In this paper we show the high impact of isothermal error on thermocouple measurements. Isothermal error is
the mismatch in temperature between a thermocouple reference junction and its associated reference junction
compensation. In many cases isothermal error dominates the overall uncertainty of the electrical instrumentation.
Therefore we examine common thermocouple instrumentation error sources and show how isothermal error compares
with other errors and how it varies depending on application parameters. We also explore application environments that
tend to increase thermal gradients and create unexpected errors. To help indentify problematic installations, we introduce
a method for characterizing isothermal error in deployed thermocouple instrumentation. Finally, we describe good
practices for minimizing isothermal error in deployed systems.