Design, construction and traceable calibration of a phosphor-based fibre-optic thermometer from 0 °C to 650 °C

Abstract

Measurement traceability, in which there is a documented chain of calibration links from a measurement back to a primary realisation of a unit, is vital for achieving the smallest uncertainty in metrology. A sensor that can provide a traceable link in harsh environments may be advantageous even if its uncertainty is larger than that of typical calibrated devices. A phosphor-based thermometer that has optical rather than electrical connections has been built and then calibrated in terms of the International Temperature Scale of 1990. Its standard uncertainty ranges from 0.5 °C at 0 °C to 2 °C at 650 °C. While this uncertainty cannot match what is possible with thermocouples or resistance thermometers, the optical basis of the sensor means it can be used in situations where electrical connections are precluded.

Electromagnetic immune phosphor-tipped fibre-optic thermometers

“…This phosphor has a 2 E → 4 A 2 energy states transition [11] with additional Stokes/anti-Stokes behaviour that results in a number of emission lines in the red part of the spectrum [12]. As well as a temperature dependence to the decay time, the relative intensity of the spectral emission peaks of this phosphor vary with temperature [13]. DTI measured the decay time of the broadband emission, while NPL measured relative changes of different emission lines.…”

“…The NPL design is described in detail in [13]. In brief, it uses a single 2 m multimode 400 µm core gold-coated fibre rated for use up to 750 • C. Figure 4 shows a schematic of the phosphor increasingly happens through thermal (phonon) processes rather than optical (photon) processes, however the sensitivity (∆R/∆T) increases as the absolute signals dropthe curve for ratio in figure 5 turns up progressively at higher temperatures.…”

“…One dry-block, with an NPL calibrated PT100 as reference, covers the range −45 • C to 100 • C. Another, with an NPL calibrated type-R thermocouple as reference, covers the range 50 • C-700 • C. A Thorlabs CCS-100 spectrometer allows phosphor emission spectra to be captured. Calibration of the NPL ratio thermometer in terms of ITS-90 has been described in detail elsewhere [13], including a complete uncertainty budget. In brief, its uncertainty ranges from about 1.5 • C at room temperature up to 4 • C at 650 • C at a confidence level of approximately 95% (that is, with a coverage factor k = 2).…”

“…Figure 5. Channel signal levels (normalised to 10 7 V A −1 ) and ratio of signals as a function of temperature for the NPL system.Reproduced from[13]. © IOP Publishing Ltd. All rights reserved.…”

A summary of new developments in phosphor thermometry

“…Finally, two studies focused on assessing the accuracy of established phosphor-based temperature sensors. The paper from Lowe et al [13] contributes to the increasing efforts of the community to establish traceable calibration links for thermometry purposes. They introduce a phosphor-based thermometer with optical rather than electrical connections, which has been calibrated in terms of following the International Temperature Scale ITS-90.…”