Article Summary
- Dissipation factor and time constant are different
- What each parameter tells you
- How mounting and measurement change results
- OEM selection and validation steps
- Conclusion
Dissipation Factor and Time Constant Are Different
NTC thermistor datasheets may specify both a dissipation factor (also called dissipation constant, usually in mW/K) and a thermal time constant (usually in seconds). They describe related but different thermal properties. Dissipation factor is not another name for time constant, and its value alone does not tell how quickly a sensor responds.
TDK defines dissipation factor as the change in power dissipated divided by the resulting change in thermistor body temperature. Its published thermal values refer to stated conditions such as still air. See the manufacturer’s NTC thermistor general technical information and check the conditions for the exact product.
What Each Parameter Tells You
Dissipation factor, δth: a steady-state heat-transfer measure. It describes how much power is associated with a given rise in thermistor body temperature above the surroundings: δth ≈ P/(Tbody − Tambient). A larger value means more power can be dissipated per kelvin of body-temperature rise under the stated test conditions. It is often relevant to self-heating and the thermistor’s voltage/current behavior.
Thermal time constant, τ: a transient-response measure. For a step change in the surrounding temperature under the stated test setup, it is commonly defined as the time for the thermistor temperature change to reach about 63.2% of its final change. A smaller τ indicates faster response for that specified mounting and medium, but field response may differ.
These properties are connected through the thermistor’s heat capacity: Cth = δth × τ, with consistent units. For illustration only, δth = 2 mW/K and τ = 10 s imply Cth = 20 mJ/K. This example is not a specification for a Starlight product; use values from the selected component’s datasheet and test conditions.
How Mounting and Measurement Change Results
Thermal values depend on body size and construction, lead arrangement, airflow or fluid, mounting surface, insulation, and the way the thermistor is attached. A value measured in still air cannot be assumed for stirred air, liquid, a bonded surface, or a finished probe. Compare datasheet numbers only when the relevant conditions and definitions match.
Measurement current also matters. Power dissipated in the thermistor can raise its body temperature and bias a sensor reading. Reduce excitation power as appropriate and validate self-heating in the intended circuit and installation. A disc or bead element’s response does not automatically represent the response of a sheathed probe or complete assembly.
OEM Selection and Validation Steps
- Identify the exact part number and datasheet revision; confirm whether the values apply to the bare element or finished probe.
- Record dissipation factor, time constant, heat capacity (if stated), units, test temperature, and surrounding medium.
- Match the measurement range, R/T curve, controller input, excitation, and required response to the application.
- Check expected self-heating under the real electrical load and mounting conditions.
- Test the installed assembly against a reference sensor using the actual medium, airflow, contact, and response criterion.
- Document the sensor configuration and acceptance limits for incoming inspection and replacements.
For practical measurement-system checks, see our NTC calibration and validation guide and NTC circuit design guide. Browse our NTC sensor range or contact us with an OEM probe requirement for configuration-specific documentation.
Conclusion
Dissipation factor describes steady-state heat transfer, while thermal time constant describes transient response under defined conditions. They are related through heat capacity but are not interchangeable. Use the selected part’s datasheet and validate the full sensor assembly in its intended environment.
