As many of the techniques used for preheat and PWHT processes inherently involve heat transfer to the metal surface by radiation, conduction and forced convection, the temperature registered by a manufactured thermocouple device may be significantly affected by the heating medium. Accordingly serious errors can occur. A technique now generally accepted within the industry is the direct welding of the two thermocouple conductor wires independently on to the metal surface, a small distance apart.
The standard unit offered by Thermal Hire for the welding of the conductor wires weighs 4kg and can be recharged for up to 450 applications from the Heat Treatment Transformer Units. An alternative mini unit is also available weighing only 0.74kg, has an approximate capacity of 250 applications and operates at 6V from AA batteries. Both units are suitable for use with Type K thermocouples of diameter 0.711mm.