Proceedings of the 2015 2nd International Symposium on Physics and Technology of Sensors, 8-10th March, 2015, Pune, India Improving a Dual-Probe Heat Pulse Based Soil Moisture Sensor Using Insulated Nichrome Wire Adhiti Raman Mechanical Engineering Indian Institute of Science Bengaluru, India araman@mecheng.iisc.ernet.in G.K. Ananthasuresh Mechanical Engineering Indian Institute of Science Bengaluru, India suresh@mecheng.iisc.ernet.in Abstract The Dual-Probe Heat-Pulse technique has been widely used for in-situ moisture sensing. In the heater probe described in our earlier work the power consumption was 350 mw--the lowest in this category of sensors. In this paper, by employing a material selection method, we further improve the performance of the sensor by employing Parylene-coated nichrome wire and different packaging methods. The power consumption is reduced to 165 mw with a temperature rise between 1 K to 6.2 K degrees in 34% wet and dry red soils, respectively. A variant of the nichrome sensor was one that employed a ceramic tube with four longitudinal holes that eliminated the need for Parylene coating. The sensor was verified in agar agar solution and calibrated with 1200 kg/m 2 red field soil. The compact packaging of the nichrome heater allowed us to provide a heat distribution of 3267 J/m on the probe surface which is two times more than the earlier attempts. Keywords Dual-Probe Heat-Pulse, Soil Moisture Sensor, Low power, Nichrome Wire, Parylene I. INTRODUCTION The Dual Probe Heat Pulse (DPHP) sensor uses two probes: the heater probe and temperature probe. Among the DPHP sensors, there are different types of heater probes. Fig 1. shows the power consumed by different heater probes mentioned in the literature. Our heater is one of the lowest surpasses marginally by [11]. In [11], a 16-mm diameter button-shaped heater combined with CMOS temperature sensor was reported. The soil needs to be disturbed while placing the button-probe in the soil compromising accuracy. Slender probe like sensors are the most effective as they cause least disturbance to soil conditions. For the heating element, it is typically better to use a wire with high electrical resistivity. However such materials are not always easy to procure. The heater probes in Jorapur and Ananthasuresh [1] and Jorapur et al. [2] employ a copper wire heating element in a steel tubing packed with MgO as a filler material. Fig. 1 Comparison of power consumption of different DPHP sensors given in the literature The choice of materials for heating element was determined by the material selection method by Nandhini [3]. For the given user specifications of ΔT (rise in temperature) and t (time), and device limitations on V (voltage), I (current), L p (length of probe) and d (diameter) and device characteristics such as R e (electrical resistivity) and R th (thermal resistivity) the parameterized solution to the optimization problem delivered a feasible map that consisted of certain copper alloys, nichrome alloys, and commercial gold. During actual construction, gold could not be considered due to its high cost. Nichrome too is difficult to work with as it is not readily available in insulated form. Copper, which is highly suitable, has low electrical resistivity and copper sensors become thick and bulky when they are packaged for higher resistances. 978-1-4673-8018-8/15/$31.00 2015 IEEE 283
Figure 2: The feasible map for materials selection of a soil-moisture sensor for the parameterized values from parameterized model taken from [3]. It shows that metals such as gold, copper alloys and nichrome alloys lie within the feasible map II. THEORY Campbell et al. [4] laid the foundation for the principle of a typical DPHP sensor. According to Campbell, the temperature rise at a distance r m, from the source of an instantaneous heat pulse depends on the volumetric heat capacity of the soil. The equation for the maximum temperature rise at r m around an instantaneously heated infinite line source is given by q ( T ) (1) e r C max rr m 2 m where q is the heat input to the line source per unit length of heater, C is the volumetric specific heat of the soil and ΔT m is the maximum temperature rise. The volumetric heat capacity of the soil depends on volumetric water content as follows: C C C (2) b s w where C is the volumetric heat capacity (Jm -3 K -1 ), r b the soil bulk density (kgm -3 ), C s the specific heat of the soil (Jkg -1 K -1 ), the volumetric moisture content, C w = r w. c w, where c w is the specific heat capacity of water, r w the density of water; and subscripts b, s and w indicate bulk soil, solid phase, and water, respectively. Further, gravimetric moisture content is related to the volumetric moisture content as follows: where d is the unit weight of dry soil (kgm -3 ) and w is unit weight of water (kgm -3 ). Volumetric moisture content θ (%) is calculated from gravimetric moisture content w (%) using Eq. (3). Practically, it is impossible to build an infinite line