Thermocouple sensors are ‘tip’ sensing devices which makes them ideal for both surface and immersion applications (Depending on the sensor style). Immersion thermocouples must be used carefully to avoid stem conduction errors, this is when heat flows to or from the sheath and in or away from the process which can result in error in the reading. As a rule of thumb the sensor should be immersed a minimum of 4 times the outside diameter of the sheath (E.g if the diamater is 3mm the minimum immersion depth would be 12mm)