Dr. Abner Mallity - a benevolently deranged local professor - needs to measure some temperatures in a liquid.  He is concerned about interaction between the liquid and the sensor.  In particular:

• The liquid could be electrically conductive - at least to a small degree.  He is worried that the electrical conductivity could allow external voltages to influence his temperature measurements.
• The liquid could corrode the metal(s) in the thermocouple.

        You need two thermocouples for this work.  One should be a thermocouple without the protective insulation (i.e. your garden-variety thermocouple) and one should be a thermocouple with protective insulation.  Compare the performance of the two kinds of thermocouples.