What Is Temperature Measurement?

Temperature measurement is the quantification of thermal energy in a body or environment using physical sensors, instruments, or radiation-based techniques. It is one of the oldest and most pervasive tasks in physical science and engineering, underpinning process control in manufacturing, calibration of scientific instruments, clinical diagnostics, and environmental monitoring. Measurement methods range from direct contact with a sensing element to non-contact detection of emitted radiation, each suited to a different temperature range or physical environment.

The field is governed by the International Temperature Scale of 1990 (ITS-90), a globally agreed set of fixed reference points and interpolation procedures that define the practical realization of thermodynamic temperature from 0.65 K to 1234.93 K. NIST's thermometry program maintains the U.S. national temperature standards and provides calibration services across this full range, including calibration of standard platinum resistance thermometers (SPRTs) at ITS-90 fixed points.

Resistance-Based Sensing

Resistance thermometry exploits the predictable change in electrical resistance of a conductor or semiconductor with temperature. The standard platinum resistance thermometer, specified by ITS-90 as the authoritative device from -196°C to 962°C, uses a high-purity platinum wire whose resistance increases nearly linearly with temperature. Resistance temperature detectors (RTDs) built from platinum, copper, or nickel provide high repeatability and stability, making them the preferred choice when accuracy is the primary requirement.

Thermistors take a complementary approach, using semiconductor oxides whose resistance changes far more steeply with temperature than metals. Most thermistors are negative-temperature-coefficient (NTC) devices: resistance drops sharply as temperature rises. This high sensitivity makes them well-suited to narrow-range medical and HVAC applications where detecting small changes matters more than covering a wide span. Thermoresistivity, the physical property underlying both RTDs and thermistors, ties the accuracy of resistance-based temperature measurement directly to the purity and stability of the sensing material.

Bolometric and Radiation Thermometry

Bolometers are radiation-absorbing detectors that infer temperature from the power of incident electromagnetic radiation rather than from direct contact with the object being measured. A bolometer absorbs incoming photons, heats up, and produces a measurable resistance change proportional to the absorbed power. This principle is used in infrared cameras, terahertz imaging systems, and radio astronomy receivers. Because bolometers require no physical contact with the target, they are indispensable for measuring surfaces in motion, extreme environments, or biological tissue without contamination.

Non-contact radiation thermometry more broadly relies on the Stefan-Boltzmann law: every object emits thermal radiation as a function of its temperature and emissivity. Infrared pyrometers and thermal cameras capture this radiation and convert it to a temperature reading. The NIST Industrial Thermometer Calibration Laboratory provides traceable calibration for radiation thermometers capable of measuring from -50°C to 150°C, supporting both laboratory and field applications.

Thermocouple Methods

Thermocouples generate a voltage at the junction of two dissimilar metals in proportion to the temperature difference between that junction and a reference point. Defined by the Seebeck effect, thermocouples cover the widest temperature range of any contact sensor: from below -200°C to above 2000°C for specialized types such as Type B (platinum-rhodium). The National Instruments guide to temperature sensor selection notes that while thermocouples sacrifice some accuracy compared to RTDs, their ruggedness, low cost, and extreme range make them dominant in industrial furnaces, turbine exhaust monitoring, and materials processing.

Applications

Temperature measurement has applications in a wide range of disciplines, including:

• Industrial process control in chemical plants, refineries, and power generation
• Clinical diagnostics and patient monitoring in medical settings
• Environmental and atmospheric monitoring for weather and climate science
• Food safety verification during processing, storage, and transport
• Cryogenic research and superconductor characterization at near-absolute-zero temperatures