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Thermal Imaging Accuracy for Furnace Applications

During discussions on thermal imaging of large furnaces/fired heaters, I get a lot of questions about the accuracy of thermal imaging cameras when measuring tube metal temperatures (TMTs). The answer is not quite as simple as some would like.

NIR-B-640-EX - James Cross BlogThankfully, AMETEK Land, is focused both on achieving the highest accuracy possible from infrared technology, but also communicating to customers the technical reality in complex applications. For example, with our NIR-B-640-EX solution, we state an accuracy of +/- 1% of the Celsius reading (e.g., +/- 8°C on a +/- 800°C surface), but with a repeatability of 1°C (which is probably more important to customers, so that they can measure and compare changes over time with a high-level confidence). 

Some customers will ask for a TMT accuracy of +/- 1°C. You can get within 1°C in a laboratory on a blackbody furnace: see the image below, where the imager is reading 820.6°C when the blackbody furnace set to 820°C, as measured by a UKAS calibrated pyrometer.

A furnace in production has a number a factors that create challenges for absolute accuracy, so we have to accept that our readings may not be within 1°C of tube’s true temperature:

  • Tubes are graybodies where emissivity can change over time
  • Some tubes have shallow viewing angles, or a limited number of pixels are visible to the camera
  • Tubes absorbs radiation from multiple surfaces of varying temperatures, but for simplicity and practicality we use a single value

NIR-B-640-EX Thermal Image & TableIn comparison, thermocouples state higher accuracies, but an unknown % of their reading will be from hot furnace gases (convection), so they are not providing true TMTs. They are also providing a single measurement point versus a the 300,000+ measurement points an imager provides.

Our solution is to be practical and focus on the camera’s value in providing relative temperature distribution, and in identifying and quantifying changes over time - which is why the tight repeatability is important. We adjust the cameras during commissioning with the Gold Cup reference pyrometer to provide the accuracy component. We can set the correct emissivity factor and background compensation settings and then validate those settings periodically.  

Learn more about our NIR-B-640-EX Thermal Imager
Learn more about our Gold Cup Reference Pyrometer

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