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NIST builds IR thermometer with 'dramatically improved' performance

16 May 2019

600mm-long device can remotely measure temperatures to a precision of thousandths of a degree Celsius.

Highly calibrated research-grade “non-contact” thermometers—which measure the infrared thermal radiation given off by objects without touching them—are critically important to many endeavors besides health care.

However, even high-end conventional radiation thermometers can produce readings with large uncertainties. But now researchers at the US National Institute of Standards and Technology have invented a portable, remarkably stable standards-quality radiation thermometer measuring just 600mm-long that can measure temperatures to a precision of within a few thousandths of a degree Celsius.

The development is described in a paper in Optics Express.

NIST has a long history of studying radiation thermometers. The new prototype instrument, which builds on that work, can measure temperatures between -50˚C to 150˚C. The corresponding infrared wavelengths are from 8 to 14µm, which NIST describes as “a sort of thermodynamic sweet spot”.

“All temperatures are equal, but some are more equal than others,” commented NIST physicist Howard Yoon, who created the thermometer design and directed the project. “That 200-degree span covers nearly all naturally occurring temperatures on Earth. If you make a big impact in measuring objects in that range, it really matters.”

Application areas

Besides clinical medicine, temperatures across this region are of particular importance in applications where contact is not appropriate or feasible. For example, surgeons need to measure the temperature of organs prior to transplant. Modern farmers need accurate temperatures when handling, storing and processing food. Satellites require non-contact thermometers for measuring temperatures on land and the surface of the sea.

The NIST design, called the Ambient-Radiation Thermometer (ART), is fitted with a suite of interior thermometers that constantly gauge temperatures at different points in the instrument. Those readings are sent to a feedback loop system, which keeps the 300mm cylinder containing the detector assembly at a constant temperature of 23˚C.

'Unprecedented stability'

The ART’s major advantage is its unprecedented stability. After it has been calibrated against standards-grade contact thermometers, the instrument can remain stable to within a few thousandths of a degree for months under continuous operation. That, says its developer, makes the system very promising for applications that involve remote sensing over long periods.

“Imagine being able to take the NIST design out in the field as traveling radiation thermometers for accurately measuring variables such as land- and sea-surface temperatures,” Yoon said. “It could serve as a trustworthy method of calibrating satellite IR sensors and validating the huge weather science programs that are used to predict, for example, the paths and strengths of hurricanes.” Its lower range of -50˚C makes it suitable for monitoring the temperature of ice over polar regions, typically in the range of -40˚C to -10˚C.

There are several methods of making very high-accuracy temperature measurements, but few are well-suited to field work. Platinum resistance thermometers are fragile and need frequent recalibration. The standard temperature source for transferring that calibration to the ART involves a heat-source cavity inside about 42 liters of liquid.

Gerald Fraser, chief of NIST’s Sensor Science Division, said that “Yoon’s innovation makes non-contact thermometry competitive with the best commercial contact thermometers in accuracy and stability in a temperature range that humans experience daily. This enables many new opportunities in product inspection and quality control and in defense and security where conventional contact methods are impractical or too expensive.”

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