What Are Thermal Sensors?

Thermal sensors are devices that convert temperature or heat flux into a measurable electrical signal. They are among the most widely deployed sensor categories in industrial, medical, aerospace, and consumer electronics applications, underpinning process control, safety monitoring, energy management, and scientific measurement. The choice of sensor type for a given application depends on temperature range, accuracy, response time, physical form factor, and cost. Thermal sensing technologies span contact-based devices that must be in thermal equilibrium with the measured object and non-contact devices that infer temperature from emitted infrared radiation.

Resistance Temperature Detectors and Thermocouples

Resistance temperature detectors (RTDs) exploit the predictable increase in electrical resistance of a pure metal with temperature. Platinum RTDs, designated Pt100 or Pt1000 based on their resistance at 0°C, offer high accuracy and excellent long-term stability over the range from roughly -200°C to 850°C. They are the reference standard for industrial process measurement and laboratory calibration. NIST's temperature measurement guidelines define the International Temperature Scale of 1990 (ITS-90) and the calibration procedures used to characterize RTDs against primary thermometry standards.

Thermocouples generate a small voltage proportional to the temperature difference between a measuring junction and a reference junction, an effect known as the Seebeck effect. They are rugged, inexpensive, and useful over a wide temperature range: Type K thermocouples cover -200°C to 1,260°C, while refractory-metal types such as Type W can measure above 2,000°C. Their voltage output requires cold-junction compensation and amplification, and their accuracy is generally lower than that of RTDs.

Thermopiles

A thermopile is an array of thermocouples connected in series, stacking their individual Seebeck voltages to produce a larger, more easily measured output. Thermopiles are commonly used in non-contact infrared temperature sensors and in radiation heat flux gauges. A typical commercial thermopile sensor absorbs incident infrared radiation on a blackened surface, raising the temperature of the hot junctions relative to the cold reference, and produces an output voltage proportional to the irradiance. Research on thermopile-based sensors in medical thermometry published in biomedical engineering literature demonstrates their use in tympanic and forehead thermometers.

Infrared Sensors

Infrared (IR) sensors detect thermal radiation emitted by objects above absolute zero without requiring physical contact. They divide into photon detectors, which respond to individual photons and require cryogenic cooling for high sensitivity in the mid-wave infrared, and thermal detectors, which respond to the heat deposited by absorbed radiation and operate at room temperature. Microbolometers, the dominant technology in uncooled thermal imaging cameras, use a thin membrane whose resistance changes with absorbed radiation. Arrays of microbolometers produce thermal images with resolutions from tens of thousands to millions of pixels. IEEE Transactions on Electron Devices publishes ongoing research on microbolometer materials, pixel architectures, and readout integrated circuit design.

Temperature Sensors in Integrated Circuits

On-chip temperature sensors are embedded in microprocessors, power management integrated circuits, and system-on-chip devices to monitor die temperature and enable thermal throttling. They typically rely on the predictable temperature dependence of a bipolar transistor's base-emitter voltage. Bandgap reference circuits produce a voltage proportional to absolute temperature, which can be digitized by an on-chip analog-to-digital converter to provide a digital temperature reading through a serial bus interface.

Applications

• Industrial process control in chemical plants, refineries, and food processing lines
• Non-contact thermometry in medical devices including ear and forehead thermometers
• Thermal imaging for building diagnostics, predictive maintenance, and security surveillance
• Die temperature monitoring in microprocessors to enable dynamic voltage and frequency scaling
• Space instrument calibration using precision platinum resistance thermometers traceable to ITS-90
• Heat flux measurement in combustion research and materials characterization