Zinc–Bismuth and Aluminum–Indium Monotectic Alloy-Based Fixed-Point Cells with Double Phase Transition for In Situ Calibration of Thermocouples

Abstract

Re-calibration of a thermocouple after it has been installed in a process is often not practical. In situ monitoring of performance is desirable and can be done with built-in reference standards based on melting or freezing phase transitions. Binary alloys with a monotectic reaction frequently have two invariant melt/freeze phase transitions taking place in the same material over a range of compositions. This makes them potentially well suited to be in situ temperature calibration artifacts, enabling correction for thermocouple drift without the need to disturb the thermocouple. A zinc–bismuth fixed-point cell was constructed and has been shown to be stable with two well-defined melting plateaus at nominally 255 $$^\circ \hbox {C}$$ and 415 $$^\circ \hbox {C}$$ . Two miniature fixed-point cells (each designed to be permanently installed with a thermocouple) based on zinc–bismuth and aluminum–indium alloys were made. Measurements have shown that the phase transitions can be identified despite the small quantity of metals used and that the alloys were sufficiently stable to have the potential to provide improved long-term confidence in process control and monitoring.