A sensor to determine calcium contents in molten metal is examined. The sensor is constituted of an oxygen sensor with an auxiliary electrode which is surrounded with a cap made of silica. 0.02 to 0.1 mass% of calcium contents in molten nickel can be detected by the sensor. Relation between calcium contents in the molten nickel and electromotive forces of the sensor can be expressed as following equation :
EMF/mV= -173log [%Ca] -423 ± 18
According to discussion on the basis of thermochemistry, it is revealed that the sensor acts as a chemical sensor of which output is determined by the thermochemical relation between calcium and oxygen.