source or generate an infinitely small heat pulse. Nevertheless, this approximation holds good for DPHPs with slender needles and short-duration heat pulses. Campbell et al. [4], Bistrow et al. [8], Valante et al. [10], and many others employ 8 s heat pulses to good accuracy. However, for large-duration heat pulses such as ours, this analytical solution is not valid. Thus we depend on numerical solution to the cylindrical transient heat conduction problem. C T k T Q (4) 2 t r where T is the temperature, t the time, k the thermal conductivity, r the radial distance, and Q rate of the heat generated per unit volume. It must be noted that to produce a significant temperature rise at a distance r m from the heater source, the heat generated at the source must be high. Instantaneous heat pulse sensors consume a large amount of power in a short duration to achieve this and typically cannot perform without a powerful external power source. By prolonging the duration of heat supplied, this power consumption was greatly reduced at the cost of sacrificing an easy analytical solution to correlate d w (3) the temperature rise to moisture content. (3) w 2 284
III. FABRICATION OF THE SENSOR The amount of heat generated at the surface of the heater depends on the resistance of the heater and the materials that compose it. The sensor in [2] employed a copper wire of 50 μm diameter, and 3.5 m long folded tightly into a 40 mm stainless steel tube with 1 mm inner diameter. The filler material used was MgO chosen for its stable thermal and physical properties. This gave a surface temperature of 12 K at 3.3 V. As, increasing the resistance decreases power. High resistance was possible with nichrome which has an electrical resistivity of 138.5 10-8 Ω-m. The first embodiment of the nichrome sensor was made of a stainless steel tube of 40 mm length 0.6 mm inner diameter and 0.28 m of wire filled with silicone thermal paste. As seen in Fig. 3(a) for a power of 127 mw at 5 V, it gave a satisfactory output of 3.3 K and 0.3 K for dry and 27% wet red soil, respectively (Fig. 3) Placing the nichrome wire inside a steel tube turned out to be problematic as the wire is not insulated. To electrically insulate the nichrome wire it was double coated with Parylene and wound tightly around an enameled copper wire as shown in Fig 4. This helix form of winding is preferable over folding for the durability it offers. Parylene is prone to chipping on abrasion even after double coating. In such a scenario, if the coating wears away, the resistance will at most only drop by an ohm at the point of contact of the live wires. The filler material used in the later prototypes was the CoolMaster Essential E2 thermal grease which possesses higher thermal conductivity than its silicon counterpart. Figure 4: 50 μm nichrome wire wrapped around a 200 μm enameled copper wire. Figure 5: The smallest sensor has a probe length of 1 cm and an overall sensor length of 2 cm Figure 3: (a) Plot of ΔT vs. time of the rise and fall of temperature for the 200 Ω nichrome sensor in dry red soil at different power ratings (b) Plot at dry soil and 27% wet Thermal paste was a preferable alternative over MgO as at such small scaled MgO is rather grainy and prone to abrasion against the delicate Parylene coated wire. Thermal paste is also more effective in slipping into all gaps and eliminating air gaps. To get 60 Ω resistance, the length of nichrome wire used is 8.5 cm. For experimentations to test the influence of various factors such as length of the heater probe and density of the soil on ΔT m, nichrome heater probes of 1 cm, 2 cm, 3 cm and 4 cm in length all having the same resistance were 285
constructed. The helical coil was distributed evenly over the length of the sensor. The packaging for the smallest sensor, which was found to be effective as compared to others spans only 1.5 cm in diameter and 1 cm in length. (shown in Fig. 5) The temperature sensor is a T type thermocouple with an accuracy of 0.1 K. The thermocouple is placed at 3 mm from the heater as in Jorapur et al. [2]. 3 mm was found by Jorapur to be an optimum distance away from the heater probe and have a significant amount of soil in between and also have a substantial rise in temperature at the sensor. In an attempt to eliminate the need for Parylene coating the nichrome wire, another variant of the heater was constructed. This employed 2 cm long off-the-shelf ceramic tubes with four longitudinal holes. An uncoated nichrome wire was passed through these holes and the holes were filled with thermal paste and sealed. This heater can be seen in Fig. 6. taken as a standard. The graph obtained from the sensor is contrasted with the COMSOL simulation of the same in water as shown in Fig 7. A usable sensor is allowed only 5% error in T B. Dependence of T on the length of heater probe For a soil with the same gravimetric moisture content of 13.5%, there was a sharp difference in the temperature rise between the 1 cm nichrome sensor and the