Triplex thermocouples are a design of thermocouple that has three individual thermocouple sensors in one sheath. They can also be called tri-level thermocouples. Most commonly the three measuring junctions of the three thermocouples are at different points along the sheath and can be used to measure different levels of a process, e.g. Tri-level thermocouples for glass furnaces. Another reason is to have 3 measuring points at the same location for resilience to failure, but this is much rarer.