Which temperature sensor has nonlinear response and high sensitivity over a limited range?

• A. Thermocouples
• B. RTDs
• C. Thermistors
• D. Infrared

Answer: (C)

This question is about a temperature sensor whose output changes nonlinearly with temperature and shows high sensitivity over a limited range. Thermistors fit this description best. They are ceramic semiconductor devices whose resistance changes very sharply with temperature, especially in a narrow temperature window. For NTC (negative-temperature-coefficient) thermistors, resistance drops quickly as temperature rises, so a small temperature change produces a relatively large change in resistance. That steep, curved resistance-versus-temperature relationship gives high sensitivity, but only around the temperatures where the device is designed to operate well—the sensitivity diminishes outside that range, making them ideal for precise measurements in a limited span rather than over a wide range.

In contrast, thermocouples generate a voltage that is proportional to temperature difference and tend to have a more linear response over a broad range with moderate sensitivity. RTDs (like platinum sensors) yield a fairly linear resistance increase with temperature and offer good stability across wide ranges, but their sensitivity per degree is typically lower than that of a thermistor in its high-sensitivity region. Infrared sensors detect radiant energy and convert it to a temperature reading through calibration; their response is not characterized by a sharply nonlinear resistance curve with high localized sensitivity.