We have developed the high temperature and longtime recording logger as a downhole tool. We can use this constructed logger for about 5.7 hours in 350 °C environments because in this constructed temperature logger, heatproof dewar flask delays thermal elevation of the tool electronics which is available only less than 125 °C. The assembly of the dewar flask and the electronics was packaged into SUS630 made tool housing whose length, maximum width or pressure capacity was 1148 mm in length, 60 mm in diameter, or 50 MPa in 350 °C, respectively. In order to estimate the available time of the constructed logger in various extremely hot environments, we carried out heating test of the constructed logger by putting it into the electric furnace. The result of the test at 350°C showed that the available time of the logger was 340 minutes. Also, the heating test can be applicable to this logger for 250 °C or 150 °C with the estimating time of 600 or 1418 minutes, respectively. The sensitivity test was performed to time for tool temperature to reach external temperature. Sensitivity tests showed the accurate temperature less than 100 °C was recorded within a minutes. And also the sensitivity tests suggested that at least 10 minutes was good enough to measure accurate temperature over 100 °C. The TRDT has sufficient function in exploring hydrothermal boreholes and lead us to understand deep seabed environments not only biologically but also other Earth scientifically.

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