other sensors (Fig. 8). As the resistance is kept constant, the increasing length of the sensor progressively decreases q, the heat supplied per unit length. Figure 8: In general over varying densities of soil, the 1 cm nichrome probe shows the highest temperature rises Figure 6: Ceramic-Nichrome Heater filled with thermal paste. (Inlaid figure) Close-up of heater end showing the nichrome wire. IV. EXPERIMENTATION AND SIMULATION A. Calibration of sensor in agar agar solution The sensor performance is verified in a 2 g/l solution of agar agar. At this concentration it is found that agar inhibited the convection property of water allowing only conduction. As the volumetric heat capacity of water is a known quantity it is C. Dependence of T on volumetric content of the water in field soil When the soil samples happened to have similar values of density temperature depended inversely on the volumetric water content as can be seen in Fig. 9. Figure 9. Temperature rise is inversely proportional to the volumetric water content in soil of same density* *These density values are not the dry bulk density of the soil. Figure 7: Verification of 1cm nichrome sensor in agar agar solution 286
D. Dependence of T on density of the soil In a sample of field soil from Gandhi Krishi Vignan Kendra (GKVK), Bengaluru, with constant gravimetric moisture content of 13.5% the temperature rise obtained across different sensors varied almost linearly with density. (See Fig. 10.) Figure 10: Variation in temperature rise with density for different sensors E. Comparision of Ceramic Tube with Copper-Steel Sensor of similar resistance The ceramic-nichrome sensor possessed a resistance of 65 Ω while the copper-steel sensor was of 63 Ω. Ceramic whose thermal conductivity is greater than that of steel, generally gives enhanced temperature rise than copper-steel but also showed the most variation in reading (See Fig 11). Figure 11: The plot shows a generally high probability of getting an error in reading in ceramic sensor V. RESULTS AND DISCUSSION Verified in agar agar solution, the nichrome-steel sensor gave a temperature rise of 2.73 K and the simulation gave a rise of 2.66 K. This shows there is a difference of 0.07 K. This amounts to an error of 2.6% in the measurement. In 30% wet and 0.8% dry red soil the sensor gave a temperature rise of 6.2 K and 1 K respectively, giving a total range of 5.2 K. The difference in temperature rise varies from 3.5 K to 1.6 K for densely packed and lightly packed soil leading to high margin for error. However, in normal field conditions, the soil densities only vary by approx. 0.1g/cc and over such a small range, the low sloping of the readings (in Fig. 10) shows that there is little variation temperature rise. The ceramic tube, while is extremely easy to construct it is not reliable. This could be attributed to the thickness of the sensor which causes it to disturb and shift the soil during insertion leading to air gaps or soil clumps around the surface. For the working temperature range of the sensor (5 C to 50 C) the temperature rise at a distance of 3 mm was largely independent of the ambient temperature. This is proved in Jorapur et al [2]. In the future work, given significant number of data points, the longevity of the sensors in different field conditions will be affirmed. Acknowledgment The authors wish to thank Viswanath Sundaram, Somanna Kollimada for their help with data processing in MATLAB and Nandhini Devi for her advice on selection of materials. We would like to thank Professor Kumaraswamy at GKVK for supplying the soil samples and Dr. Nayak at Centre for NanoScience IISc, for the ceramic tubes. References [1] N. Jorapur and G. K. Ananthasuresh, Modeling and Characterization of Heat-Pulse Soil Moisture Sensor, 6th ISSS National Conference on MEMS, Smart Materials, Structures and Systems, 2013, Pune, India, pp. 6-7. [2] N. Jorapur, V. S. Palaparthy, S. Sarik, J. John, M. Shojaei and G. K. Ananthasuresh, A Low-power, Low-cost Soil-Moisture Sensor using Dual-probe Heat-pulse Technique, Sensors and Actuators, A:Physical (in review) [3] Nandhini Devi N, 2014, A Quantitative Framework and its Implementation for Selection as an Engineering Design Paradigm. PhD Thesis, Indian Institute of Science, Bengaluru, (in review) [4] G. S. Campbell, C. Calissendorff, J. H. Williams, Probe for measuring soil specific heat using a heat-pulse method, Soil Sci. Soc. Am. J.55 (1991), 291 293. [5] J. M. Tarara, J. M. Hamm, Measuring soil water content in the laboratory and field with dual-probe heat-capacity sensors, Agronomy. 89 (1997), 535 542. [6] Y. Song, J. H. Hamm, M. B. Kirkham, G. J. Kluitenberg, Measuring soil water content under turf-grass using the dual-probe heat-pulse technique, J. Am. Soc. Hort. Sci. 123 (1998), 937 941. [7] J. M. Basinger, G. J. Kluitenberg, J. M. Ham, J. M. Frank, P. L. Barnes, and M. B. Kirkham, Laboratory evaluation of the dual-probe heat-pulse 287
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