Temperature-sensor characteristics under dynamic and static conditions are influenced by the construction, capabilities and limitations of thermocouples.
Contact temperature-sensing devices, such a thermocouples, RTDs (resistance temperature detectors), thermistors, bimetallic thermometers and liquid-filled sensors, measure the temperature of their sensing tips; that is, they do not measure the temperature of gases, liquids or solids surrounding the sensing tip unless certain requirements are met. Differences in the thermal properties of the sensor and the media surrounding the sensor (e.g., thermal conductivity, thermal diffusivity and emissivity) can produce large differences in the observed temperature readings. Additionally, isotherms (areas of constant temperature) within the gas, liquid or solid material can produce large differences between observed and actual temperatures. The test engineer (user) must consider these factors before choosing the particular device design. This article describes various situations where temperature measurements are made, and studies temperature-sensor characteristics under dynamic and static conditions. Where appropriate, typical test data are presented to illustrate the performance of temperature sensors under actual tests.