Renato Teixeira, Hamilton Vieira, Rodrigo Silva
PERFORMANCE EVALUATION OF AN OPEN ZINC CELL CONSTRUCTED AT INMETRO
The Inmetro Thermometry Laboratory uses fixed-points cells defined in the International Temperature Scale of 1990 (ITS-90) to calibrate thermocouples and standard platinum resistance thermometers. In order to meet this, the laboratory counts on reference fixed-point cells from the Argon triple point (-189,3442 ºC) to the Copper fixed-point (1084,62 °C). In a quest to improve its best measurement capability and to disseminate this, the laboratory has been investing in the manufacture of its own primary standards. This process was started with water triple point cells, mercury triple point cells and pure noble metal thermocouples which have results validated through comparisons and are part of the laboratory standards regularly used in calibrations. The following step came to be an open metal cell. Zinc was chosen for being relatively inexpensive. In late 2005 the first cell was assembled following NIST design. The methodology and the corresponding results achieved were published in 2006. Later, another zinc cell was made . However, results of both cells were considered unsatisfactory. Going on with this task, the laboratory constructed a third Zinc cell in early 2008. All Inmetro cells are open, and they were compared to the three commercially available cells the laboratory previously had. The materials, methodology, uncertainty evaluation (impurity effect estimates) and results are discussed in the present paper.
