VBEi is read. This state, where apower of approximately 80 mW is dissipated in the

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Vcb

Q1

Transistor inside

porous block

(-Vbe)Multimeter Current

Source

Keithley 220

Voltage

Source

Keithley 230

Ie

Keithley 177

Figure 2: Diagram of the measurement laboratory set-up for the soil moisture probe.

transistor, is maintained for 45 seconds. After 45 s,the value of the final VBEf is read, and the voltageand current sources are returned to their initial values.

Since a constant emitter current of 8 mA is keptduring the 45 s, the measured difference ∆VBE =VBEf−VBEi is due only to the temperature differencein the transistor caused by the power dissipated by thetransistor into the porous block+water complex. Sincethe very well known relationship for the variation ofVBE with temperature can be assumed to be linear forsmall values of ∆T [12], the probe can be calibratedusing the ∆VBE measured values, and there is no needto convert these voltage values to temperature. Thiswill make the calibration procedure for the sensor in-serted in the soil (required for all soil moisture probesensors) much easier, faster and precise.

3 Experimental Results and Discus-sions

A photograph of the a manually fabricated probe usingthe described method is shown in Fig. 3. The mea-surement set-up was used to characterize the probe,which was moisturised with several amounts of watervolumes.

To start the characterization, the probe was driedfor 12 hours inside an air oven at 100◦C, to guaranteethat the water content in the porous block was negli-gible. Then the probe was connected to the measure-ment circuit and submitted to the test cycle, to readthe value of ∆VBE = VBEf − VBEi. Next, the probewas soaked in a controlled volume of water until allthe water was absorbed by the porous block. Aftera period of 5 minutes, when the water is evenly dis-tributed inside the porous block, a new measurementcycle is executed. This procedure was repeated untilthe values of ∆VBE indicated that the porous blockwas saturated with water.

The measured results for ∆VBE as a function ofthe water content in the porous block are plotted in

Figure 3: Photograph of a manually fabricated soilmoisture sensor probe.

Fig. 4. As it can be observed, a variation of ∆VBE

in the order of tens of mV is measured for the wholeoperation range (completely dry to near water satura-tion), making it easy to be measure the soil moisturewithout amplification or sophisticated equipment.

Measured data

Figure 4: Measured results of ∆VBE as a function ofthe probe water content for the fabricated soil mois-ture probe.

It is important to notice that the measured valuesof ∆VBE show that in the region of low sensitivity(when the water content in the sensor body is high),the sensitivity of the proposed probe is almost 10times higher when compared to the standard porousceramic probes commercially available [10], since thetemperature variation observed in the proposed sensorprobe is about 7.5◦C (∆VBE = VBEf − VBEi ≈ 15mV), while the standard probe shows a temperaturevariation of only 0.8◦C.

This is explained by the fact that the transistorperforms as both heating and sensing elements, and

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the point where the temperature is being measured isexactly the same point where the heat is produced,since there is no insulating material between the heat-ing and sensing element.

This feature of the proposed sensor eliminates adrawback of the conventional sensors with separatedheating and sensing elements, which requires calibra-tion of each sensor. Individual calibration is necessarybecause of the variations in both heater and sensingelement contacts with the porous block as well as thephysical distance between heating/sensing elements,which cannot be accurately controlled in the fabrica-tion process, leading to different thermal behaviourfor each fabricated sensor.

4 ConclusionA proposal of a novel sensor for the measurement ofsoil moisture was presented. The probe uses only onebipolar transistor as the heating and sensing element,resulting in a compact, easy to fabricate and low-costmoisture sensor. Since the measured variable is the∆VBE of a bipolar transistor at a fixed collector cur-rent, which presents a well known dependence withthe temperature, the accuracy of the measured datadepends only on the measurement equipment used toread the ∆VBE values.

Measured results obtained from a manually con-structed probe showed that in wet conditions, whenthese sensors are most needed to guarantee a precisesoil humidity to the plants, the sensitivity of the pro-posed sensor is almost 10 times higher than the sen-sitivity found in similar probes with separate heatingand sensing elements.

The next steps in the development of the proposedmoisture sensor is the design and development of aportable interrogator circuit that would apply the re-quired voltage/current signals, read the ∆VBE datawith a high resolution and high precision A/D con-verter and calculate the soil humidity with microcon-troller, showing it on a display. This would allow mea-surements of soil moisture in the field, which is animportant feature for real “planted” sensors.

References:

[1] Y. Mori, J.W. Hopmans, A.P. Mortensen, G.J.Kluitenberg, Multifunctional heat pulse probefor simultaneous measurement of soil watercontent, solute concentration, and heat trans-port parameters, Vadoze Zone Journal 2, 2003,pp. 561–571.

[2] D.J. Bremer, and J.M. Ham., Soil moisture sen-sors can help regulate irrigation, Golfdom–June2003. pp. 49-54

[3] K. H. Tan, Soil Sampling, Preparation, andAnalysis, Marcel Dekker, Inc., 1996, New York,USA.

[4] S.P. Ould Mohamed, S.P. Bertuzzi, L. Raison,and L. Bruckler, Field evaluation and error anal-ysis of soil water content measurement using thecapacitance probe method, Soil Sci. Am. J. 61,1997, pp.399-408.

[5] A. Valente, R. Morais, A. Tulib, J.W. Hopmansb,and G.J. Kluitenberg, Multi-functional probe forsmall-scale simultaneous measurements of soilthermal properties, water content, and electri-cal conductivity, Sensors and Actuators A 132,2006, pp. 70–77.

[6] C. G. Topp, J. L. Davis and A. P. Annan, Electro-magnetic determination of soil water content us-ing TDR: Measurements in coaxial transmissionlines, Water Resour. Res. 16, 1980, pp 574–582.

[7] Phene, C.J., G.J. Hoffman, and S.L. Rawlins,Measuring soil matric potential in situ by sens-ing heat dissipation within a porous body: I.Theory and sensor construction, Soil Sci. Soc.Am Proc. 35, 1971, pp. 27–33.

[8] A. L. Flint, G. S. Campbell, K. M. Ellett, and C.Calissendorff, Calibration and Temperature Cor-rection of Heat Dissipation Matric Potential Sen-sors, Soil Sci. Soc. Am. J. 66, 2002, pp. 1439–1445.

[9] J. M. Ham and E. J. Benson, On the Construc-tion and Calibration of Dual-Probe Heat Ca-pacity Sensors, Soil Sci. Soc. Am. J. 68, 2004,pp. 1185–1190.

[10] Campbell Scientific, Inc., 229 Heat DissipationMatric Water Potential Sensor Instruction Man-ual, 2006.

[11] Keithley Instruments, Inc., Model 220 Cur-rent Source Manual, Model 230 Voltage SourceManual, Model 177 Microvolt DMM Manual,2001, USA.

[12] G.C.M. Meijer and K. Vingerling, Measurementof the temperature dependence of the IC(V,Jcharacteristic of integrated bipolar transistors,IEEE J. Solid-State Circ. SC-15, 1980, pp. 237–240.

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