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Case studies

Improving welding quality for submarine assembly

NPL's temperature team have developed a phosphor thermometry technique to indicate steel surface temperature, and performed a demonstration in a practical industrial setting.

Case study

The challenge

Submarines are made up of thousands of different steel parts which are welded together. The strength and quality of these steel welds are crucial for the operation of the submarine and the safety of its crew. It is well known that the weld quality depends critically on the temperature profile of the steel during the welding process. When the weld is not performed correctly, cold cracking can occur risking the integrity of the submarine.  

Until now, engineers have had to rely on thermocouple surface probes or temperature indicating crayons. For example, the probes might periodically be placed on the surface to determine if the temperature is within pre-defined limits, or a crayon drawn across the surface. However, this perturbs the temperature as the heat flows out of the surface into the probe, as well as the probe takes time to reach equilibrium. Both these factors lead to a large uncertainty for the temperature measurement and an impractical time taken to complete the welding process.

The solution

Phosphor thermometry! NPL has shown that a small amount of thermographic phosphor, painted on to the steel surface, can act as an rapid, or almost immediate, indicator of the steel temperature. This technique has been demonstrated in an industrial laboratory at BAE Systems, and shown to be practical and achieve an uncertainty of ±1 °C with only 1 second of measurement time.


The phosphor on the steel surface is excited with blue light and the emitted light is measured using a fibre optic system. This signal is related to temperature and calibrated to be traceable to the International Temperature Scale of 1990 (ITS-90). The fibre optic probe needs only to be brought close to the surface (within 10 mm) to determine the surface temperature of the steel, avoiding the major problem associated with the thermocouple surface probes. 

The impact

This new, repeatable and reliable approach will enable a reduction in weld time and therefore significant time saving in overall submarine assembly process.

The results of the measurements are also of a significantly lower uncertainty, since there is no perturbation of the surface temperature due to probe contact with the surface or user interpretation inherent in the procedure.

The technique can be used for monitoring the steel over a long duration, and the results can be automatically logged and saved for future reference.


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