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2011 International Conference on Electronic & Mechanical Engineering and Information Technology

The application of soil temperature measurement by LM35 temperature sensorsC u i h o n g L i u , W e n t a o R e n * , B e n h u a Z h a n g , C h a n g y i L v

College of Engineer ing,Shenyang Agr icultural Univers i ty

Shenyang, P . R. ChinaAuthor email :cuihongliu77@ 126.com

(*Corresponding author email :Renwentao 1958@yahoo.cn)Abstract Consider revising medium temperature used LM35temperature sensor, what is an economic and feasible method.This study mainly researches the applicability of LM35temperature sensor in soil temperature testing field. Selectedthe sensor, and based on the theoretical equation between thesensor output voltage and Celsius temperature; introducedcorrection coefficient, carried through the calibrationexperiment of the sensor; further more, it is applied to thepotted rice's soil tempera ture detection. The calibration resultsshow that , each sensor correction coefficient is different fromothers, but these numerical are close to 1, the linearrelationship was very significant between tested mediumtemperature and sensor output voltage. In the key tr ial periodof r ice potted, used LM35DZ type temperature sensor tomeasure the soil temperature. The analysis result show that,the changing trends are basically equal both soil temperatureand air temperature, and the characteristics of soiltemperatures are lag. The variance analysis shows that, thedifference was not significant paper film covered and withoutcovered on soil temp erature .

Keywords-LM35DZ temperature sensor; calibration; soiltemperature; detection

I. I N T R O D U C T I O NThe caloric conversion of soil leads to soil heat and massflow, which affects soil temperature. The soil temperature isone of the important environmental factors, with thechanging of c l imate, topography, vegetat ion, soi l type,planting form, and other factors, the soil temperature isclosely related with some processes, such as crop plantingtime, tillering growth, and wintering safety etc .The changeof soil temperature directly impact on soil nutrient absorptionand soil moisture keep and sport, The soil temperature playsa certain role on many of the physical processes of soil. The

soil water and heat migration is an important researchproblem. Therefore, the observation of soil temperature realtime and understanding of soil temperature variation havevital significance to agricultural production and scientif icresearch [2, 4, 7].The traditional measuring method is to insert the sensorinto soil before read the records. The measurement of soiltemperature has some characters, such as larger area, muchobservation points, and the time observation lasting is longer

etc, which results in detecting data inaccurate in that surveycrew visual fatigue an d other factors [5]. At presen t,commonly used soi l temperature measur ing ins trumentsmainly involves non-contact thermo-detector by infraredspectrum, the temperature sensor us ing the PN junction

transistor, and the temperature sensor using thermostats. Themeasuring precision is higher than using the infrared, but thecost is higher, too. I t needs calibration the system parametersthat measurements u s ing the transis tor PN junction andthermostats [4].This study mainly research the applicationsof LM35 integrated circuit temperature sensors in soiltemperature measurement. The sensor has some advantagessuch as lower cost, high reliability, measurement and carryeasy, as well as can be embedded within the designated soilachieve long- term observat ions .

I I. T H E W O R K I N G T H E O R Y O F LM35 T E M P E R A T U R ES E N S O R

LM35 is integrated circuit temperature sensor made byNational Semiconductor. I t has higher precision and widerrange of l inear working. The output voltage LM35 l inearproportional Celsius temperature, at ordinary temperatures, itcan provide 1/4C common precis ion of room temperaturewitho ut need additional calibration or f ine-tune [8]. The testused LM35DZ-92 type temperature sensor with plas t icpackaging. The pin functions as shown in figure 1, basicworking parameters are:Working voltage: dc 4 ~ 30V;Working current: less than 133 uA;Output voltage: + 6V 1.0V;Output impedance: 1mA load 0.1^2;Measuring precision: 0.5 C (in + 25 C);Leak current: less than 60 uA;Scale factor: linear + 10.0 mV / C;Nonlinear value: 1/4 C;Calibration means: use directly Celsius temperaturecalibration;Measuring temperature range: 0 ~ 100 C.

3 2 11 power +Vs; 2 output Vout; 3 grounding GND

Figure 1. LM35DZ temperature sensor ' s p in function.This experiment mainly research about testing crop rootlayer soi l temperature used LM35DZ temperature sensor .

