On-line version ISSN 1809-4430
TEIXEIRA, Adunias S. and COELHO, Silvana L.. Development and calibration of an electronic tensiometer for automatic reading. Eng. Agríc. [online]. 2005, vol.25, n.2, pp. 367-376. ISSN 1809-4430. http://dx.doi.org/10.1590/S0100-69162005000200010.
This paper presents the development and calibration of a tensiometer for automatic reading where mercury has been replaced by an electronic pressure sensor called the electronic tensiometer. The conventional mercury tensiometer was used as a standard for calibration. Calibration trials were conducted at the Mechanics and Electronics Laboratory of the Department of Agricultural Engineering at Ceará Federal University, Brazil. The electronic tensiometer outputs a maximum 4.5 volts and is connected to a data acquisition system (DAQ) composed of a microprocessor, a 12-bit analog to digital converter and a serial port. The DAQ outputs to a microcomputer via a serial port. The trial bench was made of three hard plastic boxes filled with sandy-loam soil. Calibration trials have run for a month and linear calibration equations of the form Count = offset + aym, were obtained, where ym represents the soil-water matric potential read at the conventional mercury tensiometer (kPa) and count, the digital output from the electronic tensiometer, with dimensionless values ranging from 0 to 4,095. Maximum and minimum values of 348.572 and 261.026 were obtained for the offset, and 36.675 kPa-1 e 34.421 kPa-1 for the angular coefficient. Statistical analysis on the offset and coefficients of the calibration equations indicated that exists a significance confidence lower than 0.1% and correlation coefficients of at least 0.9994 for any of the nine sensors tested.
Keywords : pressure sensor; automation; matric potential.