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Inicio  /  Applied Sciences  /  Vol: 9 Par: 6 (2019)  /  Artículo
ARTÍCULO
TITULO

Autonomous Soil Water Content Sensors Based on Bipolar Transistors Encapsulated in Porous Ceramic Blocks

Pedro Carvalhaes-Dias    
Flávio J.O. Morais    
Luís F. C. Duarte    
Andreu Cabot and J. A. Siqueira Dias    

Resumen

We present an autonomous sensor to measure soil water content that uses a single heat pulse probe based on a transistor encapsulated in a porous block. The sensor uses a bipolar junction transistor, which performs as both a heating and temperature-sensing element. Since the sensor depends on a porous block to measure the matric potential of the soil, it does not suffer from accuracy problems if the contact between the probe and the soil is not perfect. A prototype of the sensor showed a temperature variation of ???=2.9 ° ? T = 2.9   ° C when the porous ceramic was saturated with water. The sensor presented an almost linear behavior for small changes in the matric potential of a red latosol when tested in the 1-kPa and 35-kPa pressure range, showing a sensitivity of ??=0.015 ° S = 0.015   ° C/kPa. The ultra-low power signal conditioning circuit can read the sensor?s temperature with a resolution of approximately 0.02 ° 0.02   ° C, so the matric potential can be read in increments of at least 1.33 kPa. When powered only by a 2-F supercapacitor from the energy-harvesting system, the interrogation circuit is able to take one soil water content measurement per day, for eleven days.

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