Where RTD sensors are only partially immersed in a measured variable, stem conduction can occur. Typically your minimum immersion depth should be no less than four times the length of the detector or sensing junction.
If the immersion depth is less, stem conduction errors will occur, this is the effect of heat transfer into or away from the variable being measured via the stem or sheath for example if the sensors sheath is hotter at ambient temperature than the variable being measured heat will be transferred to the variable creating uncertainty of reading.
Stem conduction effect can be reduced by increasing the immersion length, reducing the wall thickness of the sheath or changing the sheath material.
Stem conduction errors most commonly occur under laboratory conditions; however it must be noted that they can cause significant errors in other industries.
As a rule of thumb, RTDs should be immersed 4 times the element length. (Flat film elements are typically 2-3mm, whilst wire wound elements are approximately 15mm or more)
Reference : sterlingsensors