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Spectral Analysis Boosts Accuracy of Infrared Thermometers

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A novel infrared thermometer can measure surface temperatures with unprecedented accuracy. [Image: Getty Images]

Researchers at the University of Houston, USA, have devised a noncontact optical method that can measure the temperature of hot surfaces with an accuracy better than 1°C (Device, doi: 10.1016/j.device.2024.100467). They hope that the enhanced technique will yield a better understanding of the processes inside photothermal catalysts, which exploit laser heating to initiate the chemical reaction.

The emissivity hurdle

It is widely thought that photoexcited electrons play a key role in lowering the activation energy in these catalysts, but understanding and quantifying their contribution requires precise temperature measurements during the catalytic reaction. Infrared thermometers have become the most common choice because they offer the required speed and sensitivity, while their noncontact nature also prevents any interference with the light-enabled activation process.

However, these infrared probes determine the temperature by combining the intensity of thermal radiation with the emissivity of the material―the efficiency with which it emits heat relative to a perfect blackbody. The problem is that the emissivity depends on both the wavelength and the temperature of the material, which compromises the accuracy of the derived values. Measuring the infrared emission at multiple wavelengths can reduce the uncertainty but requires complex modeling of the material's emissivity to achieve reliable results.

Greater accuracy

These measurements revealed a dramatic temperature gradient inside the catalyst, reaching more than 320°C from the surface to the thermocouple at a laser power of 500 mW.

In this new approach, the researchers address those problems by using a near-infrared spectrometer to capture the full emission spectrum between 950 to 1600 nm. To calibrate the data, they first record the spectrum at a known temperature, in this case 400°C, and compare it with the theoretical blackbody curve to compute the variation in system response with wavelength. Once this has been used to normalize the experimental data, the temperature can be determined by finding the best fit between the measured spectrum and the blackbody formula.

The researchers tested their technique using a silver heating stage, showing that it can achieve an accuracy of better than 1°C at temperatures between 200 and 550°C. They then used the method to measure the surface temperature of a powder catalyst heated by a laser, with a thermocouple buried in the sample also taking a reading 100 µm below the surface. These measurements revealed a dramatic temperature gradient inside the catalyst, reaching more than 320°C from the surface to the thermocouple at a laser power of 500 mW— a finding is consistent with simulations of the heating process.

“This technique overcomes the challenges faced by conventional thermal cameras and infrared thermometers,” comments lead author Jiming Bao. “It can help understand non-thermal contributions in photothermal reactions and solve problems in fields where accurate measurements of high surface temperatures are required.”

Publish Date: 31 July 2024

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