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.