Proper thermocouple placement is crucial for proper high-temperature repairs using portable heating systems. When heat blankets are used, multiple thermocouples are required. In general, the more thermocouples the better. This is because thermocouple failures are common and replacement during a running cure is virtually impossible. Also, temperatures can easily vary widely across a repair area during cure: 60–80ºF (35–45ºC) spreads are not uncommon. Most composite parts contain variances in thickness and/or temperature conductivity of the materials which will create this temperature variation during cure. There may also be heat sinks in the part or underlying tooling and non-uniformity in the heat blankets being used.
Do not place thermocouples directly over the repair area, due to possible thermocouple markoff. The thermocouples may “print” through the bagging schedule and even thin metal top caul plates, leaving an undesirable groove through the repair area. Even more important – thermocouples MUST be placed well inside the edge of any heating blanket. These blankets are cooler within about 5cm (2”)of the edge, and are quite cool very near the edge, within about 2cm (1”). Thermocouples placed here will naturally read cooler than they will near the middle of the heating blanket. Since the heating equipment is programmed to go to a desired temperature, it will apply too much power to the blanket in an attempt to being up these cool thermocouples to the desired temperature, and thereby overheat the repair area itself. This can burn the repair area, and even start a fire in some instances.
Published courtesy of Abaris Training Resources, Inc