978-1-61284- -8/ll/$26.00 2011 IEEE 1825 12-14 Augus t, 2011

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The biggest changing range of soil temperature is 0 ~ 40 C.According to the test requirements, the detection circuit wasdesigned,which be shown in figure 2. Three core wires ,LVVP type ,were used measurment ,and it's length is6 0 0 m m . one side of the wire connects the sensor pins, theother side connects power source and output terminal . Thesensor adopts 9Vbatteries, the sensor 's output voltage(K ou t)was detected using the type VC97 mult imeter . The theorycomputat ion formula of soil temperature (7) is:

T= V0JK t. (1 )A ccount for: TSoil temperature, C, VoutSoiltemperature mult imeter ' s output voltage, mV, K tLinearscale factor, adopt 10.0 mV /C.

+V s

L M 3 5D Z

Vout

GN DFigure 2. The principle of detection circuit about LM35DZ temperature

sensor.

I I I . LM35 TEMPERATURE SENSOR'S CALIBRATIONEXPERIMENT

calibrate to LM35DZ temperature sensor . The var iance oflinear regression analysis shows that, it is very significantl inear relat ionship between medium temperature and outputvoltage.TABLE I. THE REGRESSION ANALYSIS OF LM35DZ TEMPERATURESENSOR'S TEMPERATURE CHARACTERISTICSSe n s o r s e r i a l

n u m b e r123456789101112

C o r r e c t i o ncoeff i c i en t (K)

1.0051.0201.0091.0090.9850.9941.0310.9991.0110.9991.0120.995

Re g r e s s i o ne q u a t i o n

r=1 .005K O ut /Ktr = 1 . 0 2 0 F o u t / K tr=1 .009K O ut /Ktr=1 .009K O ut /KtT= 0.985Kout/Ktr = 0 . 9 9 4 F o u t / K tr=1 .031K O ut /Ktr = 0 . 9 9 9 F o u t / K tr=1 .011K O ut /Ktr = 0 . 9 9 9 F o u t / K tr = 1 . 0 1 2 F o u t / K tT= 0.995Kout/Kt

C o r r e l a t i o ncoeff i c i en t (R2)

0.99990.99960.99970.99990.99990.99940.99970.99990.99961.00000.99960.9999

IV. T H E T E S T O F S OIL T E MP E RAT URE B AS E D O N LM35TEMPERATURE SENSOR

A. Test materials and methodsIn order to validating the detection performance ofLM35DZ temperature sensor ,and achieving tes t the soiltemperature to multipoint repeatly, the exper iments ofdetection performance about sensor calibration must carryout. Preparating ice water mixture, f iltering on its heating,calibrating LM35DZ test results used glass mercurythermometer ; Choosing twelve LM35DZ temperaturesensors for calibration experiment, using standard glassmercury thermometer (WBG-0-2,made in hongxinginstrument and meters factory ,in W uqiang County , H eBeiProvince) , the range of temperature measurement is 0 - 5 0 C ,measurement accuracy is0.1 C. Adopt SPSS 13.0 software toanalysis the test data.B. The analysis of experiment result

Based on the theoretical equation (formula 1) betwe en thesensor output voltage and medium temperature, introducedcorrection coefficient K),revised the values of the sensor 'sassessment. Used the least squares to carry through linearregression analysis on the test data, and got the equationrelat ionship between all temperature sensors and thecorrection coefficient of temperature-voltage.As shown intable 1. Visibly, it exists good linear relationship betweenmedium temperature and sensor output voltage, Eachsensor 's correction coefficient is different from the others ,but these numer ical are close to 1. In order to gettingaccurate measurement resul ts of temperature, it mus t

A. Test planA pply ing LM35D Z temper a tur e sensor to paper f ilmcovers r ice potted exper iments , detect ing the soiltemperature of r ice . The test is w or ked in laboratory foragr icul tural machinery, Shenyang Agr icultural Univers i ty in2010. Geographical posi t ion for 4 1 0 i r - 4 3 0 2 ' N latitude,12 2 025 ' - 12304 8 ' E longitude. Toughened plas t ic soi l bin wasused, and its length is 6 0 0 m m , the w idth is 4 1 5 m m , theheight is 430mm. Tested soi l is the brown loam, naturalmois ture content is 6.35%, the total weight of soil and soilb in are 100kg. Test papers were made in Y FY Biotech Co.Ltd, J iangsu, China, and its specification islOOOOOmmx 126 0m m, the thickness is 0.1 mm. this paperwill decompose naturally after forty days.

The test carrying out under the same conditions, such asrice type, seedlings, transplanting, density, and fieldmanagement measurements e tc . Two treatments are designedin the test: F is planting r ice under paper mulching, W isplanting r ice without ov erburden, and each treatment is donethree t imes. Each tes t embedded within two L M 3 5 D Z inbasin of temperature sensors , embedding depth for 10cm.Select several key time of rice growth points, detecting dailysoil temperature of various treatments from 8:00 to 17:00,testing time interval for one hour .

