P3.4 - Long Term Stability of Miniature Fixed-Point Cells Used in Self-Calibrating Thermometers

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
P3 - Temperature / Humidity
G. Krapf, T. Froehlich, S. Augustin, H. Mammen, G. Blumroeder, M. Schalles, F. Hilbrunner - Technische Universität Ilmenau (Germany)
732 - 737


In many temperature critical and securityrelevant industrial processes the drift of the characteristics of the installed temperature sensors has a dominant effect on the resulting measurement uncertainty. Also many applications can be found in medical technology, pharmaceutics, biotechnology and food industry where an exact and traceable knowledge of the process temperature is indispensable. For this reason a frequent calibration of the used sensors becomes necessary. During this calibration process the temperature sensors usually have to be taken out of the process to be measured. In doing so, they can only be calibrated without their measurement chain. This often causes additional non-negligible contributions to the calibration uncertainty. Here and in other cases a metrological traceable in-situ calibration can improve the achievable measurement and calibration uncertainty.

Against this background, self-calibrating industrial thermometers containing miniaturised fixed-point cells were developed, improved and tested under operating conditions during the last years. Within this research a large number of miniature fixed-point cells were filled with zinc of different purities. These cells were aged under tough operating conditions partly in an oxidising atmosphere. The whole ageing process lasted more than a year. Within this timespan the zinc phase transition temperatures were monitored periodically and analysed. For this the cell temperatures were measured by mineral insulated resistance thermometers while the cells were heated up and cooled down at different heating and cooling rates. The thermometers were continuously calibrated at a traceable standard zinc fixed-point.

This article contains the collected data as well as their analysis of the ageing. It also assesses the time-dependent calibration uncertainty of selfcalibrating thermometers on the basis of miniature fixed-point cells.