Heat Inertia
Time-delayed thermal responses occur when the internal temperature of a device takes time to reach equilibrium with a changing external environment. Transient thermal lag is the duration between a shift in the ambient temperature and the moment the sensor element actually reaches that new temperature. This delay is caused by the thermal resistance of the package and the mass of the internal components.
In fast-moving control systems, this lag can lead to timing errors and incorrect data.
Thermal Mass
Heavier packages with more material take longer to heat up or cool down. Reducing transient thermal lag requires minimizing the volume of the housing and using materials with a high thermal diffusivity. This is a primary goal for sensors used in high speed aerospace applications.
Response Constant
Quantification of the delay is done by measuring the time it takes for the sensor to reach sixty-three percent of a step change in temperature. This value defines the transient thermal lag characteristic of the device. It is recorded in the technical data sheet as the thermal time constant.
Equilibrium State
True accuracy is only achieved once the internal and external temperatures are the same. Until this state is reached, transient thermal lag will cause a measurement offset. Modern software sometimes uses a predictive model to estimate the final value and speed up the response.
