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Accurate temperature measurement is crucial in industry to reduce waste and environmental impact. Industrial use of Radiation Thermometers (RTs) is becoming increasingly common due to their wide market availability, fast response time, large temperature ranges, and their ability to measure temperature without contact. With this growth in use, accurate RT measurements that are traceable to the International Temperature Scale of 1990 (ITS-90) are a growing requirement. Traceable calibrations are usually performed using horizontal Blackbody Cavity Radiation Sources (BCRSs). In the work presented, a unique vertical bath-based BCRS, constructed in-house at the National Standards Authority of Ireland (NSAI), was compared over the range from -30 °C to 150 °C, against a suite of conventional horizontal bath-based BCRSs in overseas National Metrology Institutes (NMIs) and against a previous iteration of this new vertical design. Vertical bath-based BCRSs are more flexible and economical to use than horizontal BCRSs and can take advantage of existing calibration equipment.
In the comparison of BCRSs, it was found that the vertical orientation was comparable to within 0.25 °C of standard horizontal cavities for the range from -30 °C to 150 °C. It was concluded that the vertical configuration is an economical alternative for calibration of RTs within the range assessed. A conservative evaluation of the uncertainty of measurement found that it ranged from ±0.34 °C to ±0.66 °C (𝑘 = 2). Alongside this comparison, the calibration of direct-reading, handheld Infrared RTs (often simply referred to as IRTs) was investigated. These are lower-cost instruments that read directly in temperature and do not give access to the unprocessed detector signal. IRTs are the most common type of RT used in industry. IRTs are known to suffer from poor Size-of-Source Effects (SSEs), which introduce errors caused by scattered radiation from outside the IRT’s nominal target area. Variation in readings due to changes in proximity of the detector to the source – the Distance Effect (DE) – has also been found to cause significant errors in IRTs. In the present work, best practice calibration procedures and uncertainty budgets were investigated for IRTs using a case study instrument. The instrument was calibrated over the range from -30 °C to 900 °C, and its SSE and Distance Effect (DE) were measured. The test case IRT’s SSE was measured at three different temperatures to determine its true field of view. The IRT was also tested at five target distances and using a variety of radiation sources to calibrate it and determine its DE. The IRT was found to exceed its specification by 3.7 °C when measuring an 800 °C BCRS and was out of specification across most of the rest of its range. At 500 °C, the IRT reading was found to vary by 1.75 °C across a 500 mm range of distances. The IRT reading was also found to drift by up to 2 °C when kept exposed to a 500 °C source for an hour.
The case study of the IRT highlighted the importance of providing detailed calibration conditions, particularly regarding calibration geometry, IRT housing temperature, and exposure duration. As well as aiding in the establishment of a fit-for-purpose high-level non-contact calibration capability at NSAI, the work presented details a method by which other NMIs can inexpensively develop RT calibration facilities without custombuilt baths.
Boles, Sam. (2016) Development of Traceable Capabilities in Non-Contact Thermal Metrology. Masters thesis, 2016. doi:10.21427/D7B90D