A Temperature Logging System: Development And Application
Francisco Yukio Hiodo, Tereza Higashi Yamabe, Nilton Silva, Hugo Leonardo Morais · 2009
Temperature logging equipment made in Brazil, for continuous geothermal measurement is presented. Digital and analog circuits and a temperature probe with an interchangeable negative temperature coefficient (NTC) thermistor were developed. The probe and electronic circuits are linked by a long three wire lead that makes feasible to compensate the cable effect on measured values. Cable is turned in a winch at surface where electronics circuits are fixed. Coupled to temperature sensor through cable there is a 22 bits resolution ADC delta-sigma converter MCP3551 that is controlled by a fast 16 bit digital signal processor (DSP) PIC30F3012 microcontroller. After 10 measurements in every probe position, a mean value is calculated and it can be visualized in an external LCD alpha numeric display. The probe depth value is given by a sheaver made with a small magnet fixed in a pulley. Two stable Hall magnetic sensors make possible a depth resolution of 10 cm. The measured depth values are displayed in a commercial handheld calculator directly in meters. Preliminary probe calibration curve for resistance versus temperature was determined in laboratory by using the ultra stabilized thermostatic bath HAAKE MK22. The best fit of each pair R-T was gotten with accuracy of ± 0,02oC, following the Steinhart and Hart equation. Ground water movements generate characteristic anomalies on geothermal gradients. Therefore some temperature anomalies related to ground water moving through fractured rocks can be detected at surface. Then a temperature logger is very useful to study the influence of ground water movements mainly when associated with seismicity occurring in some localities of São Paulo State. It was the main motivation for developing the present temperature logging system.