Multiple phase transitions in Al-In monotectic alloy

A multiple fixed-point temperature reference source has been constructed from a monotectic alloy ingot of indium and aluminium contained in a graphite crucible. The crucible was heated in a single zone tube furnace to above the melting temperature of pure aluminium. The temperature of the fixed-point was measured with a type-N thermocouple. Melting and freezing plateaus were seen at three distinct temperatures; 156degC, 638degC and 660degC. The lowest and highest temperatures correspond closely to the melting/freezing temperatures of pure indium and pure aluminium respectively. The third transition is close to the calculated temperature for the monotectic transition of Al-In alloy. An artefact that provides calibration points at three temperatures could simplify thermometer calibration and may have value for self-calibrating temperature sensors.