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Almost all manufacturing relies on accurate temperature measurements, but for many, a reliable knowledge of the temperature is difficult to achieve

Temperature is one of the most measured physical quantities in industry. However, for many applications and conditions a reliable knowledge of the temperature is hard to obtain.
Above 750^o C both non-contact and contact thermometry techniques are prone to large errors, and above approximately 1200^o C conventional thermocouples develop ‘drift’ in the indicated temperature that progressively worsens over time, and which can cause errors of tens to hundreds of degrees Celsius.

This has implications during, for example, aerospace and automotive heat treatment processes, where the resulting erroneous temperature indication could cause inappropriate temperature control and sub-optimal processing. In the case of engines such as gas turbines, running at a lower than optimal temperature causes loss of efficiency. Similarly, harsh environments such as high levels of ionizing radiation of electromagnetic fields can also degrade or otherwise perturb the temperature indications. For this reason, operators must periodically recalibrate (or more commonly, replace) temperature sensors which is time-consuming and costly.

Surface temperature measurement is another challenging problem in industry; radiation thermometers (e.g. thermal imagers) rely on difficult-to-obtain knowledge of the surface characteristics, while contact probes suffer from heat flux effects and rarely achieve thermal equilibrium with the surface. In both cases errors can amount to tens or hundreds of degrees Celsius.

Sensors based on stimulated optical phenomena, such as thermographic phosphor thermometry, have the potential to overcome such issues. In this technique a probe is coated with a material that, when stimulated by light at particular wavelengths, exhibits luminescence. Various characteristics of the luminescence, such as intensity at a particular wavelength, or decay time, are a function of temperature, which can be exploited to use the phosphor as a thermometer.

However, such systems are either not commercially available, lack metrological validation or are unable to reliably assess temperatures in extreme environments. Thermographic phosphors can be used to make spot temperature measurements, or two-dimensional temperature mapping.

The EMPIR project Enhancing process efficiency through improved temperature measurement 2 (17IND04, EMPRESS 2) has substantially progressed the development of techniques to overcome the above issues, and demonstrate them in industrial settings.

Building on the work of the previous EMPIR project EMPRESS three phosphor thermometers were designed and built by project partners.

• At INRiM, the National Metrology Institute (NMI) of Italy, a luminescence decay thermometer was developed based on a phosphor with a wide dynamic range, a high temperature sensitivity, and a long luminescence decay time. Testing against thermocouple measurements at 600^o C indicated that, as expected, the thermocouples underestimated the temperature by several tens of degrees and phosphor thermometry offers much lower temperature measurement uncertainty of better than 3^o C.
  The phosphor was demonstrated in on-site measurements of surface temperature in the automotive brake testing rig at an industrial facility, demonstrating its potential improved performance in terms of uncertainty over conventional measurement techniques.

• The Advanced Forming Research Centre at the University of Strathclyde (UK) developed a phosphor/pyrometer setup which was trialled on a 500 tonne hydraulic press, on an induction heating system, and on an actual forging die to measure the temperature evolution of the die surface during forging. The system has shown it can return corroborating temperatures of various metal samples (aluminium, titanium, and steel) placed on temperature controlled hydraulic press plates up to 300^o C.
   
• At NPL, the NMI of the UK, a phosphor thermometer was developed which was then trialled for a range of industrial applications including photovoltaic panel development, tensile testing and underwater welding. It has also been deployed at a nuclear decommissioning site, demonstrating its utility to a wide range of problems in nuclear decommissioning and waste storage.
  The NPL thermometer underwent comparison with Strathclyde’s setup at the Advanced Forming Research Centre (AFRC) during heat treatment of engineering alloys up to 750^o C. This resulted in a phosphor thermometer that enables reliable surface temperature mapping up to 750^o C with uncertainty better than 3^o C, over most of the temperature range.

NPL now offer a phosphor thermometry service based upon this work.

Other instrumentation developed

The EMPRESS 2 project also developed a suite of other instrumentation techniques to compliment phosphor thermometry methods including:

1. The development of a portable Standard Flame at NPL which is already implementing SI traceability in European labs.
2. The development of improved thermocouples based on rhodium and platinum – trialled in industries such as quartz glass manufacturing.
3. A low-cost multispectral flame imager – trialled at an energy supplier and a waste incineration plant.
4. Fibre-optic thermometers (for harsh environments) which were trialled at a radiotherapy hospital, a photovoltaic panel manufacturer and an offshore windfarm.

The work has also contributed to a EURAMET calibration guide on thermocouples along with thermocouple inhomogeneity data that underpins recommendations given in the guide.

The development of the phosphor thermometry technique – and introducing its traceability to the International Temperature Scale 90 (ITS-90) - will allow industries in the manufacturing sector to more accurately monitor temperature of parts undergoing forming, forging, welding, or heat treatment - with a corresponding enhancement in the efficiency of the process.

The coordinator of this highly successful project, Jonathan Pearce (NPL), said about the work:

“A theme running through all the activities in this project was practical implementation of traceable thermometry techniques to address a specific set of industrial process control challenges. The members of the consortium worked well together, and it was very pleasing to see groups across Europe with complementary expertise work together to produce effective, practical thermometry solutions to improve process assurance.”

This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.

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