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B. The analysis of experiment resultSelected three key points-in-time in the period of r ice'sgrowth stages, involving with regreening, tillering, and

booting. To analysis of the diurnal variation characteristicsof soil temperature, as shown in figure 3-5. In r ice'sregreening stage and tillering stage, soil temperature aretrends that r ising in f irst and declining then, boundary pointin 15:00 around, the changing trends are basically equal bothsoil temperature and air temperature, and the characteristicsof soil temperatures are lag. In r ice's booting stage, the ricegrows thrives, the surface of soil coverage increased, whichlead to soil temperature rises slowly. At the time froml5:00to 17:00, the soil temperatu re chan ges tend to be gen tle.

26.0

8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00t

Figure 3. The curve of diurnal change in soil temperature of regreeningstage.

8:00 10:00 12:00 14:00 16:00 18:00 20:00t

Figure 4. The curve of diurnal change in soil tempera ture of tillering stage.

22.08:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00

tFigure 5. The curve of diurnal change in soil temperature of booting stage.

Paper covering has changed the boundary layer structureof the soil, thus influences the changing trend of soiltemperature. In the regreening stage (June 10, sunnyweather), at the period from 8:00tol3:00, the soiltemperature of paper cover ing is lower than without cover ing;the soil temperatures of the two processing are basic equalafter 13:00. The probably reason is that paper covering hasslowed the heating speed of the soil. In the tillering stage(June 23, sunny weather), at the period from 8:00tol5:00, thesoil temperature of paper covering is higher than withoutcovering; the soil temperatures of the two processing arebasic equal after 15:00. The reason is to likely that papercovering has improved the physical properties of the soil. Inbooting stage (August 18, sunny weather), paper f ilm hascompletely decomposed, the soi l temperatures of the twoprocessing are basic equal. The Variance Analysis show that,it is not significant difference that of the two processing soiltemperature.

26.0

25.0

Before c alibrationAfter calibration

23.0

22.08:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00

tFigure 6. The contrast of the test results to paper covering before and aftercalibration of the sensor.

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26.0 r- Before calibrationAfter calibration

22 .0 I 1 ' ' 1 ' 1 ' ' 18:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00

tFigure 7. The contrast of the test results to without c overing before andafter calibration of the sensor.

In order to analyzing the effect of calibration experimentabout the temperature sensor, according to the test data onAugust 18, contrast the test results two circumstances beforecalibration the sensor and after calibration , as shown infigure 6 and 7. The results show that, the values of aftersensor calibration are higher than before calibration. TheVariance Analysis shows that, it is not significant that valuesof before sensor calibration and after calibration. I t is thusclear that, the application results of LM35DZ temperaturesensor are equal to that of calibration experiment.V . C O N C L U S I O N S

The cal ibrat ion exper iments of LM35DZ typetemperature sensor detection performance were carried on inthis study. Based on these experiments, applied these sensorsto the soil temperature detection. Through compared thepaper covered the rice and without covering pottedexperiments, the application effects of LM35DZ typetemperature sensor were studied. Preliminary conclusionshave been drawn as the following:(1) In order to correcting the errors of pins weldingproduction and other processes , the cal ibrat ion exper imentsof LM 35D Z temperature sensor detection per formance werecarried on. The results show that, each sensor correctioncoefficient is different from others, but these numerical areclose to 1.Visibly, the linear relationship was verysignif icant between tes ted medium temperature and sensoroutput voltage.

(2) Applying LM35 temperature sensor to paper f i lmcovers r ice potted experiments, detecting the soiltemperature of r ice. The tests results show that, the changingtrends of each treatment are basically equal both soiltemperature and air temperature, and the character is t ics ofsoil temperatures are lag.

(3) Both Calibration experiment and the soil temperaturedetecting test indicate that, the values of after sensorcalibration are higher than before calibration. But, it is notsignificant that values of before sensor calibration and aftercalibration. Obviously, in some cases what requirement isnot particularly str ict, it can be directly applied thetemperature characteristic equations before calibration, tocalculate the medium temperature.A C K N O W L E D G M E N T

This research was supported by Natural ScienceFoundation of China (No.51075283) , young scientis t fund ofShenyang Agr icultural Univers i ty (No.20091015) andproject of Liaoning agricultural mechanization keylaborator ies (No. 2009403021) .

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